Chapter 10 ppt

(1)

Biology

Sylvia S. Mader Michael Windelspecht

Chapter 10 Meiosis and

Sexual

Reproduction

Lecture Outline

See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into

PowerPoint without notes.

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Outline

• 10.1 Halving the Chromosome Number

• 10.2 Genetic Variation

• 10.3 The Phases of Meiosis

• 10.4 Meiosis Compared to Mitosis

• 10.5 Human Life Cycle

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10.1 Halving the Chromosome

Number

• Meiosis

 S

pecial type of cell division



Used only for

sexual reproduction



Halves the chromosome number prior to

fertilization

• Parents are diploid (2n)

• Meiosis produces haploid (n) gametes

• Gametes fuse in fertilization to form a diploid (2n) zygote

• The zygote becomes the next diploid (2n) generation

(4)

Halving the Chromosome

Number

• In diploid body cells, chromosomes occur in pairs • Humans have 23 different types of chromosomes

• Diploid (2n) cells have two chromosomes of each type • Chromosomes of the same type are said to be

homologous chromosomes (homologues)

 They have the same length

 Their centromeres are positioned in the same place

 One came from the father (the paternal homolog) the

other from the mother (the maternal homolog)

(5)

Homologous Chromosomes

5

a. Sister chromatids

duplication duplication

chromosome

maternal chromosome

b.

paternal chromosome

homologous pair nonsister chromatids

centromere

kinetochore

chromosome

(6)

Halving the Chromosome

Number

• Homologous chromosomes have genes controlling the same trait at the same position

 Each gene occurs in duplicate

 A maternal copy from the mother

 A paternal copy from the father

• Many genes exist in several variant forms in a large population

• Homologous copies of a gene may encode identical or different genetic information

• The variants that exist for a gene are called alleles

• An individual may have:

 Identical alleles for a specific gene on both homologs (homozygous for the trait), or

(7)

7

Halving the Chromosome Number

• Overview of Meiosis

 Meiosis I

• Chromosomes are replicated prior to meiosis I

– Each chromosome consists of two identical sister chromatids • Homologous chromosomes pair up – synapsis

• Homologous pairs align themselves against each other side by side at the metaphase plate

• The two members of a homologous pair separate

• Each daughter cell receives one duplicated chromosome from each pair

 Meiosis II

• DNA is not replicated between meiosis I and meiosis II • Sister chromatids separate and move to opposite poles

• The four daughter cells contain one daughter chromosome from each pair • Each daughter chromosome consists of a single chromatid

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Overview of Meiosis

(9)

Overview of Meiosis

9 centrioles _nucleolus

centromere

2n = 4

(10)

Overview of Meiosis

centrioles nucleolus centromere

2n = 4 2n = 4

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Overview of Meiosis

11

2n = 4

chromosome duplication

sister chromatids

synapsis

MEIOSISI

Homologous pairs

synapse and then separate. 2n = 4

(12)

Overview of Meiosis

MEIOSISI

First division

synapsis

2n = 4 2n = 4

chromosome duplication

n = 2

(13)

Overview of Meiosis

13

centrioles _nucleolus centromere

Second division First division

synapsis

2n = 4 2n = 4 chromosome

duplication

n = 2 MEIOSISI

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Overview of Meiosis

MEIOSISI Homologous pairs synapse and then separate.

MEIOSISII

Sister chromatids separate, becoming daughter chromosomes.

n = 2 Second division

First division synapsis

2n = 4 2n = 4 chromosome

duplication

n = 2

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Overview of Meiosis

15

MEIOSISI Homologous pairs synapse and then separate.

MEIOSISII

Sister chromatids separate, becoming daughter chromosomes.

n = 2

Four haploid daughter cells Second division

First division synapsis

2n = 4 2n = 4 chromosome

duplication

n = 2

(16)

10.2 Genetic Variation

• Meiosis brings about genetic variation in two key

ways:

 Crossing-over between homologous chromosomes, and

 Independent assortment of homologous chromosomes

• Crossing Over:

 Exchange of genetic material between non-sister chromatids during meiosis I

 At synapsis, a nucleoprotein lattice (called the

synaptonemal complex) appears between homologues

• Holds homologues together

• Aligns DNA of non-sister chromatids • Allows crossing-over to occur

(17)

Crossing Over Occurs During

Meiosis I

17

A

B

b B

C

c C

D

D D A A a

b

B b

c

C c

d

d d a a

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

nucleoprotein lattice sister chromatids of a chromosome

sister chromatids of its homologue

chiasmata of nonsister chromatids 1 and 3

a. b. c. d.

