Exam 3 Review – Chapter 11: Meiosis & Genetic Diversity

Fundamental Vocabulary

• Gene – A discrete unit of hereditary information consisting of a specific nucleotide sequence of DNA; codes for a polypeptide or RNA.
• Locus – The physical position of a gene on a chromosome.
• Gametes – Haploid (n) reproductive cells that unite during fertilization; in animals, sperm or eggs.
• Male gamete (sperm) – Motile micro‐gamete produced in testes; delivers paternal haploid set.
• Female gamete (egg / ovum) – Non-motile macro-gamete produced in ovaries; provides maternal haploid set and most cytoplasm/organelles.
• Somatic cells – All body cells except gametes and their precursors; diploid (2n).
• Genome – The complete complement of an organism’s genes/DNA; measured in base pairs or chromosome number.
• Asexual reproduction – Single parent produces genetically identical offspring by mitosis or budding; no fusion of gametes.
• Sexual reproduction – Two parents produce offspring with unique gene combinations via meiosis → gametes → fertilization → zygote.

Chromosome Number & Ploidy

• Human diploid number: 2n = 46 (23 homologous pairs).
• Human haploid number: n = 23.
• Chromosome – A single, long DNA molecule wound around histone proteins; visible when condensed during division.
• Somatic vs. gamete: Somatic cells have 2n chromosomes (both homologues); gametes have n chromosomes (one of each pair).
• Reproduction yielding identical offspring → asexual (mitosis/clonal).

Alternation of Generations in Sexual Life Cycles

• Sequence: Meiosis (diploid → haploid) → Gametes (n) → Fertilization (fusion) → Zygote (2n) → Mitosis to grow multicellular diploid.
• Plant/fungal variants may insert multicellular haploid phase, but core alternation of haploid (n) and diploid (2n) stages is universal.

Human Chromosome Categories

• Autosomes – 22 homologous pairs (44 chromosomes).
• Sex chromosomes – 1 pair (XX or XY).
• Description in humans:
– Sex chromosomes: 2
– Autosomes: 44

Haploid vs. Diploid – Cell-Type Checklist

• Liver cell – diploid.
• Egg cell – haploid.
• Skin cell – diploid.
• Somatic cell – diploid by definition.
• Gamete – haploid.
• Zygote – diploid.
• Sperm – haploid.
• Stem cell (body) – diploid.

Dog Chromosome Calculations (given 78 in somatic)

• Somatic/bone cell: 2n = 78.
• Haploid cell (gamete): n = 39.
• Sperm: 39.
• Zygote: 78.

Visual Guide to a Duplicated Diploid Cell

• Labels on sketch:
– Sister chromatids: duplicate copies joined at centromere.
– Centromere: constricted region holding sisters.
– Homologous chromosomes: one maternal (red) + one paternal (blue); same genes, possibly different alleles.
– Duplicated chromosome: entire two-chromatid structure.
– Maternal vs. paternal sets color-coded.
• Counting example:
– If 4 duplicated chromosomes shown → 2n = 4.
– Homologous pairs: 2.
– Chromatids: 8 (since each duplicated chromosome = 2 chromatids).
– Ploidy: diploid (2n).

Gametogenesis Sites & Process

• Male gametes produced in testes (seminiferous tubules).
• Female gametes produced in ovaries (ovarian follicles).
• Process = Meiosis (with supporting mitotic divisions in germ-line).

Purpose & Overview of Meiosis

• Halve chromosome number (prevent doubling each generation).
• Generate genetic variation (independent assortment, crossing over, random fertilization).
• One DNA replication followed by two cell divisions → four non-identical haploid daughter cells.
• Immediately after chromosome duplication (S phase) comes Meiosis I.

Detailed Stages of Meiosis

Meiosis I – Reductional Division

1. Prophase I
   • Chromosomes condense; homologs undergo synapsis (pairing) forming tetrads.
   • Crossing over occurs between nonsister chromatids → chiasmata visible.
2. Metaphase I
   • Tetrads align at metaphase plate; homolog pairs orient independently (basis of independent assortment).
   • Contrast to mitosis: chromosomes line up as pairs, not individual duplicated chromosomes.
3. Anaphase I
   • Homologous chromosomes separate; sister chromatids remain attached.
   • End of anaphase I chromosome count per pole equals n duplicated chromosomes.
4. Telophase I/Cytokinesis
   • Each daughter nucleus now haploid (chromosomes still duplicated).

Meiosis II – Equational Division

5. Prophase II–Metaphase II
   • No further DNA replication; chromosomes (with sisters) re-condense, align singly.
6. Anaphase II
   • Sister chromatids separate (now individual chromosomes).
7. Telophase II/Cytokinesis
   • Chromosomes decondense; nuclei form.
   • Final product: 4 haploid cells, each with a unique mix of alleles.

Key Definitions for Prophase I Events

• Alleles – Alternative versions of a gene; e.g., allele for brown eyes vs. blue eyes at the eye-color locus.
• Synapsis – Physical pairing of homologous chromosomes along their lengths via the synaptonemal complex.
• Crossing over – Exchange of DNA segments between nonsister chromatids of a tetrad; creates recombinant chromosomes.
• Chiasmata (sing. chiasma) – X-shaped regions where crossing over occurred; persist until anaphase I.

Quantitative Checkpoints After Meiosis I

• Chromosomes at end of anaphase I per pole = n duplicated.
• Chromosomes in each daughter cell after telophase I = n duplicated (haploid).

Fast Review Q & A

• What happens to chromosome number? → Reduced from 2n to n.
• Division reducing number? → Meiosis I.
• Purpose? → Halve ploidy & create variation.
• Chromosomes duplicate? → Once (interphase preceding meiosis).
• Cell divides times? → Twice.
• Daughter cells formed? → Four.
• Chromosome number in daughters? → n.
• Homologs? → Pair of chromosomes, one maternal + one paternal, same genes.

Mechanism of Crossing Over (Diagram Steps)

1. Homologs align; enzymes induce matching double-strand breaks in nonsister chromatids; chromatin condenses.
2. Synaptonemal complex forms, zipping homologs together.
3. DNA repair enzymes join broken ends to opposite chromatid → crossover; complex fully assembled (synapsis).
4. Complex disassembles; homologs slightly separate but remain linked at chiasmata until anaphase I.
   • Necessity of nonsister chromatids: sisters are identical copies; only nonsisters combine maternal & paternal DNA, generating new allele combos.

Comparative Table – Mitosis vs. Meiosis

• Parent cell: diploid in both.
• Synapsis: absent in mitosis, present (prophase I) in meiosis.
• Homologous chromosomes: behave independently in mitosis; pair & segregate in meiosis I.
• Chromosome duplication: once before both processes.
• Divisions: one for mitosis → 2 cells; two for meiosis → 4 cells.
• Daughter cells:
– Mitosis: 2, diploid, genetically identical.
– Meiosis: 4, haploid, genetically unique.
• Functions:
– Mitosis: growth, repair, asexual reproduction.
– Meiosis: gamete production, genetic diversity.

Key Numerical Facts & Equations

• Human: 2n = 46 \Rightarrow n = 23.
• Dog: 2n = 78 \Rightarrow n = 39.
• Generic meiosis equation:
\text{Diploid germ cell} \xrightarrow{\text{DNA S}} \text{duplicated} (2n) \xrightarrow{\text{Meiosis I}} n \text{(duplicated)} \xrightarrow{\text{Meiosis II}} n \text{(unduplicated; 4 cells)}