Cell Cycle and Checkpoints

Cell Cycle Overview

• The cell cycle is a regulated process for eukaryotic cell growth and division.
• It includes interphase (G1, S, and G2 phases) and the M phase (mitosis and cytokinesis).

Interphase - G1 Phase

• The cell grows, produces proteins, and performs normal functions.
• It prepares for DNA replication.

G1/S Checkpoint

• Regulated by Cyclin E and CDK2.
• Ensures adequate cell size, proper growth, and sufficient nutrients.
• Determines if the cell is ready for DNA replication.
• A cell that doesn't pass the G1 checkpoint may enter G0, a resting state.
• Some cells, like neurons, remain permanently in G0.

Interphase - S Phase

• DNA replication occurs, resulting in two identical sister chromatids per chromosome.

Interphase - G2 Phase

• Organelles duplicate.

G2/M Checkpoint

• Regulated by Cyclin B and CDK1.
• Verifies that DNA replication is complete and intact.
• Determines if the cell is ready for mitosis.

M Checkpoint

• Also known as the spindle checkpoint.
• Ensures all chromosomes are correctly attached to spindle fibers at the kinetochores.
• The Anaphase Promoting Complex (APC) facilitates the transition to anaphase by allowing the separation of sister chromatids.

Cell Cycle Regulation

• CDKs (cyclin-dependent kinases) and cyclins regulate the cell cycle.
• Cyclins are named for their cyclical nature; they are synthesized and degraded at different checkpoints.
• Cyclins activate CDKs to phosphorylate target proteins, pushing the cell through the cycle.

Consequences of Checkpoint Failure

• Uncontrolled cell division can lead to cancer.
• Cancer is characterized by rapid and unregulated cell proliferation.

Mitosis Overview

• Occurs in somatic cells.
• Crucial for growth, repair, and asexual reproduction.
• Includes prophase, metaphase, anaphase, and telophase.

Mitosis - Prophase

• The nuclear envelope breaks down.
• Centrioles duplicate and move to opposite poles.
• Chromatin condenses into chromosomes.
• Microtubules, composed of tubulin, begin forming spindle fibers.

Mitosis - Metaphase

• Duplicated chromosomes (sister chromatids) align at the equator.
• They attach to spindle fibers at their centromeres.
• Polar microtubules extend across the cell, stabilizing the structure.

Mitosis - Anaphase

• Spindle fibers pull sister chromatids apart, moving them toward opposite poles.
• The sliding filament hypothesis suggests that spindle elongation helps separate chromatids.
• Microtubules disassemble at the centrioles, allowing chromosome movement.

Mitosis - Telophase

• The cytoplasm divides, forming two daughter cells during cytokinesis.
• In animal cells, actin filaments create a contractile ring, forming a cleavage furrow.
• In plant cells, vesicles deliver cellulose to the cell plate, forming a new cell wall.

Meiosis Overview

• A reduction division that produces haploid gametes from a diploid cell.
• Occurs through two rounds of division: Meiosis I and Meiosis II.

Meiosis I - Prophase I

• Crossing over occurs at chiasmata, where homologous chromosomes exchange genetic material, introducing genetic diversity.

Meiosis I - Metaphase I

• Homologous pairs align at the equator.
• Independent assortment creates 2²³ (~8.4 million) possible combinations of gametes.

Meiosis I - Anaphase I

• Homologous chromosomes separate.
• The nuclear envelope does not reform.

Meiosis II

• No DNA replication occurs between telophase I and prophase II.
• Similar to mitosis.
• At the end of telophase II, four unique haploid gametes form.

Gametogenesis - Spermatogenesis

• Occurs in the spermatogonium (diploid cell).
• Produces four functional sperm cells.
• Each sperm has a 9+2 microtubule arrangement in the flagella for mobility.

Gametogenesis - Oogenesis

• Begins in the oogonium.
• Produces one viable egg and polar bodies, which disintegrate.

Significance of Meiosis

• Ensures genetic stability while allowing for genetic variation in sexually reproducing organisms.