When the sister chromatids are joined in the centromere, why is it stated that the number of chromosomes is 46 and not 72?

When the sister chromatids are joined in the centromere, why is it stated that the number of chromosomes is 46 and not 72?

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Before the DNA is replicated in a human somatic cell, the cell has 46 chromosomes. Also, after the sister chromatids are separated during Anaphase, the chromosome number in the cell doubles to 72, so when the sister chromatids are joined, why isn't the chromosome number also 72? On the internet, some sources say that one chromosome in a cell that hasn't replicated its DNA is equal to one chromatid. Is this correct, or are chromosomes and chromatids structurally different?

When I was learning genetics for the first time I have also found naming the two chromatids joined at centromere as chromosome a little bit strange. The number of DNA molecules and their behaviour in cell cycle seemed to me more important for understanding of genetics.

What you need to the term chromosome predates knowledge of DNA structure. The name literally means colored (chromo) body (soma). It is an object visible under the microscope when using specific dye. In metaphase two chromatids joined at centromere look like a single object - a chromosome. Sometimes the two chromatids of one chromosome are so close together it is difficult to tell them apart. In anaphase the chromatids separate and one object (two-chromatid chromosome) visibly splits to two objects (single-chromatid chromosomes of daughter cells)

The cell cycle, mitosis and meiosis

Living cells go through a series of stages known as the cell cycle. The cells grow, copy their chromosomes, and then divide to form new cells.

G1 phase. The cell grows.

S phase. The cell makes copies of its chromosomes. Each chromosome now consists of two sister chromatids.

G2 phase. The cell checks the duplicated chromosomes and gets ready to divide.

M phase. The cell separates the copied chromosomes to form two full sets (mitosis) and the cell divides into two new cells (cytokinesis).

The period between cell divisions is known as 'interphase'.

Cells that are not dividing leave the cell cycle and stay in G0.

When the sister chromatids are joined in the centromere, why is it stated that the number of chromosomes is 46 and not 72? - Biology

93 notecards = 24 pages ( 4 cards per page)

Campbell Biology, 11th Ed chapters 11, 12

Cytokinesis often, but not always, accompanies _____.

Chromosomes become visible during _____.

Centromeres divide and sister chromatids become full-fledged chromosomes during _____.

Spindle fibers attach to kinetochores during _____.

This animation illustrates the events of _____.

This animation illustrates the events of _____.

A) cytokinesis as it occurs in animal cells

D)cytokinesis as it occurs in plant cells

This animation illustrates the events of _____.

This animation illustrates the events of _____.

E)telophase I and cytokinesis

This animation illustrates the events of _____.

A) prophase I
B) telophase II and cytokinesis
C) anaphase II
D) prophase II
E) telophase I and cytokinesis

Gametes are produced by _____.
A) meiosis
B) fertilization
C) mitosis
D) the cell cycle
E) asexual reproductionMeiosis

A diploid organism whose somatic (nonsex) cells each contain 32 chromosomes produces gametes containing _____ chromosomes.
A) 16
B) 8
C) 64
D) 30

Meiosis I produces _____ cells, each of which is _____.

A) four . diploid
B) two. diploid
C) two. identical to the other
D) four . haploid
E) two . haploid

Meiosis II typically produces _____ cells, each of which is _____.

A) four . diploid
B) two. diploid
C) two. identical to the other
D) four . haploid
E) two . haploid

During prophase, a homologous pair of chromosomes consists of _____.

A) ONE chromosome and two chromatids

B)ONE chromosome and four chromatids

C)TWO chromosomes and two chromatids

D)TWO chromosomes and four chromatids

E)four chromosomes and two chromatids

This animation illustrates the events of _____.

This animation illustrates the events of _____.

D)cytokinesis as it occurs in plant cells

This animation illustrates the events of _____.

Which of these phases encompasses all of the stages of mitosis?

Which of these cells is (are) haploid?

Which of the following is true of kinetochores?

Which of the following is true of kinetochores?

A)They interdigitate at the cell's equator and then move apart, causing the cell to elongate.

B)They are the primary centromere structures that maintain the attachment of the sister chromatids prior to mitosis.

C)They are sites at which microtubules attach to chromosomes.

D)they attach to the ring of actin along the cytoplasmic surface of the plasma membrane, causing the actin to contract to form the cleavage furrow

E) They are located at the center of the centrosome their function is to organize tubulin into elongated bundles called spindle fibers.

In some organisms, such as certain fungi and algae, cells undergo the cell cycle repeatedly without subsequently undergoing cytokinesis. What would result from this?

A)a decrease in chromosome number

B)large cells containing many nuclei

C)division of the organism into many cells, most lacking nuclei

D)a rapid rate of gamete production

E)inability to duplicate DNA

22) Cells will usually divide if they receive the proper signal at a checkpoint in which phase of the cell cycle?

230Which of the following is true of benign tumors, but not malignant tumors?

A)They migrate from the initial site of transformation to other organs or tissues.

B)They remain confined to their original site

C)They are the result of the transformation of normal cells.

D)They can divide indefinitely if an adequate supply of nutrients is available.

E)They have an unusual number of chromosomes.

24)Through a microscope, you can see a cell plate beginning to develop across the middle of a cell and nuclei forming on either side of the cell plate. This cell is most likely

A)a plant cell in the process of cytokinesis.

B)an animal cell in the process of cytokinesis.

