Download Cellular reproduction part 2

Cellular reproduction part 2
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
• Somatic cells of each
species contain a
specific number of
chromosomes
Chromosomes
Centromere
– Human cells have
46, making up 23
pairs of homologous
chromosomes
Sister chromatids
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Figure 8.12
Homologous Chromosomes
• Humans have 23 pairs of
homologous
chromosomes
– 22 pairs – autosomes –
found in both males
and females
– 1 pair – sex
chromosomes, XX =
female,
XY= male
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Homologous Chromosomes
• Matched pairs of chromosomes
• Similar in size, shape, and banding pattern
• Both carry genes controlling the same inherited
characteristics (the version of the gene may be
different)
• The genes are located at the same locus
• One chromosome of each pair is inherited from
the mother, the other from the father
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• The human
life cycle
Haploid gametes (n = 23)
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
Figure 8.13
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Human Life Cycle
Diploid cells (2n) – cells that contain both
homologous chromosomes. In humans the
diploid number is 46.
Haploid cells (n) – cells with one copy of each
homologous chromosome. The gametes (egg
and sperm) are haploid. In humans the
haploid number is 23.
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Meiosis
Meiosis
• The division that reduces the number of
chromosomes by half.
• In animals, meiosis results in the formation
of haploid egg and sperm cells.
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.15
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Figure 8.16.2
Meiosis
Two nuclear divisions occur:
1. Meiosis I
a. During prophase I homologous chromosomes
pair – synapsis
b. During prophase I the paired chromosomes
exchange chromosome parts – crossing over
c. Homologous chromosomes are separated
d. 2 cells produced each containing one copy of
each homologous chromosome
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.16.3
Meiosis
2. Meiosis II
a. Not preceded by the replication of DNA
b. Sister chromatids of each chromosome are
separated
c. Produces 4 haploid cells
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Meiosis
Meiosis produces 4 cells that
• Are haploid
• Chromosome makeup of each is different from
each other and the parent cell
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.17
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Meiosis
Spermatogenesis
• Formation of sperm by meiosis
• Occurs in special cells (spermatogonia) in the
testes
• All 4 haploid cells become sperm
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis
Oogenesis
• Formation of an egg by meiosis
• Occurs in special cells (oogonia) in the ovaries
• Unequal divisions of the cytoplasm during
meiosis I and meiosis II result in the formation
of 1 haploid egg and 3 haploid polar bodies
• Only the egg can be fertilized
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Recombination
• Genetic Recombination – the production of
gene combinations different from those carried
by the parent
• There are 4 processes that contribute to genetic
recombination.
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.18
Independent Assortment of Chromosomes
• The large number of possible arrangements of
chromosome pairs at metaphase I of meiosis
leads to many different combinations of
chromosomes in gametes
– This results in 2n possible combinations of
gametes
– For humans 2n = 223 = 8 million possible
combinations
–
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__random_orientation_of_chromosomes_during_meiosis.html
• Random fertilization also increases variation in
offspring
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Coat-color genes
Eye-color genes
Brown
Black
C
E
c
e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Figure 8.17A, B
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C
E
C
E
c
e
c
e
Chromosomes of
the four gametes
Homologous chromosomes carry different versions
of genes
• The differences between homologous
chromosomes are based on the fact that they
can carry different versions of a gene at
corresponding loci
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Tetrad
Chaisma
Centromere
Figure 8.18A
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Coat-color
genes
• How crossing over
leads to genetic
recombination
Eye-color
genes
Tetrad
(homologous pair of
chromosomes in synapsis)
1
Breakage of homologous chromatids
2
Joining of homologous chromatids
Chiasma
3
Separation of homologous
chromosomes at anaphase I
4
Separation of chromatids at
anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Figure 8.18B
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Gametes of four genetic types
Animation
(must insert Miller and Levine
lecture cd ch11)
•Meiosis 1
•Crossing over a
closer look
•Meiosis II
•http://bcs.whfreeman.com/thelife
wire/content/chp09/0902002.html
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Figure 8.19
Genetic Recombination
Crossing over
•
The exchange of genetic information between
2 homologous chromosomes.
•
Occurs during prophase I.
Random fertilization
•
Depends on which sperm cell and its
chromosome combinations fertilizes which
egg
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• Preparation of a karyotype
Blood
culture
Packed red
And white
blood cells
Hypotonic solution
Stain
White
Blood
cells
Centrifuge
3
2
1
Fixative
Fluid
Centromere
Sister
chromatids
Pair of homologous
chromosomes
4
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5
Figure 8.19
ALTERATIONS OF CHROMOSOME NUMBER
AND STRUCTURE
A karyotype is a photographic inventory of an
individual’s chromosomes
• To study human chromosomes
microscopically, researchers stain and display
them as a karyotype
– A karyotype usually shows 22 pairs of
autosomes and one pair of sex chromosomes
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8.21 Accidents during meiosis can alter
chromosome number
• Abnormal
chromosome count
is a result of
nondisjunction
– Either
homologous
pairs fail to
separate
during
meiosis I
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
–
http://www.sumanasinc.com/webcontent/an
imations/content/mistakesmeiosis/mistakes
meiosis.swf
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Figure 8.21A
– Or sister chromatids fail to separate during
meiosis II
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n–1
n+1
n
Number of chromosomes
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n
Figure 8.21B
• Fertilization after nondisjunction in the mother
results in a zygote with an extra chromosome
Egg
cell
n+1
Zygote
2n + 1
Sperm
cell
n (normal)
Figure 8.21C
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Connection: An extra copy of chromosome 21
causes Down syndrome
• This karyotype shows three number 21
chromosomes
• An extra copy of chromosome 21 causes Down
syndrome
Figure 8.20A, B
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The chance of having a Down syndrome child goes
up with maternal age
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Figure 8.23
Alterations of Chromosomes
• In most cases abnormal chromosome number
results in spontaneous abortion long before
birth.
• Nondisjunction in the sex chromosomes has
less of an affect on survival
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Connection: Abnormal numbers of sex
chromosomes do not usually affect survival
• Nondisjunction can also produce gametes with
extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the
genetic balance less than an unusual number of
autosomes
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Table 8.1
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Connection: Alterations of chromosome structure
can cause birth defects and cancer
• Chromosome breakage can lead to
rearrangements that can produce genetic
disorders or cancer
– Four types of rearrangement are deletion,
duplication, inversion, and translocation
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Deletion
Deletion – a chromosome
breaks and a fragment is
lost. Seems to have the
greatest affect.
Duplication
Duplication – the fragment
joins to a homologous
chromosome.
Homologous
chromosomes
Inversion
Reciprocal
translocation
Nonhomologous
chromosomes
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Inversion – the fragment
reattaches to the original
chromosome but in reverse
orientation. Least likely to
produce harmful affects.
Translocation– attachment of
a chromosome fragment to a
nonhomologous
chromosome. May/may not be
harmful.
Figure 8.23A, B
Alterations of Chromosomes
Abnormalities in the structure of the
chromosome may cause disorders (Figure
8.23A)
1. Deletion – a chromosome breaks and a
fragment is lost. Seems to have the greatest
affect.
2. Duplication – the fragment joins to a
homologous chromosome.
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Alterations of Chromosomes
3. Inversion – the fragment reattaches to the
original chromosome but in reverse
orientation. Least likely to produce harmful
affects.
4. Translocation (Figure 8.23B) – attachment of
a chromosome fragment to a nonhomologous
chromosome. May/may not be harmful.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings