biology mader chapter 10 meiosis and sexual reproduction

BIOLOGYMader

Chapter 10Meiosis and Sexual

Reproduction

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Types of Reproduction

Parent

Bud

0.5 mm


Figure 13.4 Describing chromosomes

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

Centromere


• Figure 13.5 The human life cycleAt fertilization, a sperm fuses with an egg, forming a diploid zygote

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

– Repeated mitotic divisions lead to the development of a mature adult

– The adult makes haploid gametes by meiosis

– All of these processes make up the sexual life cycle of organisms


• Somatic cells of each species contain a specific number of chromosomes

– Human cells have 46, making up 23 pairs of homologous chromosomes

MEIOSIS

Chromosomes are matched in homologous pairs

Chromosomes

Centromere

Sister chromatids Figure 8.12


• Cells with two sets of chrom. are said to be diploid (2n = 46 for humans)

• Gametes are haploid, with only one set of chromosomes (n = 23 for humans)

Gametes have a single set of chromosomes


• The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene (called alleles) at corresponding loci (i.e. gene for eye color might say “blue” on dad, but “brown” on mom)

Homologous chromosomes carry different versions of genes


Figure 8.17A, B

Coat-color genes Eye-color genes

Brown Black

C E

c e

White Pink

C E

c e

C E

c e

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes

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Overview of Meiosis

n = 2 n = 2

2n = 4 2n = 4

MEIOSIS IHomologous pairs

synapse and then separate.

centrioles sister chromatidssynapsis

nucleoluscentromere

chromosomeduplication

MEIOSIS IISister chromatids separate,

becoming daughter chromosomes.

Four haploiddaughter cells

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


Interphase

Homologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologous chromosomes separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

• Meiosis, like mitosis, is preceded by chromosome duplication (during interphase)

•However, in meiosis the cell divides twice to form four daughter cells, each of which is haploid (n = 23).

Overview of meiosis: reduces chromosome number

MeiosisMen vs. Women

• Spermatogenesis:– Men– Testes– 4 cells produced– 145 rounds– 6 days

• Oogenesis– Women– Ovaries– Uneven separation

results in 4 cells, one called egg

– Born = 1-2 million oocytes (prophase I)

– Puberty = 400,000


The Meiotic Division of an Animal Cell

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tetrads line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicate Homologous chromosomes(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I


• Meiosis II is essentially the same as mitosis

– The sister chromatids of each chromosome separate

– The result is four haploid daughter cells, each of which are haploid (n = 23).


• Meiosis - 2 divisions....WHY?– In the first division, meiosis I,

homologous chromosomes are paired. (While they are paired, they can cross over and exchange genetic information)

– The homologous pairs are then separated, and two daughter cells are produced, which at this point are haploid (n = 23).

– But because each chromosome has double the genetic info (2 sister chromatids), another division is necessary.


Figure 13.8 The Meiotic Division of an Animal Cell

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still double

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Genetic Variation: Independent Assortment• Independent assortment:

– When homologues align at the metaphase plate:

• They separate in a random manner

• The maternal or paternal homologue may be oriented toward either pole of mother cell

– Causes random mixing of blocks of alleles into gametes


The independent assortment of homologous chromosomes in meiosis

Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

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Independent Assortment



What leads to variability/diversity?

• Why are we not all identical?

– Independent assortment

– Crossing over

– Random fertilization


• Random Fertilization - Each chromosome of a homologous pair comes from a different parent. (Which sperm will fertilize the egg? Or Which 2 people will produce offspring together?)

