Cell cycle & Cell division

Good morning boys.
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Today we will achieve the following Learning Outcomes:-
Students will be able to -

1. Define Cell division, cell cycle, chromosome, spindle fibres.

2. List & Enumerate various  stages involved in cell cycle.

3. Differentiate between the stages involved in MITOSIS.

4. Differentiate between the stages involved in  MEIOSIS.


GENERAL INSTRUCTIONS :-

1. Read the notes carefully.

2. There is one YouTube video link shared with this blog. Kindly watch it first before reading the notes on the blog.

3. Ask your doubts.




LET'S BEGIN...

FIRST WATCH THIS VIDEO LINK BY CLICKING ON IT :


https://youtu.be/HyJ86mS2Nao

MEIOSIS

Meiosis I

• Prophase I
• Prophase I has been further subdivided into

1. Leptotene,
2. Zygotene,

• Pachytene,

1. Diplotene and
2. Diakinesis

• In leptotene stage, the chromosomes condense and become visible.
• In zygotene, chromosomes start pairing together to form homologous chromosomes by the process of association called synapsis, which is accompanied by the formation of a complex structure called synaptonemal complex and the complex formed by a pair of synapsed homologous chromosomes called a bivalent or a tetrad.
• In pachytene, exchange of genetic material between two homologous chromosomes known as crossing over takes place between non-sister chromatids of the homologous chromosomes.
• Diplotene stage is characterized by the dissolution of the synaptonemal complex, and X-shaped structures called chiasmata form by the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers.
• In diakinesis, the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation and by the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down.

Fig. Prophase I 

• Metaphase I
• The bivalent chromosomes align on the equatorial plate.
• The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

• Anaphase I
• The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Fig. Anaphase I

• Telophase I
• The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as diad of cells.
• The stage between the two meiotic divisions is called interkinesis, which is followed by prophase II.

Fig. Telophase I 

Meiosis II

• Prophase II
• In contrast to meiosis I, meiosis II resembles a normal mitosis.
• The nuclear membrane disappears by the end of prophase II and the chromosomes again become compact.
• Metaphase II
• The chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
• Anaphase II
• The centromere of each chromosome splits, allowing them to move toward opposite poles of the cell.
• Telophase II
• The two groups of chromosomes get enclosed by a nuclear envelope.
• Cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells.

Fig. Meiosis II

THAT'S ALL FOR TODAY'S CLASS .

ASK YOUR DOUBTS??