C)a plant cell in metaphase.

D)a bacterial cell dividing.

E)an animal cell in the S phase of the cell cycle.

25)In the cells of some organisms, mitosis occurs without cytokinesis. This will result in

A)destruction of chromosomes.

B)cell cycles lacking an S phase.

D)cells that are unusually small.

E)cells with more than one

26)Which of the following does not occur during mitosis?

A)condensation of the chromosomes

B)separation of sister chromatids

C)separation of the spindle poles

27)The drug cytochalasin B blocks the function of actin. Which of the following aspects of the animal cell cycle would be most disrupted by cytochalasin B?

B)cleavage furrow formation and cytokinesis

C)spindle attachment to kinetochores

D)cell elongation during anaphase

28) Human gametes are produced by _____

29)Normal human gametes carry _____ chromosomes.

30)Of the following, a receptor protein in a membrane that recognizes a chemical signal is most similar to

A) RNA specifying the amino acids in a polypeptide.
B) genes making up a chromosome.
C) a particular metabolic pathway operating within a specific organelle.
D) the active site of an allosteric enzyme in the cytoplasm that binds to a specific substrate.
E) an enzyme with an optimum pH and temperature for activity.

31)During which stage of mitosis do spindle microtubules first attach to kinetochores?
A) metaphase
B) prophase
C) prometaphase
D) anaphase
E) telophase

32)Which of the following best describes how chromosomes move toward the poles of the spindle during mitosis?
A) Motor proteins of the kinetochores move the chromosomes along the spindle microtubules.
B) The chromosomes are "reeled in" by the contraction of spindle microtubules, and motor proteins of the kinetochores move the chromosomes along the spindle microtubules, and non-kinetochore spindle fibers serve to push chromosomes in the direction of the poles.
C) Non-kinetochore spindle fibers serve to push chromosomes in the direction of the poles.
D) The chromosomes are "reeled in" by the contraction of spindle microtubules, and motor proteins of the kinetochores move the chromosomes along the spindle microtubules.
E) The chromosomes are "reeled in" by the contraction of spindle microtubules.

33)A diploid organism whose somatic (nonsex) cells each contain 32 chromosomes produces gametes containing _____ chromosomes.

34)What number and types of chromosomes are found in a human somatic cell?

A)45 autosomes and 1 sex chromosome

B)44 autosomes and 2 sex chromosomes

C)21 autosomes and 2 sex chromosomes

D)22 autosomes and 1 sex chromosome

35)In alternation of generations, what is the diploid stage of a plant that follows fertilization called?

36)How are sister chromatids and homologous chromosomes different from each other?

A)They are not different. Homologous chromosomes and sister chromatids are both identical copies of each other.

B)Sister chromatids are only formed during mitosis. Homologous chromosomes are formed during meiosis

C)Homologous chromosomes are identical copies of each other. One sister chromatid comes from the father, and one comes from the mother.

D)Homologous chromosomes contain the same gene loci but may have different alleles of a particular gene. Sister chromatids are identical copies of each other produced during DNA replication.

E)Homologous chromosomes are closely associated with each other in both mitosis and meiosis. Sister chromatids are only associated with each other during mitosis.

37)During _____ sister chromatids separate.

38)During _____ a spindle forms in a haploid cell.
A) anaphase I
B) prophase II
C) metaphase I
D) metaphase II
E) telophase II and cytokinesis

39)During ____ the chromosomes finish their journey and two haploid daughter cells are produced, each chromosome still consists of two sister chromatids.

A) anaphase I
B) metaphase I
C) prophase I
D) telophase I and cytokinesis
E) metaphase II

40)At the end of _____ and cytokinesis, haploid cells contain chromosomes that each consist of two sister chromatids.
A) telophase
B) telophase I
C) metaphase II
D) telophase II
E) interphase

41)At the end of _____ and cytokinesis, haploid cells contain chromosomes that each consist of two sister chromatids.
A)telophase II
B)metaphase II
C)telophase I

42)Synapsis occurs during _____.
A)metaphase II
B)anaphase II
C)prophase I
D)prophase II
E)telophase I and cytogenesis

43)During _____ chromosomes align single file along the equator of a haploid cell.
A)metaphase II
B)anaphase I
C)telophase I and cytokinesis
D)metaphase I
E)prophase I

44)Homologous chromosomes migrate to opposite poles during _____.
A)metaphase II
B)anaphase I
C)metaphase I
D)telophase II and cytokinesis
E)prophase II

45)Mitosis results in the formation of how many cells meiosis results in the formation of how many cells?

A)two diploid cells . two diploid cells

B)four diploid cells . four haploid cells

C)two diploid cells . four haploid cells

D)four haploid cells . two diploid cells

E)two diploid cells . two haploid cells

46)Which of the following occurs during meiosis but not during mitosis?

B)A spindle apparatus forms.

C)Chromosomes align at the metaphase plate

E)Chromosomes migrate to opposite poles

47)Which of these gametes contains one or more recombinant chromosomes?

48)A human cell containing 22 autosomes and a Y chromosome is
A) a sperm.
B) an egg.
C) a zygote.
D) a somatic cell of a male.
E) a somatic cell of a female.

49)Homologous chromosomes move toward opposite poles of a dividing cell during
A) mitosis
B) meiosis I
C) meiosis II
D) fertilization
E) binary fission

50)Meiosis II is similar to mitosis in that
A) sister chromatids separate during anaphase
B) DNA replicated before the division
C) the daughter cells are diploid
D) homologous chromosomes synapse
E) the chromosome number is reduced.

