Download MEIOSIS AND CROSSING OVER

MEIOSIS AND CROSSING
OVER
Chromosomes are matched in
homologous pairs
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Homologous
chromosomes: the 2
members of a pair of
chromosomes—contain
genes for the same traits
Somatic cells of each
species contain a specific
number of chromosomes

Chromosomes
Human cells have 46,
making up 23 pairs of
homologous chromosomes
Sister chromatids
Paired chromosomes

Homologous chromosomes

both chromosomes of a pair carry “matching” genes

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control same inherited characters
homologous = same information
diploid
2n
2n = 4
eye color
(brown?)
eye color
(blue?)
homologous
double stranded
chromosomes homologous chromosomes
Gametes have a single set of
chromosomes


Gametes~ egg or sperm
Cells with two sets of chromosomes are
said to be diploid (2n) somatic cells(46
in humans)

Gametes are haploid, with only one set
of chromosomes, (1n)(23 in humans)
diploid = 2
copies
2n
Human female karyotype
46 chromosomes
23 pairs
XX
diploid = 2
copies 2n
Human male karyotype
46 chromosomes
23 pairs
XY
Life Cycle

At fertilization, a sperm
fuses with an egg,
forming a diploid zygote

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Repeated mitotic divisions
lead to the development
of a mature adult
The adult makes haploid
gametes by meiosis
All of these processes
make up the sexual life
cycle of organisms
Why meiosis?

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When cells divide by mitosis, the new cells have
exactly the same number and kind of
chromosomes as the original cells.
Imagine if mitosis were the only means of
cell division.
IF the parent organism has 14 chromosomes, it
would produce gametes that contained a
complete set of 14 chromosomes
The offspring would have cell nuclei with 28
chromosomes, and the next generation
would have cell nuclei with 56 chromosomes
Meiosis reduces the chromosome
number from diploid to haploid

Meiosis, like mitosis, is preceded by
chromosome duplication

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However, in meiosis the cell divides twice to form
four daughter cells
In the first division, meiosis I, homologous
chromosomes are paired

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While they are paired, they cross over and
exchange genetic information
The homologous pairs are then separated, and
two daughter cells are produced
Meiosis I

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In the first division, meiosis I, homologous chromosomes are paired
As the chromosomes coil, homologous chromosomes line up with each
other gene by gene along their length, to form a four-part structure
called a tetrad.Here synaspsis occurs: the meeting of two homologous
pairs

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While they are paired, they cross over and exchange genetic information
The homologous pairs are then separated, and two daughter cells are
produced
Division in meiosis I occurs in four phases: prophase, metaphase,
anaphase, and telophase
double
stranded
Meiosis 1 overview

Divide 1
1st division of meiosis
Copy DNA
before meiosis
prophase 1
4
chromosomes
 diploid
 2n
gamete
Line Up 1
metaphase 1
telophase 1
 2 chromosomes
 haploid
 1n
Meiosis II

Meiosis II is essentially the same as mitosis

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The sister chromatids of each chromosome
separate
The result is four haploid daughter cells
Bye Bye 2
Meiosis 2 overview
telophase 2
telophase 1

Line Up 2
2nd division of meiosis

looks like mitosis
 2 chromosomes
 haploid
 1n
metaphase 2
gametes
4
Review: A comparison of
mitosis and meiosis

For both processes, chromosomes replicate only
once, during interphase
Review: A comparison of
mitosis and meiosis
Genetic variation

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Each chromosome of a homologous pair
comes from a different parent
The large number of possible arrangements
of chromosome pairs at metaphase I of
meiosis leads to many different combinations
of chromosomes in gametes
Random fertilization also increases variation
in offspring
Crossing over further
increases genetic variability

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Crossing over is the exchange of corresponding
segments between two homologous chromosomes
Genetic recombination results from crossing
over during prophase I of meiosis

This increases variation further
Errors of Meiosis
Chromosomal Abnormalities
2006-2007
Chromosomal abnormalities

Incorrect number of chromosomes

nondisjunction
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chromosomes don’t separate properly during
meiosis
breakage of chromosomes
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deletion
duplication
inversion
translocation
ALTERATIONS OF CHROMOSOME
NUMBER AND STRUCTURE
• A karyotype is a photographic inventory of
an individual’s chromosomes
• Human female karyotype
An extra copy of chromosome
21 causes Down syndrome
• This karyotype shows
three number 21
chromosomes: trisomy
21
• An extra copy of
chromosome 21 causes
Down syndrome
• The chance of having a
Down syndrome child
goes up with maternal
age
Down syndrome & age of mother
Mother’s age
Incidence of
Down Syndrome
Under 30
<1 in 1000
30
1 in 900
35
1 in 400
36
1 in 300
37
1 in 230
38
1 in 180
39
1 in 135
40
1 in 105
42
1 in 60
44
1 in 35
46
1 in 20
48
1 in 16
49
1 in 12
Rate of miscarriage due to
amniocentesis:
 1970s data
0.5%, or 1 in 200 pregnancies
 2006 data
<0.1%, or 1 in 1600
pregnancies
Accidents during meiosis can
alter chromosome number
• Nondisjunction~
The failure of
homologous
chromosomes to
separate properly
during meiosis
• Abnormal
chromosome count
will result.
Nondisjunction
• Problems in meiosis cause errors in daughter cells
– chromosome pairs do not separate properly during Meiosis 1
– sister chromatids fail to separate during Meiosis 2
– too many or too few chromosomes
2n
n-1
n
n+1
n
Abnormal numbers of sex chromosomes
do not usually affect survival
• Nondisjunction can also produce gametes
with extra or missing sex chromosomes
• A man with Klinefelter syndrome has an
extra X chromosome
• A woman with Turner syndrome lacks an X
chromosome
Klinefelter’s syndrome
• XXY male
– one in every 2000 live births
– have male sex organs, but
are sterile
– feminine characteristics
• some breast development
• lack of facial hair
– tall
– normal intelligence
Klinefelter’s syndrome
Turner syndrome
• Monosomy X or X0
–
–
–
–
–
1 in every 5000 births
varied degree of effects
webbed neck
short stature
sterile
Nondisjunction
• When a gamete with an extra set of
chromosomes is fertilized by a normal
haploid gamete, the offspring has three sets of
chromosomes and is triploid.(3n)
• The fusion of two gametes, each with an
extra set of chromosomes, produces
offspring with four sets of
chromosomes—a tetraploid. (4n)
• This is polyploidy.
Alterations of chromosome structure
can cause birth defects and cancer
Deletion
• Chromosome breakage
can lead to
rearrangements that
can produce genetic
disorders or cancer
– Four types of
rearrangement are
deletion, duplication,
inversion, and
translocation
Duplication
Inversion
Reciprocal
translocation
Nonhomologous
chromosomes
Changes in chromosome structure
• deletion
– loss of a chromosomal segment
• duplication
– repeat a segment
• inversion
– reverses a segment
• translocation
– move segment from one chromosome to
another