What happens to the number of DNA molecules and chromosomes during meiosis?

The situation here is also similar to mitosis at initial stages.

Points to remember:

The number of chromosome = count the number of functional centromereThe number of DNA molecule = count the number of chromatids.

When the DNA molecule number change?

The number of DNA molecules increase only when DNA replicates that is only in S phase or synthetic phase.The number of DNA molecules decreases only when the cell divides.

Now when the chromosome number change?

The number of chromosomes increases only when sister chromatids separated during anaphase of mitosis and anaphase II of meiosis. Think, in anaphase I of meiosis, only homologous chromosomes are separated. Only in anaphase II sister chromatids are separated.

- As in the case of DNA molecule, reduction in chromosome number takes place only by cell division.

Re read these 4 points. Imagine and make it yours. Now you can answer any question regarding this. Now let us work out the question with the above points in mind.

At G1 phase:

No. of DNA molecule =4No. of chromosome=4 and each chromosome has a single chromatid

At G2 phase:

In S phase DNA replicates.

No. of DNA molecule in G2 will be =8No. of chromosome=4 and as two DNA molecules are held at a common centromere.Or in other words each chromosome has two chromatids.

During Meiosis, there is no change in no. of DNA molecule and chromosomes in prophase and metaphase.

At metaphase:

No. of DNA molecule =8No. of chromosome=4

During Anaphase I: homologous chromosomes separate but the no.of chromosome remains 8.

After cytokinesis at the end of meiosis I: the resulting 4 chromosomes are distributed between two cells. Now each cell has 2 chromosomes with 4 chromatids

No. of DNA molecule =4 No. of chromosome=2

No change in number occurs till metaphase II

Meiosis I

Meiosis II

At metaphase II:

No. of DNA molecule =4 No. of chromosome=2

During Anaphase II:

sister chromatids separate and the

No. of chromosome increases temporarily to 4 No. of DNA molecules remain 4.

After cytokinesis at the end of meiosis II: the resulting 4 chromosomes are distributed between two cells. Now each cell has 2 chromosomes with 2 chromatids.

No. of DNA molecule =2 No of chromosomes per cell is 2.

Now the four cells formed have half the number of chromosome to that of the parent cell, and that is the result of meiosis.

In the figure initial chromosome number is 4 (2n=4). Then the changes in no. of DNA molecules and chromosomes during mitosis and meiosis is summarised in the table.

Correlate this with figure

Cell cycle Stages Number of DNA molecules /cell Number of Chromosomes/cell

G1 4 4

G2 8 4

In Mitotic stages

Metafase 8 4

Anafase 8 8

After cytokinesis of mitosis 8 4

In Meotiotic stages

Metafase I 8 4

Metafase II 4 2

After cytokinesis of Meiosis II 2 2

Here is the table showing the summary of variation in no. of chromosomes and DNA molecule (chromatid) in human cell during mitosis and meiosis.

Characteristic in a human cell

Mitosis Meiosis I

Chromosome number in a parent cell before cell division begins

46 46

Chromatid number in a parent cell before cell division begins

92 92

Crossing over at prophase

No Yes No

Chromosome arrangement on metaphase plate

Chromosome line up Homologues pair Chromosome line up

No. of chromosomes in each daughter nucleus

46 23

No. of chromatids in each daughter nucleus

46 46 23

No. of daughter cells at the end of division

2 2 4

Chromosome notation for daughter cells

2n n

Genome notation for daughter cells

Diploid Haploid

Purpose of division Cell replacement, asexual reproduction,

Organism growth

Sexual reproduction

Genetics of daughter cells

Genetically identical Genetically variable

Types of cells where cell division occurs

Somatic cells Reproductive cells(testes, anthers, ovaries)

Types of cells produced

Somatic cells Gametes, sperms, eggs, pollen