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Chapter 13.

Meiosis &Sexual Life Cycle

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Cell reproduction Mitosis

produce cells with same information identical daughter cells

exact copies (clones) same amount of DNA

same number of chromosomes asexual reproduction

What are disadvantages of asexual reproduction?What are advantages?

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Asexual reproduction Single-celled eukaryotes

reproduce asexually yeast Paramecium Amoeba

Simple multicellulareukaryotes reproduceasexually Hydra budding

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Budding in Yeast

Binary fission in Amoeba

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Reproduction in multicellular organisms

How is it possible to maintain such geneticcontinuity in a multicellularorganism?

Baldwin brothers

Martin & Charlie Sheen, Emilio Estevez

Michael & KirkDouglas

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Sexual reproduction

What has to happento make gametes?

But how does a complex multicellularorganism reproduce? joining of gametes

egg (or ovum) sperm

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Role of Meiosis in Sexual Life Cycles Alternating processes

fertilization &meiosis alternate

meiosis reduceschromosome number diploid → haploid 2n → n

fertilization restoreschromosome number haploid → diploid n → 2n

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Meiosis &fertilization mustalternate forsexualreproduction tocontinue

Alternation of generationthe dominantmulticellular stage(n vs. 2n) differsamongst organisms

Haploid (n) organism = gametophyteDiploid (2n) organism = sporophyte

Alternation of generations:• life history having a multicellular haploid stage which alternates

with a multicellular diploid stage

Evolutionary trends:Earlier organism have dominant gametophyte (n) individual

• the gametophyte generation is the major stage in the life ofmosses, the visible stage in fungi, and is an independent plantin ferns

However, the gametophyte is only an inconspicuous structure withinthe flower or other reproductive organs of "higher" plants

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Visualizing chromosomes

karyotype

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23 pairs of chromosomes

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Human female karyotype

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Human male karyotype

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What does karyotype reveal? Mutation in chromosome number or

gross chromosomal abnormalities

Down’s syndrome

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Homologous chromosomes Paired chromosomes

both chromosomes of a pair carry genescontrolling same inherited characters homologous = same information

- 2 copies- diploid- 2n

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Sexual reproduction: Fertilization

- 2 copies- diploid- 2n

- 1 copy- haploid- 1n

What wouldhappen ifmeiosisdidn’t occur?

from Mom from Dad

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Making gametes- 2 copies- diploid- 2n

- 1 copy- haploid- 1n

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Meiosis = reduction division Meiosis

special cell division in sexuallyreproducing organisms

reduce 2n → 1n diploid → haploid (half) makes gametes (sperm, eggs)

Warning: meiosis evolved from mitosis, so stages& “machinery” are similar but the processes areradically different. Do not confuse!

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2nd division ofmeiosis separatessister chromatids

1st division ofmeiosis separateshomologous pairs

Double divisionof meiosis

DNA replication

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Steps of meiosis Meiosis 1

interphase prophase 1 metaphase 1 anaphase 1 telophase 1

Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2

2nd division ofmeiosis separatessister chromatids(1n → 1n)* just like mitosis *

1st division of meiosisseparates homologouspairs(2n → 1n)

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2n double

stranded

2n single

stranded

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 meiosisinterphase(just like in mitosis)

M1 prophase

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2n double

stranded

2n single

strandedPreparing for meiosis

prophase1

2n double

stranded

1st division ofmeiosis separateshomologous pairs

1n single

stranded

tetrad

synapsis

telophase1

metaphase1

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Meiosis 2 2nd division of meiosis separates sister

chromatids

1n double

stranded

1n single

stranded

What doesthis divisionlook like?

metaphase2

anaphase1

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Mitosis vs. Meiosis

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Mitosis vs. Meiosis Mitosis

1 division daughter cells

genetically identicalto parent cell

produces 2 cells 2n → 2n produces cells for

growth & repair no crossing over

Meiosis 2 divisions daughter cells

genetically differentfrom parent

produces 4 cells 2n → 1n produces gametes

crossing over

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The value of meiosis Meiosis introduces genetic variation

gametes of offspring do not have samegenes as gametes from parents

genetic recombination random assortment in humans produces

223 (8,388,608) different combinations

from Mom from Dad new gametesmade by offspring

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Random fertilization Any 2 parents will produce a zygote

with over 70 trillion (223 x 223) diploidcombinations

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And more variation…

tetrad

During Prophase 1 homologous pairs swap

pieces of chromosome sister chromatids intertwine crossing over

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Crossing over 3 steps

cross over breakage of DNA re-fusing of DNA

New combinationsof traits

What are advantages ofsexual reproduction?What are disadvantages?

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Genetic variation Meiosis & crossing over introduce

great genetic variation to population drives evolution

Consider the greater variation with 23 pairs of chromosomes =mixing and matching

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Sources of genetic variability Genetic variability in sexual reproduction

independent assortment homologous chromosomes in M1 sister chromatids in M2

crossing over between homologous chromosomes in

prophase1 random fertilization

random ovum by a random sperm

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Spermatogenesis Continuous &

prolific processin the adult male

Each ejaculationcontains 100–650million sperm

occurs inseminiferoustubules

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Structure of sperm

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Oogenesis At birth an ovary

contains all of theprimary oocytes it willever have ~300 eggs released

Unequal cytokinesis 1 egg 3 polar bodies polar bodies

degenerate

What is the advantage of 1 egg?