REVIEW- MITOSIS Cell Cycle Chromatin Condenses Go At Cell Maturity Chromosome now 2 sister chromatids

Chromatin, Condensed DNA, Chromosomes 95% of the time Easily Visible, 5% of the time One Chromatid

Chromosome Replication

Homologous Chromosomes vs Sister Chromatids

TWO TYPES OF CELLS: SOMATIC and GAMETES  SOMATIC:(Body Cells) (Body cells: skin/muscle/liver/bone/brain/blood) Diploid Cells (2n): The # of chromosomes in a somatic cell 2n = 46 in humans…varies/species  GAMETES:(Sex Cells) (Sex cells = Gametes = Humans?) 23 Sperm + 23 Egg = 46 Zygote (The fertilized egg) Haploid Cells (n): The # of chromosomes in a sex cell n = 23 in humans (half a complete set)

2n…n…difference? Not the NUMBER of chromosomes but the SIZE, COMPLEXITY, and amount of DNA

Chromosomes are counted on karyotypes Human Somatic Cell: 46 chromosomes  23 pairs 44 autosomes  22 pairs 2 sex chromosomes  1 pair  XX = female  XY = male Allele: Different forms of a gene (Blue, brown eyes)

Down Syndrome Trisomy 21; Nondisjunction at 21

Lab Today… DVD: Visualizing Cell Process, DNA, Stages of Mitosis Onion Root Tip……Interphase and PMAT DVD: Visualizing Cell Process- DNA- Stages of Mitosis ~30sec Interphase Telephase with cell plate Metaphase Anaphase Prophase

Meiosis- Key Differences from Mitosis 1. Meiosis reduces the # of chromosomes by half 2. Daughter cells differ from parent and each other Mitosis- Daughter Cells are Identical (Somatic) Meiosis- At end 4 different cells….Genetic Variation 3. Meiosis involves TWO divisions; Mitosis only one.

Meiosis- Key Differences from Mitosis Mitosis Creates Body Cells; Meiosis creates Sex cells 1. Meiosis I (PMAT I) involves: Prophase: homologous chromosomes pair up – called a ‘tetrad’. = Where Crossing Over of non-sister chromatids takes place. *First source of Genetic Variation- no longer identical. Metaphase I: Homologous pairs line up on the equator- double file- Mom with sister chromosome, dad with sister chromosome. (Double 23 across, not 46) *Independent Assortment- second source of genetic variation Anaphase I: Tetrad Splits- Sister chromatids do NOT separate at the centromere. (Pair of homologous chromatids split up)

Crossing Over Independent Assortment 23..STILL Replicated

MEIOSIS II: Separating Sister Chromatids * Do NOT COPY DNA again 23.. Single File All ‘n’

Crossing Over Prophase I First Source of Genetic Variation Chiasmata- sites of crossing over, occur in synapsis- exchange of genetic material between non-sister chromatids No longer Genetically Identical

Independent Assortment: Metaphase I 2 nd Source of Genetic Variation At least (2 23 ) 8 million possible combinations from EACH parent…. 64 TRILLION combinations for a diploid zygote Producing truly unique offspring

Third Source of Genetic Variation: Random Fertilization (which sperm will fertilize the egg?) Spermatogenesis vs Oogenesis