Meiosis Bio 11 Tuesday, October 7, 2008. Which one of the following does not occur during mitotic anaphase? A) The centromeres of each chromosome divide

Meiosis

Bio 11Tuesday, October 7, 2008

• Which one of the following does not occur during mitotic anaphase?

• A) The centromeres of each chromosome divide.

• B) Sister chromatids separate. • C) The chromatid DNA replicates. • D) Daughter chromosomes begin to move toward

opposite poles of the cell. • E) All of the choices occur during mitotic

anaphase.

Asexual Reproduction is a primary means of propagation for many species

Sexual reproduction generates diversity

• Variety in offspring improves chances of adaptation to changing environments

The offspring of sexual reproduction inherit parental traits

• Offspring inherit DNA from each parent

• Offspring are not the same

• DNA is from the same source

LE 8-13

Haploid gametes (n 23)

Egg cell

Sperm cell

FertilizationMeiosis

Diploidzygote

(2n 46)

n

Multicellulardiploid adults

(2n 46)

Mitosis anddevelopment

2n

nMeiosis is the process of cell division that allows gamete formationGametes are haploid (n), somatic cells are diploid (2n)

Chromosomes come in pairs• For diploid organims

(2n)• One chromosome in

a paired set comes from each parent

• #’s 1-22 are autosomes

• X and Y are sex chromosomes

LE 8-12

Homologous Chromosomes

Centromere

Sister chromatids

Homologous chromosomes• Pairs of chromosomes

are homologous• The site for particular

genes are called loci (singular: locus)

• Identical strands of the same chromosome are sister chromatids

Loci

LE 13-6

Key

HaploidDiploid

Gametesn

Diploidmulticellularorganism(sporophyte)

MitosisDiploidmulticellular

organism

FERTILIZATIONMEIOSIS

Zygote

n

n

2n 2n

Animals Plants and some algae Most fungi and some protists

n n

n

n n

nn

n

n

nFERTILIZATION

FERTILIZATION

MEIOSIS

MEIOSIS

Gametes Gametes

Zygote

ZygoteMitosis

Mitosis Mitosis Mitosis Mitosis

2n2n

2n

Spores

Haploid multicellular organism (gametophyte)

Haploid multicellular organism

Overview of Meiosis

• Meiosis consists of 2 major stages, Meiosis I and Meiosis II

• Cell copies DNA once, divides twice, creating 4 haploid cells

• In meiosis I, homologous pairs of chromosomes are grouped, recombined, and then segregated into two intermediate cells

• In meiosis II, those cells are divided

The Stages of Meiosis

• In (meiosis I), homologous chromosomes separate

• Meiosis I results in two haploid daughter cells with replicated chromosomes

• In the second cell division (meiosis II), sister chromatids separate

• Meiosis II results in four haploid daughter cells with unreplicated chromosomes

LE 8-14a

INTERPHASE PROPHASE METAPHASE ANAPHASE

MEIOSIS

Centrosomes(with centriolepairs)

Sites of crossing over

Spindle

Microtubulesattached tokinetochore

Metaphaseplate

Sister chromatidsremain attached

Homologouschromosomes separate

Centromere(with kinetochore)

TetradSisterchromatidsChromatin

Nuclearenvelope

: Homologous chromosome separate

LE 13-8aa

Centrosomes(with centriole pairs)

Nuclearenvelope

Chromatin

Chromosomes duplicate

INTERPHASE

MEIOSIS I: Separates homologous chromosomes

METAPHASE I ANAPHASE I(Interphase precedes meiosis, of course)

• Division in meiosis I occurs in four phases:Prophase IMetaphase IAnaphase ITelophase I

Prophase I• >90% of the time required for

meiosis• Chromosomes condense• Synapsis: homologous

chromosomes pair up• Crossing over: nonsister

chromatids exchange DNA segments

• Tetrad: four chromatids• Chiasmata: X-shaped regions

where crossing over occurred

Crossing-Over in Prophase I

LE 8-18a

Tetrad Chiasma

Centromere

TE

M

2,20

0

Crossing-over in Prophase I

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle


Metaphaseplate

Homologouschromosomesseparate


Microtubuleattached tokinetochore

Tetrad


PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Metaphase I

• Tetrads line up at the metaphase plate

• Microtubules from one pole are attached

• Microtubules from the other pole are attached to the kinetochore of the other chromosome

Animation: Metaphase I

Independent Assortment in Metaphase I

• Random alignment of maternal/paternal chromosomes at the metaphase plate

• Produces genetic variability within populations



Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

• Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

• The number of combinations possible when chromosomes assort independently into gametes is 2n-

1, where n is the haploid number• For humans (n = 23), there are more than 8 million (222)

possible combinations of chromosomes

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle


Metaphaseplate

Homologouschromosomesseparate


Microtubuleattached tokinetochore

Tetrad


PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Anaphase I

• homologous chromosomes separate

• One chromosome moves toward each pole

• Sister chromatids remain attached at the centromere

Meiosis II

• Resembles mitosis in its mechanisms• However, only half the chromosomes are

present in meiosis II compared with mitosis

LE 8-14b

Cleavagefurrow

TELOPHASE PROPHASE METAPHASE ANAPHASE

TELOPHASE

Sister chromatidsseparate

Haploid daughtercells forming

MEIOSIS : Sister chromatids separate

AND CYTOKINESIS AND CYTOKINESIS

LE 13-8b

Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I ANDCYTOKINESIS

TELOPHASE II ANDCYTOKINESIS


Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Metaphase II

• At metaphase II, the sister chromatids are arranged at the metaphase plate

• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

• The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Animation: Metaphase II

Spindle Microtubules bind to the Kinetochore

• Prometaphase marks the binding of spindle fibers to kinetochore

• Chromosomes will be pulled apart in anaphase

LE 13-8b

Cleavagefurrow








Anaphase II

• At anaphase II, the sister chromatids separate• The sister chromatids of each chromosome

now move as two newly individual chromosomes toward opposite poles

Animation: Anaphase II

LE 13-8b

Cleavagefurrow








Which one of the following statements is false? A) A typical body cell is called a somatic cell. B) Gametes are haploid cells. C) Somatic cells are diploid. D) Gametes are made by mitosis. E) A zygote is

a fertilized egg.