Daughter chromosomes Crossing-over has occurred Bivalent forms

(18)

Genetic Variation

• Independent assortment



When homologous chromosome pairs

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

(19)

Independent Assortment

19

Combination 1

Combination 2 _{Combination 4} _{Combination 6}

Combination 8 Combination

7 Combination 5

(20)

Genetic Variation

• Fertilization

– union of male and female

gametes



Chromosomes donated by the parents are

combined



In humans, (2

23

₎

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

(21)

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

(22)

10.3 The Phases of Meiosis

• Meiosis I:

 Prophase I

• A spindle forms

• The nuclear envelope fragments • The nucleolus disappears

• Each chromosome is duplicated (consists of two identical sister chromatids)

• Homologous chromosomes pair up and physically align themselves against each other side by side (synapsis)

• Synapsed homologs are referred to as a bivalent (two homologues) or a tetrad (four chromatids)

 Metaphase I

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The Phases of Meiosis

• Meiosis I



Anaphase I

• Homologous chromosomes of each bivalent separate from one another

• Homologues move towards opposite poles • Sister chromatids do not separate

• Each is still a duplicated chromosome with two chromatids



Telophase I

• Daughter cells have one duplicated chromosome (n) from each homologous pair

(24)

The Phases of Meiosis

• Interkinesis



Two haploid (n) daughter cells, each with one

duplicated chromosome of each type



Interkinesis is similar to mitotic interphase

except

• It is usually shorter

(25)

The Phases of Meiosis

• Meiosis II (mitosis of two haploid cells)



Prophase II – Chromosomes condense



Metaphase II – Chromosomes align at

metaphase plate



Anaphase II

• Centromere dissolves

• Sister chromatids separate and become daughter chromosomes



Telophase II and cytokinesis

• Four haploid (n) cells all genetically unique

(26)

(27)

(28)

(29)

(30)

10.4 Meiosis Compared to

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

• 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

(31)

Meiosis Compared to Mitosis

(32)

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Meiosis Compared to Mitosis

Anaphase I

Homologous chromosomes separate and move toward the poles.

Anaphase

Sister chromatids separate and become daughter chromosomes. Metaphase

Chromosomes align at the metaphase plate. MITOSIS Prophase MEIOSISI Prophase I Synapsis and crossing-over occur. MetaphaseI Homologous pairs align independently at the metaphase plate.

(34)

Meiosis Compared to Mitosis

Telophase I Daughter cells are forming and will go on to divide again.

MEIOSIS I cont'd

Sister chromatids separate and become daughter chromosomes.

Daughter cells

n = 2

MEIOSIS II

Daughter cells Telophase Two diploid daughter cells.

Four haploid daughter cells. Their nuclei are genetically different from the parent cell.

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10.5 The Cycle of Life

• A life cycle is all the reproductive events that occur from one

generation to the next similar generation

• In plants, haploid multicellular “individuals” alternate with diploid multicellular “individuals” (alternation of generations)

• The haploid individual:

 Is called the gametophyte

 May be larger or smaller than the diploid individual

• The diploid individual:

 Is called the sporophyte

 May be larger or smaller than the haploid individual

• Mosses are haploid for most of their life cycle

• In fungi and most algae, only the zygote is diploid

• Ferns & higher plants are diploid for most of their life cycles • In plants, algae, and fungi gametes are produced by haploid

individuals

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The Cycle of Life

• In animals:

 “Individuals” are diploid and produce haploid gametes

 The only haploid part of the life cycle is the gametes

 The products of meiosis are always gametes

 Meiosis occurs only during gametogenesis

• Production of sperm

– Spermatogenesis

– All four cells become sperm • Production of eggs

– Oogenesis

– One of the four nuclei receives the majority of the cytoplasm

(37)

The Cycle of Life

• Human Life Cycle:

 Sperm and egg are produced by meiosis

 A sperm and egg fuse at fertilization

 Results in a zygote

• The one-celled stage of an individual of the next generation • Undergoes mitosis