– Each chromosome thus differs at many points from the other member of the pair (different alleles)

• Crossing over

• Independent assortment

Processes allowing for increased GENETIC DIVERSITY

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Genetic Variation: Significance

• Asexual reproduction produces genetically identical clones

• Sexual reproduction cause novel genetic recombinations

• Asexual reproduction is advantageous when environment is stable

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

– Offspring adapt to that environment

Random Fertilization

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Genetic Variation: Fertilization

• When gametes fuse at fertilization:

– Chromosomes donated by the parents are combined

– In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible

• If crossing-over occurs only once

– (423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible


Figure 13.10 The independent assortment of homologous chromosomes in meiosis

Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

Crossing over further increases genetic variability

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Meiosis vs. Mitosis Meiosis

Requires two nuclear divisions

Chromosomes synapse and cross over

Centromeres survive Anaphase I

Halves chromosome number

Produces four daughter nuclei

Produces daughter cells genetically different from parent and each other

Used only for sexual reproduction

Mitosis Requires one nuclear

division Chromosomes do not

synapse nor cross over Centromeres dissolve in

mitotic anaphase Preserves chromosome

number Produces two daughter

nuclei Produces daughter cells

genetically identical to parent and to each other

Used for asexual reproduction and growth


Figure 8.15

MITOSIS MEIOSIS

PARENT CELL(before chromosome replication)

Site ofcrossing over MEIOSIS I

PROPHASE ITetrad formedby synapsis of homologous chromosomes

PROPHASE

Duplicatedchromosome(two sister chromatids)

METAPHASE

Chromosomereplication

Chromosomereplication

2n = 4

ANAPHASETELOPHASE

Chromosomes align at the metaphase plate

Tetradsalign at theMetaphase plate

METAPHASE I

ANAPHASE ITELOPHASE ISister chromatids

separate duringanaphase

Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together

No further chromosomal replication; sister chromatids separate during anaphase II

2n = 4 2n = 4

Daughter cellsof mitosis

Daughter cells of meiosis II

MEIOSIS II

Daughtercells of meiosis I

Haploidn = 2

n n n n

Haploidn = 2

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Changes in Chromosome Number

Euploid is the correct number of chromosomes in a species.

Aneuploid is change in the chromosome number Results from nondisjunction

Monosomy - only one of a particular type of chromosome,

Trisomy - three of a particular type of chromosome

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Changes in Chromosome

a. b.

pair ofhomologouschromosomes

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

normal

normal

pair ofhomologouschromosomes

Meiosis I

Meiosis II

Fertilization

Zygote

nondisjunction

nondisjunction


2n - 12n - 1

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Trisomy

Trisome 21 Occurs when an individual has three of a

particular type of chromosome The most common autosomal trisomy seen

among humans Also called Down syndrome Recognized by these characteristics:

short stature eyelid fold flat face stubby finger wide gap between first and second toes

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Changes in Sex Chromosome

Result from inheriting too many or too few X or Y chromosomes

Nondisjunction during oogenesis or spermatogenesis Turner syndrome (XO)

Female with single X chromosome Short, with broad chest and widely spaced nipples Can be of normal intelligence and function with hormone

therapy Klinefelter syndrome (XXY) – a male

Male with underdeveloped testes and prostate; some breast overdevelopment

Long arms and legs; large hands

Near normal intelligence unless XXXY, XXXXY, etc.

No matter how many X chromosomes, presence of Y renders individual male

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Changes in Chromosome Structure

Changes in chromosome structure include: Deletions

One or both ends of a chromosome breaks off Two simultaneous breaks lead to loss of an internal

segment Duplications

Presence of a chromosomal segment more than once in the same chromosome

Translocations

A segment from one chromosome moves to a non-homologous chromosome

Follows breakage of two nonhomologous chromosomes and improper re-assembly

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Changes in Chromosome Structure

Duplication A segment of a chromosome is repeated in the

same chromosome Inversion

Occurs as a result of two breaks in a chromosome The internal segment is reversed before re-

insertion Genes occur in reverse order in inverted segment

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Types of Chromosomal Mutation

c. Inversion d. Translocation

b. Duplicationa. Deletion

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b: Courtesy The Williams Syndrome Association

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translocation

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b: American Journal of Human Genetics by N. B. Spinner. Copyright 1994 by Elsevier Science & Technology Journals. Reproduced with permission of Elsevier Science & Technology Journals in the format Textbook via Copyright Clearance Center

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