Download Ch 7- The Cellular Basis of Inheritance

11/6/2016
Ch. 7: The Cellular
Basis of Inheritance
Fig. 12.15, p. 205
Slide 19
Sexual reproduction
• A diploid organism produces haploid sex cells (gametes)
• The union of two gametes from opposite sexes (fertilization)
makes a new diploid individual
• A special type of nuclear division called meiosis yields
variations in offspring, which provides a species survival
advantage
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•
An overview of mitosis vs. meiosis
•
Mitosis produces
– 2 identical diploid daughter cells for development,
growth and repair (in multicellular organisms such as
animals)
Meiosis produces
– 4 non-identical
haploid gametes for
sexual reproduction
Meiosis
•
•
Occurs in testes and ovaries
Chromosome number is reduced by half
•
After the chromosome duplication that occurred during interphase, there are
2 nuclear divisions called meiosis I and meiosis II
•
A mixing of gene combinations occurs via crossing over and independent
assortment (more on this soon)
– Diploid cells produce haploid gametes (sperm cells and egg cells)
– Each of these has its own prophase, metaphase, anaphase, and telophase
– Remember, genes code for proteins that can effect various aspects of an
individual’s biology (e.g. physical traits, physiological traits, metabolic traits, etc.)
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Synapsis
• A duplicated chromosome consists of 2 sister
chromatids
• During metaphase I, synapsis occurs: a tight pairing of
(physical connection between) homologous duplicated
chromosomes
– The 4 sister chromatids together are called a tetrad
– Genes on homologous chromosomes are precisely aligned
with each other…
Tetrad
Synapsis
Sister
chromatids
Crossing over
• Occurs during synapsis
• Non-sister homologous
chromatids exchange a
few genes
• May occur at several
points for each pair
• Significance: crossing
over ultimately results in
recombinant
chromosomes and
gametes with different
gene combinations
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Independent assortment of chromosomes
• Refers to the
random
orientation of
paternal and
maternal
chromosomes at
metaphase I
• Further
increases
the number
of gene
combinations in
gametes
Genetic variability of offspring
• There are 2 sources of variability from meiosis:
– 1. Independent assortment
• Consider the possible outcomes…
– There are 2 possible arrangements (maternal/paternal) for each
pair of chromosomes at metaphase I, and there are 23 pairs in
humans
» So there are 223 options for chromosome orientation at
metaphase I
» I.e., there are 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2
x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 8,333,608 possible
orientations of your maternal and paternal chromosomes at
metaphase I
» Which means, once fertilization occurs, there are 223 x 223 = 70
trillion diploid combinations for the new individual!
– 2. Crossovers
• There are an average of 2-3 crossovers per chromosome,
which increases the possible diploid combinations even
more…140 to 200 trillion!
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Comparison of reproductive strategies:
asexual vs. sexual
• Asexual:
– Two parents
– Offspring are a random
genetic mixture of the parents
(and, by extension, the
grandparents)
– Each parent usually has
different alleles (versions of
the same gene)
– Offspring inherit new
combinations of alleles,
resulting in the appearance of
new traits
• New traits mean changes in
individual behavior,
appearance, physiology, etc.
A review of
mitosis vs. meiosis
– Single “parent”
– Offspring are genetically
identical to the parent and
each other
– Offspring inherit the same
alleles (versions of the
same gene) that the
parent has, so there’s less
chance for the appearance
of new traits
• Sexual:
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Errors in meiosis
• Occur when chromosomes behave abnormally
during meiosis
• There are 2 types of errors:
– Abnormal chromosome number
– Chromosome structural rearrangements
• A karyogram (= an organized, visual display/chart
of a karyotype) provides information about
chromosome number and structure
Abnormalities in chromosome number
• Include the
duplication or loss of
entire (whole)
chromosomes
• Are caused by
nondisjunction:
– Failure of
chromosomes to
separate during
either meiosis I or
meiosis II
• The risk of
nondisjunction
increases with age of
the parents (see the
graph on the next
page)
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Chromosome number terminology
• Euploid: has the normal number of chromosomes
• Aneuploid: has an abnormal normal chromosome number
– Polyploid: has more than the correct number of complete
chromosome sets (e.g. more than 2 sets for a diploid species)
• Rare in animals
• Common in plants
– Monosomy: loss of one
chromosome – fail to
develop to birth
– Trisomy: gain of one
extra chromosome
• Most fail to develop
to birth, however…
• Some trisomies can
result in offspring that
survive for several
weeks to many years
• Down syndrome
(trisomy 21) is a wellknown example
More on chromosome number:
sex chromosomes in humans
• Two X sex chromosomes (XX) results in a
female; XY is a male
• Males operate with only one X chromosome
• So do females…
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X inactivation (females only)
•
•
•
Early in female
mammal
development, one of
the two X
chromosomes in
each cell (either the
maternally derived
one or the paternally
derived one) is
randomly inactivated
(condensed into a
nonfunctional
structure called a
Barr body)
Each cell expresses
the genes on the
remaining X
chromosome
E.g. if you see a
“tortoiseshell” cat,
you know it’s
female!
Chromosome
structural rearrangements
• Include:
–
–
–
–
Deletions
Inversions
Translocations
Partial duplications
• Are usually caused by
DNA breakage and/or
errors during DNA
replication and repair
• Effects vary…
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Deletion
• Is the loss of a chromosomal
segment
• E.g. cri-du-chat:
– Deletion of most of the small arm of
chromosome 5
– Causes
nervous
system
abnormalities
and characteristic physical
features
Inversion
• Is the detachment, 180°
rotation, and reinsertion of
a chromosomal segment
• Has little to no effect if the
gene sequence is not
altered
• Is associated with primate
evolution: chromosome 18
inversion
– It’s one of the main
differences between
humans and chimpanzees,
our closest genetic
relatives
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Translocation
• Is the detachment of a
chromosomal segment
followed by its
reattachment to a different
chromosome
• May or may not have an
effect (it depends on how
the gene position changes
with respect to its
regulatory sequences)
• Specific translocations
have been associated with
several cancers and
schizophrenia
Partial duplication
• Is the duplication of a
chromosomal segment
• Subsequent mutations to
the duplicated segment
allows the original segment
to work normally while the
new one may code for a
new protein, for example…
– Antarctic icefish
• The duplication of a digestive
gene appears to have mutated
to code for an antifreeze
glycoprotein, allowing these
fish to live in water below 0°C
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