MEIOSISA type of cell division that produces gametes

or sex cells.

It is a reduction division because it produces four daughter cells each with half the number of chromosomes, the haploid

number (n)

How many chromosomes in the case of humans?

Why is meiosis necessary?

• It is necessary for sexual reproduction. It is not necessary for the life of an individual but for survival of the species as a whole.

• Meiosis makes gametes (eggs and sperms). When one reproduces an individual passes on its genetic material to its offspring.The two parents contribute its chromosomes.

Where does meiosis occur?

• It occurs in the ovaries and testes to produce eggs and sperm.

• All other cells in the body (somatic cells) reproduce by mitosis

Asexual reproduction

• Asexual reproduction requires only one parent • Examples: Bacteria (prokaryotes) reproduce by binary

fission.

A Eukaryotic protist such as a paramecium reproduces by mitotic division ( two cells from one).

Yeast and Hydra reproduce by budding ( grows a bud that then breaks off).

Flat worms reproduce by fragmentation ( breaks into pieces) and some organisms can develop new ones from unfertilized eggs (some fish and lizards)

Sexual Reproduction

• Involves

– Meiosis

– Gamete production

– Fertilization

• Produces genetic variation among offspring

Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits

• This variation in traits is the basis for evolutionary change

Variation• Sexual reproduction results in greater variation among

offspring than does asexual reproduction.• Two parents give rise to offspring that have unique

combinations of genes inherited from the parents.• Offspring of sexual

reproduction vary genetically from their siblings and from both parents.

What is sexual reproduction?

Each parent produces a gamete (egg, sperm) and the two parents contribute

chromosomes to the offspring.

During fertilization the two gametes unite

What would happen to the number of chromosomes if the gametes of each parent

contained the same number of chromosomes as the somatic cells?

– Sexual life cycles• Involve the

alternation ofhaploid and diploid stages

Mitosis and development

Multicellulardiploid adults

(2n = 46)

Diploidzygote

(2n = 46) 2n

Meiosis Fertilization

Egg cell

Sperm cell

n

Haploid gametes (n = 23)

n

Figure 8.13

Gamete Formation• Gametes are sex cells (sperm, eggs)

• Arise from germ cells

testes

ovaries

anther ovary

Meiosis reduces the chromosome number from diploid to haploid– Meiosis, like mitosis

• Is preceded by chromosome duplication

– But in meiosis• The cell divides twice to form four daughter cells

Chromosome Number

• Sum total of chromosomes in a cell

• Germ (ovaries and testes) cells are diploid

(2n)

• Gametes (eggs, sperm) are haploid (n)

• Meiosis halves chromosome number

Gametes have a single set of chromosomes– Cells (somatic or body cells) with two sets of

chromosomes• Are diploid

– Gametes, eggs and sperm, are haploid• With a single set of chromosomes

Human Chromosome Number• Diploid chromosome number (2n) = 46

• Two sets of 23 chromosomes each– One set from father– One set from mother

• Mitosis produces cells with 46 chromosomes--two of each type

MEIOSIS AND CROSSING OVER• Chromosomes are matched in homologous

pairs– The somatic (body) cells of each species

• Contain a specific number of chromosomes

– For example human cells have 46• Making up 23 pairs of homologous chromosomes

• In humans, each somatic cell has 46 chromosomes.– Each chromosome can be distinguished by its size, position of

the centromere, and by pattern of staining with certain dyes.

• These homologous chromosome pairs carry genes that control the same inherited characters at the same place or locus.

(Locus or loci means place)

What are homologous chromosomes?

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

What are homologous pairs of chromosomes?

Humans have 46 chromosomes. 23 from the mother and 23 from the father. We get a complete set of genetic information from each parent.

• The chromosomes of these pairs that contain similar genetic material and similar size and shape are called homologous pairs of chromosomes.

– The chromosomes of a homologous pair• Carry genes for the same characteristics at the same

place, or locus

Chromosomes

Centromere

Sister chromatidsFigure 8.12

Homologous Chromosomes May Carry Different Alleles

• Cell has two of each chromosome

• One chromosome in each pair from mother,

other from father

• Paternal and maternal chromosomes carry

different alleles

What is fertilization?What is a zygote?

• Fertilization is the union of two gametes, one from each parent.

• Each gamete is haploid. During fertilization the diploid number is restored.

Ex: in humans the egg and the sperm each contains 23 chromosomes so after fertilization the fertilized egg will have 46 chromosomes

A zygote is a fertilized egg. It is diploid.

