Biology 120 Notes - CH. 13: Meiosis

Meiosis and Gametes

• Meiosis: nuclear division leading to halving chromosomes

• Gametes: reproductive cells (eggs + sperm) that unite during fertilization

• Humans have 46 chromosomes (23 from each parent)

Chromosomes

• Karyotype: visualization of all chromosomes; includes sex chromosomes (XX/XY)

• Homologous chromosomes: same type, same genes, different alleles (also called homologs)

• Ploidy: refers to sets of chromosomes; humans are diploid (2n)

• Haploid number (n): indicates number of chromosome sets; for humans, it's (2n)/ gametes (n)

Meiosis Process

• 2 divisions from a diploid parent cell:

• 1st division: 2 (n) daughter cells -> sister chromatids

• 2nd division: 4 different daughter cells with varied combinations of alleles

• Stages: prophase, metaphase, anaphase, telophase

Prophase I

• Nuclear envelope breaks down; chromosomes condense

• Synapsis: homologous chromosomes pair up to form bivalents

• Crossing-over: exchange of genetic material occurs at chiasmata

Metaphase I

• Homologous chromosomes align on the metaphase plate; alignment is random

Anaphase I

• Homologous chromosomes are pulled apart to opposite poles

Telophase I and Cytokinesis

• Results in two cells with reduced chromosomes

• Subsequent stages (Prophase II, Metaphase II, etc.) pull sister chromatids apart

Mitosis vs. Meiosis

• Mitosis: single division creates 2 identical diploid daughter cells

• Meiosis: results in 4 unique daughter cells due to shuffling of alleles

Evolution and Natural Selection

• Evolution affects populations over time; natural selection explains its mechanism

• Variability in genetics is crucial for survival and reproduction

• Adaptations: heritable traits that enhance survival

Genetic Variation

• Arises from crossing over, independent assortment, and random fertilization

Down Syndrome

• Trisomy of chromosome 21 due to nondisjunction, leading to extra chromosome

Reproductive Strategies

• Sexual reproduction more efficient; introduces genetic variance resistant to diseases

• Asexual reproduction quickly replicates but risks total loss from diseases due to genetic uniformity

• Purifying Selection: natural selection against harmful genetic variants more prevalent in asexual populations.