Chapter 9

Meiosis And Sexual

Reproduction

Genes and Alleles

Heredity

• The transmission of traits from one generation to

next is called inheritance , or heredity

Inheritance of Genes

• Genes are segments of DNA that encode heritable

information about 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

Sexual Reproduction

Meiosis

• Nuclear Division process that halves the chromosome number in half

• Basis of sexual reproduction

Fertilization

• Fusion of a sperm nucleus and an egg nucleus result in the formation a single - celled zygote

Gamete

• Reproductive cells

• Vehicles that transmit genes from one generation to next

• Male and female gametes (Sperm and egg)

Where Gametes Form

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

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

fertilizationzygote

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 non-identical sex chromosomes (X and Y)

– Same length, shape, genes

• All pairs interact at meiosis

– One chromosome of each type sorts into gametes

Homologous Chromosomes

p.141b

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

p.141c

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

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 formingmicrotubules ofthe spindle

breakupof nuclearenvelope

centrosome witha pair of centrioles,moving to oppositesides of nucleus

Chromosomes were duplicated earlier, ininterphase.

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 duplicatedchromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole.

One of each typeof chromosome hasarrived at a spindlepole. In most species,the cytoplasm dividesat this time.

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

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

How Meiosis Introduces Variation

in Traits

Prophase I: Crossing Over

• Non sister 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

Fig. 9.6c, p.144

Fig. 9.6d, p.144

Fig. 9.6d, p.144

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. 9.6f, p.144

Metaphase I: Chromosome Shuffling

• Homologous chromosomes align randomlyduring 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

From Gametes to Offspring

• Multicelled diploid and haploid bodies are typical in life cycles of plants and animals

• Gamete Formation in Plants

– Sporophyte: A multicelled plant body (diploid) that makes haploid spores

– Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form)

meiosisDIPLOID

fertilization

zygote

gametes spores

HAPLOID

meiosis

meiosis

meiosis

Plant life cycle

(2n)

(2n)

(n)

multicelledgametophyte

multicelledsporophyte

(n) (n)

From Gametes to Offspring

Gamete Formation in 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

meiosis DIPLOID

fertilization

zygote

HAPLOID

meiosis

(2n)

(2n)

(n)gametes

multicelledbody

Animal life cycle

Gamete Formation in Animals

• In male animals

• Germ cell develops into a primary sporocyte

• After meiosis, 4 haploid cells are formed and

develop into spermatids

• Then it becomes sperm, a type of mature

male gamete

Sperm Formation in Animals

Gamete Formation in Animals

• In Female Animals

• Germ cell becomes primary oocyte

• Meiosis I followed by unequal cytoplasmic division produces a small polar body and a large secondary oocyte. Both are haploid

• Meiosis II and unequal cytoplasmic division produces one large secondary oozyte and three small polar bodies

• Secondary oozyte is also called ovum

Egg Formation in Animals

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

no interphase

and no DNA

replication

between the

two nuclear

divisions

Prophase II Metaphase II Anaphase II Telophase II

All chromosomes still

duplicated. New spindle

forms in each nucleus,

tethers chromosomes

to spindle poles.

All chromosomes

aligned at the

spindle equator.

Sister chromatids of

each chromosome

moved to opposite

spindle poles.

Four haploid (n) nuclei

form. After cytoplasmic

division, haploid cells

function as gametes

or spores.