Meiosis Sexual Reproduction Mitosis Review Mitosis division of

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. – 2 n = 2 n when n = a single chromosome • Ie: n = 23 chromosomes and 2 n = 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” 2 n 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 (2 n)

How about the rest of us? • 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 egg + 46 92 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 diploid 2 n 2 n = 4 single stranded homologous chromosomes 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 meiosis 46 egg zygote 23 46 23 23 sperm gametes fertilization

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

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

Preparing for meiosis • 1 st 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) M 1 prophase 2 n = 6 single stranded 2 n = 6 double stranded

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

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

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 synapsis tetrad prophase 1

Mitosis vs. Meiosis

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