Meiosis

Cell Division Cell Division occurs in two ways: Mitosis produces genetically identical daughter cells Meiosis produces genetically different offspring

Chromosome Changes Most normal cells start with “n” number of pairs of chromosomes n = number of kinds of chromosomes 2n = total number of chromosomes Examples: n 2n Onion 4 8 Fly 8 16 Human 23 46 How Many Chromosomes Does It Have? Fathers donate sex cell with half a set of chromosomes = 23 Mothers donate sex cell with half a set of chromosomes = 23 Offspring receives full set of chromosomes = 46/23 pairs Diploid cells – 2n chromosomes Haploid cells – n chromosomes, ½ set

Quick Quiz!! If a certain cell has 10 total chromosomes, what number of types of chromosomes does it have? 10 5 2.5 20 1 2 3 4 5

Quick Quiz!! If the cells of a sexually reproducing organism have 16 chromosomes, how many were likely to be donated from it’s father? 32 16 8 4 1 2 3 4 5

Quick Quiz!! If you have a diploid cell with 12 kinds of chromosomes, how many total chromosomes are in the cell? 24 12 13 6 1 2 3 4 5

Quick Quiz!! If you have a haploid cell with 12 kinds of chromosomes, how many total chromosomes are in the cell? 24 12 13 6 1 2 3 4 5

Meiosis Sperm Ovum/Egg Only used to create sex cells cells have n number of chromosomes Human = 23 Onion = 4 Sperm Ovum/Egg

They result in new species They have half the number of chromosomes Quick Quiz!! How are sex cells genetically different from other body cells? They are not different They result in new species They have half the number of chromosomes 1 2 3 4 5

Stages of Meiosis Meiosis occurs over two separate sets of stages, Meiosis 1 and Meiosis 2. Let’s examine how this works with a cell that has 6 chromosomes

Prophase I Homologous chromosomes form, find each other and become tetrads Spindle forms Nucleus breaks up

Metaphase I Tetrads line up in the middle of the cell Spindle fibers attach to centromeres

Anaphase I Tetrads split Homologous chromosomes move to opposite ends of the cell Cell begins cytokinesis

Telophase I Two haploid daughter cells are created Meiosis 2 may begin immediately

Quick Quiz!! In the previous slide, what does n=? 1 2 3 4 6 8 10 12 1 2 3 4 5

Prophase II Spindle fibers form Nucleus breaks apart

Metaphase II Individual Chromosomes line up in the center of the cell Spindle fibers attach to centromeres

Quick Quiz!! How is Metaphase II different from Metaphase I of meiosis? It isn’t, they are exactly the same In Metaphase II, the chromosomes don’t line up In Metaphase I, tetrads lined up, not individual chromosomes In Metaphase I, the spindle fibers did not attach 1 2 3 4 5

Anaphase II Sister chromatids separate Chromatids move to opposite ends of the cell Spindles shorten Cytokinesis begins

Telophase II Cytokinesis ends Four haploid cells are left In most males, all four will become gametes In females, only one will

Gametogenesis Gametogenesis = formation of gametes Gametes: haploid sex cells, contain n number of chromosomes Occurs slightly differently in males and females Spermatogenesis = sperm Oogenesis = egg

Gametogenesis Spermatogenesis Oogenesis X X x x X X x x I I I Gametes 3 nonfunctional polar bodies I I I I Sperm Egg or Ovum I I Fertilization: sperm DNA enters egg, completing the set of chromosomes

Quick Quiz!! After fertilization, what is the n number of the cell in the last slide? 2 4 8 16 1 2 3 4 5