51)Look at the cell in the figure. Based on this figure, which of the following statements is true?

C)It is impossible to tell whether the cell is haploid or diploid.

52)Which life cycle stage is found in plants but not animals?

53)Why do some species employ both mitosis and meiosis, whereas other species use only mitosis?

A)A single-celled organism only needs mitosis.

B)They need meiosis if the cells are producing organs such as ovaries

C)They need only meiosis if they produce egg cells.

D)They need only mitosis to make large numbers of cells such as sperm.

E)They need both if they are producing animal gametes.

54) Nucleoli are present during _____.

55)During _____ both the contents of the nucleus and the cytoplasm are divided.

56)During _____ the cell grows and replicates both its organelles and its chromosomes.

57)Which of the following correctly matches a phase of the cell cycle with its description?

B)G1: follows cell division

D)S: immediately precedes cell division

E)All of the above are correctly matched.

58)One difference between cancer cells and normal cells is that cancer cells

A)cannot function properly because they are affected by density-dependent

B)are arrested at the S phase of the cell cycle.

C)are always in the M phase of the cell cycle.

D)are unable to synthesize DNA.

E)continue to divide even when they are tightly packed together.

59)A particular cell has half as much DNA as some other cells in a mitotically active tissue. The cell in question is most likely in

60)The drug cytochalasin B blocks the function of actin. Which of the following aspects of the animal cell cycle would be most disrupted by cytochalasin B?

C)cleavage furrow formation and cytokinesis

D)spindle attachment to kinetochores

E)cell elongation during anaphase

61)Asexual reproduction _____.

A)produces offspring genetically identical to the parent

B)requires both meiosis and mitosis

D)is limited to single-cell organisms

E)leads to a loss of genetic material

62)For what purpose(s) might a karyotype be prepared?

A)for prenatal screening, to determine if a fetus has the correct number of chromosomes

B)to determine whether a fetus is male or female

C)to detect the possible presence of chromosomal abnormalities such as deletions, inversions, or translocations

D)The first and second answers are correct

E)The first three answers are correct.

63)In alternation of generations, what is the diploid stage of a plant that follows fertilization called?

A) gametophyte
B) chiasmata
C) sporophyte
D) karyotype
E) spore

64)This chromosome has two chromatids, joined at the centromere. What process led to the formation of the two chromatids?

A)The two chromatids were formed by synapsis and the formation of a synaptonemal complex.

B)The two chromatids were formed by fertilization, bringing together maternal and paternal chromatids.

C)The two chromatids were formed by duplication of a chromosome.

65)Two sister chromatids are joined at the centromere prior to meiosis. Which statement is correct

A)These chromatids make up a diploid chromosome.

B) The cell that contains these sister chromatids must be haploid.

C)Barring mutation, the two sister chromatids must be identical

What is the best evidence telling you whether this cell is diploid or haploid?

A)The cell is haploid because the chromosomes are not found in pairs.

B)The cell is diploid because it contains two sets of chromosomes.

C)The cell is diploid because each chromosome consists of two chromatids

67)Identify all possible products of meiosis in plant and animal life cycles.

C)Multicellular adult organisma

81)What is crossing over?
A)-a direct consequence of the separation of sister chromatids
B)-also referred to as the "independent assortment of chromosomes"
C)-the movement of genetic material from one chromosome to a nonhomologous chromosome
D)-making an RNA copy of a DNA strand
E)-the exchange of homologous portions of nonsister chromatids

87)Which of the following defines a genome?
A)-the complete set of an organism's polypeptides
B)-the complete set of an organism's genes
C)-a karyotype
D)-the complete set of a species' polypeptides
E)-representation of a complete set of a cell's polypeptides

88)Which of the following is true of a species that has a chromosome number of 2n = 16?
A)-During the S phase of the cell cycle there will be 32 separate chromosomes.
B)-A gamete from this species has four chromosomes.
C)-The species is diploid with 32 chromosomes per cell.
D)-The species has 16 sets of chromosomes per cell.
E)-Each cell has eight homologous pairs.

89)Assume that an organism exists in which crossing over does not occur, but that all other processes associated with meiosis occur normally. Consider how the absence of crossing over would affect the outcome of meiosis.

If crossing over did not occur, which of the following statements about meiosis would be true? Select all that apply.

A)The two sister chromatids of each replicated chromosome would no longer be identical.

B)Independent assortment of chromosomes would not occur.

C)There would be less genetic variation among gametes.

D)The two daughter cells produced in meiosis I would be identical.

90)The shuffling of chromosomes that occurs during both fertilization and _____ can lead to genetic variation.

91)Heritable variation is required for which of the following?

C)the production of a clone
E)asexual reproduction

92)Homologous chromosomes move toward opposite poles of a dividing cell during
A)meiosis I.
C)meiosis II.
E)binary fission.

93)Meiosis II is similar to mitosis in that
A)the chromosome number is reduced.
B)the daughter cells are diploid.
C)homologous chromosomes synapse.
D)sister chromatids separate during anaphase.
E)DNA replicates before the division.

94)We can see that the chromosomes are duplicated and lined up by homologous pair.

