Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

Meiosis: GOAL!!!!!!!!!!!!!!! In the sex organs (gonads), cells undergo a variation of cell division (meiosis) which yields four daughter cells, each with half the chromosomes of the parent.

In humans, meiosis reduces the number of chromosomes from 46 to 23 Chromosomes #1 through 22 – autosomal Chromosome #23 – sex Meiosis - formation of gametes

Goal 2: So much Diversity!!!!

Meiosis - formation of gametes diploid A cell that contains both sets of chromosomes (1 from each parent ) is said to be diploid (2n) haploid Cells containing 1 set of chromosomes are said to be haploid (n)

It produces 4 haploid cells that are genetically different from each other and from the diploid parent 2 parts: Meiosis I – separation of homologues Meiosis II – separation of sister chromatids Meiosis

Prophase I Everything that happens in Prophase of mitosis also happens in Prophase I of meiosis Chromosomes find their pairs to form a tetrad (process called synapsis) They can exchange genetic info (crossing over) Site of crossing over is the chiasmata

Metaphase I Same as Metaphase of mitosis Tetrads line up at the equator *Homologous Chromosomes: matching pair

Anaphase I Same as Anaphase of mitosis Homologous chromosomes separate and move to the poles

Telophase I Same as Telophase of mitosis Instead of having two genetically identical cells, the chromosomal number has been halved (2n to n) Chromosomes are still double stranded (sister chromatids still attached)

Meiosis II No replication occurs Mitosis resembles meiosis II more than meiosis I Sister chromatids are separated to make daughter cells that have a single set (n) of single stranded chromosomes

Prophase II Same as prophase of mitosis

Metaphase II and Anaphase II Double stranded (not homologous) chromosomes align along the equator in Metaphase II

Telophase II and cytokinesis At the end of meiosis, there are four haploid daughter cells

Mitosis and meiosis have several key differences. The chromosome number is reduced by half in meiosis, but not in mitosis. Mitosis produces daughter cells that are genetically identical to the parent and to each other. Meiosis produces cells that differ from the parent and each other.

Mitosis produces two identical daughter cells, but meiosis produces 4 genetically different cells.

Sexual vs. Asexual Reproduction In asexual reproduction, a single individual passes along copies of all its genes to its offspring Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis

Sexual vs. Asexual Reproduction Sexual reproduction results in greater variation among offspring than does asexual reproduction Offspring of sexual reproduction vary genetically from their siblings and from both parents

Sexual vs. Asexual Reproduction Three mechanisms contribute to genetic variation: independent assortment crossing over (Prophase I) random fertilization – each zygote is the result of 1 of 70 trillion possible chromosomal combos (2 23 x 2 23 )