Cell Division: Mitosis and Meiosis (Chapters 9 & 10) Study Guide

Cell Division: Mitosis and Meiosis

Overview

Cell division is a fundamental process in biology, essential for growth, development, and reproduction. This guide covers the mechanisms and significance of mitosis and meiosis, focusing on their stages, outcomes, and roles in genetic continuity and variation.

Mitosis

Chromosome Structure and Terminology

• Sister chromatids: Identical copies of a chromosome joined together after DNA replication.

• Cohesins: Protein complexes that hold sister chromatids together until they are separated during mitosis.

• Centromere: The region where sister chromatids are most tightly attached; serves as the site for kinetochore formation.

• Kinetochore: Protein structure assembled on the centromere where spindle microtubules attach during cell division.

• Duplicated chromosome: Consists of two sister chromatids.

• Unduplicated chromosome: Consists of a single chromatid.

• Human somatic cell chromosome number: 46 (23 pairs).

The Cell Cycle

• Phases: The cell cycle consists of Interphase (G1, S, G2) and the Mitotic phase (Mitosis and Cytokinesis).

• Interphase: The longest phase, where the cell grows, replicates DNA, and prepares for division.

• G1 phase: Cell grows and copies organelles.

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

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

Stages of Mitosis

1. Prophase: Chromosomes condense, the mitotic spindle forms, and the nucleolus disappears. Chromosome number: 46 duplicated chromosomes.

2. Prometaphase: Nuclear envelope breaks down; spindle microtubules attach to kinetochores.

3. Metaphase: Chromosomes align at the metaphase plate (the cell's equatorial plane).

4. Anaphase: Sister chromatids separate and are pulled toward opposite poles by motor proteins along microtubules. Chromosome number: 92 unduplicated chromosomes (temporarily, as chromatids are now considered individual chromosomes).

5. Telophase: Nuclear envelopes reform around the chromosomes, which decondense.

Cytokinesis

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

• Animal cells: Cytokinesis occurs via a cleavage furrow that pinches the cell in two.

• Plant cells: Cytokinesis occurs via formation of a cell plate that develops into a new cell wall.

• Result: Two genetically identical daughter cells.

Mitotic Spindle and Microtubules

• Mitotic spindle: Structure made of microtubules that segregates chromosomes during mitosis.

• Centrosome: Microtubule-organizing center in animal cells.

• Kinetochore microtubules: Attach to chromosomes at kinetochores.

• Nonkinetochore microtubules: Overlap and push the cell apart during division.

Cell Cycle Regulation

• Cell cycle control system: Network of regulatory proteins that control progression through the cell cycle.

• G1 checkpoint: Determines whether the cell will divide or enter a nondividing state (G0).

• G0 phase: Nondividing, resting state.

• M checkpoint: Ensures all chromosomes are properly attached to the spindle before separation.

• Growth factors: Proteins that stimulate cell division.

• Density-dependent inhibition: Cells stop dividing when crowded.

• Anchorage dependence: Cells must be attached to a substrate to divide.

Prokaryotic Cell Division

• Binary fission: A form of asexual reproduction in prokaryotes where the cell divides into two genetically identical cells.

Meiosis

Sexual and Asexual Reproduction

• Asexual reproduction: One parent produces genetically identical offspring.

• Sexual reproduction: Two parents produce offspring with genetic variation.

Chromosome Terminology

• Homologous chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Locus: Specific location of a gene on a chromosome.

• Alleles: Different versions of a gene.

• Somatic cells: Body cells; diploid (2n = 46 in humans).

• Gametes: Sex cells (sperm and egg); haploid (n = 23 in humans).

• Karyotype: Organized display of an individual's chromosomes.

• Autosomes: Non-sex chromosomes (22 pairs in humans).

• Sex chromosomes: X and Y chromosomes (1 pair in humans).

• Ploidy: Number of sets of chromosomes in a cell.

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes.

Meiosis Overview

• Definition: Cell division that reduces chromosome number by half, producing four haploid gametes.

• Number of divisions: Two consecutive divisions: Meiosis I and Meiosis II.

Stages of Meiosis

Meiosis I

1. Prophase I: Homologous chromosomes pair up and exchange genetic material through crossing over.

2. Metaphase I: Homologous pairs align at the metaphase plate (key difference from mitosis: pairs, not individual chromosomes, align).

3. Anaphase I: Homologous chromosomes separate; sister chromatids remain together.

4. Telophase I: Two haploid cells form, each with duplicated chromosomes.

Result of Meiosis I: Two haploid cells.

Meiosis II

1. Prophase II: Spindle apparatus forms in each haploid cell.

2. Metaphase II: Chromosomes align at the metaphase plate.

3. Anaphase II: Sister chromatids separate and move to opposite poles.

4. Telophase II: Four haploid cells are produced, each genetically distinct.

Final result of meiosis: Four genetically unique haploid gametes.

Sources of Genetic Variation

• Independent assortment: Random orientation of homologous pairs during Metaphase I leads to different combinations of maternal and paternal chromosomes in gametes.

• Crossing over: Exchange of genetic material between homologous chromosomes during Prophase I creates new allele combinations.

• Random fertilization: Any sperm can fertilize any egg, increasing genetic diversity.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic identity	Identical to parent	Genetically unique
Role	Growth, repair, asexual reproduction	Sexual reproduction, genetic diversity

Key Terms and Definitions

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

• Centrosome: Organelle that serves as the main microtubule organizing center.

• Metaphase plate: Imaginary plane equidistant between the spindle's two poles where chromosomes align during metaphase.

• Cleavage furrow: Indentation that begins the process of cytokinesis in animal cells.

• Cell plate: Structure that forms during cytokinesis in plant cells, leading to cell wall formation.

• G0 phase: Resting phase where cells exit the cell cycle and do not divide.

Equations and Notation

• Diploid number (2n): $2n = 46$ (in humans)

• Haploid number (n): $n = 23$ (in humans)

Summary

Mitosis and meiosis are essential processes for life. Mitosis ensures genetic continuity for growth and repair, while meiosis introduces genetic diversity through sexual reproduction. Understanding their mechanisms and regulation is fundamental to cell biology and genetics.