Meiosis Sexual Reproduction

Mitosis Review Mitosis: division of somatic (body) cells during the cell cycle. Product = 2 identical daughter cells. Start with 46 chromosomes and ends with 46 chromosomes. 2n = 2n when n = a single chromosome Ie: n = 23 chromosomes and 2n = 46 chromosomes or 23 pairs of chromosomes.

Notes Meiosis is: The division of sex cells (gametes) Humans have 46 chromosomes Fruit fly has 8 chromosomes Diploid = “two sets” 2n Haploid = “one set” n Meiosis is division of gametes, while mitosis is division of somatic (body) cells. There are 2 cell divisions during meiosis. Gametes (sex cells): Males = XY Females = XX Fertilizations: Joining of a sperm (n) and egg (n) to create a zygote (2n)

How about the rest of us? + 46 46 92 What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis? No! if we did, then…. 46 + 46 92 egg sperm zygote Doesn’t work!

Karyotype A “picture” of our chromosomes used by genetists. If you remember: our parents gave each of us 1/2 set of chromosomes (23 of them to be exact) to pair up and complete the set (46). Used by scientists and doctors to look for abnormalities and gender Pictures are taken during the prophase/metaphase stage of Mitosis. When chromosomes become visible. Each chromosome is paired up with it’s match and then laid out in order by size and shape.

Homologous chromosomes Paired chromosomes both chromosomes of a pair carry “matching” genes control same inherited characters homologous = same information single stranded homologous chromosomes diploid 2n 2n = 4 double stranded homologous chromosomes

How do we make sperm & eggs? Must reduce 46 chromosomes  23 must reduce the number of chromosomes by half 23 46 23 zygote 46 egg 23 meiosis 46 23 fertilization sperm gametes

Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n  1n) “reduction division” 2nd division of meiosis separates sister chromatids (1n  1n) * just like mitosis *

Overview of meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1 metaphase 1 anaphase 1 n = 2 n = 2 prophase 2 metaphase 2 anaphase 2 telophase 2 n = 2 telophase 1

Preparing for meiosis 1st step of meiosis Duplication of DNA Why bother? meiosis evolved after mitosis convenient to use “machinery” of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) 2n = 6 single stranded 2n = 6 double stranded M1 prophase

Meiosis 1 1st division of meiosis separates homologous pairs synapsis single stranded 1st division of meiosis separates homologous pairs 2n = 4 double stranded prophase 1 synapsis 2n = 4 double stranded metaphase 1 tetrad reduction 1n = 2 double stranded telophase 1 Repeat after me! I can’t hear you!

4 Meiosis 2 2nd division of meiosis separates sister chromatids double stranded prophase 2 What does this division look like? 1n = 2 double stranded metaphase 2 1n = 2 single stranded telophase 2 4

Meiosis 1 & 2

Trading pieces of DNA Crossing over during Prophase 1, sister chromatids intertwine homologous pairs swap pieces of chromosome DNA breaks & re-attaches prophase 1 synapsis tetrad

Mitosis vs. Meiosis

Mitosis vs. Meiosis Mitosis Meiosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n  2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n  1n produces gametes crossing over

Pedigrees Pedigrees are family trees that show specific traits passed down through each generation. Ms. Greb’s family tree:

How to read a pedigree