The Cellular Basis of Reproduction
and Inheritance : part C
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Mitosis vs Meiosis
!  To summarize what we have seen before
Mitosis duplicates cells (chromosomes = 2n, diploid cells) into two
daughter cells with the same number of chromosomes (they remain
diploid cells)
Meiosis creates 4 daughter cells (haploid cells, chromosomes = n), where
each cell contains half the number of chromosomes of the original
diploid cell
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1
Mitosis where 2n=4
!  Can you name
the different
phases here ?
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Meiosis where 2n=2
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2
Genetic Variation
!  Genetic variation in gametes results from
–  independent orientation at metaphase I resulting in
different possibilities of gametes
–  If the number of homologous chromosomes = n, the
different possible gametes can be 2n
–  So for humans , n= 23.. Thus possible different
combinations of chromosomes in gametes is 223
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!  This diploid cell has 2 pair of homologous chromosomes…
thus n = 2
!  Possible combination in gametes = 2n =22 = 4
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
3
8.16 Homologous chromosomes may carry
different versions of genes
Coat-color
genes
Eye-color
genes
Brown
C
Black
E
c
White
e
Pink
Locus for eye color
C
E
C
E
c
e
c
e
Different versions for eye
color
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
If we use this example above, and assume that the diploid cell only
has 1 pair of chromosomes, the possible gametes = 2
The next slide shows the meiosis end result of possible gametes.
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Meiosis
C
c
C
c
E
e
E
e
C
C
c
c
E
E
e
e
Chromosomes of the four gametes. Note that there are
actually only 2 different kinds of gametes.
4
8.17 Crossing over : further increases genetic
variability
!  An additional mechanisms, called genetic
recombination, increases genetic variability
among the gametes.
!  Genetic recombination is the production of new
combinations of genes due to crossing over.
!  Crossing over is an exchange of corresponding
segments between separate, non-sister
chromatids on homologous chromosomes.
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8.17 Crossing over : further increases genetic
variability
!  A reminder of the definitions
Sister chromatids
Sister chromatids
Pair of homologous
chromosomes
NON- Sister chromatids
!  Crossing over exchanges DNA segments
between separate, non-sister chromatids on
homologous chromosomes.
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8.17 Crossing over : further increases genetic
variability
!  In late prophase I, homologous chromosomes
pair laterally, or side-by-side. At this time they are
said to be in synapsis, forming tetrads.
!  During synapsis, cross-connections are formed
from breakage and rejoining between sister
chromatids.
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8.17 Crossing over : further increases genetic
variability
!  First nonsister chromatids will join at a point called
chiasma (plural, chiasmata), the site of
attachment and crossing over.
!  Corresponding amounts of genetic material are
exchanged between maternal and paternal
(nonsister) chromatids.
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8.17 Crossing over : further increases genetic
variability
Piece of DNA that has
‘crossed’ over
8.17 Crossing over : further increases genetic
variability
!  In figure below, following crossing over, the blue
and red chromosomes, which originally carried AA
and aa alleles, respectively, now carry Aa alleles in
both chromosomes at the end of prophase I.
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!  Using our previous example, crossing over now
creates different kind of gametes
C
c
C
c
C
c
E
e
E
e
E
e
Crossing over during synapsis
C
C
c
c
E
e
E
e
Chromosomes of the four gametes: note that now
we have 4 different kind of gametes instead of 2 !
C
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(pair of homologous
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
8
Figure 8.17B_2
C
E
Chiasma
c
e
Separation of homologous
chromosomes at anaphase I
3
Figure 8.17B_3
C
E
C
e
c
E
c
e
C
E
C
c
e
E
c
e
4
Separation of chromatids at
anaphase II and
completion of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
End result : Gametes of 4 genetic types
9
ALTERATIONS OF
CHROMOSOME NUMBER
AND STRUCTURE
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8.18 A karyotype is a photographic inventory of
an individual’s chromosomes
!  A karyotype is an ordered display of magnified
images of an individual’s chromosomes arranged
in pairs.
!  Karyotypes
–  are often produced from dividing cells arrested at
metaphase of mitosis and
–  allow for the observation of
–  homologous chromosome pairs,
–  chromosome number, and
–  chromosome structure.
