Cell cycle

What is cell cycle?

The cell cycle is a series of ordered events that a cell goes through from the time it is formed until it divides into two daughter cells. It encompasses the cell's growth, DNA replication, preparation for division, and the actual division process.

The cell cycle is essential for cell growth, development, and reproduction. It ensures that genetic material is accurately duplicated and distributed to daughter cells. The cell cycle is divided primarily into two broad phases:

1. Interphase

• The longest phase, sometimes called the "resting phase" but actually very active.

• The cell grows and prepares for division.

• Interphase is subdivided into three stages:

  • G1 phase (Gap 1): The cell grows physically, produces RNA, proteins, organelles, and carries out normal metabolic activities. It prepares for DNA replication but does not replicate DNA yet.

  • S phase (Synthesis): DNA replication occurs, duplicating the cell's chromosomes. Each chromosome now consists of two sister chromatids.

  • G2 phase (Gap 2): The cell continues to grow and produces proteins and organelles necessary for mitosis. The cell also begins reorganizing its contents to prepare for division.

Cells that do not divide can enter a resting phase called G0, where they remain metabolically active but do not progress through the cell cycle unless stimulated.

2. M Phase (Mitotic Phase)

• The phase of actual cell division.

• It includes:

  • Mitosis: Division of the cell’s nucleus and its genetic content into two nuclei.

  • Cytokinesis: Division of the cytoplasm, creating two daughter cells.

Checkpoints in the Cell Cycle

The cell cycle has built-in checkpoints to ensure the process proceeds correctly:

• G1 checkpoint: Determines if the cell is ready for DNA synthesis.

• G2 checkpoint: Checks for DNA damage and completeness of DNA replication before mitosis.

• Metaphase checkpoint (during M phase): Ensures chromosomes are properly aligned before division proceeds.

Major events of the cell cycle

Phase	Description	Key Activities
G1 Phase	Cell growth and metabolism	RNA and protein synthesis; organelle production
S Phase	DNA replication	Duplication of chromosomes
G2 Phase	Preparation for mitosis	Synthesis of proteins, organelles; cell growth
M Phase	Cell division (mitosis + cytokinesis)	Nuclear division and cytoplasmic division
G0 Phase	Resting/non-dividing phase	Cells exit cycle temporarily or permanently

Cell cycle regulation read here

Cell division read here

Cell division

What is Cell Division?

Cell division is the process by which a parent cell divides into two (or more) daughter cells. It is essential for growth, tissue repair, and reproduction. This process ensures that organisms can develop, repair damage, and reproduce efficiently. Cell division typically forms part of a larger cell cycle, during which the cell grows, duplicates its DNA, and then divides.

There are two main types of cell division in eukaryotic cells:

• Mitosis: Vegetative division resulting in identical cells.

• Meiosis: Responsible for producing gametes (sperm and egg), resulting in genetic variation.

Mitosis Cell Division:

Mitosis is the process through which a single cell divides to produce two genetically identical daughter cells. This type of division maintains the chromosome number from the parent cell in each of the daughter cells.

Key Features of Mitosis:

• Occurs in somatic (body) cells.

• Results in two identical diploid daughter cells.

• Fundamental for growth, repair, and asexual reproduction.

• Ensures cells have the same genetic content as the parent cell.

Stages of Mitosis:

1. Prophase: Chromosomes condense, the nuclear membrane breaks down, and spindle fibers form.

2. Metaphase: Chromosomes align at the cell's equatorial plate.

3. Anaphase: Sister chromatids are pulled apart to opposite poles.

4. Telophase: Nuclear membranes reform around each set of chromosomes; the cell starts to divide.

5. Cytokinesis: Division of the cytoplasm, resulting in two distinct cells.

        Mitosis ensures continuity and genetic consistency across cells in tissues and organs.

Meiosis Cell Division:

Meiosis is the specialized process that produces gametes—eggs or sperm—with half the genetic material of the parent cell.

Key Features of Meiosis:

• Occurs only in germ cells (reproductive cells).

• Results in four genetically unique haploid daughter cells.

• Reduces chromosome number by half (from diploid to haploid).

• Introduces genetic variation through crossing-over and independent assortment.

Stages of Meiosis:

Meiosis occurs in two sequential divisions—Meiosis I and Meiosis II.

Meiosis I:

• Prophase I: Chromosomes condense, homologous chromosomes pair up, and crossing-over occurs (exchange of genetic material).

• Metaphase I: Homologous pairs align at the equator.

• Anaphase I: Homologous chromosomes separate.

• Telophase I and Cytokinesis: Two cells form, each with half the chromosome number.

Meiosis II:

• Similar to mitosis, including prophase II, metaphase II, anaphase II, and telophase II.

• Chromatids in each cell are separated, resulting in four haploid daughter cells.

        Meiosis ensures genetic diversity and is fundamental to sexual reproduction.

Comparison between mitosis and meiosis cell division

Characteristic features	Mitosis	Meiosis
Cell divisions occur	1	2
Daughter cell produced	2	4
Genetic similarity of divided cells	Identical	Genetically different
Chromosome numbers in daughter cells	Same as parent(2n/diploid)	Half of the parent (n/haploid)
Purpose of cell division	Vegetative growth, repair, asexual multiplication	Sexual reproduction, gamete production
Genetic recombination	No	Yes
Crossing over	No	Yes, the Pachytene stage of Prophase I