The Cell Cycle: Structure, Function, and Phases

The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a fundamental process in eukaryotic organisms, governing how cells grow, replicate their genetic material, and divide to produce new cells. Understanding the cell cycle is essential for comprehending growth, development, tissue repair, and reproduction in living organisms.

• Cell division is crucial for asexual reproduction, growth and development, and tissue renewal.

• Proper distribution of genetic material ensures genetic continuity between generations of cells.

Key Terms in the Cell Cycle

Definitions and Concepts

• DNA (Deoxyribonucleic Acid): The molecule that carries genetic instructions in all living things.

• Genome: The complete set of genetic material in an organism.

• Chromosome: A structure composed of DNA and proteins that contains genetic information. Eukaryotic cells have multiple linear chromosomes.

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes within the nucleus.

• Somatic cell: Any cell of a living organism other than the reproductive cells (gametes).

• Gamete: A reproductive cell (sperm or egg) that contains half the genetic material of a somatic cell.

• Ploidy: The number of sets of chromosomes in a cell (e.g., diploid, haploid).

Example: In humans, somatic cells are diploid (2n = 46 chromosomes), while gametes are haploid (n = 23 chromosomes).

Roles of Cell Division

Biological Importance

• Asexual reproduction: Single-celled organisms reproduce by dividing into two identical daughter cells.

• Growth and development: Multicellular organisms grow by increasing their cell number through division.

• Tissue renewal: Damaged or dead cells are replaced by new cells produced through division.

Example: Skin cells are constantly replaced by new cells formed through mitosis.

Distribution of Genetic Material During Eukaryotic Cell Division

Chromosome Structure and Segregation

During cell division, genetic material must be accurately distributed to ensure each daughter cell receives a complete set of chromosomes.

• Chromosome: Consists of two sister chromatids joined at a region called the centromere after DNA replication.

• Chromosome duplication: Each chromosome is replicated to form two identical sister chromatids.

• Separation of sister chromatids: During mitosis, sister chromatids are separated and distributed to two daughter cells.

Diagram Explanation: The process involves chromosome duplication, alignment, and separation, ensuring each new cell receives an identical set of chromosomes.

Phases of the Cell Cycle

Overview of Cell Cycle Phases

The cell cycle consists of a series of phases that prepare the cell for division and ensure accurate replication and segregation of genetic material.

• Interphase: The period of cell growth and DNA replication, comprising three subphases:

  • G1 phase (First Gap): Cell grows and carries out normal functions.

  • S phase (Synthesis): DNA is replicated, resulting in duplicated chromosomes.

  • G2 phase (Second Gap): Cell prepares for division by producing proteins and organelles needed for mitosis.

• Mitotic (M) phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm), resulting in two genetically identical daughter cells.

Example: In a typical human cell, interphase accounts for about 90% of the cell cycle, while the M phase is relatively short.

Chromosome Duplication and Segregation

Detailed Steps

1. Chromosome duplication: Each chromosome is copied during the S phase, forming two sister chromatids.

2. Alignment: Chromosomes align at the cell's equator during metaphase of mitosis.

3. Separation: Sister chromatids are pulled apart during anaphase, ensuring each daughter cell receives one copy of each chromosome.

Equation for DNA content during the cell cycle:

$\text{DNA content after S phase} = 2 \times \text{DNA content before S phase}$

Summary Table: Phases of the Cell Cycle

Phase	Main Events
G1 phase	Cell growth, normal metabolic roles
S phase	DNA replication (chromosome duplication)
G2 phase	Preparation for mitosis, synthesis of proteins
M phase	Mitosis (nuclear division) and cytokinesis (cytoplasmic division)

Additional info:

• Microtubules and microfilaments play essential roles in chromosome movement and cytokinesis during mitosis (not detailed in the provided slides but relevant to the topic).

• Checkpoints within the cell cycle ensure that each phase is completed accurately before the next begins.