Meiosis and Sexual Reproduction

Chapter 9

9.1 Genes and Alleles

Genes

• Sequences of DNA that encode heritable traits

Alleles

• Slightly different forms of the same gene

• Each specifies a different version of gene product

Sexual and Asexual Reproduction

Asexual reproduction (1 parent)

• Offspring inherit parent’s genes

• Clones (identical copies of parent)

Sexual reproduction (2 parents)

• Offspring differ from parents and each another

• Different combinations of alleles

• Different details of shared traits

Sexual Reproduction

Meiosis, gamete formation, and fertilization

occur in sexual reproduction

Meiosis and fertilization shuffle parental alleles

• Offspring inherit new combinations of alleles

Key Concepts:

SEXUAL VS. ASEXUAL REPRODUCTION

By asexual reproduction, one parent alone

transmits its genetic information to offspring

By sexual reproduction, offspring typically inherit

information from two parents that differ in their

alleles

Alleles are different forms of the same gene;

they specify different versions of a trait

9.2 What Meiosis Does

Meiosis

• Nuclear division mechanism that precedes

gamete formation in eukaryotic cells

• Halves parental chromosome number

Fertilization

• Fusion of two gamete nuclei

• Restores parental chromosome number

• Forms zygote (first cell of new individual)

Fig. 9.12, p.150

gametes gametes

germ cell germ cell

each chromosome

duplicated during

interphase

MEIOSIS II

separation of

sister chromatids

MEIOSIS I

separation of

homologues

diploid number

restored at

fertilization zygote

2n

2n

n

Homologues

Sexual reproducers inherit pairs of chromosomes

• 1 from maternal parent, 1 from paternal parent

The pairs are homologous (“the same”)

• Except nonidentical sex chromosomes (X and Y)

• Same length, shape, genes

All pairs interact at meiosis

• One chromosome of each type sorts into gametes

9.3 Tour of Meiosis

All chromosomes are duplicated during

interphase, before meiosis

Two divisions, meiosis I and II, divide the

parental chromosome number by two

Each forthcoming gamete is haploid (n)

Meiosis I

The first nuclear division

Each duplicated chromosome lines up with its

homologous partner

The two homologous chromosomes move apart,

toward opposite spindle poles

Prophase I

Chromosomes condense and align tightly with

their homologues

Each homologous pair undergoes crossing over

Microtubules form the bipolar spindle

One pair of centrioles moves to the other side of

the nucleus

Prophase I (cont.)

Nuclear envelope breaks up

• Microtubules growing from

each spindle pole penetrate

the nuclear region

Microtubules tether one or

the other chromosome of

each homologous pair

Metaphase I

Microtubules from both poles position all pairs of

homologous chromosomes at the spindle

equator

Anaphase I

Microtubules separate each

chromosome from its

homologue, moving to

opposite spindle poles

Other microtubules overlap

midway between spindle

poles, slide past each other

to push poles farther apart

As anaphase I ends, one set

of duplicated chromosomes

nears each spindle pole

Telophase I

Two nuclei form

• Typically, the cytoplasm

divides

All chromosomes are still

duplicated

• Each still consists of

two sister chromatids

Meiosis II

The second nuclear

division

Sister chromatids of

each chromosome are

pulled away from each

other

Each is now an

individual chromosome

Prophase II Metaphase II

Anaphase II and Telophase II

In anaphase II, one

chromosome of each

type is moved toward

opposite spindle poles

• Occurs in both nuclei

formed in meiosis I

By the end of telophase

II, there are four haploid

nuclei, each with

unduplicated

chromosomes

Anaphase II Telophase II

plasma membrane

spindle equator (midway between the two poles)

one pair of homologous chromosomes

Prophase I Metaphase I Anaphase I Telophase I

Meiosis I

Fig. 9.5a, p.142

newly forming microtubules of the spindle

breakup of nuclear envelope

centrosome with a pair of centrioles, moving to opposite sides of nucleus

Chromosomes were duplicated earlier, in interphase.

Prior to metaphase I, one set of microtubules had tethered one chromosome of each type to one spindle pole and another set tethered its homologue to the other spindle pole.

One of each duplicated chromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole.

One of each type of chromosome has arrived at a spindle pole. In most species, the cytoplasm divides at this time.

Fig. 9.5b, p.142

Prophase II Metaphase II Anaphase II Telophase II

Meiosis II

there is

no DNA

replication

between

the two

divisions

Fig. 9.5b, p.142

In each cell, one of two

centrioles moves to the

opposite side of the

cell, and a new bipolar

spindle forms.

By now,

microtubules from

both spindle poles

have finished a tug-

of-war.

The sister chromatids

of each chromosome

move apart and are

now individual,

unduplicated

A new nuclear envelope

encloses each parcel of

chromosomes, so there

are now four nuclei.

