Cell Cycle and Cell Division Notes for NEET, Download PDF!
Important notes Cell Cycle and Cell Division
In this article, we are discussing Cell Cycle and Cell division for the NEET 2020 examination. This is an important section to pay attention from the Unit Cell Structure and Function from which questions are asked every year. Let's begin with a brief intro of Cell Cycle first.
Cell Cycle and Cell Division Notes
A cell cycle is a sequence of events that occurs during cell growth and division.
The cell cycle is divided into two phases-
- Interphase, and
- M Phase or Mitosis Phase.
INTERPHASE- Phase between two successive cell division
- G1 PHASE (First gap phase) - lasts about 1/3rd time of the complete cell cycle
- Grows in mass and prepares itself for replication
- Synthesizes proteins and mRNA that are essential for synthesizing DNA.
- S PHASE (Synthesis phase)
- This is the period during which DNA replication takes place along with duplication of the centriole.
- The DNA synthesis takes place in a short span of time.
- G2 PHASE (Pre-mitotic gap phase/ Second gap phase)
- Cell continues to increase in size and prepare itself for Division (for mitosis or the first stage of mitosis i.e., prophase)
- Organelles like mitochondria, chloroplast duplicate.
If any cell in G1 phase does not divide, it leaves the G1 phase and does not enter the synthesis phase instead goes into G0 phase i.e., a dormant stage.
Cell division may be direct or indirect.
1. Direct division- Amitosis
- Rapid process.
- Nucleus first constricts in middle and then divides. The cytoplasm divides thereafter. The nuclear membrane remains throughout the division.
- Irregular division of chromatin in the daughter cells.
- Does not occurs in mammalian species.
2. Indirect division- Mitosis and Meiosis.
- Mitosis occurs in somatic & immature germ cells.
- The daughter cells formed have equal no. of chromosomes and gene distribution is exactly as that in the mother cell.
- Mitosis has 2 phases: Karyokinesis- Division of the nucleus; Cytokinesis- Division of cytoplasm
- PROPHASE (takes about 1.5hr to complete)
- Chromatin fibres shorten and thicken and form chromosomes.
- Chromosomes undergo coiling and dehydration and keep on becoming thick and short.
- Nuclear membrane and nucleolus start to disintegrate in late prophase.
- The 2 centrioles begin to move away from each other.
- METAPHASE (20mins)
- The chromosomes (condensed and thick) arrange along the equatorial plate.
- 2. Colourless, bipolar spindle fibres made up of 97% tubulin proteins and 3% RNA are formed that attach to the kinetochores.
- Kinetic spindles composed of interpolar and proteinaceous chromosomal fibres.
- Spindle fibres radiate as aster in animals from centriole. In plants, these asters are absent.
- Cell division arrested at metaphase by colchicine that prevents the formation of spindle microtubules
- Centromeres split longitudinally thus separating the two chromatids and forming two new chromosomes.
- The spindle fibres contract and the 2 newly formed chromosomes (chromatid) separate and move to the opposite poles of the spindle.
- The formation of chromosomes by separation of chromatids takes place by microtubule contraction.
- The MT of centrioles on both sides move inward and form midbody in the centre of the cell.
- Each separated chromatid is known as daughter chromosome
- The chromosomes reach on opposite poles of spindle fibre and group into 2.
- Chromosomes begin to uncoil and the new daughter chromosomes are enveloped by the nuclear membrane.
- Nucleolus reappears.
- The daughter chromosomes uncoil and undergo hydration thus forming chromatin network.
- The cytoplasm divides and 2 new cells are thus formed.
- The spindle fibers disappear.
- Meiosis occurs in sex cells (gamete formation)
- Also known as reductional division.
- A diploid parent cell produces four haploid daughter cells.
- Consists of 2 divisions, Meiosis I and II, occurring one after the other.
- Karyokinesis and cytokinesis occur here as well.
1. Meiosis I
- Karyokinesis I involves division of nucleus by the passage of the cell through prophase I, metaphase I, anaphase I and telophase I.
- Cytokinesis I may be or may not be present.
- PROPHASE I
The longest phase of meiosis 1. Divided into 5 stages
- Leptotene: Chromosomes appear as long threads with one end of each attached to the nuclear membrane. Lampbrush chromosomes are seen. Chromatin fibres shorten and thicken and form chromosomes.
- Zygotene: Homologous chromosomes arranged in pair lengthwise. One member is maternal and the other is of paternal origin in the homolog. Paired chromosomes are k/bivalents.
- Pachytene: Crossing over occurs in this stage. Tetrad formation occurs. The 2 similar chromatid of the same chromosome is k/a sister chromatids whereas those of homologous chromosomes are k/w non-sister chromatid.
- Diplotene: Chiasma formation occurs wherein the non-sister chromatids cross each other at one or more points. Chromosomes are attached only at the chiasmata. Desynapsis starts to occur wherein the paired chromosomes begin to separate. (Genetic information is exchanged)
- Diakinesis: Chiasmata moves towards the end of chromosomes. Homologous centromeres pull apart and the chromosomes re-condense. Nucleolus and nuclear membrane disappear. Spindle formation occurs. Tetrad shifts to metaphase plate.
Prophase I ends with diakinesis.
2. METAPHASE I
3. ANAPHASE I
4. TELOPHASE I
2. MEIOSIS II- equating division/ homotypic division. Meiosis II is similar to mitosis.
From meiosis I each daughter cell enters Metaphase II of meiosis II. There is no DNA replication between the 2 meiotic divisions.
- Karyokinesis II- the 2 chromatids of each chromosome separate and move to form separate cells. Karyokinesis II is almost similar to mitosis.
- Cytokinesis II- occurs always. In the plant, it occurs by cell plate formation and in animals by cell furrow formation.
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