Why do you share some but not all characters of each parent? Meiosis Why do you share some but not all characters of each parent?

Heredity Heredity Passing of genetic traits from parent to offspring Chromosome theory of heredity Chromosomes carry genes Gene = unit of heredity

What Meiosis is all About Meiosis allows the creation of unique individuals through sexual reproduction.

Two different types of cells Somatic Cells are “body” cells and contain the normal number of chromosomes . Examples would be … skin cells, brain cells, etc. Diploid Gametes are the “sex” cells and contain only ½ the normal number of chromosomes. Haploid Sperm cells and ova are gametes.

Homologous Chromosomes Pair of chromosomes (maternal and paternal) similar in shape and size. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Homologous pairs (tetrads are duplicated) carry genes controlling the same inherited traits.

Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color locus hair color Paternal Maternal

Autosomes In Humans the “Autosomes” are sets 1 – 22 (The Autosomes code for most of the offspring’s traits)

In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male

Sex Chromosomes the 23rd set

Boy or Girl? The Y Chromosome Decides X chromosome Y chromosome

In The Beginning Two Reproduction where the genetic material combined is called sexual reproduction Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved

Two Important Accomplishments 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). In meiosis, one diploid cells produces four haploid cells.

Two Important Accomplishments 2) Meiosis scrambles the genes that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This happens through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division –4 gamete cells with ½ the original number of chromosomes

Gametes Are Haploid Gametes have exactly one set of chromosomes, this state is called haploid (1n) Regular cells have two sets of chromosomes, this state is called diploid (2n) Why?

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis Meiosis 1st division

Synapsing Unlike mitosis, homologous chromosomes line up next to each other during prophase This process is called synapsing Lined up homologues are called tetrads

Prophase I - Synapsis Longest and most complex phase. spindle fiber 90% of the meiotic process is spent in Prophase I Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). centrioles spindle fiber homologues

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Prophase 1– Crossing Over Homologous chromosomes connect at areas called chiasmata Segments break and reform at similar locations. This results in new genetic combinations

Crossing Over Non-sister chromatids Tetrad variation chiasmata: site of crossing over Tetrad variation

Check out the Tetrads

Crossing-Over

Metaphase I INDEPENDENT ASSORTMENT Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT metaphase plate OR metaphase plate

Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 223 = 6,000,0000. That’s a lot of diversity by this mechanism alone

Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

In Telophase 1, two daughter cells are formed. They are NOT identical In Telophase 1, two daughter cells are formed. They are NOT identical! Why?

Meiosis II No interphase II (or very short - no more DNA replication)

Prophase II same as prophase in mitosis spindle reforms and chromosomes move toward the metaphase plate

Metaphase II same as metaphase in mitosis sister chromatids lined up on the metaphase plate metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids are separated and pulled toward opposite poles of the cell

Telophase II Same as telophase in mitosis Nuclei form, Cytokinesis occurs. Four haploid daughter cells produced. Telophase II

In Humans Example: 23 chromosomes in haploid n = 23 2n = 223 = ~ 6 million possible combinations!

Random fertilization At least 6 million combinations from Mum and another 6 million from Dad … >36 trillion combinations for a diploid zygote!!!

Meiosis in Male and Female In males is called spermatogenesis and produces sperm. In females is called oogenesis and produces ova.

Secondary Spermatocyte Spermatogenesis n=23 sperm meiosis II n=23 meiosis I Primary Spermatocyte n=23 2n=46 Secondary Spermatocyte n=23 4 sperm cells are produced from each primary spermatocyte n=23

Spermatogenesis The male gamete is called a spermatozoa Four spermatocytes are formed during meiosis Men are busy - meiosis produces roughly 250,000 sperm a day.

Oogenesis The female gamete is called an ovum At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.

Oogenesis Only one ovum is produced during meiosis Oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.

Any Questions?