Web sites to check

• http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Terms to know• Meiosis

• Genome

• Gametes

• Diploid chromosome number

• Haploid chromosome number

• Zygote

• Karyotype

• Homologous chromosomes

• Non-disjunction

Meiosis: Two Divisions

• Two consecutive nuclear divisions

– Meiosis I

– Meiosis II

• DNA is NOT duplicated between divisions

• Four haploid nuclei are formed

– The first division, meiosis I• Starts with synapsis, the pairing of homologous

chromosomes

– In crossing over• Homologous chromosomes exchange corresponding

segments

– Meiosis I separates each homologous pair• And produce two daughter cells, each with one set of

chromosomes

Meiosis I

Each homologue in the cell pairs with its partner,

then the partners separate

Meiosis II• The two sister chromatids of each

duplicated chromosome are separated from each other

one chromosome (duplicated)

two chromosomes (unduplicated)

Stages of MeiosisMeiosis I

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

Meiosis II

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

– The stages of meiosis

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes (with centriole pairs)

Sites of crossing over

Spindle

Microtubulesattached to kinetochore

Metaphaseplate

Sister chromatids remain attached

Nuclearenvelope Chromatin

Sisterchromatids Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

Figure 8.14 (Part 1)

PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

MEIOSIS II: Sister chromatids separate

Figure 8.14 (Part 2)

– Random arrangements of chromosome pairs at metaphase I of meiosis

• Lead to many different combinations of chromosomes in eggs and sperm

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probablearrangements of chromosomes at

metaphase I

Possibility 1 Possibility 2

Figure 8.16

Prophase I• Each duplicated chromosome pairs with its

homologue

• Homologues swap segments

• Each chromosome becomes attached to microtubules of spindle

Metaphase I• Chromosomes are pushed and pulled into

the middle of cell

• Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole

• The spindle is now fully formed

Anaphase I

• Homologous chromosomes segregate

(move away) from each other

• The sister chromatids of each chromosome

remain attached

Telophase I• The chromosomes arrive at opposite poles

• The cytoplasm divides

• There are now two haploid cells

• This completes Meiosis I

Meiosis II - Stages

Prophase II Metaphase II Anaphase II Telophase II

Prophase II• Microtubules attach to the kinetochores of

the duplicated chromosomes

• Motor proteins drive the movement of chromosomes toward the spindle’s equator

Metaphase II

• All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles

Anaphase II• Sister chromatids separate to become

independent chromosomes

• Motor proteins interact with microtubules to move the separated chromosomes to opposite poles

Telophase II• The chromosomes arrive at opposite ends of

the cell

• A nuclear envelope forms around each set of chromosomes

• The cytoplasm divides

• There are now four haploid cells

Overview of Meiosis stages

• Prophase ISpindle begins to form Homologous chromosomes pair up and exchange

pieces of genetic material. This is crossing over.

• Metaphase IHomologous pairs align in the center of the cell (tetrads)

• Anaphase IHomologous chromosomes move toward opposite poles. Centromeres

do not divide and sister chromatids stay together.

• Telophase IChromosomes gather together at the two poles

Meiosis stages (continuation)• Prophase IIChromosomes re-condense and spindle forms again

• Metaphase IIChromosomes align along the center of the cell

• Anaphase IISister chromatids separate and move toward opposite

poles

• Telophase IIChromatids arrive at each pole, cytokynesis begins

Mitosis Meiosis

Parent cell(before chromosome replication)

Chromosome replication

Chromosome replication

Chromosomes align at themetaphase plate

Tetradsalign at themetaphase plate

Sister chromatidsseparate during anaphase

Homologous chromosomesseparate duringanaphase I;sister chromatidsremain together

No furtherchromosomalreplication; sisterchromatidsseparateduringanaphase II

Prophase

Metaphase

AnaphaseTelophase

Duplicated chromosome(two sister chromatids)

Daughter cellsof mitosis

2n 2n

Daughtercells of

meiosis I

n n nn

2n = 4

Tetrad formedby synapsis ofhomologouschromosomes

Meiosis i

Meiosis ii

Prophase I

Metaphase I

Anaphase ITelophase I

Haploidn = 2

Daughter cells of meiosis II

•Review: A comparison of mitosis and meiosis

Figure 8.15

Crossing over• Crossing over is the exchange of genetic

material (DNA) between homologous chromosomes.

• During prophase I of meiosis, the homologous portions of two non-sister chromatids exchange places.

• They get so close to each other that chromosomes form connections called chiasmata and exchange sections of DNA

Why is crossing over important?