95)Nucleotides can be radiolabeled before they are incorporated into newly forming DNA and, therefore, can be assayed to track their incorporation. In a set of experiments, a studentfaculty research team used labeled T nucleotides and introduced these into the culture of dividing human cells at specific times.

The research team used their experiments to study the incorporation of labeled nucleotides into a culture of lymphocytes and found that the lymphocytes incorporated the labeled nucleotide at a significantly higher level after a pathogen was introduced into the culture. They concluded that _____.

A)infection causes cell cultures in general to reproduce more rapidly

B)infection causes lymphocytes to divide more rapidly

C)infection causes lymphocyte cultures to skip some parts of the cell cycle

D)the presence of the pathogen made the experiments too contaminated to trust the results

Campbell Biology: Ninth Edition - Chapter 12: The Cell Cycle Flashcards

Chapter 12
Cell Division / Mitosis
Vocabulary: gene, cell division, chromosomes, somatic cells, gametes, chromatin, sister chromatids, centromere, mitosis, cytokinesis, meiosis, mitotic phase, interphase, centrosome, aster, kinetochore, cleavage furrow, cell plate, mitotic spindle, binary fission, transformation, benign tumor, malignant tumor, metastasis
After attending lectures and studying the chapter, the student should be able to:
1. Define gene as it relates to the genetic material in a cell.
2. Describe the composition of the genetic material in bacteria, in archaea, and in eukaryotic cells.
3. State the location of the genetic material in prokaryotic and eukaryotic cells.
4. Distinguish between the structure of the genetic material as chromatin and as
5. Distinguish between the function of the genetic material as chromatin and as
6. Relating to eukaryotic cells:
a. Describe the centromere region in the genetic material.
b. State the role of cohesins in duplicated genetic material.
c. Describe the sister chromatids of a duplicated chromosome.
d. State the role of the kinetochores on the chromatids at the centromere of a duplicated
e. Describe spindle fibers and state their role in the separation of chromosomes during eukaryotic cell division.
f. Describe the role of centrosomes in the formation of the spindle apparatus.
g. Distinguish between a gene and an allele.
h. Describe homologous chromosomes.
i. Distinguish between an individual's genome and karyotype.
j. State the number of chromosomes in human haploid cells and in human diploid cells.
k. State which cells in humans are haploid, which cells are diploid, and which cells are neither.
7. State the two major parts of the cell cycle.
8. Describe the differences of growth characteristics between a cancerous (transformed) cell and a normal cell.
8. Relating to the prokaryotic cell cycle:
a. State the number of chromosomes in a prokaryotic cell.
b. State the cellular activities that occur during interphase.
c. Show the process of binary fission that is prokaryotic cell division.
9. Relating to the eukaryotic cell cycle:
a. Distinguish between interphase and cell division.
b. Distinguish between the G1, S, and G2 phases of interphase.
c. Define karyokinesis and cytokinesis.
d. State the two types of karyokinesis.
e. Distinguish between the M and C phases of cell division.
f. State when in the cell cycle duplication of the genetic material occurs.
10. Relating to cell division involving mitosis (mitosis + cytokinesis):
a. Define mitosis.
b. Explain why mitosis is sometimes considered "duplication division".
c. State what 1 human diploid cell becomes after mitosis plus cytokinesis.
d. State the reason humans undergo cell division involving mitosis.
e. State which cells in humans undergo cell division involving mitosis.
f. Be able to describe, draw, and recognize the 4 stages of mitosis.
g. Describe the cleavage-furrow process of cytokinesis in animal cells.
h. Describe the cell-plate process of cytokinesis in plant cells.

Cell division: mitosis and meiosis

Cell division cycle, figure from Wikipedia. Cells that stop dividing exit the G1 phase of the cell cycle into a so-called G0 state.

Cells reproduce genetically identical copies of themselves by cycles of cell growth and division. The cell cycle diagram on the left shows that a cell division cycle consists of 4 stages:

  • G1 is the period after cell division, and before the start of DNA replication. Cells grow and monitor their environment to determine whether they should initiate another round of cell division.
  • S is the period of DNA synthesis, where cells replicate their chromosomes.
  • G2 is the period between the end of DNA replication and the start of cell division. Cells check to make sure DNA replication has successfully completed, and make any necessary repairs.
  • M is the actual period of cell division, consisting of prophase, metaphase, anaphase, telophase, and cytokinesis.


Chromosomes were first named by cytologists viewing dividing cells through a microscope. The modern definition of a chromosome now includes the function of heredity and the chemical composition. A chromosome is a DNA molecule that carries all or part of the hereditary information of an organism. In eukaryotic cells, the DNA is packaged with proteins in the nucleus, and varies in structure and appearance at different parts of the cell cycle.
Chromosomes condense and become visible by light microscopy as eukaryotic cells enter mitosis or meiosis. During interphase (G1 + S + G2), chromosomes are fully or partially decondensed, in the form of chromatin, which consists of DNA wound around histone proteins (nucleosomes).

In G1, each chromosome is a single chromatid. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister. The sister chromatids are joined at their centromeres, as shown in the image below. A pair of sister chromatids is a single replicated chromosome, a single package of hereditary information.

