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, which includes mitosis and cytokinesis.

• 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: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Cell Cycle Checkpoints

Checkpoints are regulatory mechanisms that ensure the cell cycle progresses only when certain conditions are met. They help maintain genetic integrity and prevent uncontrolled cell division.

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

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

• M checkpoint: Spindle assembly checkpoint; mitosis will not continue if chromosomes are not properly aligned.

DNA Replication

Mechanism and Importance

DNA replication is the process by which a cell copies its DNA before cell 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.

Equation:

Transmission of Genetic Information

DNA replication allows genetic information to be inherited from parent to daughter cells during mitosis and from generation to generation during meiosis.

• Each gene is a unit of hereditary information.

• Replication produces two DNA molecules, each distributed to daughter cells.

Chromosome Structure and Organization

Chromosomes and Chromatids

Chromosomes are long 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 DNA replication.

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

Chromosome Condensation

During mitosis, chromosomes condense to become visible and distinguishable under a microscope. This facilitates their proper segregation during cell division.

Mitosis

Purpose and Outcomes

Mitosis is the process by which a eukaryotic cell divides its nucleus, ensuring that each daughter cell receives an identical set of chromosomes. It is essential for growth, repair, and asexual reproduction.

• Results in two genetically identical daughter cells.

• Maintains chromosome number (diploid, 2n).

Stages of Mitosis

Mitosis is divided into 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 are pulled to opposite poles.

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

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

Microscopic View of Mitosis

Cells undergoing mitosis can be observed under a microscope, showing condensed chromosomes and dividing nuclei.

Cell Cycle Regulation and Cancer

Checkpoints and Cell Cycle Control

Cell cycle checkpoints ensure that each stage is completed correctly before the cell proceeds. Regulatory molecules at these checkpoints allow the cell to "decide" whether to continue. Cancer cells often bypass these checkpoints, leading to uncontrolled division.

• G1 checkpoint: Checks for DNA damage after mitosis.

• G2 checkpoint: Ensures DNA replication is complete and undamaged.

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

Example: If DNA is damaged at the G1 checkpoint, the cell may undergo apoptosis to prevent propagation of mutations.

Summary Table: Chromosome Types and Cell Cycle Phases

Additional info:

• Most somatic cells are diploid (2n), containing two sets of chromosomes—one from each parent.

• DNA replication is semiconservative, meaning each new DNA molecule contains one parental and one new strand.

• Cell cycle checkpoints are critical for preventing mutations and cancer.