• It produces VARIETY in the offspring

Crossing Over

•Each chromosome

becomes zippered to its

homologue

•All four chromatids are

closely aligned

•Non-sister chromosomes

exchange segments

Effect of Crossing Over

• After crossing over, each chromosome

contains both maternal and paternal

segments

• Creates new allele combinations in

offspring

Homologous chromosomes carry different versions of genes– The differences between homologous chromosomes

• Are based on the fact that they can bear different versions of a gene at corresponding loci (means places)

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

ec

EC

White Pink ec

ec

EC

EC

Meiosis

BlackBrown

Chromosomes of the four gametes

Eye-colorgenes

Coat-colorgenes

Brown coat (C); black eyes (E) White coat (C); pink eyes (e)

Figure 8.17A

Figure 8.17B

•Crossing over further increases genetic variability– Genetic recombination

• Which results from crossing over during prophase I of meiosis, increases variation still further

Figure 8.18A

ChiasmaTetrad

Centromere

TE

M 2

,200

Coat-colorgenes

Eye-colorgenes

Tetrad (homologous pair of chromosomes

in synapsis)

C E

c e

C E

c e

C E

c e

Chiasma

C E

C e

c E

c e

C E

C e

c E

c e

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

– How crossing over leads to genetic variation

Breakage of homologous chromatids1

Joining of homologous chromatids2

3Separation of homologous chromosomes at anaphase I

4

Separation of chromatids at anaphase II and completion of meiosis

Figure 8.18B

Random Alignment • At the beginning of metaphase I,

microtubules from spindle attach to kinetochores of chromosomes

• Initial contacts between microtubules and chromosomes are random

Possible Chromosome Combinations

As a result of random alignment, the number of possible combinations of chromosomes

in a gamete is:

2n

(n is number of chromosome types)

Possible Chromosome

Combinations or

or

or

1 2 3

Spermatogenesis

GrowthMiosis I,

Cytoplasmic divisionMeiosis II,

Cytoplasmic division

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

spermato-gonium

(diploid male reproductive

cell)

Oogenesis

Growth Mitosis I,Cytoplasmic division

Meiosis II,Cytoplasmic division

ovum (haploid)

primary oocyte (diploid)

oogonium (diploid

reproductive cell) secondary

oocyte haploid)

first polar body

haploid)

three polar bodies

haploid)

Fertilization• Male and female gametes unite and nuclei

fuse

• Fusion of two haploid nuclei produces

diploid nucleus in the zygote

• Which two gametes unite is random

– Adds to variation among offspring

– Random fertilization of eggs by sperm• Greatly increases this variation

Factors Contributing to Variation Among Offspring

• Crossing over during prophase I

• Random alignment of chromosomes at

metaphase I

• Random combination of gametes at

fertilization

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

What is a karyotype?

•A karyotype is a photograph of an individual’s chromosomes arranged in order

•A karyotype is a picture, a display of the 46 chromosomes. Shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.

Blood culture

Fluid

Centrifuge

Packed red andwhite blood cells

Hypotonicsolution

Fixative

White blood cells

Stain

Centromere

Pair of homologouschromosomes

Sisterchromosomes

2,6

00X

A bloodculture is centrifuged to separate the blood cells from the culture fluid.

1 The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out.

2 Another centrifugation step separates the swollen whiteblood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixedwith the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained.

3

The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape.

4 The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram).

5

– Preparation of a karyotype from a blood sample

Figure 8.19

Karyotypes Go to activities (on left hand side) and click

on “karyotyping”

• http://www.biology.arizona.edu/cell_bio/cell_bio.html

Sex chromosome pair

• Male pair is: XY• Female pair is: XX

How is sex determined?

Down syndrome

• An extra copy of chromosome 21 causes Down syndrome– A person may have an abnormal number of

chromosomes• Which causes problems

– Down syndrome is caused by trisomy 21• An extra copy of chromosome 21

5,00

0

Figure 8.20A Figure 8.20B

• One aneuploid condition, Down syndrome, is due to three copies of chromosome 21.

– It affects one in 700 children born in the United States.

• Although chromosome 21 is the smallest human chromosome, it severely alters an individual’s phenotype.

– The chance of having a Down syndrome child• Goes up with maternal age

Age of mother

45 50353025 4020

90

0

10

20

30

40

50

60

70

80

Infa

nts

with

Dow

n sy

ndro

me

(per

1,0

00 b

irth

s)

Figure 8.20C

•Accidents during meiosis can alter chromosome number– Abnormal chromosome count is a result of

nondisjunction• The failure of homologous pairs to separate during meiosis I• The failure of sister chromatids to separate during meiosis II

aneuploidy• have an abnormal chromosome number

_________________________________– Trisomic cells have three copies of a particular chromosome

type and have 2n + 1 total chromosomes.

