MEIOSIS AND CROSSING OVER

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MEIOSIS AND CROSSING OVER
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Chromosomes are matched in homologous pairs
Chromosomes

• Homologous chromosomes the 2 members of a pair
  of chromosomescontain genes for the same traits
• Somatic cells of each species contain a specific
  number of chromosomes
• Human cells have 46, making up 23 pairs of
  homologous chromosomes

Sister chromatids
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Paired chromosomes

• Homologous chromosomes
• both chromosomes of a pair carry matching genes
  
• control same inherited characters
• homologous same information

diploid2n 2n 4
eye color (brown?)
eye color (blue?)
homologouschromosomes
double strandedhomologous chromosomes
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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)

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Human female karyotype
diploid 2 copies
2n
46 chromosomes 23 pairs
XX
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Human male karyotype
diploid 2 copies
2n
46 chromosomes 23 pairs
XY
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Life Cycle

• At fertilization, a sperm fuses with an egg,
  forming a diploid zygote
• 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

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Why meiosis?

• 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

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Meiosis reduces the chromosome number from
diploid to haploid

• Meiosis, like mitosis, is preceded by chromosome
  duplication
• However, in meiosis the cell divides twice to
  form four daughter cells
• In the first division, meiosis I, homologous
  chromosomes are paired
• While they are paired, they cross over and
  exchange genetic information
• The homologous pairs are then separated, and two
  daughter cells are produced

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Meiosis I

• 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
• 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

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Meiosis 1 overview
doublestranded
Divide 1

• 1st division of meiosis

Copy DNA before meiosis
Line Up 1
prophase 1
metaphase 1

• 4 chromosomes
• diploid
• 2n

telophase 1

• 2 chromosomes
• haploid
• 1n

gamete
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Meiosis II

• Meiosis II is essentially the same as mitosis
• The sister chromatids of each chromosome separate
• The result is four haploid daughter cells

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Meiosis 2 overview
Bye Bye 2
telophase 2
telophase 1
Line Up 2

• 2nd division of meiosis
• looks like mitosis

metaphase 2

• 2 chromosomes
• haploid
• 1n

gametes
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Review A comparison of mitosis and meiosis

• For both processes, chromosomes replicate only
  once, during interphase

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Review A comparison of mitosis and meiosis
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Genetic variation

• 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

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Crossing over further increases genetic
variability

• 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

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Errors of Meiosis Chromosomal Abnormalities
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Chromosomal abnormalities

• Incorrect number of chromosomes
• nondisjunction
• chromosomes dont separate properly during
  meiosis
• breakage of chromosomes
• deletion
• duplication
• inversion
• translocation

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ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

• A karyotype is a photographic inventory of an
  individuals chromosomes
• Human female karyotype

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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

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Down syndrome age of mother
Mothers age Incidence of Down Syndrome
Under 30 lt1 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 data0.5, or 1 in 200 pregnancies
• 2006 datalt0.1, or 1 in 1600 pregnancies

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Accidents during meiosis can alter chromosome
number

• Nondisjunction The failure of homologous
  chromosomes to separate properly during meiosis
• Abnormal chromosome count will result.

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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
n1
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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

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Klinefelters 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

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Klinefelters syndrome
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Turner syndrome

• Monosomy X or X0
• 1 in every 5000 births
• varied degree of effects
• webbed neck
• short stature
• sterile

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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 chromosomesa tetraploid. (4n)
• This is polyploidy.

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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
Reciprocaltranslocation
Nonhomologouschromosomes
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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