Mitosis and Meiosis

Introduction

This module covers the fundamental processes of mitosis and meiosis, which are essential for cell division and genetic inheritance in eukaryotic organisms. Understanding these processes is crucial for comprehending how genetic material is transmitted from one generation to the next and how genetic variation arises.

Cell Structure and Organization

Generalized Animal (Eukaryotic) Cell

Eukaryotic cells are complex structures containing various organelles that perform specialized functions. The genetic material is housed within the nucleus, and the cell is surrounded by a plasma membrane.

• Nucleus: Contains the cell's DNA organized as chromatin; site of transcription.

• Nucleolus: Region within the nucleus responsible for ribosome synthesis.

• Chromatin: DNA-protein complex that condenses to form chromosomes during cell division.

• Endoplasmic Reticulum (Rough and Smooth): Involved in protein and lipid synthesis.

• Ribosomes: Sites of protein synthesis; can be free or bound to the rough ER.

• Golgi Body: Modifies, sorts, and packages proteins and lipids.

• Mitochondrion: Site of cellular respiration and energy (ATP) production.

• Centriole: Involved in organizing microtubules during cell division.

• Lysosome: Contains digestive enzymes for breaking down waste.

• Plasma Membrane: Regulates entry and exit of substances.

Example: During mitosis, the centrioles help form the spindle apparatus that separates chromosomes.

Genetic Material and Chromosomes

Organization of Genetic Material

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Chromosome: A DNA molecule with part or all of the genetic material of an organism.

• Chromatin: The less condensed form of DNA present during interphase.

• Homologous Chromosomes: Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location but may carry different alleles.

• Sister Chromatids: Identical copies of a chromosome connected by a centromere after DNA replication.

Chromosome Structure and Types

• Centromere: The region of a chromosome where the two sister chromatids are joined and where spindle fibers attach during cell division.

• Chromosome Types by Centromere Position:

  • Metacentric: Centromere in the middle.

  • Submetacentric: Centromere slightly off center.

  • Acrocentric: Centromere near one end.

  • Telocentric: Centromere at the very end.

Example: Human chromosomes can be classified based on centromere position for karyotyping.

Cell Division in Prokaryotes and Eukaryotes

Prokaryotic Cell Division

Prokaryotes (e.g., bacteria) divide by binary fission, a simpler process than mitosis.

• DNA is located in a single, circular chromosome in the nucleoid region.

• Cell grows, DNA replicates, and the cell divides into two identical daughter cells.

Eukaryotic Cell Division

Eukaryotic cells divide by mitosis (for somatic cells) or meiosis (for gametes).

• Mitosis: Produces two genetically identical diploid (2n) cells.

• Meiosis: Produces four genetically unique haploid (n) gametes.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication.

• Interphase: Period of cell growth and DNA replication; consists of:

  • G1 phase (First Gap): Cell grows and performs normal functions.

  • S phase (Synthesis): DNA is replicated.

  • G2 phase (Second Gap): Preparation for mitosis; further growth and regulation.

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

Key Point: Most of the cell cycle (about 90%) is spent in interphase.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the proper progression of the cell cycle.

• G1 Checkpoint: Determines if the cell is ready to proceed to DNA synthesis.

• G2 Checkpoint: Ensures DNA replication is complete and undamaged before mitosis.

• M Checkpoint: Ensures all chromosomes are properly attached to the spindle before anaphase.

Example: If a cell does not receive a go-ahead signal at the G1 checkpoint, it may enter a non-dividing state (G0 phase).

Mitosis

Overview and Stages

Mitosis is the process by which a eukaryotic cell separates its duplicated genome into two identical sets, resulting in two genetically identical daughter cells.

• Prophase: Chromosomes condense and become visible; spindle fibers form; nuclear envelope breaks down.

• Prometaphase: Nuclear envelope disappears; spindle fibers attach to kinetochores on chromosomes.

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

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

• Telophase: Nuclear envelopes reform around the two sets of chromosomes; chromosomes decondense.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• Animal Cells: Cytokinesis occurs via cleavage furrow formation.

• Plant Cells: Cytokinesis occurs via formation of a cell plate.

Key Point: Mitosis is an asexual process; daughter cells are genetically identical to the parent cell.

Meiosis

Overview and Purpose

Meiosis is a two-division process that reduces the chromosome number by half, producing haploid gametes or spores. It introduces genetic variation through recombination and independent assortment.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Stages of Meiosis

• Prophase I: Homologous chromosomes pair (synapsis) and exchange genetic material (crossing over at chiasmata).

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologous chromosomes separate (disjunction) to opposite poles.

• Telophase I: Two haploid cells form; chromosomes may decondense.

• Meiosis II: Similar to mitosis; sister chromatids separate, resulting in four haploid cells.

Genetic Variation in Meiosis

• Independent Assortment: Random orientation of homologous pairs during metaphase I leads to genetic diversity. For humans, the number of possible combinations is .

• Crossing Over: Exchange of genetic material between non-sister chromatids during prophase I increases genetic variability.

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

Gametogenesis

Spermatogenesis and Oogenesis

• Spermatogenesis: Occurs in the testes; produces four viable sperm cells from each precursor cell.

• Oogenesis: Occurs in the ovaries; typically produces one viable egg and polar bodies from each precursor cell.

Fertilization: Fusion of haploid gametes (n) restores the diploid (2n) chromosome number in the zygote.

Comparison of Mitosis and Meiosis

Key Differences

Key Terms and Definitions

• Histone: Protein that helps package DNA into nucleosomes.

• Nucleosome: DNA wrapped around a core of histone proteins.

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

• Centromere: Region joining sister chromatids.

• Karyotype: The number and appearance of chromosomes in a cell.

• Monad: A single chromatid.

• Dyad: A pair of sister chromatids.

• Tetrad: Four chromatids (two homologous chromosomes) during meiosis I.

• Synapsis: Pairing of homologous chromosomes during prophase I of meiosis.

• Chiasma: Site of crossing over between homologous chromosomes.

• Zygote: Fertilized egg cell (diploid).

Summary

• Mitosis and meiosis are essential for growth, repair, and reproduction in eukaryotes.

• Mitosis produces genetically identical cells; meiosis produces genetically diverse gametes.

• Genetic variation arises from independent assortment, crossing over, and random fertilization.