Sexual Reproduction and Meiosismmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · Sexual life cycle •Made up of meiosis and fertilization •Diploid cells –Somatic cells

Sexual Reproduction

and Meiosis

Chapter 11

1

Sexual life cycle

• Made up of meiosis and fertilization

• Diploid cells

– Somatic cells of adults have 2 sets of chromosomes

• Haploid cells

– Gametes have only 1 set of chromosomes

• Offspring inherit genetic material from 2 parents

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Haploid sperm

Haploid egg

Diploid zygote

Fertilization

Paternal

homologue

Maternal

homologue

4

• Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages

• Some life cycles include longer diploid phases, some include longer haploid phases

• In most animals, diploid state dominates

– Zygote first undergoes mitosis to produce diploid cells

– Later in the life cycle, some of these diploid cells

undergo meiosis to produce haploid gametes

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n

2n

Sperm

(haploid) n

Egg

(haploid) n

Zygote

(diploid) 2n

Somatic

cells

Germ-line

cells

Adult male

(diploid) 2nAdult female

(diploid) 2n

MITOSIS

MITOSIS

Germ-line

cells

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Features of Meiosis

• Meiosis includes two rounds of division

– Meiosis I and meiosis II

– Each has prophase, metaphase, anaphase, and

telophase stages

• Synapsis

– During early prophase I

– Homologous chromosomes become closely

associated

– Includes formation of synaptonemal complexes

• Formation also called tetrad or bivalents

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• First meiotic division is termed the “reduction

division”

– Results in daughter cells that contain one homologue

from each chromosome pair

• No DNA replication between meiotic divisions

• Second meiotic division does not further reduce

the number of chromosomes

– Separates the sister chromatids for each homologue

The Process of Meiosis

• Meiosis I

– Prophase I

– Metaphase I

– Anaphase I

– Telophase I

• Meiosis II

– Prophase II

– Metaphase II

– Anaphase II

– Telophase II

• Meiotic cells have an

interphase period that

is similar to mitosis

with G1, S, and G2

phases

• After interphase,

germ-line cells enter

meiosis I

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Kinetochore

Sister chromatids

Homologues

a.

Centromere

Synaptonemal

complex


Prophase I

• Chromosomes coil tighter and become visible,

nuclear envelope disappears, spindle forms

• Each chromosome composed of 2 sister

chromatids

• Synapsis

– Homologues become closely associated

– Crossing over occurs between nonsister chromatids

– Remain attached at chiasmata

• Chiasmata move to the end of the chromosome

arm before metaphase I

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Crossing over

• Genetic recombination between nonsister

chromatids

• Allows the homologues to exchange

chromosomal material

• Alleles of genes that were formerly on

separate homologues can now be found

on the same homologue

• Chiasmata – site of crossing over

– Contact maintained until anaphase I

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Crossing over

Cohesin proteins

Prophase I Metaphase I Anaphase I

Site of crossover


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Chromosome (replicated)

Spindle

Chiasmata

In prophase I, the chromosomes

begin to condense, and the

spindle of microtubules begins

to form. The DNA has been

replicated, and each

chromosome consists of two

sister chromatids attached at

the centromere. The cell

illustrated here has four

chromosomes, or two pairs of

homologues. Homologous

chromosomes pair along their

entire length during synapsis.

Crossing over occurs, forming

chiasmata, which hold

homologous chromosomes

together.

Paired homologous

chromosomes

Sister

chromatids


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Metaphase I

• Terminal chiasmata hold homologues together following crossing over

• Microtubules from opposite poles attach to each homologue

– Not each sister chromatid as in mitosis

• Homologues are aligned at the metaphase plate side-by-side

• Orientation of each pair of homologues on the spindle is random

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16

Kinetochore microtubule

In metaphase I, the pairs of

homologous chromosomes

align at the equator of the cell.

Chiasmata keep homologous

pairs together and microtubules

from opposite poles attach to

sister kinetochores of each

homologue, producing tension.

A kinetochore microtubule from

one pole of the cell attaches to

one homologue of a

chromosome, while a

kinetochore microtubule from

the other cell pole attaches to

the other homologue of a pair.

Homologue pair

on metaphase plate


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Anaphase I

• Microtubules of the spindle shorten

– Chiasmata break

• Homologues are separated from each other and

move to opposite poles

– Sister chromatids remain attached to each other at

their centromeres

• Each pole has a complete haploid set of

chromosomes consisting of one member of each

homologous pair

• Independent assortment of maternal and

paternal chromosomes

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Homologous chromosomes

Sister chromatids

In anaphase I, kinetochore

microtubules shorten, and

homologous pairs are pulled

apart. One duplicated homologue

goes to one pole of the cell, and

the other duplicated homologue

goes to the other pole. Sister

chromatids do not separate. This

is in contrast to mitosis, in which

duplicated homologues line up

individually on the metaphase

plate, and sister chromatids are

pulled apart in anaphase.


