The situation here is also similar to
mitosis at initial stages.
Points to remember:
- The number of
chromosome = count the number of functional centromere
- The 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 =4
- No. 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 =8
- No. 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 =8
- No. 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
|
||
Metaphase
|
8
|
4
|
Anaphase
|
8
|
8
|
After cytokinesis of Mitosis
|
4
|
4
|
In Meiotic Stages
|
||
Metaphase I
|
8
|
4
|
Metaphase II
|
4
|
2
|
After cytokinesis of Meiosis II
|
2
|
2
|
Characteristic in a human cell
|
Mitosis
|
Meiosis I
|
Meiosis II
|
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
|
|
