Introduction, Phase, Mechanism and Significance

Introduction

• Cell cycle is series of events.
• It occurs in cell which leads the cell to its maturation and subsequent division.
• Discovered by Prevost and Dumas in 1824.
• This entire process where a new cell population grows and develops with the help of one single parent cell is termed as the cell cycle.

Interphase

• It in the period of cell cycle in which animal cell prepared itself for mitosis.
• Important events occurs during this period is DNA replication,
• Synthesis of large amount of protein which in needed for division.
• Interphase in divided into 3-subphases.

G1 phase

• It is the period in which, animal cell prepared for DNA replication.
• Rate of transcription is very high.
• At the end of G1 phase, Go phase is present.
• In case of animal, it is called restriction point
• If growth factor / mitogens and nutrient condition is high then, animal cell exit from the Go phase and enter in S-phase.
• In yeast, Go phase is called start point.
• It is functionally similar to restriction point and controlled by mating factor and nutrient availability.

S-phase (Synthetic phase)

• It is active phase of Interphase.
• DNA replication occur in this phase.
• Number of genetic quantity just doubled but character remain same.
• Centrioles doubled in this phases.
• S-phase is easily studied by measuring the amount of nucleotide incorporation by using radiolabeled / fluorescently labelled nucleotide and fluorescent microscope.
• S-phase specific cln-cdk complex in synthesized in Late G1 phase but due to sik-protein, it is functionally inactive.
• Late G1 phase specific cln-cdk complex phosphorylate the inhibitor protein and trigger its degradation by proteosomal complex.
• S-phase specific cln-cdk complex phosphorylate the pre RC complex and triggered DNA replication.

G2-phase

• It is the period between S and M-phase.
• Animal cell prepare itself for mitosis.
• Rate of transcription and translation in very high.
• Cell size Increased.
• It is the Last phase of interphase.

M-phase (mitotic phase)

• It is the most active phase of the cell cycle.
• It is divided into two sub-phases.
  • Karyokinesis.
  • Cytokinesis.
• Division occur in M-phase.

Mechanism of Cell division

• It is proposed by W. Fleming in 1882.
• In mitosis, one round of DNA replication and only one round of chromosome segregation and generating two identical daughter cells.
• It is a continuous process.

Stages of mitosis:

It start with Karyokinesis and end with cytokinesis.

Prophase

• Chromatin condensation.
• Chromatin duplicated during S phase and consist two sister Chromatid and both sister chromatid, are held by centromere.
• At late prophase, microtubules are polymerized and forms mitotic plate.
• In thin phase nuclear membrane NPC, ER, Golgi body fragmented.

Metaphase

• Two kinetochores at each chromosome, attached on opposite sides.
• At metaphase chromosomes are properly aligned at equator site and form mitotic plate.
• Microtubule show polarity.
• Microtubule dependent motor protein play important role in arrangement of chromosome on equator line.

Anaphase

• In this phase, Kinetochore microtubule pull the two chromosome at opposite pole.
• Best phase for analysis of shape of chromosome.
• During metaphase sister Chromatids are not separate because they are held by SMC and Cohesion.
• During Anaphase APC causes-activation of separase and separase degrade SMC and cohesin. That’s why it allow separation of sister chromatid.
• Anaphase in divided into two sub-phases

Anaphase-A

• • It allow microtubule depolymerization by using motor protein.
  • kinesis-13.

Anaphase B

• • It allow active polward movement by using Dynein motor protein.
  • During late anaphase, cellular organelles also move at polar site.

Telophase

• Chromatid move at opposite pole.
• Kinetochores and microtubules are disappeared.
• Chromatin decondensation.
• Cellular organelle permanently move at polar region.
• Nuclear membrane and NPC reappeared.
• ER and GB reappeared.

Cytokinesis

• It is completed by two different mechanism.
  • In animal cells it in completed by cleavage and furrow.
• Cytokinesis is initiated by actin filament and form contractile rings.
  • Myosin and Rho-GTPase protein play important role in cleavage and furrow formation.
  • In plants due to presence of rigid cell wall, Ridge & furrows is not formed, that’s why cell separate with each other by cell plate formation.
  • At late anaphase golgi body forms fragmoplast which forms cell plate.
  • By fragmoplast it forms plasmodesmeta that connect cell electrically and mechanically.

Significance of mitosis

• Maintain chromosome number.
• Responsible for formation of somatic body.
• It play important role in regeneration.
• In unicellular organism, it is responsible for increase population size.

Meiosis

• Proposed by Farmer and Moore.
• One round of DNA replication followed by two round of chromosome segregation.
• It is completed into two steps:

Meiosis-I

• Homologous character are separate and called reductional division because number of chromosome is exactly half.

Meiosis- II

• Sister chromatid are separate and actual haploidy is synthesized, that’s why it is called equational division.

Difference between Mitosis and Meiosis

References and Sources:

• https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/meiosis
• https://www.khanacademy.org/science/high-school-biology/hs-reproduction-and-celldivision/hs-the-cell-cycle-and-mitosis/a/hs-the-cell-cycle-and-mitosis-review
• https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell-cycle/cellcycle/a/cell-cycle-phases
• https://quizlet.com/266870446/ch11-meiosis-mitosis-flash-cards/

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