Human karyotype “painted” using fluorescent DNA probes. These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs (e.g., 2 copies of chromosome 1) have been lined up next to each other. Image from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157

Humans are diploid, meaning we have two copies of each chromosome. We inherited one copy of each chromosome from other mother, and one copy of each from our father. Gametes (sperm cells or egg cells) are haploid, meaning that they have just one complete set of chromosomes.
Chromosomes that do not differ between males and females are called autosomes, and the chromosomes that differ between males and females are the sex chromosomes, X and Y for most mammals. Humans most commonly have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY), for a total of 46 chromosomes. We say that humans have 2N = 46 chromosomes, where N = 23, or the haploid number of chromosomes.
Cells with complete sets of chromosomes are called euploid cells with missing or extra chromosomes are called aneuploid. The most common aneuploid condition in people is variation in the number of sex chromosomes: XO (having just one copy of the X), XXX, or XYY. Having no X chromosome results in early embryonic death.
The two copies of a particular chromosome, such as chromosome 1, are called homologous. The karyotype image above shows the homologous pairs for all the autosomes. Homologous chromosomes are not identical to each other, unlike sister chromatids. They frequently have different variants of the same hereditary information – such as blue eye color vs brown eye color, or blood type A versus blood type B.
Mitosis produces two daughter cells that are genetically identical to each other, and to the parental cell. A diploid cell starts with 2N chromosomes and 2X DNA content. After DNA replication, the cells is still genetically diploid (2N chromosome number), but has 4X DNA content because each chromosome has replicated its DNA. Each chromosome now consists of a joined pair of identical sister chromatids. During mitosis the sister chromatids separate and go to opposite ends of the dividing cell. Mitosis ends with 2 identical cells, each with 2N chromosomes and 2X DNA content. All eukaryotic cells replicate via mitosis, except germline cells that undergo meiosis (see below) to produce gametes (eggs and sperm).

  • prophase – chromosomes condense each chromosome consists of a pair of identical sister chromatids joined at the centromere.
  • metaphase – chromosomes line up at the middle of the cell, along the plane of cell division, pushed and pulled by microtubules of the spindle apparatus
  • anaphase – sister chromatids separate and migrate towards opposite ends of the cell
  • telophase – chromatids cluster at opposite ends of the cell and begin to decondense
  • cytokinesis – the membrane pinches in to divide the two daughter cells

Here is a simplified diagram illustrating the overall process and products of mitosis:

Source: Wikimedia Commons (

Questions or points to ponder or note about the figure above (answers at bottom of page):

  1. are the two daughter cells the same or different from each other, and from the parent cell at the start?
  2. why does the cartoon illustration of the chromosomes change (from a single rod to joined double rods) after DNA replication, and again (back to single rods) during mitosis?
  3. does the figure show 2 different chromosomes or a single pair of homologous chromosomes?
  4. can haploid cells undergo mitosis? what about triploid cells (cells that have 3N chromosomes)?

This animation below shows the packaging of DNA and condensation of chromosomes as a cell undergoes mitosis.

The video narration has a major error at time 1:22: chromosomes exist throughout the entire cell cycle (at all times in a cell’s life) they are visible in their condensed form only during mitosis and meiosis.


This is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells. It starts with a diploid cell that has undergone chromosomal DNA replication: 2N chromosomes, 4X DNA content. Two successive divisions, with no additional DNA replication, results in 4 haploid gametes: 1N chromosomes, 1X DNA content.
NOVA has a good interactive side-by-side comparison of mitosis and meiosis on this page: How cells divide
Meiosis sets the stage for Mendelian genetics. Students need to know that most of the genetics action occurs in the first meiotic division:

  • homologous chromosomes pair up and align end-to-end (synapsis) in prophase I
  • crossing over occurs between homologous chromosomes in prophase I, before chromosomes line up at the metaphase plate
  • homologous chromosomes separate to daughter cells (sister chromatids do not separate) in the first division, creating haploid (1N) cells
  • the separation of each pair of homologous chromosomes occurs independently, so all possible combinations of maternal and paternal chromosomes are possible in the two daughter cells – this is the basis of Mendel’s Law of Independent Assortment
  • the first division is when daughter cells become functionally or genetically haploid

The last point appears to be the most difficult for students to grasp. Consider the X and Y chromosomes. They pair in prophase I, and then separate in the first division. The daughter cells of the first meiotic division have either an X or a Y they don’t have both. Each cell now has only one sex chromosome, like a haploid cell.
One way of thinking about ploidy is the number of possible alleles for each gene a cell can have. Right after meiosis I, the homologous chromosomes have separated into different cells. Each homolog carries one copy of the gene, and each gene could be a different allele, but these two homologs are now in two different cells. Though it looks like there are two of each chromosome in each cell, these are duplicated chromosomes ie, it is one chromosome which has been copied, so there is only one possible allele in the cell (just two copies of it).
The second meiotic division is where sister (duplicated) chromatids separate. It resembles mitosis of a haploid cell. At the start of the second division, each cell contains 1N chromosomes, each consisting of a pair of sister chromatids joined at the centromere.
Here is a simplified diagram illustrating the overall process and products of meiosis:

Meiosis Overview from Wikipedia by Rdbickel

And here is a video that walks through the steps of meiosis:

It is very important that you recognize how and why cells become haploid after meiosis I.
To confirm for yourself that you understand meiosis, work through one or more of these interactive tutorials:

  • The U. Arizona Cell Biology Project’s Meiosis tutorial has a click-through animation of meiosis, with 10 thought-provoking problem questions.
  • Jung Choi’s interactive flash tutorial, programmed by Pearson, uses human chromosome 7, with wild type and cystic fibrosis alleles for CFTR, to track segregation through meiosis, with and without crossing over: Meiotic Segregation tutorial

Chromosomes, chromatids, what is the difference and how many chromosomes are there at different times of the cell cycle and after mitosis and meiosis?

