AP Biology 2007-2008

Meiosis & Sexual Reproduction

AP Biology

Cell division / Asexual reproduction § Mitosis

u produce cells with same information §  identical daughter cells

u exact copies §  clones

u same amount of DNA §  same number of chromosomes §  same genetic information

Aaaargh! I’m seeing

double!

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Asexual reproduction § Single-celled eukaryotes

u yeast (fungi) u Protists

§ Paramecium § Amoeba

§ Simple multicellular eukaryotes u Hydra

What are the disadvantages of

asexual reproduction?

What are the advantages?

budding

budding

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How about the rest of us? § What if a complex multicellular organism

(like us) wants to reproduce? u  joining of egg + sperm

§ Do we make egg & sperm by mitosis?

46 46 + 92

egg sperm zygote

What if we did, then….

Doesn’t work!

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Human female karyotype

46 chromosomes 23 pairs

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Human male karyotype

46 chromosomes 23 pairs

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Homologous chromosomes §  Paired chromosomes

u  both chromosomes of a pair carry “matching” genes §  control same inherited characters §  homologous = same information

diploid 2n

2n = 4

single stranded homologous

chromosomes

double stranded homologous chromosomes

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How do we make sperm & eggs? § Must reduce 46 chromosomes → 23

u must half the number of chromosomes

23

23 46

egg

sperm

46

meiosis 46

fertilization

23

23

gametes

zygote

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Meiosis: production of gametes §  Alternating stages

u  chromosome number must be reduced §  diploid → haploid §  2n → n

w humans: 46 → 23 §  meiosis reduces

chromosome number §  makes gametes

u  fertilization restores chromosome number §  haploid → diploid §  n → 2n

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Sexual reproduction lifecycle

§  1 copy §  haploid §  1n

§  2 copies §  diploid §  2n

§  1 copy §  haploid §  1n

meiosis fertilization

We’re mixing things up here! A good thing?

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Meiosis §  Reduction Division

u  special cell division in sexually reproducing organisms

u  reduce 2n → 1n u  diploid → haploid

§  “half” u  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!

AP Biology

Overview of meiosis I.P.M.A.T.P.M.A.T

interphase 1 prophase 1 metaphase 1 anaphase 1

telophase 1

prophase 2 metaphase 2 anaphase 2 telophase 2

2n=4

n=2

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Repeat after me! I can’t

hear you!

2nd division of meiosis separates sister chromatids

1st division of meiosis separates homologous pairs

Double division of meiosis

DNA replication

Meiosis 1

Meiosis 2

AP Biology 2n = 6 double

stranded

2n = 6 single

stranded

Preparing for meiosis §  1st step of meiosis

u Duplication of DNA u 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)

M1 prophase

AP Biology Repeat

after me!

2n = 4 single

stranded Meiosis 1

2n = 4 double

stranded prophase 1

§  1st division of meiosis separates homologous pairs

tetrad

synapsis

1n = 2 double

stranded

telophase 1

2n = 4 double

stranded metaphase 1

I can’t hear you!

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Meiosis 2

1n = 2 double

stranded metaphase 2

1n = 2 single

stranded

telophase 2

prophase 2

1n = 2 double

stranded

§  2nd division of meiosis separates sister chromatids

What does this division look like?

4

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Steps of meiosis § Meiosis 1

u  interphase u prophase 1 u metaphase 1 u anaphase 1 u  telophase 1

§ Meiosis 2 u prophase 2 u metaphase 2 u anaphase 2 u  telophase 2

2nd division of meiosis separates sister chromatids (1n → 1n) * just like mitosis *

1st division of meiosis separates homologous pairs (2n → 1n) “reduction division”

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Trading pieces of DNA § Crossing over

u  during Prophase 1, sister chromatids intertwine §  synapsis §  homologous pairs swap

pieces of chromosome w DNA breaks & re-attaches

tetrad

synapsis

prophase 1

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Crossing over §  3 steps

u cross over u breakage of DNA u  re-fusing of DNA

§ New combinations of traits

What are the advantages of

sexual reproduction?

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Meiosis 1

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Meiosis 2

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Mitosis vs. Meiosis

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Mitosis vs. Meiosis §  Mitosis

u  1 division u  daughter cells

genetically identical to parent cell

u  produces 2 cells u  2n → 2n u  produces cells for

growth & repair u  no crossing over

§  Meiosis u  2 divisions u  daughter cells

genetically different from parent

u  produces 4 cells u  2n → 1n u  produces gametes

u  crossing over

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mitosis zygote

Putting it all together…

23

23 46

egg

sperm

46

meiosis 46 23

23

fertilization mitosis & development

meiosis → fertilization → mitosis + development

46

46

46

46

46

46 46 46

gametes

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The value of sexual reproduction §  Sexual reproduction introduces genetic variation

u  genetic recombination during meiosis §  independent assortment of chromosomes

w  random alignment of homologous chromosomes in Meiosis 1 u  crossing over

§  mixing of alleles across homologous chromosomes u  random fertilization

§  which sperm fertilizes which egg? §  Driving evolution

u  variation for natural selection

metaphase1

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Variation from genetic recombination §  Independent assortment of chromosomes

u meiosis introduces genetic variation u 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 Dad from Mom offspring new gametes made by offspring

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Variation from crossing over § Crossing over creates completely new

combinations of traits on each chromosome u creates an infinite

variety in gametes

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Variation from random fertilization § Sperm + Egg = ?

u any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations

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Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences.

Martin & Charlie Sheen, Emilio Estevez

Michael & Kirk Douglas

Baldwin brothers

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Sperm production Epididymis Testis

Coiled seminiferous tubules

Vas deferens

Cross-section of seminiferous tubule

spermatozoa

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

germ cell (diploid)

MEIOSIS II

MEIOSIS I

§ Spermatogenesis u  continuous & prolific process u  each ejaculation =

100-600 million sperm

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Egg production

Meiosis 1 completed during egg maturation

Meiosis 2 completed triggered by fertilization

ovulation

§ Oogenesis u  eggs in ovaries halted

before Anaphase 1 u  Meiosis 1 completed

during maturation u  Meiosis 2 completed

after fertilization u  1 egg + 2 polar bodies

What is the advantage of this development system?

unequal division

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Oogenesis

MEIOSIS I

MEIOSIS II

first polar body

second polar body

ovum (haploid)

secondary oocyte

(haploid)

primary oocyte

(diploid)

germinal cell (diploid)

primary follicles

mature follicle with secondary oocyte

ruptured follicle (ovulation)

corpus luteum

developing follicle

fertilization fallopian tube

after fertilization

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Differences across kingdoms §  Not all organisms use haploid & diploid

stages in same way u  which one is dominant (2n or n) differs u  but still alternate between haploid & diploid

§  must for sexual reproduction

AP Biology 2007-2008

What are the DISadvantages of

sexual reproduction?

Any Questions??