Cell Cycle Overview

Phases of the Cell Cycle

The cell cycle is the series of events that cells undergo to grow and divide. It consists of interphase (G1, S, G2) and the mitotic (M) phase.

• G1 phase: Cell growth and preparation for DNA replication.

• S phase: DNA synthesis (replication) occurs, resulting in duplicated chromosomes.

• G2 phase: Further growth and preparation for cell division.

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Cell Cycle Checkpoints

Checkpoints are regulatory mechanisms that ensure the cell cycle progresses only when conditions are favorable.

• G1 checkpoint: Checks for DNA damage and sufficient growth signals. If DNA is damaged, apoptosis may occur.

• G2 checkpoint: Ensures DNA has been replicated properly before mitosis. Apoptosis occurs if DNA is damaged and cannot be repaired.

• M checkpoint: Ensures chromosomes are properly aligned before cell division proceeds.

DNA Replication

Mechanism and Importance

DNA replication is the process by which a cell copies its entire genome before division. This ensures that each daughter cell receives a complete set of genetic instructions.

• Semiconservative replication: Each new DNA molecule consists of one old (parental) strand and one new (daughter) strand.

• Origins of replication: Multiple sites where replication begins in eukaryotic chromosomes.

• Replication fork: The area where the DNA double helix is unwound and new strands are synthesized.

Transmission of Genetic Information

DNA replication allows genetic information to be inherited from parent to daughter cells.

• Genes: Units of hereditary information located on chromosomes.

• Chromosome condensation: After replication, chromosomes condense to facilitate segregation during mitosis.

Chromosome Structure and Organization

Chromosomes and Chromatids

Chromosomes are linear DNA molecules wrapped around proteins. After DNA replication, each chromosome consists of two sister chromatids joined at the centromere.

• Sister chromatids: Genetically identical copies formed during S phase.

• Centromere: Region where chromatids are joined and spindle fibers attach during mitosis.

Genome and Ploidy

• Genome: One complete set of genetic information.

• Ploidy (n): Number of unique sets of chromosomes. Most somatic cells are diploid (2n), containing one set from each parent.

Mitosis

Purpose and Outcomes

Mitosis is the process by which a cell divides its nucleus and distributes identical genetic material to two daughter cells.

• Functions: Growth, asexual reproduction, cell replacement, and healing.

• Result: Two genetically identical daughter cells.

Stages of Mitosis

Mitosis consists of several distinct stages:

• Prophase: Chromosomes condense, spindle fibers form, and centrosomes migrate to opposite poles.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to chromosomes at the centromere.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope re-forms, chromosomes decondense.

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

Microscopic View of Mitosis

Microscopy can be used to observe cells undergoing mitosis, revealing the distribution of chromosomes.

Summary Table: Chromosome Types

Comparison of Homologous Chromosomes and Sister Chromatids

Additional info:

• Cell cycle checkpoints are critical for preventing uncontrolled cell division, which can lead to cancer.

• DNA replication is semiconservative, as demonstrated by the Meselson-Stahl experiment.

• Chromosome condensation is necessary for accurate segregation during mitosis.