Meiosis Mitosis Cuts chromosome number in half Forms sex cells or gametes (sperm and egg) Mitosis equal division forms body or somatic cells

Human’s have 23 homologous pairs of chromosomes Figure: 09-07 Title: A Karyotype Displays a Full Set of Chromosomes Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Notice that this karyotype set is from a human male; there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). A female would have two X chromosomes. All the chromosomes are in the duplicated state.

One DNA double helix, one chromosome, one chromatid Review from DNA Replication Lecture Old half of DNA Old half of DNA New nucleotides Two DNA double helices, one chromosome, two chromatids Review from Mitosis Lecture Chromosome with two chromatids

Ploidy = number of chromosome sets in a cell Haploid (1N) = one set of chromosomes Diploid (2N) = two sets of chromosomes

Each member of the homologous pair was inherited from either a person’s male and female parent. Figure: 09-07 Title: A Karyotype Displays a Full Set of Chromosomes Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Notice that this karyotype set is from a human male; there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). A female would have two X chromosomes. All the chromosomes are in the duplicated state.

23 + 23 = 46 homologous chromosomes Sperm, 1N egg, 1N 23 chromosomes 23 chromosomes 23 + 23 = 46 homologous chromosomes Zygote = 2N cell

Same cell after DNA replication Sperm, 1N egg, 1N 1 1’ 2 2’ 1’ 1 Eye Dark Eye light 1 and 1’ = homologous chromosomes Same cell after DNA replication 1’ 1’ 1 1 Zygote = 2N cell 2 2’ 2 2’

Zygote (2n) Mitosis Adult Cells (2n) Meiosis Meiosis Fertilization Human = 46 chromosomes Mitosis Adult Cells (2n) Human = 46 Meiosis Meiosis Fertilization Sperm (1n gamete) Egg (1n gamete) 23 + 23 Sperm = 23 chromosomes Egg = 23 chromosomes

Meiosis = One 2N cell forms four 1N cells Mitosis = One 2N cell forms two 2N cells

Mitosis Meiosis 1 1’ 1 2N 2’ 2 2N 2 1’ 1 1’ 2’ 1N 1N 2 2’ 2’ 2 2’ 2 1’

Crossing over = Exchange of genetic material between chromatids on homologous chromosomes …... Only in meiosis

Homologous Chromosomes Recombinant Chromatids 1 Homologous Chromosomes 1’ 1 Crossing over 1’ Recombinant Chromatids

Chiasmata = cross-overs

Which one can have crossing over? Mitosis Meiosis 1 1’ Which one can have crossing over? 1 2N 2’ 2 2N 2 1’ 1 1’ 2’ 1N 1N 2 2’ 2’ 2 2’ 2 1’ 1 2’ 1 1 2 1’ 2’ 2 1 1’ 1’ 1N 1N 1N 1N 2N 2N

Crossing over occurs in meiosis because homologous chromosomes are next to each other. Review

Summary Mitosis – 1, 2N cell makes 2, 2N Cells Review Summary Mitosis – 1, 2N cell makes 2, 2N Cells Meiosis – overall; 1, 2N cell makes 4, 1N cells