Sexual Reproduction  Heredity genes  Heredity - passing down of characteristics (genes)  Alleles  Alleles- different forms of the same gene

Sexual Reproduction  Why sexual reproduction? shuffles alleles; new combinations provides genetic variation in species

Meiosis cell division  Type of cell division One germ * cell makes 4 gametes (egg/sperm) with ½ the # of chromosomes. Occurs only in germ * cells of gonads ○ testes /ovaries Occurs in flowers ○ ovary and anther * “Germ” here doesn’t mean bacteria. It’s a term used for the type cells that gives rise to gametes in organisms.

While females are born with all the eggs they will ever have (around 400,000), only about 400 will ever mature and ovulate between puberty and menopause.

 Matrue males produce about 100 million sperm each day.

Sperm surrounding an egg

This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes.

Homologous Chromosomes  Pair of chrom. similar in shape, size, and types of genes. Each locus (location of the gene) in same position on chrom. homologues  Humans have 23 pairs of homologues Housefly – 6 prs Housefly – 6 prs Chicken – 39 prs Chicken – 39 prs Apple – 17 prs Apple – 17 prs Dog – 39 prs Dog – 39 prs Cat – 19 prs Cat – 19 prs This is a karyotype (an image of an organism’s chromosomes) This is a karyotype of a normal human male

Chromosome numbers:  However many “types” of chrom. an organism has, that number is the “n” number of chrom. it has. Body cellsEgg/Sperm cells Called:Somatic cellsGametes Term for chrom. # Diploid (2 sets of chrom.) Haploid (1 set of chrom.) “n” number2n1n For Humans:46 chrom.23 chrom.

Homologous Chromosomes Paternal (from Dad) Maternal (from Mom) eye color locus eye color locus hair color locus hair color locus

Animation  Meiosis A animation Meiosis A  Meiosis B animation Meiosis B  Meiosis C animation Meiosis C

Prophase I  Longest and most complex phase ( 90% of meiosis ).  DNA condenses into chromosomes.  Synapsis - homologous chrom. tetrad  Synapsis - a process: when homologous chrom. come together, pair up, form a tetrad.

Prophase I - Synapsis Nonsister chromatids sister chromatids Tetrad

Prophase I - Crossing Over  Crossing over may occur in the tetrad: between nonsister chromatids, ends break and reattach

Crossing Over - Provides Variation nonsister chromatids Chiasma: site of crossing over variation Tetrad

Metaphase I  Shortest phase; paired homologues align.  INDEPENDENT ASSORTMENT  INDEPENDENT ASSORTMENT occurs  pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation.

Anaphase I  Homologous chromosomes separate towards the poles (Tetrads separate)  Sister chromatids  Sister chromatids remain attached

Telophase I  Each pole now has haploid set of chromosomes (however – still doubled).  Cytokinesis  Cytokinesis occurs: two haploid daughter cells formed.

Meiosis II  No interphase II DNA replication ( no more DNA replication)  Remember: Meiosis II mitosis  Remember: Meiosis II is similar to mitosis Prophase II / Metaphase II Anaphase II

Telophase II telophasemitosis  Same as telophase in mitosis.  Nuclei form.  Cytokinesis  Cytokinesis occurs (2 nd time).  Four haploid daughter cells produced (chromosomes now back to single condition). gametes ~ sperm or egg; ovule or pollen grain

Gamete Formation in Animals  Diff. bet. male and female gametes. Male: spermatogenesis all 4 develop into sperm cells. Female: oogenesis cytokinesis is uneven. most cytoplasm goes to 1 of the 4 “eggs” (forms 1 large egg cell) 3 other cells are small “polar bodies” which die

Spermatogenesis 2n = 46 human germ cell in testes diploid (2n) n=23 Still doubled chromosomes n=23 Still doubled chromosomes meiosis I n=23 sperm haploid (n) meiosis II

Oogenesis 2n = 46 human germ cell in ovary diploid (2n) n=23 Still doubled n=23 Still doubled meiosis I 23 Ovum n=23 Polar Bodies meiosis II

Mitosis vs Meiosis  Mitosis Body (somatic cells) 2 daughter cells made (identical) Each w/ same # & kind of chrom. as parent cell 1 division process 1 cytokinesis No synapsis or crossing over Are diploid (2n)  Meiosis Germ cells of gonads 4 gamete cells made (all different) Each w/ ½ chrom. # as parent cell 2 divisions 2 cytokineses events Synapsis & crossing over occurs in Prophase 1 Are haploid (n)

Nondisjunction  When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes.  Lethal most of the time

Karyotype… normal male

Karyotype… normal female

 Downs Syndrome  Trisomy 21  47, XY, +21  The only trisomy survivable to adulthood