Nuclear Division

MEIOSIS AND CROSSING OVER

2012 Pearson Education, Inc.

In humans, ___________________have

23 pairs of homologous chromosomes and

one member of each pair from each parent.

The human _______________ X and Y differ in

8.11 Chromosomes are matched in homologous pairs

The human _______________ X and Y differ in

size and genetic composition.

The other 22 pairs of chromosomes are

____________ with the same size and genetic

composition.


Centromere

Sisterchromatids

Pair of homologous

Karyotype- an ordered display of magnified images of an individuals chromosomes arranged in pairs (starting with longest)

Pair of homologouschromosomes

___________________________are matched in

length,

centromere position, and

gene locations.

8.11 Chromosomes are matched in homologous pairs

gene locations.

A _________ (plural, loci) is the position of a gene.

Different versions of a gene may be found at the

same locus on maternal and paternal

chromosomes.


Figure 8.11

Pair of homologouschromosomes

Locus

Centromere

Applying Your Knowledge Humans have 46

chromosomes; how many homologous pairs does that represent?

Centromere

Sisterchromatids

One duplicatedchromosome

If there is one pair of sex chromosomes, how many pairs of autosomes are found in humans?

Meiosis is a process that converts diploid nuclei to haploid nuclei.

__________ cells (2n) have two homologous sets of chromosomes.

__________ cells (n) have one set of chromosomes.

8.12 Gametes have a single set of chromosomes

__________ cells (n) have one set of chromosomes.

Meiosis occurs in the sex organs, producing gametessperm and eggs.

Fertilization is the union of sperm and egg.

The zygote has a diploid chromosome number, one set from each parent.


Figure 8.12A

Haploid gametes (n ==== 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Figure 8.12A The human life cycle

Ovary Testis

Diploidzygote

(2n ==== 46)2n

MitosisKey

Haploid stage (n)

Diploid stage (2n)Multicellular diploidadults (2n ==== 46)

All sexual life cycles include an alternation

between

a diploid (2n) stage and a haploid (n) stage

Producing haploid (n) gametes prevents the

8.12 Gametes have a single set of chromosomes

Producing haploid (n) gametes prevents the

chromosome number from doubling in every

generation.


Figure 8.12B

Sisterchromatids

1 2 3

INTERPHASE MEIOSIS I MEIOSIS II

A pair ofhomologouschromosomesin a diploidparent cell

A pair ofduplicatedhomologouschromosomes

Figure 8.12B How meiosis halves chromosome number

____________ is a type of cell division that

produces haploid gametes in diploid organisms.

Two haploid gametes combine in fertilization to

restore the diploid state in the zygote.

8.13 Meiosis reduces the chromosome number from diploid to haploid


Meiosis and mitosis are preceded by the duplication

of chromosomes. However,

meiosis is followed by two consecutive cell divisions

mitosis is followed by only one cell division


In meiosis, one duplication of chromosomes is

followed by two divisions, producing _____ daughter

cells, each has a __________ set of chromosomes.


Figure 8.13_left

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Spindle microtubulesattached to a kinetochore

Sister chromatidsremain attached

Chromosomes duplicate Prophase I Metaphase I Anaphase I

INTERPHASE:MEIOSIS I: Homologous chromosomes separate

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Centromere(with akinetochore)

Metaphaseplate Homologous

chromosomesseparate

Figure 8.13_right

Cleavagefurrow

Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II

MEIOSIS II: Sister chromatids separate

Telophase IIand Cytokinesis

Sister chromatidsseparate

Haploid daughtercells forming

Figure 8.13_1

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Chromosomes duplicate Prophase I

INTERPHASE:MEIOSIS I

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Figure 8.13_2

Spindle microtubulesattached to a kinetochore

Sister chromatidsremain attached

Metaphase I Anaphase I

MEIOSIS I

Centromere(with akinetochore)

Metaphaseplate Homologous

chromosomesseparate

Figure 8.13_3

Cleavagefurrow

Telophase I and Cytokinesis

Figure 8.13_4

Prophase II Metaphase II Anaphase II

MEIOSIS II: Sister chromatids separate

Telophase IIand Cytokinesis

Sister chromatidsseparate

Haploid daughter

cells forming

Meiosis I Prophase I

Chromosomes coil and become compact.


Homologous chromosomes come together as pairs by ____________.

Each pair, with four chromatids, is called a ________.

Nonsister chromatids exchange genetic material by _______________.


Applying Your Knowledge

Human cells have 46 chromosomes. At the end

of prophase I


of prophase I

How many chromosomes are present in

one cell?

