Meiosis and Sexual Life Cycles

Overview: Variations on a Theme

Living organisms are distinguished by their ability

Fig. 13-1

Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

In a

Inheritance of Genes

Genes are the units of heredity, and are made

Comparison of Asexual and Sexual Reproduction

In asexual reproduction, one parent

Fig. 13-2

(a) Hydra

(b) Redwoods

Parent

Bud

0.5 mm

Fig. 13-2a

(a) Hydra

0.5 mm

Bud

Parent

Fig. 13-2b

(b) Redwoods

Concept 13.2: Fertilization and meiosis alternate in sexual life cycles

A life

Sets of Chromosomes in Human Cells

Human somatic cells (any cell other

Fig. 13-3

APPLICATION

TECHNIQUE

Pair of homologous
replicated chromosomes

5 µm

Centromere

Sister
chromatids

Metaphase
chromosome

Fig. 13-3a

APPLICATION

Fig. 13-3b

TECHNIQUE

Pair of homologous
replicated chromosomes

Centromere

Sister
chromatids

Metaphase
chromosome

5 µm

The sex chromosomes are called X and Y
Human females have a

Each pair of homologous chromosomes includes one chromosome from each parent
The

In a cell in which DNA synthesis has occurred, each chromosome

Fig. 13-4

Key

Maternal set of
chromosomes (n = 3)

Paternal set of
chromosomes (n =

A gamete (sperm or egg) contains a single set of chromosomes,

Fertilization is the union of gametes (the sperm and the egg)
The

At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are

Fig. 13-5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Egg (n)

Sperm (n)

MEIOSIS

FERTILIZATION

Ovary

Testis

Diploid
zygote
(2n =

The Variety of Sexual Life Cycles

The alternation of meiosis and fertilization

In animals, meiosis produces gametes, which undergo no further cell division

Fig. 13-6

Key

Haploid (n)

Diploid (2n)

n

n

Gametes

n

n

n

Mitosis

MEIOSIS

FERTILIZATION

MEIOSIS

2n

2n

Zygote

2n

Mitosis

Diploid
multicellular
organism

(a) Animals

Spores

Diploid
multicellular
organism
(sporophyte)

(b) Plants and some algae

2n

Mitosis

Gametes

Mitosis

n

n

n

Zygote

FERTILIZATION

n

n

n

Mitosis

Zygote

(c) Most fungi

Fig. 13-6a

Key

Haploid (n)

Diploid (2n)

Gametes

n

n

n

2n

2n

Zygote

MEIOSIS

FERTILIZATION

Mitosis

Diploid
multicellular
organism

(a) Animals

Plants and some algae exhibit an alternation of generations
This life cycle

Each spore grows by mitosis into a haploid organism called a

Fig. 13-6b

Key

Haploid (n)

Diploid (2n)

n

n

n

n

n

2n

2n

Mitosis

Mitosis

Mitosis

Zygote

Spores

Gametes

MEIOSIS

FERTILIZATION

Diploid
multicellular
organism
(sporophyte)

Haploid multi-
cellular organism
(gametophyte)

(b) Plants and some algae

In most fungi and some protists, the only diploid stage is

Fig. 13-6c

Key

Haploid (n)

Diploid (2n)

Mitosis

Mitosis

Gametes

Zygote

Haploid unicellular or
multicellular organism

MEIOSIS

FERTILIZATION

n

n

n

n

n

2n

(c) Most fungi and some

Depending on the type of life cycle, either haploid or diploid

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid

The Stages of Meiosis

In the first cell division (meiosis I), homologous

Fig. 13-7-1

Interphase

Homologous pair of chromosomes
in diploid parent cell

Chromosomes
replicate

Homologous pair of replicated

Fig. 13-7-2

Interphase

Homologous pair of chromosomes
in diploid parent cell

Chromosomes
replicate

Homologous pair of replicated

Fig. 13-7-3

Interphase

Homologous pair of chromosomes
in diploid parent cell

Chromosomes
replicate

Homologous pair of replicated

Meiosis I is preceded by interphase, in which chromosomes are replicated

Fig. 13-8

Prophase I

Metaphase I

Anaphase I

Telophase I and
Cytokinesis

Prophase II

Metaphase II

Anaphase II

Telophase II

Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase

Metaphase I

Fig. 13-8a

Prophase I

Anaphase I

Telophase I and
Cytokinesis

Centrosome
(with centriole pair)

Sister
chromatids

Chiasmata

Spindle

Homologous
chromosomes

Fragments
of nuclear
envelope

Centromere
(with kinetochore)

Metaphase
plate

Microtubule
attached

Prophase I
Prophase I typically occupies more than 90% of the time

In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes

Metaphase I
In metaphase I, tetrads line up at the metaphase plate,

Fig. 13-8b

Prophase I

Metaphase I

Centrosome
(with centriole pair)

Sister
chromatids

Chiasmata

Spindle

Centromere
(with kinetochore)