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

opposite poles• Nuclei form, and the chromosomes begin

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

cells, each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell

Animation: Telophase II and Cytokinesis

LE 8-21a

Number of chromosomes

Gametes

Normalmeiosis Normalmeiosis

Nondisjunctionin meiosis

n 1 n 1 n 1 n 1

LE 8-21b

Number of chromosomes

Gametes

Normalmeiosis

Nondisjunctionin meiosis

n 1 n 1 n n

LE 8-21c

Egg cell

Sperm cell

n (normal)

n 1

Zygote 2n 1

Downs syndrome is caused by a nondisjunction (trisomy) at chromosome 21

LE 8-20c

Infa

nts

wit

h D

ow

n s

yn

dro

me

(pe

r 1

,00

0 b

irth

s)

Age of mother20 25 30 35 40 45 50

10

20

30

40

50

60

70

80

90

0

Figure 8.20A

LE 8-19

Packed red andwhite blood cells

Hypotonicsolution

CentrifugeBloodculture

Fluid

Fixative

White blood cells

Stain

Centromere

Sisterchromatids

Pair of homologouschromosomes

2,60

0

Comparing mitosis and meiosis

Property Mitosis Meiosis

DNA replication

Divisions

Synapsis and crossing over

Daughter cells, genetic composition

Role in animal body

Property Mitosis Meiosis

DNA replication

During interphase

During interphase

Divisions One Two

Synapsis and crossing over

Do not occur In prophase I

Daughter cells, genetic composition

Two diploid, identical to parent cell

Four haploid, different from parent cell and each other

Role in animal body

Produces cells for growth and tissue repair

Produces gametes

LE 13-9

Propase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

2n = 6

Parent cell(before chromosome replication)

Chromosomereplication

MITOSIS MEIOSIS

Chiasma (site ofcrossing over) MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Tetradspositioned at themetaphase plate

Metaphase IChromosomes positioned at themetaphase plate

Metaphase

AnaphaseTelophase

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Sister chromatidsseparate duringanaphase

Daughtercells of

meiosis I

Haploidn = 3

Anaphase ITelophase I

MEIOSIS IIDaughter cells

of mitosis

2n2n

n

Sister chromatids separate during anaphase II

n n nDaughter cells of meiosis II

Review of Meiosis

Spermatogenesis vs. Oogenesis

Meiosis• One spermatocyte

produces 4 sperm

• One oocyte produces 1 viable egg and 3 non-viable polar bodies

Variation from sexual reproduction contributes to evolution

• Reshuffling of different versions of genes (alleles) during sexual reproduction produces genetic variation

LE 13-10

Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1 Possibility 2

Combination 2Combination 1 Combination 3 Combination 4

Daughtercells

Metaphase II

Two equally probablearrangements ofchromosomes at

metaphase I

LE 13-11Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells

Random Fertilization

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

• The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations

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

Evolutionary Significance of Genetic Variation Within Populations

• Natural selection results in accumulation of genetic variations favored by the environment

• Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

Explain how junior and sis are not identical.

LE 8-17a

Coat-colorgenes

Eye-colorgenes

Brown

Meiosis

White

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes

Black

C E

ec

C

C

c

c

E

e

E

ePink

LE 8-18bCoat-color

genesEye-color

genes

(homologous pair ofchromosomes in synapsis)

Tetrad

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase

Separation of chromatids atanaphase andcompletion of meiosis

Parental type of chromosome

Recombinant chromosome


C

C

E

E

e

e

c

c

c

c

e

E

C

C E

e

e

E

c

C

EC

c e

EC

c e


Gametes of four genetic types

LE 8-18ba

Coat-colorgenes

Eye-colorgenes

(homologous pair ofchromosomes in synapsis)

Tetrad

Breakage of homologous chromatids

EC

c e

EC

c e


LE 8-18bb


Chiasma

Separation of homologouschromosomes at anaphase


EC

c

e

EC

c e

e

C

c

E

LE 8-18bc






Gametes of four genetic types

EC

c

eC

c

E

e

Table 8.22

Review of mitosis and meiosis

LE 8-un149

INTERPHASE (cell growth and chromosome duplication)

S(DNA synthesis)

Cytokinesis Mitosis

Geneticallyidentical“daughtercells”

MITOTIC PHASE (M)

G1

G2

(division ofcytoplasm)

(division of nucleus)

LE 8-un149s

Haploid gametes (n 23)

Egg cell

Sperm cell

n

FertilizationMeiosis

n

Multicellulardiploid adults

Diploidzygote

(2n 46) (2n 46)

HUMANLIFE

CYCLE

Mitosis and development

2n

LE 8-un1

Sister chromatids

Sister chromatids

Homologouspair ofchromosomes

Documents

Meiosis Bio 11 Tuesday, October 7, 2008. Which one of the following does not occur during mitotic anaphase? A) The centromeres of each chromosome divide