How many chromatids are present in one

cell?


Meiosis I Metaphase I

_________ align at the cell equator.

Meiosis I Anaphase I


Meiosis I Anaphase I

_________________ separate and move toward opposite poles of the cell.



Human cells have 46 chromosomes. At the end of Metaphase I

How many chromosomes are present in


How many chromosomes are present in one cell?

How many chromatids are present in one cell?


Meiosis I Telophase I

Duplicated chromosomes have reached the poles.

A nuclear envelope re-forms around chromosomes in some species.

Each nucleus has the haploid number of


Each nucleus has the haploid number of chromosomes.



After telophase I and cytokinesis

How many chromosomes are present in one

human cell?


human cell?

How many chromatids are present in one human

cell?


Meiosis II follows meiosis I without

chromosome duplication.

Each of the two haploid products enters meiosis II.


Meiosis II Prophase II

Chromosomes coil and become compact (if uncoiled after telophase I).

Nuclear envelope, if re-formed, breaks up again.


Meiosis II Metaphase II

Duplicated chromosomes align at the cell

equator.


Meiosis II Anaphase II

Sister chromatids separate

chromosomes move toward opposite poles.


Meiosis II Telophase II

Chromosomes have reached the

poles of the cell.

A nuclear envelope forms around


A nuclear envelope forms around

each set of chromosomes.

With cytokinesis, four haploid cells

are produced.



After telophase II and cytokinesis

How many chromosomes are present in one

human cell?


human cell?

How many chromatids are present in one human

cell?


Similarities:

begin with diploid parent cells that

have chromosomes duplicated during the previous interphase.

8.14 Mitosis and meiosis have important similarities and differences

However the end products differ.

Mitosis produces 2 genetically identical diploid somatic daughter cells.

Meiosis produces 4 genetically unique haploid gametes.


Figure 8.14

Prophase

Metaphase

Duplicated

chromosome(two sister

chromatids)

MITOSIS

Parent cell

(before chromosome duplication)

Chromosomeduplication


Site of

crossing

over

2n ==== 4

Chromosomesalign at the

metaphase plate

Tetrads (homologouspairs) align at the

metaphase plate

Tetrad formedby synapsis of

homologous

chromosomes

Metaphase I

Prophase I

MEIOSIS I

Anaphase

Telophase

Sister chromatids

separate duringanaphase

2n 2n

Daughter cells of mitosis

No further

chromosomalduplication;

sister

chromatidsseparate during

anaphase II

n n n n

Daughter cells of meiosis II

Daughter

cells ofmeiosis I

Haploid

n ==== 2

Anaphase I

Telophase IHomologous

chromosomesseparate during

anaphase I;

sister

chromatids

remain together

MEIOSIS II

Figure 8.14_1

Prophase

MITOSIS

Parent cell(before chromosome duplication)



Site ofcrossing

over

2n ==== 4 Tetrad

Prophase I

MEIOSIS I

Metaphase

2n ==== 4

Chromosomesalign at the

metaphase plate

Tetrads (homologouspairs) align at the

metaphase plate

Tetrad

Metaphase I

Which characteristics are similar for mitosis and meiosis?

Ans: ______ (no. of) duplication of chromosomes

Which characteristics are unique to meiosis?

8.14 Mitosis and meiosis have important similarities and differences

Which characteristics are unique to meiosis?

Ans:

_____ (no. of) divisions of chromosomes

P_________g of homologous chromosomes

Exchange of genetic material by ________ _______


Genetic variation in gametes results from

independent orientation at metaphase I

random fertilization

8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

crossing over


Independent assortment at metaphase I

Each pair of chromosomes independently aligns at the cell equator.

There is an equal probability of the maternal or


There is an equal probability of the maternal or paternal chromosome facing a given pole.

The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes.



Animation: Genetic VariationRight click on animation / Click play

Figure 8.15_s3

Possibility A

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility B

Metaphase II

Gametes

Combination 3 Combination 4Combination 2Combination 1

Figure 8.15 Results of the independent orientation of chromosomes at metaphase I

Separation of homologous chromosomes can lead

to genetic differences between gametes.

Homologous chromosomes may have different versions of a gene at the same locus.

One version was inherited from the maternal parent and One version was inherited from the maternal parent and the other came from the paternal parent.

Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or

paternal version of the gene.


Figure 8.16

Coat-colorgenes

Eye-colorgenes

Brown Black

Meiosis

Brown coat (C);black eyes (E)

EC

e

E

E

c

C

C

White Pink

Tetrad in parent cell

(homologous pair of

duplicated chromosomes)

Chromosomes of

the four gametes White coat (c);pink eyes (e)

ec ec

Figure 8.16 Differing genetic information (coat color and eye color) on homologous chromosomes

Random fertilization

The combination of each unique sperm with each unique egg increases genetic variability.



