Cell cycle and Division

1. Interphase

G1 Phase (Gap 1)

Early G1

Cells exit the previous cell cycle and enter a quiescent state.

Mid G1

Cells increase in size and carry out normal metabolic activities.

Late G1

Cells prepare for DNA replication by synthesizing proteins and organelles.

S Phase (Synthesis)

Replication Initiation

DNA replication begins at multiple origins of replication along the chromosomes.

Replication Elongation

DNA polymerase synthesizes new DNA strands in a semiconservative manner.

Replication Termination

DNA replication is completed, resulting in the formation of sister chromatids.

G2 Phase (Gap 2)

Preparation for Mitosis

Cells continue to grow and synthesize proteins necessary for mitosis.

Checkpoint Control

Checkpoints monitor DNA integrity and ensure proper completion of DNA replication before entering mitosis.

2. M Phase

Mitosis

Prophase

Early Prophase

Chromosomes condense, becoming visible under a light microscope.

Centrioles

Centrioles begin to migrate to opposite poles of the cell, forming the spindle apparatus.

Cyclin-CDK complexes

Cyclin-CDK complexes phosphorylate target proteins, initiating chromosome condensation and spindle formation.

Checkpoints

Checkpoints monitor the integrity of DNA and ensure proper condensation and alignment of chromosomes.

Late Prophase

The nuclear envelope ,ER ,Golgi body, breaks down, and the mitotic spindle forms.

Kinetochore

Kinetochore proteins assemble on the centromeres of chromosomes, facilitating their attachment to spindle fibers.

Chromosome Alignment

Chromosomes align along the metaphase plate, ready for proper segregation during metaphase.

Prometaphase

Chromosomes attach to spindle fibers at their kinetochores and begin to move toward the metaphase plate.

Metaphase

Chromosomes align along the metaphase plate, establishing a bipolar spindle.

Anaphase

Anaphase A

Sister chromatids separate and move toward opposite poles of the cell due to spindle fiber shortening.

Anaphase B

The spindle poles move apart, elongating the cell.

Based on shaped formed due to shapes of centromere :

1. Metacentric : inverted V shaped

2. Submetacentric: inverted L shaped

3. Acrocentric:inverted J shaped

4. Telocentric:Inverted I shaped

Telophase

Early Telophase

Chromosomes decondense, and nuclear envelopes form around each set of chromosomes.

Late Telophase

The mitotic spindle disassembles, and cytokinesis initiates.

In case of Meiosis

Meiosis I:

1. Prophase I:

  Leptotene: Chromosomes condense and become visible under the microscope.
  Zygotene: Homologous chromosomes pair up to form bivalents or tetrads. This pairing process is facilitated by the synaptonemal complex.
  Pachytene: Crossing over occurs between non sister chromatids of homologous chromosomes. This results in the exchange of genetic material, increasing genetic diversity.
  Diplotene: Synaptonemal complex dissolves, but homologous chromosomes remain attached at points called chiasmata, where crossing over occurred.
  Diakinesis: Chromosomes further condense, and the nuclear envelope breaks down. Spindle fibers start to form, and chromosomes begin to move towards the metaphase plate.

2.Metaphase I:

Orientation:

Bivalents align randomly along the metaphase plate, with each pair of homologous chromosomes attached to spindle fibers from opposite poles.

Independent Assortment:

The orientation of each bivalent is independent of other bivalents, contributing to genetic variation.

3. Anaphase I:

Separation of Homologous Chromosomes:

Spindle fibers contract, pulling homologous chromosomes of each bivalent towards opposite poles of the cell. Sister chromatids remain attached at the centromere.

4. Telophase I:

Cytokinesis:

The cytoplasm divides, resulting in two haploid daughter cells, each containing one set of chromosomes.

Nuclear Envelope Formation:

A new nuclear envelope forms around each set of chromosomes.

Interkinesis:

In some organisms, a short interphase like stage may occur, although DNA replication does not occur during this stage. Stage between Meiosis I and meiosis II

Meiosis II:

1.Prophase II:

Chromosomes Condense:

Chromosomes condense again after briefly decondensing in interkinesis.

Nuclear Envelope Breakdown:

The nuclear envelope dissolves, and spindle fibers begin to form.

2. Metaphase II:

Chromosome Alignment:

Chromosomes align along the metaphase plate of each haploid daughter cell.

3. Anaphase II:

Separation of Sister Chromatids:

Centromeres divide, and sister chromatids are pulled towards opposite poles of the cells by spindle fibers.

4. Telophase II:

Formation of Four Haploid Daughter Cells:

Cytokinesis occurs again, resulting in the formation of four haploid daughter cells, each containing a single set of chromosomes.

Nuclear Envelope Formation:

A new nuclear envelope forms around the chromosomes in each daughter cell.

Cytokinesis

Cytokinesis begins in anaphase and ends in telophase

Contractile Ring Formation

Actin and myosin filaments assemble at the equatorial plane.

Cleavage Furrow Ingrowth

The contractile ring contracts, pinching the cell membrane inward.

Cell Separation

The cytoplasm divides, resulting in two daughter cells, each with its nucleus.

Additional Terms

Cyclins

Proteins that regulate the progression of the cell cycle by activating cyclin-dependent kinases (CDKs).

Cyclin-dependent kinases (CDKs)

Enzymes that regulate the cell cycle by phosphorylating target proteins.

Cyclin-CDK complexes

Active complexes formed by the binding of cyclins to CDKs, which phosphorylate target proteins to drive cell cycle progression.

Checkpoints

Control mechanisms that monitor the integrity of DNA and other cellular components at key stages of the cell cycle, ensuring proper progression or halting the cycle if abnormalities are detected.

Apoptosis

Programmed cell death, which plays a crucial role in removing damaged or unwanted cells from the organism.

Centrioles

Microtubule-organizing centers that play a role in spindle formation during cell division.