– Monosomic cells have only one copy of a particular chromosome type and have 2n - 1 chromosomes.

• If the organism survives, aneuploidy typically leads to a distinct phenotype.

Nondisjunction

Nondisjunction in meiosis I

Normal meiosis II

Gametes

n 1 n 1 n 1 n 1

Number of chromosomes

Nondisjunction in meiosis II

Normal meiosis I

Gametes

n 1 n 1 n n

Number of chromosomes

Figure 8.21A

Figure 8.21B

– Fertilization after nondisjunction in the mother

Sperm cell

Egg cell

n (normal)

n + 1

Zygote2n + 1

Figure 8.21C

Abnormal numbers of sex chromosomes

Abnormal numbers of sex chromosomes do not usually affect survival– Nondisjunction can also produce gametes with

extra or missing sex chromosomes• Leading to varying degrees of malfunction in humans but

not usually affecting survival

Figure 8.22A

Poor beard growth

BreastDevelopment

Under-developedtestes

Figure 8.22B

Characteristic facialfeatures

Web of skin

Constrictionof aorta

Poor breastdevelopment

Under developedovaries

– Human sex chromosome abnormalities

• Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live births.– These individuals have male sex organs, but are sterile.– There may be feminine characteristics, but their intelligence

is normal.

• Males with an extra Y chromosome (XYY) tend to somewhat taller than average.

• Trisomy X (XXX), which occurs once in every 2000 live births, produces healthy females.

• Monosomy X or Turner’s syndrome (X0), which occurs once in every 5000 births, produces phenotypic, but immature females.

• Alterations of chromosome structure can cause birth defects and cancer– Chromosome breakage can lead to

rearrangements• That can produce genetic disorders or, if the

changes occur in somatic cells, cancer

– Deletions, duplications, inversions, and translocations

Deletion

Duplication

Inversion

Homologouschromosomes

Reciprocaltranslocation

Nonhomologouschromosomes

“Philadelphia chromosome”

Chromosome 9

Chromosome 22Reciprocaltranslocation

Activated cancer-causing geneFigure 8.23A

Figure 8.23C

Figure 8.23B

Chromosomal translocations between

non-homologous chromosome are also associated with human disorders.

• implicated in certain cancers, including chronic myelogenous leukemia (CML).– CML occurs when a fragment of chromosome 22

switches places with a small fragment from the tip of chromosome 9.

• Some individuals with Down syndrome have the normal number of chromosomes – all or part of a third chromosome 21 attached to

another chromosome by translocation.

• Deletions, even in a heterozygous state, cause severe physical and mental problems.

• cri du chat, – results from a specific deletion in chromosome 5.– These individuals are mentally retarded, have a

small head with unusual facial features, and a cry like the mewing of a distressed cat.

– This syndrome is fatal in infancy or early childhood.

Meiosis compared to mitosis

MitosisIn somatic cells• One duplication of

chromosomes followed by one division (duplication takes place during “S”phase before mitosis starts)

• Results in two daughter cells identical to the parent cell and to each other

MeiosisIn sex cells• One duplication of

chromosomes followed by two divisions (duplication takes place during “S”phase before meiosis starts)

• Results in four daughter cells with half the number of chromosomes.

• Variety or new combinations result.

Mitosis• Functions

– Asexual reproduction

– Growth, repair

• Occurs in somatic cells

• Produces clones

Mitosis & Meiosis Compared

Meiosis• Function

– Sexual reproduction

• Occurs in germ cells

• Produces variable offspring

MITOSIS AND MEIOSIS COMPARED

• Occurs in somatic cells

• One duplication one division

• Result in two diploid (2N) daughter cells

• Daughter cells are identical to parent cell

• Occurs in gametes only

• One duplication two divisions

• Result in four haploid (N) daughter cells

• Daughter cells are different to parent cell. Introduces variety.

How is genetic variation introduced?

1. Crossing over between homologous chromosomes during prophase I of meiosis

2. Independent assortment of homologous chromosomes during metaphase I as they attach to spindle and move to either pole

3. Random fertilization of an ovum (egg) by random sperm

Results of Mitosis and Meiosis• Mitosis

– Two diploid cells produced

– Each identical to parent

• Meiosis

– Four haploid cells produced

– Differ from parent and one another

Web sites to check

• http://www.pbs.org/wgbh/nova/baby/divi_flash.html