meiosis & sexual reproduction. asexual reproduction the prokaryotic chromosome is a ring of dna...

Meiosis & Sexual Reproduction

Asexual Reproduction

• The prokaryotic chromosome is a ring of DNA

• Binary fission– Splitting in two– Two replicate chromosomes are distributed to two

daughter cells– Produces two daughter cells identical to original cell –

asexual reproduction

Chapter 9 Section 4

Binary FissionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1. Attachment of chromosome toa special plasma membranesite indicates that thisbacterium is about to divide.

2. The cell is preparing for binaryfission by enlarging its cell wall,plasma membrane, and overallvolume.

3. DNA replication has producedtwo identical chromosomes.Cell wall and plasma membrane begin to grow inward.

4. As the cell elongates, thechromosomes are pulled apart.Cytoplasm is being distributedevenly.

5. New cell wall and plasmamembrane has divided thedaughter cells.

chromosome

cell wall

plasmamembrane

cytoplasm

SEM 2,345X© Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Sexual Reproduction

• Two parents

• Genetic variation

• Haploid gametes

• Unique offspring– Similar to parents– Similar to each other

Haploid vs. Diploid

HAPLOID• 1n

• One copy of each chromosome

DIPLOID• 2n

• Two copies of each chromosome

• Homologous chromosomes

Homologous Chromosomes

6

a. Sister chromatids

duplicationduplication

chromosome

maternal chromosome

b.paternal chromosome

homologous pair

nonsisterchromatids

centromere

kinetochore

chromosome

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a: © L. Willatt/Photo Researchers, Inc.

Overview of Meiosis

7

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2n = 4

• Diploid parent cell divides twice to make haploid gametes

• Homologous chromosomes

• Sister chromatids

Overview of Meiosis

8

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centrioles nucleoluscentromere

MEIOSISIHomologous pairs

synapse and then separate.

2n = 4 2n = 4

chromosomeduplication

Overview of Meiosis

9

centrioles nucleoluscentromere

sister chromatids

MEIOSISIHomologous pairs

synapse and then separate.

MEIOSISIISister chromatids separate,

becoming daughter chromosomes.

n = 2

Four haploiddaughter cells

Second divisionFirst division

synapsis

2n = 4 2n = 4

chromosomeduplication

n = 2

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Genetic VariationFor humans - 23 pairs of chromosomes:

• Independent Assortment – 50/50 chance that gamete gets maternal/ paternal homologous chromosome– # of possible gametes = 223

– >8 million possible gamete combinations

• Random Fertilization– (8 million)(8 million) = 64 trillion possible offspring

• Crossing over – increases genetic variation even more

Independent Assortment

11

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Combination 1

Combination 2 Combination 4 Combination 6 Combination 8

Combination7Combination 5Combination 3

Crossing Over Occurs During Meiosis I

12

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A

B

bB

C

c C

DD D

A Aa

b

B b

c

C c

dd d

a a

1 2 3 4 1 2 3 4 1 2 3 4

nucleoprotein lattice sister chromatidsof a chromosome

sister chromatidsof its homologue

chiasmata ofnonsisterchromatids1 and 3

a. b. c. d.

Daughterchromosomes

Crossing-overhas occurred

Bivalentforms

a: Courtesy Dr. D. Von Wettstein

Crossing Over

• Increases genetic variation

• Synapsis• Tetrad• Chiasma

• Happens during Prophase I

Genetic Variation

• Significance of genetic variation:

– Asexual reproduction produces genetically identical clones

– Sexual reproduction causes genetic recombinations among members of a population

– Asexual reproduction is advantageous when the environment is stable

– However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous

• Some offspring may have a better chance of survival

14

Karyotyping

Mitosis vs. Meiosis

Meiosis Compared to Mitosis

• Meiosis– 2 nuclear divisions– Chromosomes cross over– Halves chromosome

number– Produces 4 genetically

different daughter cells – Used only for sexual

reproduction

• Mitosis– 1 nuclear division– Chromosomes don’t cross

over– Preserves chromosome

number– Produces 2 genetically

identical daughter cells– Used for asexual

reproduction & growth

17

Life Cycles

Life Cycle of Humans

19

MEIOSISFERTILIZATION

2n

MITOSIS

2n

2n

2n

MITOSIS

egg

n

n

zygote

sperm

haploid (n)n = 23

2n = 46diploid (2n)

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Spermatogenesis and Oogenesis in Mammals

n

n

OOGENESISprimaryoocyte

Meiosis I

firstpolarbody

Meiosis II

Fertilization

secondaryoocyte

Meiosis II is completedafter entry of sperm

(fertilization)

egg

Sperm nucleus

fusion of spermnucleus andagg nucleus

zygote

2n

n

n

n

secondpolarbody

2n2n

n

n

n

Metamorphosisand maturation

Meiosis II

Meiosis I

SPERMATOGENESIS

primaryspermatocyte

secondaryspermatocytes

spermatids

sperm

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Changes in Chromosome Number & Structure

• Euploidy: correct number of chromosomes

• Aneuploidy: change in the chromosome number– Results from nondisjunction

• Monosomy - 1• Trisomy - 3

21

Nondisjunction

22

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pair ofhomologouschromosomes

nondisjunction

nondisjunction

pair ofhomologouschromosomes

normalMeiosis I

normalMeiosis II nondisjunction

Nondisjunction

23

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pair ofhomologouschromosomes

nondisjunction

a.

2n + 1 2n + 1 2n - 1 2n - 1

b.

2n 2n 2n + 1 2n - 1

pair ofhomologouschromosomes

normalMeiosis I

normalMeiosis II

Fertilization

Zygote

nondisjunction

Trisomy 21

24

a. b.

extra chromosome 21

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a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers

Changes in Sex Chromosome

25

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a. Turner syndrome missingchromosome X

b. Klinefelter syndrome extrachromosome X

a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz,http://klinefeltersyndrome.org; a, b(bottom): © CNRI/SPL/Photo Researchers, Inc

Types of Chromosomal Mutation

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c. Inversion d. Translocation

b. Duplicationa. Deletion

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