_____________________ is the production of

new combinations of genes due to crossing over.

Crossing over is an exchange of corresponding

segments between nonsister chromatids on

homologous chromosomes.

8.17 Crossing over further increases genetic variability

homologous chromosomes.

Nonsister chromatids join at a ___________ (plural, chiasmata), the site of attachment and crossing over.

Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids.



Animation: Crossing OverRight click on animation / Click play

Figure 8.16Q

Sister chromatids

Sister chromatids

Pair of homologouschromosomesSister chromatids chromosomes

Figure 8.16Q Diagram distinguishing homologous chromosomes from sister chromatids

Figure 8.17A

Chiasma

Tetrad

Figure 8.17A Chiasmata, the sites of crossing over

Figure 8.17BTetrad

(pair of homologous

chromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

1

2

3

C

c e

E

C

c e

E

c e

C E

Figure 8.17B How crossing over leads to genetic recombination

chromosomes at anaphase I

Separation of chromatids atanaphase II andcompletion of meiosis

Parental type of chromosome

Recombinant chromosome



Gametes of four genetic types

4

C E

C e

ec

c E

C E

C e

c E

ec

Breakage of homologous chromatids

Coat-colorgenes

Eye-colorgenes

C

(homologous pair ofchromosomes in synapsis)

E

c e

Tetrad

C E

1

c e

Joining of homologous chromatids2

C E

c e

Chiasma

Separation of homologous chromosomes at anaphase I

C E

c e

Chiasma

Separation of chromatids at

C E

c e

c E

C e

3

Separation of chromatids at anaphase II and

completion of meiosis

c e

c e

c E

C E

C e


Gametes of four genetic types




4

Trisomy 21

involves the inheritance of three copies of chromosome 21 and

is the most common human chromosome abnormality.

8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome


FYI

Figure 8.19A

Trisomy 21, called Down

syndrome, produces a

characteristic set of symptoms,

which include:

mental retardation,

characteristic facial features,

FYI

short stature,

heart defects,

susceptibility to respiratory infections, leukemia, and Alzheimers disease, and

shortened life span.

Figure 8.19B

40

50

60

70

80

90

Infa

nts

wit

h D

ow

n s

yn

dro

me

(pe

r 1

,00

0 b

irth

s)

The incidence increases with the

age of the mother.

Age of mother

504540353025200

10

20

30

40

Infa

nts

wit

h D

ow

n s

yn

dro

me

(pe

r 1

,00

0 b

irth

s)

FYI

Nondisjunction is the failure of chromosomes or

chromatids to separate normally during meiosis.

This can happen during

Fertilization after nondisjunction yields zygotes with

altered numbers of chromosomes.

8.20 Accidents during meiosis can alter chromosome number

altered numbers of chromosomes.


FYI

Figure 8.20A_s3

Nondisjunction

MEIOSIS I

MEIOSIS II

NormalNormalmeiosis II

Gametes

Number ofchromosomes

Abnormal gametes

n ++++ 1 n ++++ 1 n 1 n 1

FYI

Figure 8.20B_s3

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametes Normal gametes

n ++++ 1 n 1 n n

FYI

Table 8.21

FYI

Can you identify the mitotic phases?

Figure 8.UN03

Number of chromosomalduplications

Number of cell divisions

Number of daughter cells

produced

Number of chromosomes in

Mitosis Meiosis

Number of chromosomes inthe daughter cells

How the chromosomes lineup during metaphase

Genetic relationship of the

daughter cells to the parent cell

Functions performed in thehuman body

1. Compare the parent-offspring relationship in asexual and sexual reproduction.

2. Describe the stages of the cell cycle.

3. List the phases of mitosis and describe the events characteristic of each phase.

You should now be able to

characteristic of each phase.

4. Compare cytokinesis in animal and plant cells.

5. Describe the functions of mitosis.

6. Explain how chromosomes are paired.

7. Distinguish between somatic cells and gametes and between diploid cells and haploid cells.


8. Explain why sexual reproduction requires meiosis.

9. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.

10. Compare mitosis and meiosis noting similarities and differences.

You should now be able to

differences.

11. Explain how genetic variation is produced in sexually reproducing organisms.

12. Define the following terms: chiasma*, chromosome, chromatid, centromere, crossing over, homologous chromosome pair, and spindle*


The END

Nuclear Division

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