Download Errors in Genes and Chromosomes

Errors in Genes and
Chromosomes
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Genes are portions of
DNA at a specific site
called a locus within a
chromosome.
The genes at a specific
locus encode for a
particular function.
The genetic sequence
could encode for
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Enzymes
Hormones
Structural proteins
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At times, errors or mutations in a gene or
chromosome may occur during:
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Transcription
Chromosome separation during Mitosis or Meiosis
Mutations that occur in gamete cells:
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will be present in the organism and
be passed on to the next generation.
Causes of Mutations
Mutagenic Agents
 These are agents that
cause mutations.
These include:
 Radiation (UV, X-rays)
 Temperature extremes
 Exposure to chemicals
(pesticides
Mutations are divided into two categories:
1. Point mutations
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At a single gene
Alterations may occur in the sequence or number of
nucleotides
2. Chromosomal mutations:
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More extensive alteration with a part of or entire
chromosome
Point or Gene Mutations:
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Occur when DNA is transcribed into RNA
There are two types:
‘The child walked down the
street’
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What do you notice about….
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“The child wapked down the street”
1. Substitution:
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One nucleotide is
being substituted or
replaced with another.
End result is a
different nucleotide
sequence than the
original DNA
sequence
‘The child walked down the
street’
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What do you notice about….
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“The child walkxe ddow nth estreet”
X
2. Frame-shift Mutation
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During transcription
nucleotide base pairs
may be inserted or
deleted from the DNA
sequence
Point mutations can lead to the
following outcomes:
Silent Mutations
 Have no effect on the operation of the cell (do not
change the amino acid sequence)
 Usually occurs in the noncoding regions of DNA
Why are they silent?
 Introns are cut out of the mRNA transcript during
transcription, thus mutations never surfaces.
 Genetic code has a redundancy in nature (Ex.
UUU and UUC both code for phenylalanine
Missense Mutations
 Occurs when a change in the base sequence
of DNA alters a codon, therefore a different
amino acid is placed in the protein
sequence.
 E.g. sickle cell anemia (see next slide)
Ex. Sickle Cell Anemia
Nonsense Mutations
 Arises when a change in the DNA sequence
causes a stop codon to replace a codon specifying
an amino acid
 Causes translation to stop short of the end of the
full mRNA.
 Therefore, only the part of the protein that
precedes the stop codon is produced (the
fragment may be digested by cell proteases)
 Are often lethal to the cell
Chromosomal Mutations
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Portions of a chromosome may break off
and rejoin leading to an interruption in the
sequence of genetic information.
There are 4 types:
‘The child walked down the
street’
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What do you notice about….
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‘The child down walked the street’
1. Inversion
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A segment of DNA
will break off and be
re-inserted in the
same location but
‘flipped’
This could result in a
drastically changed
nonfunctional protein.
‘The child walked down the
street’
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What do you notice about….
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‘walked down the street’
‘It was a sunny day. The child’
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2. Translocation
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A segment of DNA
breaks off a chromosome
and is inserted into
another chromosome.
At times, portions of
DNA can be exchanged
between two
chromosomes (not only
one a one way process)
‘The child walked down the
street’
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What do you notice about….
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‘The down the street’
3. Deletion
Loss of a chromosome
segment
 The effects could be
lethal if the deleted
segment codes for vital
proteins.
Ex. Cri-du-chat (The loss of
a portion of chromosome
5, causes an abnormally
developed larynx; makes
the affected infant’s cry
sound like the meowing
of a cat)
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‘The child walked down the
street’
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What do you notice about….
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‘The child child child walked down the
street’
4. Duplication:
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Within a
chromosome,
repeated segments of
DNA are seen.
Nondisjunction
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Improper separation of chromosomes
during:
Meiosis I
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Meiosis II or Mitosis
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(homologous chromosomes do not separate)
(sister chromatids do not separate)
Result?
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Excess or lack of chromosomes
Nondisjunction in autosomal
chromosomes
p.173
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During gamete formation, if nondisjunction
occurs with a chromosome pair the resulting
gametes will have one extra or one less
chromosome.
Thus, when fertilization occurs, the zygote will
have:
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3 copies of one chromosome called TRISOMY or
1 copy of a chromosome called MONOSOMY
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Ex. Down syndrome
occurs because of an
extra chromosome 21
(trisomic condition)
Nondisjunction in sex
chromosomes
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Nondisjunction in sex chromosomes during
Meiosis will lead to an additional X or Y
chromosome in the offspring.
This may result in disorders such as Turner
and Klinefelter syndrome
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Sometimes an entire set of chromosomes
do not separate during Meiosis.
Result?
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The gamete will be diploid
Upon fertilization the zygote will have 3
sets of chromosomes (3n)
Rare in animals, but common in plants
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Polypoidy: A cell or an organism in which
the number of complete sets of
chromosomes is greater than two.
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Ex. Seedless
Watermelon
Breeders cross a
diploid male with a
tetraploid female (4n)
Result,
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Sterile offspring (no
seeds)
MUTATION REPAIR MECHANISMS
1) DIRECT REPAIR / PROOFREADING DURING
REPLICATION
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During DNA replication, an incorrect base may be
added to the growing polynucleotide chain. DNA
Polymerase I performs a proofreading function.
When a mispairing of bases occur during the
replication process, then the enzyme will remove the
improperly placed base and try again. (Helicase, DNA
ligase and other proteins also play a role in this
mechanism
MISMATCH REPAIR
 If a mispairing of bases occurred during
DNA replication and ‘Proofreading’ wasn’t
effective at correcting this error, then
mismatch repair will take place.
 Proteins will excise the mismatched base
and DNA polymerase will add the correct
bases.
3) EXCISION REPAIR
 During the life of a cell, DNA may become damaged due
to hazards such as high-energy radiation, chemicals that
induce mutations, and random spontaneous chemical
reactions.
 Therefore, the cell will rely on excision repair, where
certain enzymes will ‘inspect’ the cell’s DNA.
 When they find mispaired bases, chemically modified
bases or points at which one strand has more bases than
the other, these enzymes cut the defective strand.
 Other enzymes will cut away at the adjacent bases and
DNA polymerase and DNA ligase synthesize and seal up
a new piece to replace the excised one.
Recap
Errors or Mutations
Chromosomal mutation
Gene/point mutation
4 types:
Nondisjunction
Inversion
Autosomal chromosomes
2 Types:
Translocation
Sex chromosomes
Substitution
Deletion
Polyploidy
Frame-shift mutation
Duplication
Questions
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1. A diploid cell (2n) undergoes Meiosis I and II.
Nondisjunction of one pair of chromosomes
occurs during Meiosis I. What are the number of
chromosomes that result in the new gametes?
Answer:
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diploid (46) haploid (23 + 1) = 24 chromosomes
Diploid (46)haploid (23 – 1) = 22 chromosomes
Question:
2. When fertilized, which gamete will produce a
trisomic condition and a monosomic condition?
Explain
 Answer: Trisomic Condition - Gamete with 24
chromosomes will have 3 of the same
chromosomes.
Monosomic Condition – Gamete with 22
chromosomes will have only one chromosome
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Homework:
Grade 12 Text: p. 263 #1-4, 6-7 (Use the
genetic code found on p. 240 to answer #6
and 7)