Haploid Daughter Cells

When cytoplasm divides, four haploid cells result

One or all may serve as gametes or, in plants,

as spores that lead to gamete-producing bodies

Key Concepts:

STAGES OF MEIOSIS

Diploid cells have a pair of each type of

chromosome, one maternal and one paternal

Meiosis, a nuclear division mechanism, reduces

the chromosome number

Meiosis occurs only in cells set aside for sexual

reproduction

Key Concepts:

STAGES OF MEIOSIS (cont.)

Meiosis sorts out a reproductive cell’s

chromosomes into four haploid nuclei

Haploid nuclei are distributed to daughter cells

by way of cytoplasmic division

9.4 Meiosis Introduces Variation in Traits

Two events in meiosis cause variation in traits in

sexually reproducing species

• Crossing over during prophase I of meiosis

• Chromosome shuffling during metaphase I of

meiosis

Prophase I: Crossing Over

Nonsister chromatids of homologous

chromosomes undergo crossing over

• They exchange segments at the same place

along their length

Each ends up with new combinations of alleles

not present in either parental chromosome

Crossing Over

Fig. 9.6, p.144

a A maternal chromosome (purple)

and paternal chromosome (blue)

were duplicated earlier, during

interphase. They become visible in

microscopes early in prophase I,

when hey star to condense to

threadlike form.

b Each chromosome

and its homologous

partner zipper

together, so all four

chromatids

are tightly aligned.

mom’s

allele

B

mom’s

allele

A

mom’s

allele

A

mom’s

allele

A

mom’s

allele

B

dad’s

allele

a

dad’s

allele

b

dad’s

allele

b

c Here is a simple way to

think about crossing over.

(Chromosomes are still c

ondensed and threadlike,

and each is tightly aligned

with its homologous

partner.)

d Their intimate contact

promotes crossing over

at different places along

the length of nonsister

chromatids.

e At the crossover

site, paternal and

maternal chromatids

exchange

corresponding

segments.

f Crossing over mixes

up maternal and paternal

alleles on homologous

chromosomes.

Fig. 9.6a, p.144

Fig. 9.6b, p.144

Fig. 9.6c, p.144

Fig. 9.6d, p.144

Fig. 9.6e, p.144

Fig. 9.6f, p.144

Metaphase I: Chromosome Shuffling

Homologous chromosomes align randomly

during metaphase I

Microtubules can harness either a maternal or

paternal chromosome of each homologous pair

to either spindle pole

Either chromosome may end up in any new

nucleus (gamete)

Chromosome Shuffling:

Random Alignment

Key Concepts: CHROMOSOME

RECOMBINATION AND SHUFFLING

During meiosis, each pair of maternal and

paternal chromosomes swaps segments and

exchanges alleles

Pairs get randomly shuffled, so forthcoming

gametes end up with different mixes of maternal

and paternal chromosomes

9.5 From Gametes to Offspring

Multicelled diploid and haploid bodies are typical

in life cycles of plants and animals

Plants

• Sporophyte: A multicelled plant body (diploid) that

makes haploid spores

• Spores give rise to gametophytes (multicelled

plant bodies in which haploid gametes form)

From Gametes to Offspring

Animals

• Germ cells in the reproductive organs give rise to

sperm or eggs

• Fusion of a sperm and egg at fertilization results

in a zygote

Comparing Plant And Animal Life Cycles

meiosis DIPLOID

fertilization

zygote

gametes spores

HAPLOID

Fig. 9.8a, p.146

meiosis

meiosis

meiosis

a Plant life cycle

(2n)

(2n)

(n)

multicelled gametophyte

multicelled sporophyte

(n) (n)

Fig. 9.8b, p.146

meiosis DIPLOID fertilization

zygote

HAPLOID

meiosis

(2n)

(2n)

(n) gametes

multicelled body

b Animal life cycle

Introducing Variation in Offspring

Three events cause new combinations of alleles

in offspring:

• Crossing over during prophase I (meiosis)

• Random alignment of maternal and paternal

chromosomes at metaphase I (meiosis)

• Chance meeting of gametes at fertilization

All three contribute to variation in traits

Sperm Formation in Animals

Egg Formation in Animals

Key Concepts: SEXUAL

REPRODUCTION IN LIFE CYCLES

In animals, gametes form by different

mechanisms in males and females

In most plants, spore formation and other events

intervene between meiosis and gamete

formation

9.6 Comparing Mitosis and Meiosis

Both mitosis and meiosis require bipolar spindle

to move and sort duplicated chromosomes

Some mechanisms of meiosis resemble those of

mitosis, and may have evolved from them

• Example: DNA repair enzymes function in both

Differences in Mitosis and Meiosis

Mitosis maintains parental chromosome number

• Duplicates genetic information

• Occurs in body cells

Meiosis halves chromosome number

• Introduces new combinations of alleles in

offspring

• Occurs only in cells for sexual reproduction

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Key Concepts:

MITOSIS AND MEIOSIS COMPARED

Recent molecular evidence suggests that

meiosis originated through mechanisms that

already existed for mitosis and, before that, for

repairing damaged DNA