Cell Division and Meiosis Overview

Cell Division

• Binary fission is DNA duplication followed by cell splitting in two.
• Prokaryotes use this simple reproductive strategy.
• The cell cycle has distinct phases.
• Interphase consists of G1 (growth, checkpoint), S (DNA duplication), and G2 (preparation).
• Mitosis is the nuclear division phase.
• Checkpoints (G1, G2, M) regulate cell cycle progression.
• G0 is the resting phase, where cells stop dividing (e.g., neurons).
• Cytokinesis is the division of the cytoplasm; methods differ in organisms (plants, animals, fungi).
• Density-dependent inhibition stops cells from dividing when crowded.
• Anchorage dependence requires cell attachment for division.
• Cancer cells lack density-dependent inhibition, exhibiting uncontrolled growth.
• Transformation is the process of normal cells becoming cancerous.

Meiosis

• Meiosis is crucial for sexual reproduction, generating gametes.
• Diploid cells (full set) are crucial starting material.
• Haploid cells (half set) form as a result of meiosis.
• Meiosis I separates homologous chromosomes, including crossing over in Prophase I.
• Random alignment of homologous pairs in Metaphase I increases diversity.
• Meiosis II separates sister chromatids to make four haploid cells.
• Gamete formation varies in males (4 sperm per diploid cell) and females (1 egg + polar bodies).
• Crossing over in Prophase I exchanges genetic material between homologous pairs.
• Nondisjunction results in abnormal chromosome numbers (e.g., Down syndrome).

Genetics

• Genes are DNA sequences coding for proteins.
• Locus is the specific location of a gene on a chromosome.
• Sexual reproduction enhances adaptability.
• Asexual reproduction produces clones efficiently but lacks diversity.
• Homologous chromosomes have corresponding lengths, centromere positions, and gene types.
• Autosomes are non-sex chromosomes (22 pairs).
• Sex chromosomes determine sex (XX = female, XY = male).
• X-linked traits are carried on the X chromosome (e.g., colorblindness).
• Hemizygous refers to males having one X chromosome copy for traits on that chromosome.
• Chromosomal mutations (deletions, translocations) are often severe or lethal.
• Syndromes result from specific chromosomal mutations (e.g., Turner, Klinefelter syndromes).
• XXY is Klinefelter syndrome, with male characteristics and some female traits.

Mendelian and Molecular Genetics

• Linked genes are inherited together (due to proximity on the same chromosome).
• Crossing over can separate linked genes.
• Chi-Square Analysis is a statistical method to determine if observed data matches expected data. Formula ( \chi^2 = \sum \frac{(O - E)^2}{E} ).
• Fruit flies (Drosophila) are widely used in genetic studies, due to rapid reproduction, easily observable traits, and simple genetic makeup.
• XX and XY determine fruit fly sex.

Important Terms

• Aneuploidy: Abnormal chromosome number (e.g., Down syndrome - trisomy 21).
• Chiasmata: Crossover points during homologous chromosome pairing.
• Barr body: Inactivated X chromosome in females.
• Tetrad: Group of four chromatids during meiosis.