Gametes and Chromosomes

• Gametes, also known as sex cells (egg and sperm cells), and spores.
• Somatic cells, or body cells, comprise most of the body's tissues and organs.
• Somatic cell DNA is not passed to offspring.
• Germ cells, located in reproductive organs (ovaries or testes), develop into gametes.
• Germ cell DNA is passed to offspring.
• Homologous chromosomes are two chromosomes, one from each parent, that are the same size and have copies of the same genes.
• Each species has a characteristic number of chromosomes per cell, usually given for body cells.
• The first 22 pairs of chromosomes are autosomal.
• Sex chromosomes (X and Y) determine gender in mammals.
• Fertilization occurs between egg and sperm in sexual reproduction.
• Body cells are diploid.
• Gametes are haploid.
• Gametes have 22 autosomes and 1 sex chromosome.

Mitosis vs. Meiosis

• Mitosis produces genetically identical diploid cells, takes place throughout an organism's lifetime, and is involved in asexual reproduction.
• Meiosis produces genetically unique haploid cells, takes place at certain times in an organism's life cycle, and is involved in sexual reproduction.

Meiosis Overview

• Meiosis starts with a diploid germ cell that has undergone DNA replication.
• Meiosis involves one DNA replication followed by two cell divisions, resulting in four haploid cells.
• Meiosis is a form of nuclear division that creates four haploid cells from one diploid cell.
• Each round of meiosis (Meiosis I and II) has four phases.
• Homologous chromosome pairs separate in Meiosis I.
• Sister chromatids divide in Meiosis II.
• Sister chromatids are copies of the same chromosome.

Meiosis I Stages

• Prophase I: The nuclear membrane breaks down, chromosomes condense, and homologous chromosomes pair up, where crossing over occurs.
• Metaphase I: Chromosomes line up along the middle of the cell.
• Anaphase I: Homologous chromosomes separate, while sister chromatids remain attached.
• Telophase I: Cytokinesis separates the cells, resulting in haploid cells with duplicated chromosomes.
• Meiosis I separates homologous chromosome pairs.

Meiosis II Stages

• Prophase II: The nuclear membrane breaks down, and cells prepare to divide.
• Metaphase II: Chromosomes line up along the middle of the cell.
• Anaphase II: Sister chromatids separate and move to opposite sides of the cell.
• Telophase II: Nuclear membranes form again, resulting in four haploid cells with a combination of chromosomes from both parents.
• DNA is not replicated between Meiosis I and II.
• In Anaphase I, sister chromatids stay together.

Gametogenesis

• Gametogenesis is the production of gametes; it differs between females and males.
• Spermatogenesis results in four sperm cells.
• Oogenesis results in one egg cell and polar bodies, which are smaller cells that contain little more than DNA and are eventually broken down.
• The haploid cells resulting from meiosis require further changes to form mature gametes capable of fertilization.

Genetic Diversity Factors

• Independent assortment, crossing over, and random fertilization contribute to genetic diversity.
• A major advantage of sexual reproduction is the genetic variation within a species.
• There are about 8 million (2^23) combinations of chromosomes that can be produced through meiosis from one human cell due to 23 chromosomes in haploid cells.
• Random fertilization of gametes yields over 70 trillion (2^23 x 2^23) different combinations.
• Independent assortment describes how chromosomes assort independently of one another during gamete formation.
• Unique phenotypes may give a reproductive advantage to some organisms.
• Phenotype refers to the physical appearance of a trait.

Crossing Over & Genetic Linkage

• Crossing over exchanges chromosome segments between homologous chromosomes. Resulting in genetic recombination, mixing parental alleles.
• Alleles are representations of a gene.
• Crossing over occurs in Prophase I of Meiosis I.
• Closer genes are more likely to be inherited together.
• Linked genes (genetic linkage) are genes on the same chromosome located close to each other.
• Chromosomes contain many genes.
• The farther apart two genes are on a chromosome, the more likely they are to be separated by crossing over and independent assortment.
• Genetic linkage allows calculation of the distance between two genes.