Chromosomes by definition contain the DNA that makes up the fundamental genome of the cell. In a prokaryote, the genome is usually packaged into one circular chromosome consisting of a circular DNA molecule of a few million base pairs (Mbp). In eukaryotes, the genome is packaged into multiple linear chromosomes, each consisting of a linear DNA molecule of tens or hundreds of Mbp. Chromosomes exist at all different phases of the cell cycle. They condense and become visible to light microscopy in prophase of mitosis or meiosis, and they decondense during interphase, in the form of chromatin (DNA wrapped around nucleosomes, like “beads on a string”).
The chromosome number, N, in eukaryotes, refers to the number of chromosomes in a haploid cell, or gamete (sperm or egg cell). Diploid cells (all the cells in our body except our gametes) have 2N chromosomes, because a diploid organism is created by union of 2 gametes each containing 1N chromosomes. In terms of chromosome number (ploidy), it’s useful to think of chromosomes as packages of genetic information. A pair of sister chromatids is one chromosome because it has genetic information (alleles) inherited from only one parent. A pair of homologous chromosomes, each consisting of a single chromatid in a daughter cell at the end of mitosis, has alleles from the father and from the mother, and counts as 2 chromosomes.
This chromosome number stays the same after chromosome replication during S phase: each chromosome entering cell division now consists of a pair of sister chromatids joined together at the centromere. Then in mitosis, the sister chromatids of each chromosome separate, so each daughter cell receives one chromatid from each chromosome. The result of mitosis is two identical daughter cells, genetically identical to the original cell, all having 2N chromosomes. So during a mitotic cell cycle, the DNA content per chromosome doubles during S phase (each chromosome starts as one chromatid, then becomes a pair of identical sister chromatids during S phase), but the chromosome number stays the same.
A chromatid, then, is a single chromosomal DNA molecule. The number of chromatids changes from 2X in G1 to 4X in G2 and back to 2X, but the number of chromosomes stays the same.
The chromosome number is reduced from 2N to 1N in the first meiotic division, and stays at 1N in the second meiotic division. Because homologous chromosomes separate in the first division, the daughter cells no longer have copies of each chromosome from both parents, so they have haploid genetic information, and a 1N chromosome number. The second meiotic division, where sister chromatids separate, is like mitosis. Chromosome number stays the same when sister chromatids separate.
Using the information above, compare these two simplified diagrams of mitosis and meiosis to visualize why cells are haploid after meiosis I. Specifically, compare the chromosomes in cells at the end of mitosis vs the end of meiosis I, recognizing that the diagram of mitosis tracks just a single pair of homologous chromosomes, whereas the diagram of meiosis tracks two pairs of homologous chromosomes (one long chromosome and short chromosome):

Meiosis Overview from Wikipedia by Rdbickel

The video below is geared toward a high school audience, but it does present a helpful way for recognizing how many chromosomes are present in a cell (and thus the ploidy level of that cell). While watching, see if you can recognize why the products of meiosis 1 are haploid cells:

Search of Genetic Material

Transforming Principle – It was an early name for DNA. In the year 1928, scientists were not aware that DNA carried genetic information but they were aware that there was something that causes bacteria to transform from one form to another.

Frederick Griffith in the year 1928 carried out an experiment on pneumococcus bacteria. These bacteria were of two types, that is. smooth type(S) and rough type (R) . When Streptococcus pneumoniae bacteria were grown on a culture plate, some produced rough colonies while some produced smooth shiny colonies. This is so because the S strain bacteria consist of polysaccharide (mucous) coat, while R was not having any coat. When a mouse was infected with S strain, they died due to pneumonia infection but mice infected with R strain do not developed any infection (Please refer the image below) . Thus,

S Strain → Inject into mice → Mice die

R Strain → Inject into mice → Mice live

Further, Griffith killed bacteria by heating them. He found that when heat – killed S Strain bacteria was injected, mice was alive (Please refer the image below) . Further, after injecting a mixture of heat – killed S and live R bacteria, the mice died. Added to this, he was able to recover living S bacteria from the dead mice. Thus,

S Strain (Heat – killed) → Inject into mice → Mice live

S Strain (Heat + killed) + R Strain (live) → Inject into mice → Mice die

Thus, from the above experiment, Griffith reached to the conclusion that –

“R Strain bacteria had somehow transformed by heat – killed S strain bacteria. This was guided by some “transforming principle.” However, till this experiment, the biochemical nature of genetic material was not defined.”

Following diagram pictorially represents the entire experiment of Griffith. The above stated four reactions display the various conditions of mice in different stages .

R Strain → Inject into mice → Mice live

S Strain → Inject into mice → Mice die

S Strain (Heat – killed) → Inject into mice → Mice live

S Strain (Heat + killed) + R Strain (live) → Inject into mice → Mice die

Biochemical Characterization of Transforming Principle

Initially, protein was considered as genetic material. In the year 1933 – 44, Oswald Avery, Colin Macleod and Maclyn McCarty worked on the objective of determining the biochemical nature of “transforming principle.” So, they purified biochemicals like DNA, RNA, proteins, etc. from S cells in order to know which one can transform live R cells to S cells. They found –

  • DNA from S bacteria is enough for transforming R bacteria.
  • Proteases (protein digesting enzymes) and RNases (RNA digesting enzymes) have no effect on transformation. Thus, transforming substance was not RNA or protein.

