Bell Ringer + = 46 CHROMOSOMES + 46 CHROMOSOMES = 92 CHROMOSOMES What is this process called? Is this scenario reasonable? Why or why not?

Bell Ringer + = = 23 CHROMOSOMES + 23 CHROMOSOMES 46 CHROMOSOMES =

MEIOSIS & SEXUAL REPRODUCTION

Meiosis Overview The purpose of meiosis is to create gametes (sperm and eggs) with half the original number of chromosomes. Meiosis begins with one diploid cell (2 of each type of chromosome) and ends with four haploid cells (one of each chromosome). 4 sperm cells in males 1 egg cell with 3 polar bodies in females

Chromosomes Chromosome: A structure that carries genes. Each human cell has 46 chromosomes (23 pairs) Each human gamete has 23 chromosomes (0 pairs)

Meiosis Meiosis: occurs in testes of males occurs in ovaries of females Involves two divisions with one DNA replication.

Interphase Before meiosis begins, DNA must replicate itself so enough copies will be available for the four new cells being created. This happens in Interphase. Chromosomes in the replicated state are made of two SISTER CHROMATIDS that are connected by a CENTROMERE.

Meiosis I Occurs in Four Phases Prophase I- Homologous chromosomes pair up. Crossing over may occur in this phase.

Metaphase I Homologous Chromosomes line up at the spindle equator. The random arrangement of pairs on either side of the spindle equator results in variation in the genotypes found in the gametes.

Anaphase I Homologous chromosomes are pulled to opposite ends of the cell by the spindle fibers.

Telophase I The cell begins to pinch together and form two distinct haploid cells. How many chromosomes were in the original cell? A short Interphase occurs but no DNA replication

Meiosis II Prophase II- No pairing of homologues (they are in separate cells). Metaphase II- Sister chromatids line up at the spindle equator. Anaphase II- Sister chromatids are pulled to opposite ends of the two cells. Telophase II- Cytokinesis occurs in both cells resulting in 4 haploid cells.

Gametogenesis The purpose of meiosis is to create gametes. The production of male gametes is called spermatogenesis

Spermatogenesis

Oogenesis The production of female gametes is called oogenesis.

Oogenesis

Oogenesis

SEXUAL REPRODUCTION Definition: the union of two gametes (fertilization), thereby combining the genes from two different organisms. It is responsible for the abundant variation that can be seen in organisms that reproduce in this way.

Fertilization The gametes that are formed by meiosis may randomly combine to form offspring that contain a variety of traits.

Summary Meiosis results in the production of gametes which contain a variety of combinations of the parental genes. Variation in gametes results from meiosis I when: 1) In prophase I when crossing over may occur between homologues. 2) During metaphase as homologues randomly line up on either side of the spindle equator.

Fertilization contributes to variation when the winner of the sperm marathon hits the target egg and claims its prize!