Question

I don't understand the stages of the cell cycle
interphase g1, s, g2, pro phase meta phase, an a phase telophase

Answer 1

      CELL CYCLE can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next. Cell cycle consist of interphase and mitotic phase. Howard and Pele (1953) have divided cell cycle into 4 stages- G1, S, G2 and M phase. G1,S,G2 combine to form interphase.

INTERPHASE- Though the interphase is called is called " resting phase ", but in fact it is period of great activity.

General events of interphase: 1. The nuclear envelope remains intact The chromosomes occurs in the form of diffused, long, coiled and indistinctly visible chromatin fibres.

2. The DNA amount becomes double. Replication of DNA along with the synthesis of nuclear proteins such histones takes place.

3. Due to accumulation of rRNA and ribosomal proteins in the nucleolus, the size of the nucleolus is greatly increased.

4. In animal cells, duplication of a centriole takes place by the outgrowth of daughter centrioles from the parent centrioles, which are at right angle to each other. Thus, an interphase cell has two pairs of centrioles.

5. Synthesis of energy rich compounds takes place, which provide energy for mitosis, and synthesis of proteins also takes place at the end of the interphase.

DIPLOID Nucleus - DAUGHTER Ceaus F. INTER PHASE

1. G1 phase- The first phase G1 is also called first gap phase, since no DNA synthesis takes place during this stage; currently G1 phase is called first growth phase, since it involves synthesis of RNA, proteins and membranes which lead to the growth of nucleus and cytoplasm of each daughter cell towards their mature size.

                   During G1 phase, chromatin is fully extended and not distinguishable as discrete chromosomes with the light microscope. This is the time of resumption of normal cell metabolism which has slowed down during the previous cell division. Thus, G1 involves transcription of three types of RNAs, namely rRNA, tRNA and mRNA; rRNA synthesis is indicated by the appearance of nucleolus in the interphase nucleus. Protein synthesized during G1 phase (1) regulatory protein which control various events of mitosis; (2) enzymes necessary for DNA synthesis of the next stage; and (3) tubulin and other mitotic apparatus proteins.

                   G1 phase is most variable as to duration; it occupies 30 to 50% of the total time of the cell cycle or lacks entirely in rapidly dividing cells.

2. S phase- During S phase or the synthetic phase of interphase, replication of DNA and synthesis of histone protein occur. New histones are required in massive amounts immediately at the beginning of the S period of DNA synthesis to provide the new DNA with nucleolus. Thus, at the end of the S phase, each chromosome has two DNA molecules and a duplicate set of genes. S phase occupies roughly 35 to 45 % of cell cycle.

3. G2 phase- This is the second gap or the growth phase or resting phase of the interphase. During G2 phase, synthesis of RNA and protein continues which is required for cell growth. It may occupy 10 to 20% of the cell cycle. As the G2 phase draws to a close, the cell enters the M phase.

M PHASE or THE MITOTIC PHASE- The mitosis occurs in the somatic cells and is meant for the multiplication of cell number during embryogenesis and blastogenesis of plants and animals. Mitosis starts at the culmination point of the interphase i.e G2 phase. It is short period of chromosome condensation, segregation and cytoplasmic division. Mitosis is important for the replacement of cells lost natural friction, wear and tear for wound healing.

                   Mitosis is called equational division. Te mitotic cycle is divided into 4 phases; prophase, metaphase, anaphase and telophase.

1. PROPHASE- Prophase is the first stage of mitosis. In prophase

i. chromosomes condense and becomes visible.

ii. spindle fibres emerge from the chromosomes

iii. nuclear envelopes break down

iv. nucleolus disappears (late prophase)

CYTO PLASM -CELL WALL NUCLEAR MEMBRANE CHROMATID - NUCLEO Lus Coor coord Occcaccoco CHROMOSOME -DIPLOID NELLLEUS CENTROMERE A. PROPHASE B. LATE PROPHASE

2. METAPHASE-. In metaphase

i. mitotic spindle is fully developed, centrosomes are at opposite poles of the cell

ii. chromosomes are lined at the metaphase plate.

iii. each sister chromatids is attached to the spindle fibre originating from opposite poles.

-- SPINDLE FIBRES Tillid C. METAPHASE

3. ANAPHASE- In anaphase

i. cohesion proteins binding the sister chromatids together break down

ii. sister chromatids are pulled towards opposite poles.

iii. non - kinetochore spindle fibres lengthens, elongating the cell.

\ון, / 7 והי, 7 ון ), 77 גן רוץ A אית///// א D. ANAPHASE

4. TELOPHASE- In telophase

i. The end of the polar migration of the daughter chromosomes marks the beginning of the telophase; which in turn is terminated by the reorganisation of two new nuclei and their entry into the G1 phase.

ii. The events of prophase occur in reverse sequence during this phase.

iii. A nuclear envelope ressembles around each group of chromosomes to form two daughter nuclei.

iv.The mitotic apparatus except the centrioles dissappear; high viscocity of the cytoplasm decreases; the chromosomes resume their long, slender, extended form as their coils relax; and RNA- synthesis restarts causing the nucleolus to reappear.

- CELL PLATE E. TELOPHASE