Thus, they concluded that DNA was the only genetic material.

The Genetic Material is DNA (The Hershey – Chase experiments)

Alfred Hershey and Martha Chase in 1952 studied bacteriophage that proved that “DNA is the genetic material.” Bacteriophages are the viruses that can infect bacteria. These bacteriophages attaches to bacteria and its genetic material, thereby entering the bacterial cell. These cells treat viral genetic material and manufacture more virus particles. Hershey and Chase wanted to know whether it was DNA or protein from viruses that entered bacteria. Therefore, they grew some viruses in a special medium with radioactive phosphorus and some in the medium with radioactive sulfur.

Those radioactive phages were then allowed to associate to E.Coli bacteria and as the infection increased, the viral coats were eliminated from bacteria agitating them in blender. Thereby, separation of virus particles was done by spinning them in a centrifuge.

Those, bacteria which were infected by virus with radioactive DNA were radioactive, that helped in concluding that DNA is the material that was transferred from virus to bacteria. Added to this, the bacteria infected by radioactive protein were not radioactive. Thereby, this concluded that protein in not transferred from virus to bacteria.

Following figure shows all the steps of the Hershey – Chase experiments, which include infection at first, followed by blending and centrifugation. This experiment helped in concluding that DNA is the genetic material while protein in not the material that passed.

RNA World

RNA was the first genetic material. There are several evidences that suggest that essential life processes are evolved around RNA. RNA used to act as catalyst and genetic material. But RNA being a catalyst was reactive and therefore, unstable. This resulted in the evolution of DNA which is more stable. DNA is double stranded and resists change by evolving the process of repair.

What is DNA Replication?

“DNA replication is the process by which DNA makes a copy of itself during cell division.”

  • At the first step, DNA ‘unzips’ its double helical structure. This is carried out by an enzyme called helicase. This helps in breaking of hydrogen bonds that hold commentary bases of DNA.
  • The separation of two strands creates ‘Y’ shape called a replication fork. These two separated strands act as a template for making the new strands of DNA.
  • One of strand is oriented in 3’ to 5’ direction which is referred as leading strand, while other strand is oriented in 5’ to 3’ direction referred as lagging strand . Due to this different orientation, the two strands replicated differently.

Following figure shows the replication of leading and lagging strands of DNA as discussed.


Microscopy can be used to visualize condensed chromosomes as they move through meiosis and mitosis. [4]

Various DNA stains are used to treat cells such that condensing chromosomes can be visualized as the move through prophase. [4]

The giemsa G-banding technique is commonly used to identify mammalian chromosomes, but utilizing the technology on plant cells was difficult due to the high degree of chromosome compaction in plant cells. [5] [4] G-banding was fully realized for plant chromosomes in 1990. [6] During both meiotic and mitotic prophase, giemsa staining can be applied to cells to elicit G-banding in chromosomes. [2] Silver staining, a more modern technology, in conjunction with giesma staining can be used to image the synaptonemal complex throughout the various stages of meiotic prophase. [7] To perform G-banding, chromosomes must be fixed, and thus it is not possible to perform on living cells. [8]

Fluorescent stains such as DAPI can be used in both live plant and animal cells. These stains do not band chromosomes, but instead allow for DNA probing of specific regions and genes. Use of fluorescent microscopy has vastly improved spatial resolution. [9]

Prophase is the first stage of mitosis in animal cells, and the second stage of mitosis in plant cells. [10] At the start of prophase there are two identical copies of each chromosome in the cell due to replication in interphase. These copies are referred to as sister chromatids and are attached by DNA element called the centromere. [11] The main events of prophase are: the condensation of chromosomes, the movement of the centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli break down. [3]

Condensation of chromosomes Edit

DNA that was replicated in interphase is condensed from DNA strands with lengths reaching 0.7 μm down to 0.2-0.3 μm. [3] This process employs the condensin complex. [11] Condensed chromosomes consist of two sister chromatids joined at the centromere. [12]

Movement of centrosomes Edit

During prophase in animal cells, centrosomes move far enough apart to be resolved using a light microscope. [3] Microtubule activity in each centrosome is increased due to recruitment of γ-tubulin. Replicated centrosomes from interphase move apart towards opposite poles of the cell, powered by centrosome associated motor proteins. [13] Interdigitated interpolar microtubules from each centrosome interact with each other, helping to move the centrosomes to opposite poles. [13] [3]

Formation of the mitotic spindle Edit

Microtubules involved in the interphase scaffolding break down as the replicated centrosomes separate. [3] The movement of centrosomes to opposite poles is accompanied in animal cells by the organization of individual radial microtubule arrays (asters) by each centromere. [13] Interpolar microtubules from both centrosomes interact, joining the sets of microtubules and forming the basic structure of the mitotic spindle. [13] Planet cells do not have centrosomes and the chromosomes can nucleate microtubule assembly into the mitotic apparatus. [13] In plant cells, microtubules gather at opposite poles and begin to form the spindle apparatus at locations called foci. [10] The mitotic spindle is of great importance in the process of mitosis and will eventually segregate the sister chromatids in metaphase. [3]

