cell cycle and cell division

Question 1

explain the cell cycle?

Answer 1

consist of
G1–growth
S– DNA syenthesis
G2– growth and preparation of mitosis
M– mitosis (cell division

Question 2

explain mitosis.

Answer 2

occurs in somatic cells.
period between two mitotic division= interphase

1) Prophase–nucleolus disappears.
Chromosomes becomes coiled as they condense, shorten and thicken.
Each chormosomes now consist of two parallel subunits called chromatids, that remain joined to each other by centromere.
But the chromatids cannot be recognised.

2) Prometaphase– Chromosomes become recognisable.
Centrioles move to opposite poles.

3) Metaphase– Nuclear membrane dissappears.
The chromosomes lines up at the equatorial plane of the spindle and get attached to the microtubules of the spindle extending between two centrioles, one at each pole.

4) Anaphase– Centromeres of each chromosomes splits and the two chromosomes seperated from each other.
Now called daughter chromosomes.
Spindle fibres attached to the centromere of the chromosome contracts, pulling the daughter
chromosomes towards poles.
Due to pull on centromeres, daughter chromosome becomes V-shaped with their arms trailing as they move towards the pole.

Telophase– seperated chromatids migrated to the opposite poles of the spindle.
Spindle fibres dissappear and nuclear membrane appears around each polar group of daughter chromosomes.
Chromosomes uncoil and become less compact.
Nucleolus reappears.
Appearance of cleavage furrow beneath the equator that deepens and seperates the 2 daughter cells (cytokinesis)

Question 3

explain meiosis.

Answer 3

First Meiotic Division
1) Prophase– divided into 5 stages.
Leptotene: chromosomes appear as slender threads
not visible at this stage

Zygotene: lengthwise pairing of homologous chromosomes begin.
One each is from paternal and maternal.
This event is called synapsis
Each synapsis is called bivalent.

Pachytene: chromatids of each chromosome become visible seperately
Each bivalent chromosome appears to have four chromatids called tetrahed.
Each chromatid pair is united by a kinetochore.
There are two central chromatids and 2 peripheral chromatids (one from each chromosome)
The two central chromatids (one belonging to each chromosome) of tetrahed coil over each other so that they cross over a number of points (crossing over).
Due to crossing over, central chromatids present a cross-like configuration called chiasmata.

Diplotene: paired homologue of tetrahed starts seperating.
During this process, central chromatids break at the point of crossing over and unite to the opposite chromatid.
Causes exchange of genetic material between these chromatids.

Diakinesis: chromosomes become more contracted and migrate towards nuclear membrane.
At the end of prophase, nuclear membrane disappears.

Metaphase– homologous pair of chromosomes become arranged on the equatorial plane of spindle.

Anaphase– Homologous chromosomes migrate to opposite pole of spindle.
Shorter chromosomes move earlier than longer chromosomes.

Telophase–Nuclear membrane is formed around the polarized group of chromosomes
Cell membrane constricts and two daugter cells are formed (cytokinesis).
Each daughter cells thus forming only half he number of chromosomes (haploid) with exchanged genetic material

Second meiotic division
essentially similar to mitosis but the only difference is that DNA does not duplicate.
By this time, two daughter cells of first meiotic division form 4 daughter cells, each with haploid number of chromosomes.