The Cell Cycle
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
Mitosis |
Meiosis |
| Takes place in somatic cells | Take place in germ cells |
| Nucleus divides only once | Nucleus divides twice |
| Formation of two daughter cells | Formation of four daughter cells. |
| Diploid daughter cells | Haploid daughter cells |
| More frequent | Less frequent |
| Formation of somatic organs from daughter cells | Formation of germ cells. |
| Only one prophase, metaphase, Â anaphase and telophase | Two of each phase occurs and five sub-phases in prophase 1 |
| Number of chromosome not changed | Chromosome number reduced to half |
| No crossing over | Crossing over occurs |
| Equational division | Reductional division |
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/