Cell Division --Meiosis--

Meiosis How does it differ from mitosis? Do all cells do it? Why is it done?

The Cell Cycle

Mitosis In mitosis, chromosomes are duplicated once and the cell divides once. Each daughter cell has the same number of chromosomes as the parent cell.

Mitosis vs Meiosis In mitosis, chromosomes are duplicated once and the cell divides once. In meiosis, chromosomes are duplicated once and the cell divides twice.

Meiosis In meiosis, chromosomes are duplicated once and the cell divides twice. Each daughter cell has half the number of chromosomes as the parent cell.

Do all cells undergo meiosis? NO!! Only sex cells undergo meiosis Why only sex cells? So each sex cell (gamete) has the haploid (1n) number of chromosomes. So each gamete has half the number of chromosomes. Male gamete + femal gamete = zygote 1n + 1n = 2n

Adult Male 2n Sexual Reproduction Cycle Adult Female 2n gametes meiosis meiosis n n zygote 2n 2n = diploid n = haploid mitosis New Adult 2n

PRACTICE n 2n Penicillium 1 Fruit flies 4 Gorillas 24 Pea plants 14 Goldfish 47 Some ferns 1000

PRACTICE n 2n Penicillium 1 2 Fruit flies 4 Gorillas 24 48 Pea plants 7 14 Goldfish 47 94 Some ferns 500 1000

Where does meiosis occur? Meiosis occurs in the reproductive structures and organs. Meiosis occurs in the testes of the male and is called spermatogenesis. Male gametes are sperm Meiosis occurs in the ovaries of the female and is called oogenesis. Female gametes are eggs

Gamete production in males (spermatogenesis)

Gamete production in females (oogenesis)

Where does meiosis occur? Special cells in the reproductive structures called germ cells undergo meiosis The germ cells produce haploid gametes Eggs and sperm are haploid (1n) gametes When gametes join, they produce a diploid (2n) zygote

Me I o s I s

Analogy Imagine you have two sets of books on how to build a house; each set has 23 books. One set was given to you by your mother; the other set by your father. You must have all 46 books to build your house, even though you may not use all of the information in all the books.

Each set of books is analogous to the genetic material received from one parent. One set is the information in the egg and one set is the information in the sperm. Humans have 46 chromosomes; each gamete has 23 chromosomes.

Meiosis Prophase I Prophase II Meiosis consists of two stages: MEIOSIS I & MEIOSIS II. Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II

Meiosis 2n 2n Meiosis 1 End of Meiosis I n Meiosis II End of Meiosis II n

Meiosis Step by Step

Interphase Meiosis I Prophase I Metaphase I Anaphase I Telophase I

Results of Meiosis I At the beginning of Meiosis I there was one diploid cell. At the end there are two haploid cells. 2n 1n

Meiosis II The next set of cell divisions will separate the chromatids. This process is very similar to the process of mitosis

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Results of Meiosis II At the beginning of meiosis II, there were 2 haploid cells At the end of meiosis, 4 new haploid cells have been formed formed. 1n 1n

ONE diploid cell prophase I (CROSSING-OVER OCCURS) metaphase I anaphase I telophase I / cytokinesis prophase II metaphase II anaphase II telophase II cytokinesis FOUR haploid cells

Cell Division--Meiosis End

Prophase I Telophase I Metaphase I Anaphase I

Prophase II Metaphase II Anaphase II Telophase II