Meiosis Sexual Reproduction Ch 13 Cell division Asexual

Meiosis & Sexual Reproduction (Ch. 13)

Cell division / Asexual reproduction • Mitosis – produce cells with same information • identical daughter cells – exact copies • clones – same amount of DNA • same number of chromosomes • same genetic information Aaaargh! I’m seeing double!

Asexual reproduction • Single-celled eukaryotes – yeast (fungi) – Protists • Paramecium • Amoeba • Simple multicellular eukaryotes – Hydra What are the disadvantages of asexual reproduction? What are the advantages? budding

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! What if we did, then…. 46 egg + 46 92 sperm zygote Doesn’t work!

Human female karyotype 46 chromosomes 23 pairs

Human male karyotype 46 chromosomes 23 pairs

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 zygote 23 egg 46 23 23 sperm gametes fertilization

Meiosis: production of gametes • Meiosis – chromosome number must be reduced • diploid haploid • 2 n n – humans: 46 23 • makes gametes – fertilization restores chromosome number • haploid diploid • n 2 n haploid diploid

Sexual reproduction lifecycle § 2 copies § diploid § 2 n § 1 copy § haploid § 1 n gametes fertilization meiosis In the next generation… We’re mixing things up here! A good thing? § 1 copy § haploid § 1 n gametes

Meiosis • Reduction Division – special cell division for sexual reproduction – reduce 2 n 1 n – diploid haploid • “two” “half” – makes gametes • sperm, eggs Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!

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

Double division of meiosis DNA replication 1 st division of meiosis separates homologous pairs 2 nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 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 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

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 *

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

Crossing over • 3 steps – cross over – breakage of DNA – re-fusing of DNA What are the advantages of crossing over in sexual reproduction? • New combinations of traits

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

Putting it all together… meiosis fertilization mitosis + development gametes 46 meiosis 46 23 egg 23 23 23 zygote fertilization sperm 46 46 46 mitosis development

The value of sexual reproduction • Sexual reproduction introduces genetic variation – genetic recombination • independent assortment of chromosomes – random alignment of homologous chromosomes in Metaphase 1 – crossing over • mixing of alleles across homologous chromosomes – random fertilization • which sperm fertilizes which egg? • Driving evolution – providing variation for natural selection metaphase 1

Variation from genetic recombination • Independent assortment of chromosomes – meiosis introduces genetic variation – gametes of offspring do not have same combination of genes as gametes from parents • random assortment in humans produces 223 (8, 388, 608) different combinations in gametes from Mom from Dad offspring new gametes made by offspring

Variation from crossing over • Crossing over creates completely new combinations of traits on each chromosome – creates an infinite variety in gametes

Variation from random fertilization • Sperm + Egg = ? – any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations (not even counting crossing over!!!!!)

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) Vas deferens spermatids (haploid) spermatozoa • Spermatogenesis – continuous. Cross-section & prolificofprocess seminiferous tubule – each ejaculation = 100 -600 million sperm MEIOSIS II

Egg production • Oogenesis – eggs in ovaries halted before Anaphase 1 – Meiosis 1 completed during maturation – Meiosis 2 completed after fertilization unequal divisions – 1 egg + 2 polar bodies Meiosis 1 completed during egg maturation What is the advantage of this development system? Meiosis 2 completed triggered by fertilization ovulation

Oogenesis Putting all your egg in one basket! germinal cell (diploid) primary follicles fallopian tube fertilization primary oocyte (diploid) MEIOSIS I secondary oocyte (haploid) first polar body MEIOSIS II after fertilization second polar body ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum

Differences across kingdoms • Not all organisms use haploid & diploid stages in same way – which one is dominant (2 n or n) differs – but still alternate between haploid & diploid • must for sexual reproduction

Eggs are Precious, Sperm is Cheap. Any Questions? ? What are the DISadvantages of sexual reproduction?

Review Questions

1. How do cells at the completion of meiosis compare with cells that have replicated their DNA and are just about to begin meiosis? A. They have twice the amount of cytoplasm and half the amount of DNA. B. They have half the number of chromosomes and half the amount of DNA. C. They have the same number of chromosomes and half the amount of DNA. D. They have half the number of chromosomes and onefourth the amount of DNA. E. They have half the amount of cytoplasm and twice the amount of DNA.

2. Which number represents G 2? * A. I B. II C. III D. IV E. V

3. . Which number represents the DNA content of a sperm cell? A. I B. II C. III D. IV E. V

4. Which of the following would not be considered a haploid cell? A. daughter cell after meiosis II B. gamete C. daughter cell after mitosis in gametophyte generation of a plant D. cell in prophase I E. cell in prophase II

5. A cell in G 2 before meiosis compared with one of the four cells produced by that meiotic division has A. twice as much DNA and twice as many chromosomes. B. four times as much DNA and twice as many chromosomes. C. four times as much DNA and four times as many chromosomes. D. half as much DNA but the same number of chromosomes. E. half as much DNA and half as many chromosomes.