Biology: Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction

• Sexual reproduction provides greater variation in heritable traits in populations, allowing for adaptation to changing environments (abiotic or biotic).
• This leads to some forms being more adaptive, and adaptive traits spreading faster.

Chromosome Number

• Diploid number (2n) is the number of chromosomes in a diploid cell, with two sets of each type (e.g., humans have 46 chromosomes).
• Haploid number (n) is the number of chromosomes in a haploid cell, with a single set of each type (e.g., humans have 23 chromosomes).

Alleles and Genes

• Alleles are different forms of the same gene, influencing specific traits.
• In asexual reproduction, genes are identical to the parent (clones), except for mutations.
• In sexual reproduction, pairs of genes (maternal and paternal) are combined, resulting in new combinations of alleles in offspring.

Meiosis

• Meiosis involves the production of reproductive cells (gametes) with a haploid number of chromosomes.
• Meiosis maintains the chromosome number, producing four haploid daughter cells.
• The process of meiosis involves two nuclear divisions, with no S phase between them.

Meiosis I

• Prophase I is the most complex stage of meiosis, involving the condensation of chromosomes, nuclear membrane breakdown, and the formation of a spindle.
• Prophase I has five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• During prophase I, homologous chromosomes pair up and exchange genetic material through crossing-over.

Meiosis II

• Metaphase II involves the alignment of chromosomes on the metaphase plate.
• Anaphase II involves the separation of sister chromatids, moving to opposite poles of the cell.
• Telophase II involves the uncoiling of chromosomes, reformation of nuclear envelopes, and the reappearance of nucleoli.

Cytokinesis

• Cytokinesis occurs after both meiosis I and II, resulting in four haploid cells.
• In some cases, nuclei may degenerate or be ejected as polar bodies, resulting in a single uninucleate ovum.

Variation in Traits

• Crossing over during meiosis I results in the exchange of genetic material between paternal and maternal chromosomes, leading to increased variation in traits.
• The separation of chromosome pairs during meiosis I is independent and random, resulting in a huge number of possible combinations (e.g., 8,388,608 in humans).
• The combination of these unique sets of genes at fertilization results in an even greater variation in traits.

Life Cycles

• Meiosis and cytoplasmic division result in haploid gametes, which fuse to form a diploid zygote.
• Many variations exist in life cycles, including the production of gametes, size, and shape, as well as different types of life cycles (unicellular, multicellular, diploid, haploid generations).