Unit 13 Bio 1201

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of gametes (sperm and eggs).
Key processes in meiosis generate genetic diversity through mechanisms such as independent assortment and crossing over.

Genes: Fundamental units of heredity in living organisms that are passed from parents to offspring, influencing traits and characteristics.
Asexual Reproduction: A form of reproduction involving a single organism producing offspring that are genetically identical to itself.
Cloning: A method of asexual reproduction where a genetically identical copy of an organism is produced.
Sexual Reproduction: Involves the combination of genetic material from two parents, leading to offspring with genetic variation.

Karyotype: A visual representation of an individual's complete set of chromosomes, arranged in homologous pairs.
- Consists of duplicated chromosomes, centromeres, and sister chromatids.
- Example: A pair of homologous duplicated chromosomes where the centromere is located at a specific position.
- Size of human chromosomes can vary, with an example size being 5 μm.

Diploid Cells (2n): Cells containing two sets of chromosomes, one from each parent.
- For human somatic cells, this is 2n = 46.
Haploid Cells (n): Cells containing a single set of chromosomes, typical of gametes.
- For human gametes, this is n = 23.
Examples of Diploid and Haploid Cells:
- Egg (n), sperm (n) produced in the ovaries and testis respectively.
- Zygote (2n) results from fertilization, containing genetic material from both parents.

Meiosis consists of two main divisions: Meiosis I and Meiosis II.
- Meiosis I: Separates homologous chromosomes.
- Meiosis II: Separates sister chromatids.
Stages of Meiosis I:
- Prophase I: Homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over).
- Metaphase I: Homologous pairs line up along the metaphase plate.
- Anaphase I: Homologous chromosomes are drawn apart, while sister chromatids remain attached.
- Telophase I and Cytokinesis: Two haploid cells form, each containing duplicated chromosomes.
Stages of Meiosis II:
- Prophase II: Chromosomes condense again; no need for DNA replication.
- Metaphase II: Chromosomes line up at the metaphase plate as individual chromatids.
- Anaphase II: Sister chromatids are separated and pulled towards opposite poles.
- Telophase II and Cytokinesis: Results in four haploid daughter cells, each with unduplicated chromosomes.

Key Differences from Mitosis:
- DNA Replication: Occurs once before meiosis I; does not occur before meiosis II.
- Synapsis: Homologous chromosomes pair during prophase I, a step not found in mitosis.
- Genetic Variation: Meiosis produces genetically unique gametes, whereas mitosis produces identical cells.
- Number of Daughter Cells: Meiosis results in four daughter cells that are haploid (n), compared to two diploid (2n) cells from mitosis.

Random Mutations in Genes: Changes in the DNA sequence can lead to genetic variation.
Independent Assortment: Each pair of homologous chromosomes sorts into daughter cells independently from other pairs. For example, if n = 23, there are $(2^n) = 2^{23}$ possible combinations.
- For humans, this results in approximately 8 million combinations.
Crossing Over: During prophase I, homologous chromosomes exchange segments leading to genetic recombination.
Random Fertilization: Any sperm can fuse with any ovum, creating a vast array of possible combinations in the resulting zygote, approximately 64 trillion diploid combinations.

Mitosis:
- Occurs for growth, repair, and asexual reproduction.
- One division, produces two identical diploid daughter cells.
Meiosis:
- Produces gametes; reduces chromosome number by half.
- Two divisions, producing four genetically diverse haploid cells.