Metaphase
plate

Homologous
chromosomes

Fragments
of nuclear
envelope

Microtubule
attached to
kinetochore

Anaphase I
In anaphase I, pairs of homologous chromosomes separate
One chromosome moves

Telophase I and Cytokinesis
In the beginning of telophase I, each half

In animal cells, a cleavage furrow forms; in plant cells, a

Fig. 13-8c

Anaphase I

Telophase I and
Cytokinesis

Sister chromatids
remain attached

Homologous
chromosomes
separate

Cleavage
furrow

Division in meiosis II also occurs in four phases:
– Prophase II
– Metaphase II
– Anaphase

Fig. 13-8d

Prophase II

Metaphase II

Anaphase II

Telophase II and
Cytokinesis

Sister chromatids
separate

Haploid daughter cells
forming

Prophase II
In prophase II, a spindle apparatus forms
In late prophase II,

Metaphase II
In metaphase II, the sister chromatids are arranged at the

Fig. 13-8e

Prophase II

Metaphase II

Anaphase II
In anaphase II, the sister chromatids separate
The sister chromatids of

Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite

Cytokinesis separates the cytoplasm
At the end of meiosis, there are four

Fig. 13-8f

Anaphase II

Telephase II and
Cytokinesis

Sister chromatids
separate

Haploid daughter cells
forming

A Comparison of Mitosis and Meiosis

Mitosis conserves the number of chromosome

Fig. 13-9

MITOSIS

MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Homologous
chromosome
pair

Chromosome
replication

Parent cell

2n = 6

Chromosome
replication

Replicated chromosome

Prophase

Metaphase

Metaphase I

Anaphase I

Telophase I

Haploid

n

Fig. 13-9a

MITOSIS

MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosome
replication

Homologous
chromosome
pair

Chromosome
replication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase

Metaphase I

Anaphase I

Telophase I

Haploid

Fig. 13-9b

SUMMARY

Meiosis

Mitosis

Property

DNA
replication

Number of
divisions

Occurs during interphase before
mitosis begins

One, including prophase, metaphase,
anaphase, and

Three events are unique to meiosis, and all three occur in

Sister chromatid cohesion allows sister chromatids of a single chromosome to

Fig. 13-10

EXPERIMENT

RESULTS

Shugoshin+ (normal)+

Spore case

Fluorescent label

Metaphase I

Shugoshin–

Anaphase I

Metaphase II

Anaphase II

Mature
spores

OR

Spore

Two of three

Fig. 13-10a

EXPERIMENT

Shugoshin+ (normal)

Spore case

Fluorescent label

Metaphase I

Anaphase I

Metaphase II

Anaphase II

Mature
spores

Spore

OR

Two of three

Fig. 13-10b

RESULTS

Shugoshin+

Shugoshin–

Spore cases (%)

100

80

60

40

20

0

Concept 13.4: Genetic variation produced in sexual life cycles contributes to

Origins of Genetic Variation Among Offspring

The behavior of chromosomes during meiosis

Independent Assortment of Chromosomes

Homologous pairs of chromosomes orient randomly at metaphase

The number of combinations possible when chromosomes assort independently into gametes

Fig. 13-11-1

Possibility 1

Possibility 2

Two equally probable
arrangements of
chromosomes at
metaphase I

Fig. 13-11-2

Possibility 1

Possibility 2

Two equally probable
arrangements of
chromosomes at
metaphase I

Metaphase II

Fig. 13-11-3

Possibility 1

Possibility 2

Two equally probable
arrangements of
chromosomes at
metaphase I

Metaphase II

Daughter
cells

Combination 1

Combination

Crossing Over

Crossing over produces recombinant chromosomes, which combine genes inherited from

In crossing over, homologous portions of two nonsister chromatids trade places
Crossing

Fig. 13-12-1

Prophase I
of meiosis

Pair of
homologs

Nonsister
chromatids
held together
during synapsis

Fig. 13-12-2

Prophase I
of meiosis

Pair of
homologs

Nonsister
chromatids
held together
during synapsis

Chiasma

Centromere

TEM

Fig. 13-12-3

Prophase I
of meiosis

Pair of
homologs

Nonsister
chromatids
held together
during synapsis

Chiasma

Centromere

Anaphase I

TEM

Fig. 13-12-4

Prophase I
of meiosis

Pair of
homologs

Nonsister
chromatids
held together
during synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

TEM

Fig. 13-12-5

Prophase I
of meiosis

Pair of
homologs

Nonsister
chromatids
held together
during synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

Daughter
cells

Recombinant chromosomes

TEM

Random Fertilization

Random fertilization adds to genetic variation because any sperm can

Crossing over adds even more variation
Each zygote has a unique genetic

The Evolutionary Significance of Genetic Variation Within Populations

Natural selection results in

Fig. 13-UN1

Prophase I: Each homologous pair undergoes
synapsis and crossing over between

Fig. 13-UN2

F

H

Fig. 13-UN3

Fig. 13-UN4