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10
Packed red
and white
blood cells
Blood
culture
Hypotonic
solution
Centrifuge
2
Fixative
Stain
White
blood
cells
3
Fluid
1
Human karyotype
Centromere
Sister
chromatids
Pair of
homologous
chromosomes
5
Sex chromosomes
11
8.20 Accidents during meiosis can alter normal
chromosome number
!  Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis. This
can happen during
–  meiosis I, if both members of a homologous pair go to
one pole or
–  meiosis II if both sister chromatids go to one pole.
!  Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes.
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MEIOSIS I
The diploid cell has 2n =4
Nondisjunction
MEIOSIS II
Normal
meiosis II
Gametes should have n =2
Gametes
Number of
chromosomes
n+1
n+1
n-1
n-1
Abnormal gametes
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MEIOSIS I
Normal
meiosis I
MEIOSIS II
Nondisjunction
n+1
n-1
Abnormal gametes
n
n
Normal gametes
Abnormalities in karyotypes
!  Trisomy 21, called Down syndrome,
–  involves the inheritance of three copies of chromosome
21 and
–  is the most common human chromosome abnormality.
!  Trisomy 21, produces a characteristic set of
symptoms, which include:
–  characteristic facial features and short stature,
–  mental retardation,heart defects,
–  susceptibility to respiratory infections, leukemia, and Alzheimer’s
disease, and shortened life span.
!  The incidence increases with the age of the mother.
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Figure 8.19A
Trisomy 21
Figure 8.19B
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
Age of mother
45
50
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Abnormalities in karyotypes
!  Other trisomies include
!  Trisomy 18, called Edwards syndrome,
–  Very low survival rate due to multiple organ disorders.
!  Trisomy 13, Patau syndrome
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8.21 CONNECTION: Abnormal numbers of sex
chromosomes do not usually affect survival
!  While an extra chromosome in the autosomal
chromosomes can be quite dramatic, sex
chromosome abnormalities tend to be less severe,
perhaps because of
–  the small size of the Y chromosome and/or
–  X-chromosome inactivation.
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8.21 CONNECTION: Abnormal numbers of sex
chromosomes do not usually affect survival
!  The following table lists the most common human
sex chromosome abnormalities. In general,
–  a single Y chromosome is enough to produce
“maleness,” even in combination with several X
chromosomes, and
–  the absence of a Y chromosome yields “femaleness.”
–  a single X chromosome is a viable outcome (Turner
syndrome) ; a single Y chromosome is non-viable.
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Table 8.21
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8.22 EVOLUTION CONNECTION: New species
can arise from errors in cell division
!  Errors in mitosis or meiosis may produce polyploid
species, with more than two chromosome sets.
!  The formation of polyploid species is
–  widely observed in many plant species but
–  less frequently found in animals.
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Figure 8.22
The gray tree frog (Hyla versicolor), a tetraploid organism
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8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
!  Chromosome breakage can lead to
rearrangements that can produce
–  genetic disorders or,
–  if changes occur in somatic cells, cancer.
–  All cancers are due to genetic chromosomal changes
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8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
!  These rearrangements may include
–  a deletion, the loss of a chromosome segment,
–  a duplication, the repeat of a chromosome segment,
–  an inversion, the reversal of a chromosome segment,
or
–  a translocation, the attachment of a segment to a
nonhomologous chromosome that can be reciprocal.
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8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
!  Chronic myelogenous leukemia (CML)
–  is one of the most common leukemias,
–  affects cells that give rise to white blood cells
(leukocytes), and
–  results from part of chromosome 22 switching places
with a small fragment from a tip of chromosome 9.
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Chromosome 9
Chromosome 22
Reciprocal
translocation
Activated cancer-causing gene
“Philadelphia chromosome”
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Figure 8.UN03
Mitosis
Meiosis
Number of chromosomal
duplications
Number of cell divisions
Number of daughter cells
produced
Number of chromosomes in
the daughter cells
How the chromosomes line
up during metaphase
Genetic relationship of the
daughter cells to the parent cell
Functions performed in the
human body
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