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Telophase I

• Nuclear envelope re-forms around each

daughter nucleus

• Sister chromatids are no longer identical

because of crossing over (prophase I)

• Cytokinesis may or may not occur after

telophase I

• Meiosis II occurs after an interval of

variable length

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Chromosome

Nonidentical sister chromatids

In telophase I, the separated

homologues form a cluster at

each pole of the cell, and the

nuclear envelope re-forms

around each daughter cell

nucleus. Cytokinesis may occur.

The resulting two cells have

half the number of

chromosomes of the original

cell: In this example, each

nucleus contains two

chromosomes (versus four in

the original cell). Each

chromosome consists of two

sister chromatids, but sister

chromatids are not identical

because crossing over has

occurred.

Homologous

chromosomes


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22

Meiosis II

• Resembles a mitotic division

• Prophase II: nuclear envelopes dissolve and new spindle apparatus forms

• Metaphase II: chromosomes align on metaphase plate

• Anaphase II: sister chromatids are separated from each other

• Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows

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Spindle

Nuclear membrane breaking down

Following a brief interphase,

with no S phase, meiosis II

begins. During prophase II,

a new spindle apparatus forms

in each cell, and the nuclear

envelope breaks down. In some

species the nuclear envelope

does not re-form in telophase I,

obviating the need for nuclear

envelope breakdown.


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Sister chromatids

Chromosome

In metaphase II, chromosomes

consisting of sister chromatids

joined at the centromere align

along the metaphase plate in

each cell. Now, kinetochore

microtubules from opposite

poles attach to kinetochores of

sister chromatid, as in mitosis.


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Sister chromatids

When microtubules shorten in

anaphase II, sister chromatids

are pulled to opposite poles

of the cells, as in mitosis.

Kinetochore

microtubule


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In telophase II, the nuclear

membranes re-form around

four different clusters of

chromosomes. After

cytokinesis, four haploid cells

result. No two cells are alike

due to the random alignment of

homologous pairs at

metaphase I and crossing over

during prophase I.

Nuclear

membrane

re-forming


Final result

• Four cells containing haploid sets of

chromosomes

• In animals, develop directly into gametes

• In plants, fungi, and many protists, divide

mitotically

– Produce greater number of gametes

– Adults with varying numbers of gametes

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Errors in Meiosis

• Nondisjunction – failure of chromosomes

to move to opposite poles during either

meiotic division

• Aneuploid gametes – gametes with

missing or extra chromosomes

• Most common cause of spontaneous

abortion in humans

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Meiosis vs. Mitosis

Meiosis is characterized by 4 features:

1. Synapsis and crossing over

2. Sister chromatids remain joined at their

centromeres throughout meiosis I

3. Kinetochores of sister chromatids attach

to the same pole in meiosis I

4. DNA replication is suppressed between

meiosis I and meiosis II

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Meiosis I Mitosis

Metaphase I

Anaphase I

Metaphase

Anaphase

Crossovers and

sister chromatid

cohesion lock

homologues

together.

Microtubules

connect to the

kinetochores of

sister chromatids so

that homologues are

pulled toward

opposite poles.

Microtubules pull

the homologous

chromosomes

apart, but sister

chromatids are

held together at

the centromere.

Homologues do

not pair;

kinetochores of

sister chromatids

remain separate;

microtubules

attach to both

kinetochores on

opposite sides of

the centromere.

Microtubules

pull sister

chromatids

apart.

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Parent cell (2n)

MEIOSIS I

Prophase I Metaphase I Anaphase I Telophase I

Prophase Metaphase Anaphase Telophase

Homologous chromosomes

do not pair.Individual homologues align

on metaphase plate.

Paternal

homologue

Homologous

chromosomes

Homologous chromosomes pair;

synapsis and crossing over occur.

Paired homologous chromosomes

align on metaphase plate.

Maternal

homologue

MITOSIS

Sister chromatids separate, cytokinesis occurs, and two cells

result, each containing the original number of homologues.

Two

daughter

cells

(each 2n)

Homologous chromosomes separate;

sister chromatids remain together.

Chromosome

replication

Chromosome

replication

33


MEIOSIS II

Prophase II Metaphase II Anaphase II Telophase II

Chromosomes align, sister chromatids separate, and four haploid cells result,

each containing half the original number of homologues.

Four

daughter

cells

(each n)

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Sexual Reproduction and Meiosismmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · Sexual life cycle •Made up of meiosis and fertilization •Diploid cells –Somatic cells