Chapter No.7 : Cell Cycle - Botany And Zoology- For HSC Part 2 (XII) - Science Group

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Chapter No.7
CELL CYCLE

Botany And Zoology

For HSC Part 2 (XII) Science Group

Q.5: Define Meiosis. Explain the various stages of Prophase -I of meiosis.
Ans: MEIOSIS:
Meiosis is a cell division in which one cell divides into four cells and the number of the chromosomes is reduced to half. Meiosis occurs in diploid cells during gametogenesis. Meiosis consists of two successive divisions, Division I and Division II.
Division I is reduction division, during which the chromosomes number (2n) in both daughter cells is reduced to half (n).
Division II is simple mitotic in nature.
Each division passes through four phases, as:

1^st Meiotic Division

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I

2^nd Meiotic Division

1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II

PROPHASE I:
Prophase of first meiotic division is longer in duration and complicated. It passes through five sub-stages.

1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis

Leptotene = (Slender):

• This is the first stage of meiosis.
• The chromatin material condenses to from thread like structures called chromosomes.
• Before meiosis starts, the chromosomes become duplicated which is DNA replication, but the two chromosomes are so close that they look like a thread.
• The identical pairs, Maternal and Paternal chromosomes remain unevenly distributed.
• The centrioles move to the opposite poles.

Zygotene = (Joining):

• The chromosomes shorten and thicken.
• The homologous (maternal and paternal) chromosomes attracts each other and start pairing, the process is called SYNAPSIS.
• The pairing is very exact and is point to point and gene to gene.
• The two synapped chromosomes are called BIVALENTS.

Pachytene = (Thick):

• After pairing, the chromosomes become thick and short.
• The homologous chromosomes twine around each other and each starts splitting into two sister chromatids by a longitudinal splitting except in the centromere region.
• Due to this splitting, the bivalent becomes tetravalent (with four chromatids).

Diplotene = (Double):

• The synaptic force of attraction between each bivalent consisting of four chromatids decrease.
• The homologous chromosomes start repelling each other but mostly uncoiled i.e. overlapping areas remain contact in one or more points.
• This point of contact is called chiasma (Plural: Chiasmata).
• The overlapping occurs at non-sister chromatids.
• At chiasmata the chromatids breaks. This breakage is accompanied by reunion in such a way that corresponding fragments are interchanged between the two non-homologous chromosomes, this physical change is called crossing over.
• Crossing over is significant in heredity.

Diakinesis = (Through movement):

• Diakinesis is characterized by the disappearing of the nuclear membrane, nucleolus and completion of spindle apparatus.
• The separation of bivalent is completed by the process called terminalization.
• The movement of the chiasmata from the centromere towards the ends of the chromosome arms take place like a zipper.
• At the end of stage, the two chromatids held together at their ends and the bivalent become more thickened, contracted and prominently visible.