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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.
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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.
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
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
2012 Pearson Education, Inc.
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
8.13 Meiosis reduces the chromosome number from diploid to haploid
In meiosis, one duplication of chromosomes is
followed by two divisions, producing _____ daughter
cells, each has a __________ set of chromosomes.
2012 Pearson Education, Inc.
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.
8.13 Meiosis reduces the chromosome number from diploid to haploid
Homologous chromosomes come together as pairs by ____________.
Each pair, with four chromatids, is called a ________.
Nonsister chromatids exchange genetic material by _______________.
2012 Pearson Education, Inc.
Applying Your Knowledge
Human cells have 46 chromosomes. At the end
of prophase I
8.13 Meiosis reduces the chromosome number from diploid to haploid
of prophase I
How many chromosomes are present in
one cell?
How many chromatids are present in one
cell?
2012 Pearson Education, Inc.
Meiosis I Metaphase I
_________ align at the cell equator.
Meiosis I Anaphase I
8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis I Anaphase I
_________________ separate and move toward opposite poles of the cell.
2012 Pearson Education, Inc.
Applying Your Knowledge
Human cells have 46 chromosomes. At the end of Metaphase I
How many chromosomes are present in
8.13 Meiosis reduces the chromosome number from diploid to haploid
How many chromosomes are present in one cell?
How many chromatids are present in one cell?
2012 Pearson Education, Inc.
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
8.13 Meiosis reduces the chromosome number from diploid to haploid
Each nucleus has the haploid number of chromosomes.
2012 Pearson Education, Inc.
Applying Your Knowledge
After telophase I and cytokinesis
How many chromosomes are present in one
human cell?
8.13 Meiosis reduces the chromosome number from diploid to haploid
human cell?
How many chromatids are present in one human
cell?
2012 Pearson Education, Inc.
Meiosis II follows meiosis I without
chromosome duplication.
Each of the two haploid products enters meiosis II.
8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis II Prophase II
Chromosomes coil and become compact (if uncoiled after telophase I).
Nuclear envelope, if re-formed, breaks up again.
2012 Pearson Education, Inc.
Meiosis II Metaphase II
Duplicated chromosomes align at the cell
equator.
8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis II Anaphase II
Sister chromatids separate
chromosomes move toward opposite poles.
2012 Pearson Education, Inc.
Meiosis II Telophase II
Chromosomes have reached the
poles of the cell.
A nuclear envelope forms around
8.13 Meiosis reduces the chromosome number from diploid to haploid
A nuclear envelope forms around
each set of chromosomes.
With cytokinesis, four haploid cells
are produced.
2012 Pearson Education, Inc.
Applying Your Knowledge
After telophase II and cytokinesis
How many chromosomes are present in one
human cell?
8.13 Meiosis reduces the chromosome number from diploid to haploid
human cell?
How many chromatids are present in one human
cell?
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
Figure 8.14
Prophase
Metaphase
Duplicated
chromosome(two sister
chromatids)
MITOSIS
Parent cell
(before chromosome duplication)
Chromosomeduplication
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)
Chromosomeduplication
Chromosomeduplication
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 ________ _______
2012 Pearson Education, Inc.
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
2012 Pearson Education, Inc.
Independent assortment at metaphase I
Each pair of chromosomes independently aligns at the cell equator.
There is an equal probability of the maternal or
8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
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.
2012 Pearson Education, Inc.
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
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.
8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
2012 Pearson Education, Inc.
_____________________ 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.
2012 Pearson Education, Inc.
2012 Pearson Education, Inc.
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
Recombinant chromosome
Parental type of 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
Parental type of chromosome
Gametes of four genetic types
Recombinant chromosome
Parental type of chromosome
Recombinant chromosome
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
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
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.
2012 Pearson Education, Inc.
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*
2012 Pearson Education, Inc.
The END