Introduction to Genetic Function and Cell Division

Genetic information, carried by DNA, directs the growth, development, and reproduction of organisms. This information is passed from cell to cell and from generation to generation, ensuring continuity of life. Cell division is central to these processes, occurring via two main mechanisms: mitosis and meiosis.

Prokaryotic vs. Eukaryotic Cells

Key Differences

• Prokaryotic cells have a single circular chromosome located in the nucleoid region, not surrounded by a membrane. Examples: Bacteria, Archaea.

• Eukaryotic cells possess multiple linear chromosomes contained within a membrane-bound nucleus. Examples: Animals, Plants, Fungi.

• Genetic material in eukaryotes is complexed with proteins (histones), forming chromatin.

• Organelles such as mitochondria and chloroplasts also contain their own DNA.

Chromosomes: Structure and Classification

Chromosome Structure

• Each chromosome consists of a single, continuous DNA molecule complexed with proteins.

• Genes are specific sequences of DNA that code for proteins or RNAs; their positions are called loci (singular: locus).

• The centromere is a specialized region required for accurate segregation during cell division.

• Chromosomes are classified by centromere position:

  • Metacentric: centromere near the center

  • Submetacentric: centromere slightly off-center

  • Acrocentric: centromere close to one end

  • Telocentric: centromere at the very end

Homologous Chromosomes

• In diploid organisms, chromosomes exist in pairs called homologous chromosomes, one inherited from each parent.

• Homologs have the same morphology and similar genetic content but may carry different alleles.

• The complete set of chromosomes in a cell is called the genome.

Chromosome Number

• Diploid (2n): two sets of chromosomes (e.g., human somatic cells have 46 chromosomes).

• Haploid (n): one set of chromosomes (e.g., human gametes have 23 chromosomes).

Cell Cycle and Cell Division

The Cell Cycle

• The cell cycle is the life of a cell from its formation to its own division.

• It consists of interphase (G1, S, G2) and the mitotic phase (M).

• G1 phase: cell growth and preparation for DNA replication.

• S phase: DNA synthesis (replication).

• G2 phase: preparation for mitosis.

• Cells may enter a non-dividing state called G0.

Regulation of the Cell Cycle

• Progression through the cell cycle is tightly regulated by cyclin-dependent kinases (CDKs) and cyclins.

• Checkpoints ensure proper division and prevent uncontrolled cell proliferation.

Mitosis

Overview

Mitosis produces two genetically identical daughter cells from a somatic cell, maintaining chromosome number. It is essential for growth, repair, and asexual reproduction.

Phases of Mitosis

1. Prophase: Chromosomes condense and become visible; spindle fibers form.

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

3. Metaphase: Chromosomes align at the metaphase plate.

4. Anaphase: Sister chromatids separate and move to opposite poles (disjunction).

5. Telophase: Chromosomes decondense; nuclear envelope reforms.

Cytokinesis

• Division of the cytoplasm, resulting in two separate cells.

• In animal cells: cleavage furrow forms.

• In plant cells: cell plate forms, becoming the new cell wall.

Meiosis

Overview

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

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate.

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

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

  • Anaphase I: Homologs separate (disjunction).

  • Telophase I: Two haploid cells form.

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

  • Prophase II: Chromosomes condense again.

  • Metaphase II: Chromosomes align at the metaphase plate.

  • Anaphase II: Sister chromatids separate.

  • Telophase II: Four haploid cells result.

Genetic Variation in Meiosis

• Crossing over (recombination) between non-sister chromatids increases genetic diversity.

• Independent assortment of chromosomes during metaphase I further increases variation.

Comparison of Mitosis and Meiosis

Key Terms and Definitions

• Genome: The complete set of genetic material in an organism.

• Locus (loci): The specific location of a gene on a chromosome.

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

• Bivalent: A pair of homologous chromosomes physically associated during meiosis I.

• Tetrad: Four chromatids (two homologous chromosomes, each with two sister chromatids) aligned during prophase I of meiosis.

• Chiasma (chiasmata): The site where crossing over occurs between non-sister chromatids.

• Synapsis: The pairing of homologous chromosomes during meiosis.

• Disjunction: The separation of homologous chromosomes or sister chromatids during cell division.

• Cohesin: Protein complex that holds sister chromatids together.

• Kinetochores: Protein structures on chromatids where spindle fibers attach during cell division.

• Checkpoints: Regulatory points in the cell cycle ensuring proper division.

Gametogenesis: Spermatogenesis vs. Oogenesis

Overview

• Spermatogenesis: Formation of sperm cells; produces four functional sperm from each precursor cell.

• Oogenesis: Formation of egg cells; typically produces one functional ovum and polar bodies (which degenerate).

• Both processes involve meiosis but differ in timing, outcome, and cytoplasmic division.

Important Equations

• Diploid chromosome number:

• Haploid chromosome number:

• DNA content changes during cell cycle:

  • After S phase: DNA content doubles, but chromosome number remains the same.

  • After mitosis or meiosis II: DNA content and chromosome number are halved in daughter cells.

Summary Table: Chromosome and DNA Content During Cell Cycle

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Tables and diagrams referenced in the original notes were recreated in text and HTML table format.