Beginning of nucleoli breakdown Edit

The nucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production. [3] This indicates a redirection of cellular energy from general cellular metabolism to cellular division. [3] The nuclear envelope stays intact during this process. [10]

Meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II. [12] Prophase I is the most complex phase in all of meiosis because homologous chromosomes must pair and exchange genetic information. [3] : 98 Prophase II is very similar to mitotic prophase. [12]

Prophase I Edit

Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in mitotic prophase, several crucial events occur within these phases such as pairing of homologous chromosomes and the reciprocal exchange of genetic material between these homologous chromosomes. Prophase I occurs at different speeds dependent on species and sex. Many species arrest meiosis in diplotene of prophase I until ovulation. [3] : 98 In humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to ovulation. [12]

Leptotene Edit

In the first stage of prophase I, leptotene (from the Greek for "delicate"), chromosomes begin to condense. Each chromosome is in a haploid state and consists of two sister chromatids however, the chromatin of the sister chromatids is not yet condensed enough to be resolvable in microscopy. [3] : 98 Homologous regions within homologous chromosome pairs begin to associate with each other. [2]

Zygotene Edit

In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived chromosomes have found their homologous partner. [3] : 98 The homologous pairs then undergo synapsis, a process by which the synaptonemal complex (a proteinaceous structure) aligns corresponding regions of genetic information on maternally and paternally derived non-sister chromatids of homologous chromosome pairs. [3] : 98 [12] The paired homologous chromosome bound by the synaptonemal complex are referred to as bivalents or tetrads. [10] [3] : 98 Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are homologous. [3] : 98

The nucleolus moves from a central to a peripheral position in the nucleus. [14]

Pachytene Edit

The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis. [3] : 98 Chromatin has condensed enough that chromosomes can now be resolved in microscopy. [10] Structures called recombination nodules form on the synaptonemal complex of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the synaptonemal complex in an event known as crossing-over or genetic recombination. [3] : 98 Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome. [13] : 681

Diplotene Edit

In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), crossing-over is completed. [3] : 99 [10] Homologous chromosomes retain a full set of genetic information however, the homologous chromosomes are now of mixed maternal and paternal descent. [3] : 99 Visible junctions called chiasmata hold the homologous chromosomes together at locations where recombination occurred as the synaptonemal complex dissolves. [12] [3] : 99 It is at this stage where meiotic arrest occurs in many species. [3] : 99

Diakinesis Edit

In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four sister chromatids can be seen in bivalents with microscopy. The rest of the phase resemble the early stages of mitotic prometaphase, as the meiotic prophase ends with the spindle apparatus beginning to form, and the nuclear membrane beginning to break down. [10] [3] : 99

Prophase II Edit

Prophase II of meiosis is very similar to prophase of mitosis. The most noticeable difference is that prophase II occurs with a haploid number of chromosomes as opposed to the diploid number in mitotic prophase. [12] [10] In both animal and plant cells chromosomes may de-condense during telophase I requiring them to re-condense in prophase II. [3] : 100 [10] If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the model organism Arabidopsis. [10]

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis. [15] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefor present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The adaptive significance of prophase I arrest is still not fully understood. However, it has been proposed that the arrest of ooctyes at the four genome copy stage may provide the informational redundancy needed to repair damage in the DNA of the germline. [15] The repair process used appears to be homologous recombinational repair [15] [16] Prophase arrested oocytes have a high capability for efficient repair of DNA damages. [16] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility. [16]

The most notable difference between prophase in plant cells and animal cells occurs because plant cells lack centrioles. The organization of the spindle apparatus is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference is preprophase, an additional step in plant mitosis that results in formation of the preprophase band, a structure composed of microtubules. In mitotic prophase I of plants, this band disappears. [10]

Prophase I in meiosis is the most complex iteration of prophase that occurs in both plant cells and animal cells. [3] To ensure pairing of homologous chromosomes and recombination of genetic material occurs properly, there are cellular checkpoints in place. The meiotic checkpoint network is a DNA damage response system that controls double strand break repair, chromatin structure, and the movement and pairing of chromosomes. [17] The system consists of multiple pathways (including the meiotic recombination checkpoint) that prevent the cell from entering metaphase I with errors due to recombination. [18]

Chapter Six - Chromatin Reorganization Through Mitosis

Chromosome condensation is one of the major chromatin-remodeling events that occur during cell division. The changes in chromatin compaction and higher-order structure organization are essential requisites for ensuring a faithful transmission of the replicated genome to daughter cells. Although the observation of mitotic chromosome condensation has fascinated Scientists for a century, we are still far away from understanding how the process works from a molecular point of view.

In this chapter, I will analyze our current understanding of chromatin condensation during mitosis with particular attention to the major molecular players that trigger and maintain this particular chromatin conformation. However, within the chromosome, not all regions of the chromatin are organized in the same manner. I will address separately the structure and functions of particular chromatin domains such as the centromere. Finally, the transition of the chromatin through mitosis represents just an interlude for gene expression between two cell cycles. How the transcriptional information that governs cell linage identity is transmitted from mother to daughter represents a big and interesting question. I will present how cells take care of the aspect ensuring that mitotic chromosome condensation and the block of transcription does not wipe out the cell identity.

Watch the video: ΓΙΑ ΤΗΝ ΑΔΕΡΦΟΥΛΑ ΜΟΥ ΠΟΥ ΛΑΤΡΕΥΩ!!!!! (November 2022).