• Cell Cycle, Mitosis, and Meiosis

Overview of Topics

• Mitosis, meiosis, and the cell cycle

• Mendelian principles of inheritance, cell cycle, and binomial equation

• Mendelian and human pedigrees

• Chromosome structure, allele variation, and DNA mutations

• Transmission and translation

• All content covered up to week 6 and all readings

Mitosis, Meiosis, and the Cell Cycle

Introduction

The cell cycle is the process by which cells grow, replicate their DNA, and divide. Mitosis and meiosis are two distinct types of cell division, each with unique roles in growth, repair, and reproduction.

Phases of the Cell Cycle

• G1 phase: Cell growth and metabolism, organelles and protein production. 2n = 46 in humans.

• S phase: DNA replication; each chromosome is duplicated, resulting in sister chromatids. The amount of DNA doubles, but chromosome number remains the same.

• G2 phase: Preparation for cell division; final checks and repairs.

• M phase (Mitosis or Meiosis): Division of the nucleus and cytoplasm.

Homologous chromosomes: Contain different versions (alleles) of the same genes.

Diploid (2n): Two sets of chromosomes (one from each parent). Haploid (n): One set of chromosomes.

Cell Division Types

• Mitosis: Produces two genetically identical diploid cells. Used for growth, repair, and asexual reproduction in eukaryotes.

• Meiosis: Produces four genetically unique haploid cells (gametes). Used for sexual reproduction.

Eukaryotes: DNA in nucleus. Prokaryotes: DNA in cytoplasm.

Interphase and Chromosome Structure

Interphase

Interphase consists of G1, S, and G2 phases. DNA is replicated during S phase. Cells not actively cycling may exit to G0 phase.

Chromosome Structure

• Sister chromatids: Identical copies of a chromosome, joined at the centromere.

• Centromere: Region where sister chromatids are joined.

• Kinetochore: Protein complex at the centromere, attaches to spindle microtubules.

• Spindle: Microtubule structure that separates chromosomes during cell division.

Mitosis

Phases of Mitosis

• Prophase: Chromosomes condense, spindle forms.

• Prometaphase: Nuclear envelope breaks down, spindle attaches to kinetochores.

• Metaphase: Chromosomes align at the cell center.

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

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

Key Points

• Mitosis produces two genetically identical diploid cells.

• Used for growth, repair, and asexual reproduction.

Cell Cycle Regulation

Cyclins and CDKs

Cell cycle progression is regulated by cyclin-dependent kinases (CDKs) and cyclins. These proteins form complexes that control transitions between cell cycle phases.

• G1/S cyclin-CDK: Prepares cell for DNA replication.

• S cyclin-CDK: Initiates DNA synthesis.

• M cyclin-CDK: Prepares cell for mitosis.

Cell Cycle Checkpoints

• DNA damage checkpoint (G1): Checks for damaged DNA before S phase.

• DNA replication checkpoint (G2): Ensures all DNA is replicated before mitosis.

• Spindle assembly checkpoint (M): Ensures all chromosomes are attached to spindle before anaphase.

Failure in checkpoint control can lead to uncontrolled cell division and cancer.

Cancer and Cell Cycle

Tumor Suppressors and Cancer Progression

• Tumor suppressor genes: Encode proteins that inhibit cell division.

• Cancer: Results from multiple mutations that disrupt normal cell cycle regulation.

• Metastasis: Spread of cancer cells to new sites.

Meiosis

Introduction

Meiosis is a specialized form of cell division that reduces chromosome number by half, producing four genetically unique haploid gametes. It consists of two sequential divisions: Meiosis I and Meiosis II.

Comparison: Mitosis vs. Meiosis

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate (similar to mitosis).

Meiosis I

1. Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis), crossing over occurs (exchange of genetic material).

2. Metaphase I: Homologous pairs align at the cell center.

3. Anaphase I: Homologous chromosomes separate to opposite poles.

4. Telophase I and Cytokinesis: Two haploid cells form.

Meiosis II

1. Prophase II: Chromosomes condense in each haploid cell.

2. Metaphase II: Chromosomes align at the center.

3. Anaphase II: Sister chromatids separate.

4. Telophase II and Cytokinesis: Four genetically unique haploid cells result.

Crossing Over and Genetic Variation

• Crossing over: Exchange of genetic material between homologous chromosomes during prophase I, creating new allele combinations.

• Independent assortment: Random orientation of homologous pairs during metaphase I increases genetic diversity.

Kinetochore Attachment in Mitosis and Meiosis

• In mitosis, kinetochores attach to microtubules from opposite poles (bi-orientation).

• In meiosis I, kinetochores of sister chromatids attach to the same pole (mono-orientation), while homologous chromosomes attach to opposite poles.

Important Differences in Meiosis

• One round of DNA replication, two rounds of division.

• Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.

• Results in four genetically unique haploid cells.

Key Terms and Definitions

• Chromatid: One of two identical halves of a duplicated chromosome.

• Centromere: Region joining two sister chromatids.

• Kinetochore: Protein structure on the centromere where spindle fibers attach.

• Synapsis: Pairing of homologous chromosomes during meiosis I.

• Chiasma (plural: chiasmata): Site of crossing over between homologous chromosomes.

• Allele: Different versions of a gene.

Summary Table: Mitosis vs. Meiosis

Key Equations

• DNA content after S phase:

• Number of possible gamete combinations (independent assortment): where = number of chromosome pairs

Example

Example: In humans (2n = 46), after S phase, each cell has 46 chromosomes and 92 chromatids. After mitosis, each daughter cell has 46 chromosomes. After meiosis, each gamete has 23 chromosomes.