Chapter 11: Meiosis and Sexual Reproduction Flashcards
What is meiosis?
A specialized cell division process that produces haploid gametes (sperm and eggs) with half the chromosome number of the parent cell. Essential for sexual reproduction to ensure genetic diversity.
List and describe the stages of Meiosis I.
Prophase I: Homologous chromosomes pair (synapsis), forming tetrads. Crossing over occurs at chiasmata (exchange of genetic material). Sub-stages: Leptotene (chromosomes condense), Zygotene (synapsis begins), Pachytene (crossing over), Diplotene (chiasmata visible), Diakinesis (nuclear envelope breaks). Metaphase I: Homologous pairs align randomly at the metaphase plate (independent assortment). Anaphase I: Homologs separate and move to opposite poles. Telophase I: Chromosomes decondense; two haploid cells form (each with sister chromatids still attached).
How does Meiosis II differ from Meiosis I?
Prophase II: Chromosomes recondense; spindle reforms. Metaphase II: Chromosomes align singly at the metaphase plate. Anaphase II: Sister chromatids separate. Telophase II: Nuclear envelopes reform; four haploid daughter cells result. No DNA replication occurs between Meiosis I and II.
What are synapsis and crossing over?
Synapsis: Pairing of homologous chromosomes via the synaptonemal complex during Prophase I. Crossing Over: Exchange of genetic material between non-sister chromatids at chiasmata, creating recombinant chromosomes. Significance: Increases genetic diversity by shuffling alleles.
Explain independent assortment and its impact.
During Metaphase I, homologous pairs align randomly at the metaphase plate. Each pair’s orientation is independent of others, producing 2^n possible chromosome combinations (n = haploid number). In humans, 2^23 = 8.4 million combinations.
Compare male and female gamete formation.
Spermatogenesis: Occurs in testes. Produces 4 functional sperm per meiosis. Continuous from puberty. Oogenesis: Occurs in ovaries. Produces 1 viable egg and 3 polar bodies (discarded). Arrested in Prophase I until puberty; completes only if fertilized.
What is nondisjunction? Provide examples of disorders.
Nondisjunction: Failure of homologs (Meiosis I) or sister chromatids (Meiosis II) to separate. Results in aneuploidy (abnormal chromosome number). Examples: Trisomy 21 (Down syndrome: 47 chromosomes). Monosomy X (Turner syndrome: 45, X). Klinefelter syndrome (47, XXY).
Define haploid (n) and diploid (2n).
Haploid: Cells with one set of chromosomes (e.g., gametes: n = 23 in humans). Diploid: Cells with two sets (e.g., somatic cells: 2n = 46). Fertilization combines n + n to restore diploidy.
Name three sources of genetic diversity in meiosis.
Crossing Over (Prophase I). Independent Assortment (Metaphase I). Random Fertilization (any sperm + any egg = 8.4×10^6 × 8.4×10^6 combinations).
Differentiate homologous chromosomes and sister chromatids.
Homologs: Paired chromosomes (one maternal, one paternal) with the same genes but different alleles. Sister Chromatids: Identical copies of a chromosome (connected at centromere) formed during S phase.
What are chiasmata? How do they relate to recombinant DNA?
Chiasmata: Physical sites where crossing over occurs between non-sister chromatids. Recombinant DNA: New allele combinations on chromatids, enhancing genetic diversity.
Describe meiosis in the human life cycle.
Diploid (2n) adults → meiosis → haploid (n) gametes. Fertilization (sperm + egg) → diploid zygote (2n) → mitosis → adult.
Why is sexual reproduction advantageous?
Generates genetic diversity, enabling populations to adapt to environmental changes, resist diseases, and reduce harmful recessive allele expression.
How does meiosis differ in plants?
Plants alternate between sporophyte (diploid) and gametophyte (haploid) phases: Sporophyte undergoes meiosis → haploid spores. Spores grow into gametophytes → gametes via mitosis. Animals lack the gametophyte phase.
How is meiosis regulated?
Controlled by: Checkpoints (e.g., spindle assembly checkpoint). Cohesin proteins: Hold sister chromatids together. Shugoshin: Protects centromeric cohesin in Meiosis I. APC/C (Anaphase-Promoting Complex): Triggers chromatid separation.
What is a zygote?
The diploid cell formed by the fusion of two haploid gametes (sperm and egg) during fertilization. Develops into an embryo via mitosis.
Name disorders caused by meiotic errors.
Trisomy 21 (Down syndrome). Turner syndrome (45, X). Klinefelter syndrome (47, XXY). Edwards syndrome (Trisomy 18).
Compare mitosis and meiosis.
Mitosis: 1 division → 2 diploid cells. No crossing over. Maintains genetic stability. Somatic cells. Meiosis: 2 divisions → 4 haploid cells. Crossing over in Prophase I. Increases genetic diversity. Gametes.
What are polar bodies?
Small, non-functional cells produced during oogenesis to discard excess genetic material, ensuring the egg retains most cytoplasm for embryo development.
Why is genetic diversity critical for species survival?
Allows populations to adapt to environmental stressors (e.g., disease, climate change) and reduces the risk of extinction from deleterious mutations.