 Results in a multicellular embryo that gradually takes

on features determined when the zygote was formed

 All growth occurs as mitotic division

 As a result of mitosis, each somatic cell in the body

• Has same number of chromosomes as zygote

• Has the same genetic makeup, which was determined when the zygote was formed

(38)

Life Cycle of Humans

(39)

Life Cycle of Humans

39

MEIOSIS

2n 2n

n sperm

(40)

Life Cycle of Humans

MEIOSIS

2n 2n

n

(41)

Life Cycle of Humans

41

MEIOSIS FERTILIZATION

2n

2n 2n

egg n

n zygote

sperm

(42)

Life Cycle of Humans

2n

MITOSIS

2n

2n 2n

n

n zygote

(43)

Life Cycle of Humans

43

2n

MITOSIS

2n

2n 2n

MITOSIS

egg n

n zygote

sperm

(44)

Life Cycle of Humans

2n

MITOSIS

2n

2n 2n

MITOSIS

n

n zygote

haploid (n) n = 23 2n = 46 diploid (2n)

(45)

Spermatogenesis and

Oogenesis in Mammals

n n OOGENESIS primary oocyte Meiosis I first polarbody Meiosis II Fertilization secondary oocyte

Meiosis II is completed after entry of sperm

(fertilization)

egg

Sperm nucleus

fusion of sperm nucleus and agg nucleus zygote 2n n n n second polarbody 2n 2 n n n n Metamorphosis and maturation Meiosis II Meiosis I SPERMATOGENESIS primary spermatocyte secondary spermatocytes spermatids sperm

(46)

10.6 Changes in Chromosome

Number and Structure

• Euploidy

is the correct number of

chromosomes in a species.

• Aneuploidy

is

a

change in the

chromosome number



Results from

nondisjunction

• Monosomy - only one of a particular type of chromosome,

(47)

Nondisjunction

47

pair of

homologous chromosomes

pair of

(48)

Nondisjunction

pair of

nondisjunction

pair of

homologous chromosomes normal

(49)

Nondisjunction

49

pair of

nondisjunction

nondisjunction pair of

normal Meiosis I

normal

(50)

Nondisjunction

pair of

nondisjunction

pair of

normal Meiosis II

Fertilization

(51)

Nondisjunction

51

pair of

Fertilization

Zygote nondisjunction

(52)

Nondisjunction

pair of

nondisjunction

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

pair of

Fertilization

Zygote

(53)

Changes in Chromosome

Number and Structure

• Trisomy occurs when an individual has

three of a particular type of chromosome

• The most common autosomal trisomy

seen among humans is Trisomy 21



Also called Down syndrome



Recognized by these characteristics:

• short stature • eyelid fold • flat face

• stubby fingers

• wide gap between first and second toes

(54)

Trisomy 21

a. b.

extra chromosome 21

(55)

Changes in Chromosome

Number and Structure

• Changes in sex chromosome

number:



Results from inheriting too many or too few X

or Y chromosomes



Nondisjunction during oogenesis or

spermatogenesis



Turner syndrome (XO)

• Female with a single X chromosome

• Short, with broad chest and widely spaced nipples • Can be of normal intelligence and function with

hormone therapy

(56)

Changes in Chromosome

Number and Structure

• Changes in sex chromosome

number:



Klinefelter syndrome (XXY)

• Male with underdeveloped testes and prostate; some breast overdevelopment

• Long arms and legs; large hands

• Near normal intelligence unless XXXY, XXXXY, etc.

(57)

Changes in Sex Chromosome

57

a. Turner syndrome missing

chromosome X b. Klinefelter syndrome _{chromosome X}extra

(58)

Changes in Chromosome

Number and Structure

• Changes in chromosome structure include:

 Deletion

• One or both ends of a chromosome breaks off

• Two simultaneous breaks lead to loss of an internal segment

 Duplication

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

 Translocation

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

(59)

Changes in Chromosome

Number and Structure



Changes in chromosome structure include:

• 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 the inverted segment

(60)

Types of Chromosomal Mutation

b. Duplication a. Deletion + a b c d e f g a b c d e f g h a b c d e f g a b e f g l m n o p q r a b c d e f l m n o p a b c d e f g a b c d e f g b c d e f g g h q r d e a c d

(61)

Deletion

61

deletion _lost

a. b.

+ h

a b c d e f g h a b c d e f g