Lecture on Meiosis and Sexual Reproduction

Definition: Asexual reproduction is the process whereby offspring arise from a single parent, resulting in genetically identical offspring.
Methods of Asexual Reproduction:
- Binary Fission: This is the process in which a single organism divides in half to form two new organisms.
- Budding: A new organism develops as an outgrowth, or bud, from a parent organism.
- Parthenogenesis: This is the growth and development of an embryo without fertilization by a male.

Definition: Sexual reproduction involves the mixing of DNA from two individuals, producing offspring that are genetically distinct from each other and from both parents.
Parental Genetic Composition:
- Parents are typically diploid (2n), meaning they possess two sets of chromosomes.
- Germ Cells (Gametes): Specialized reproductive cells involved in sex reproduction.
- Gametes are haploid (n), meaning they have one set of chromosomes.

Definition: Meiosis is the division of specialized diploid primordial germ cells to generate four haploid germ cells.
Process Overview:
- Involves one round of DNA replication followed by two successive nuclear divisions.

Meiosis:
- Generates genetically nonidentical haploid cells.
- Duplicated homologs pair and recombine.
Mitosis:
- Results in genetically identical diploid cells.

In mitosis, duplicated chromosomes line up in a single file at the metaphase plate.
In meiosis, duplicated homologous chromosomes pair, with four chromatids sticking together to form a bivalent or tetrad.
Each homologous chromosome pair is shuffled, leading to haploid cells with a unique combination of genes.

Definition: Crossing-over is the process of swapping chromosomal segments between maternal and paternal homologs in a bivalent.
Formation of Synaptonemal Complex:
- A complex of proteins that holds the bivalent together, aligning homologs for strand exchange between non-sister chromatids.
Chiasma:
- The point where crossing-over occurs, which can happen at multiple points. This ensures proper segregation of homologs, keeping maternal and paternal chromosomes bundled until spindle fibers separate them in anaphase I.

Meiosis I:
- Homologous chromosomes are arranged on opposite sides of the metaphase plate.
- Homologs are pulled to opposite poles.
- At the end, nuclear envelopes form, and cytoplasm divides.
Meiosis II: Similar to mitosis.
- A single cell produces four cells.
- Chromosomes condense and spindle fibers grow and attach to centromeres.
- Chromosomes line up at the metaphase plate, with sister chromatids oriented towards opposite poles.
- Cohesins are degraded, allowing the separation of sister chromatids, leading to the formation of nuclei and final division of cytoplasm.

Based on independent assortment alone, each individual could produce up to 2^n genetically different gametes.
Humans: This results in 2^{23} = 8.4 imes 10^6 different gametes due to the independent assortment of chromosomes.

Nondisjunction: This is the failure of homologs or sister chromatids to separate properly during meiosis, which occurs at relatively high frequencies, particularly in female gametes (about 10%).
Consequences:
- Results in aneuploidy, an abnormal number of chromosomes.
- High rates of miscarriages in early pregnancy in humans may be attributed to this phenomenon.