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• the source of the altered versions of genes that provide the raw material for evolution.
• Most mutations have no effect on the organism, especially among the eukaryotes, because a large portion of the DNA is not in genes and thus does not affect the organism’s phenotype.
• Of the mutations that do affect the phenotype, the most common effect of mutations is lethality, because most genes are necessary for life.
• Only a small percentage of mutations causes a visible but non-lethal change in the phenotype.
• Change in structure or amount of an organism’s genetic material
Significance of Mutations • Most are neutral
• Eye color • Birth marks
• Some are harmful • Sickle Cell Anemia • Down Syndrome
• Some are beneficial • Sickle Cell Anemia to Malaria • Immunity to HIV
What Causes Mutations?
• There are two ways in which DNA can become mutated:
– Mutations can be inherited. • Parent to child
– Mutations can be acquired. • Environmental damage • Mistakes when DNA is copied
DNA (antisense strand)
mRNA
Polypeptide
Normal gene GGTCTCCTCACGCCA
↓ CCAGAGGAGUGCGGU
Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
The antisense strand is the DNA strand which acts as the template for mRNA transcription
Transcription
Translation
Gene Mutations • Point Mutations – changes in one or a few
nucleotides • The most common type of point mutation is a transition, where either a purine is mutated to other purine or a pyrimidide is changed to the other pyrimidine (ex: A to G or C to T[U])
Most transitions results in silent mutation
• The other type of mutation is transversion, where a pyrimidine changes to a purine or vice versa (ex: A to T[U], A to C, G to C, G to T[U])
Transversion more likely to result in missense mutation
Substitutions Substitution mutation
GGTCACCTCACGCCA ↓
CCAGUGGAGUGCGGU
↓ Pro-Arg-Glu-Cys-Gly
Normal gene GGTCTCCTCACGCCA
↓ CCAGAGGAGUGCGGU
Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Missense mutations. • Substitutions will only affect a single codon. Substitute one amino acid for another. • Some missense mutations have very large effects, while others have minimal or no effect. It depends on whether they affect an amino acid that is essential for the structure and function of the finished protein molecule (e.g. sickle cell anaemia).
The genetic code is degenerate
A mutation to have no effect on the phenotype
Changes in the third base of a codon often have no effect.
No change Normal gene
GGTCTCCTCACGCCA ↓
CCAGAGGAGUGCGGU Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Substitution mutation GGTCTTCTCACGCCA
↓ CCAGAAGAGUGCGGU
↓ Pro-Glu-Glu-Cys-Gly
Silent mutations • Since the genetic code is degenerate, several codons produce the same amino acid. Especially, third base changes often have no effect on the amino acid sequence of the protein. • These mutations affect the DNA but not the protein. Therefore they have no effect on the organism’s phenotype.
Disaster Normal gene
GGTCTCCTCACGCCA ↓
CCAGAGGAGUGCGGU Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Substitution mutation GGTCTCCTCACTCCA
↓ CCAGAAGAGUGAGGU
↓ Pro-Glu-Glu-STOP
• Nonsense mutations convert an amino acid into a stop codon. The effect is to shorten the resulting protein. • Sometimes this has only a little effect, as the ends of proteins are often relatively unimportant to function. • However, often nonsense mutations result in completely non-functional proteins.
Inversion Normal gene
GGTCTCCTCACGCCA ↓
CCAGAGGAGUGCGGU Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Inversion mutation GGTCCTCTCACGCCA
↓ CCAGGAGAGUGCGGU
↓ Pro-Gly-Glu-Cys-Gly
Inversion mutations, also, only affect a small part of the gene
Additions
Normal gene GGTCTCCTCACGCCA
↓ CCAGAGGAGUGCGGU
Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Addition mutation GGTGCTCCTCACGCCA
↓ CCACGAGGAGUGCGGU
↓ Pro-Arg-Gly-Val-Arg
A frame shift mutation
Original: The fat cat ate the wee rat.
Frame Shift (“a” added): The fat caa tet hew eer at.
Deletions
Normal gene GGTCTCCTCACGCCA
↓ CCAGAGGAGUGCGGU
Codons ↓
Pro-Glu-Glu-Cys-Gly Amino acids
Deletion mutation GGTC/CCTCACGCCA
↓ CCAGGGAGUGCGGU
↓ Pro-Gly-Ser-Ala-Val
A frame shift mutation
Deletion THE FAT CAT ATE THE RAT THE FAT ATE THE RAT
Frameshifts and Reversions • Translation occurs codon by codon, examining nucleotides in groups of 3. If a
nucleotide or two is added or removed, the groupings of the codons is altered. This is a “frameshift” mutation, where the reading frame of the ribosome is altered.
• Frameshift mutations result in all amino acids downstream from the mutation site being completely different from wild type. These proteins are generally non-functional.
– example Hb-α Wayne. The final codons of the alpha globin chain are usually AAA UAC CGU UAA G, which code for lysine-tyrosine-arginine-stop. In the mutant, one of the A’s in the first codon is deleted, resulting in altered codons: AAU ACC GUU AAG, for asparagine-threonine-valine-lysine. There are also 5 more new amino acids added to this, until the next stop codon is reached.
• A “reversion” is a second mutation that reverse the effects of an initial mutation, bringing the phenotype back to wild type (or almost).
• Frameshift mutations sometimes have “second site reversions”, where a second frameshift downstream from the first frameshift reverses the effect.
– Example: consider Hb Wayne above. If another mutation occurred that added a G between the 2 C’s in the second codon, the resulting codons would be: AAU ACG CGU UAA, or asparagine-threonine-arginine-stop. Note that the last 2 codons are back to the original. Two amino acids are still altered, but the main mutational effect has been reverted to wild type.
Trinucleotide Repeats • A fairly new type of mutation has been described, in which a particular codon is
repeated.
• During replication, DNA polymerase can “stutter” when it replicates several tandem copies of a short sequence. For example, CAGCAGCAGCAG, 4 copies of CAG, will occasionally be converted to 3 copies or 5 copies by DNA polymerase stuttering.
• Outside of genes, this effect produces useful genetic markers called SSR (simple sequence repeats).
• Within a gene, this effect can cause certain amino acids to be repeated many times within the protein. In some cases this causes disease.
• For example, Huntington’s disease is a neurological disease that generally strikes in middle age, producing paranoia, uncontrolled limb movements, psychosis, and death. Woody Guthrie, a folk singer from the 1930’s, had this disease.
• The Huntington’s disease gene normally has between 11 and 33 copies of CAG (codon for glutamine) in a row. The number occasionally changes. People with HD have 37 or more copies, up to 200). The rate of copy number change is much higher in HD people--too many copies makes the repeated sequence more subject to DNA polymerase stuttering during meiosis.
• Interestingly, the age of onset of the disease is related to the number of CAG repeats present: the more repeats, the earlier the onset.
Mutation caused by mismatch wobble base pairing.
mRNA Problems • Although many mutations affect the protein sequence directly, it is
possible to affect the protein without altering the codons.
• Splicing mutations. Intron removal requires several specific sequences. Most importantly, introns are expected to start with GT and end in AG. Several beta globin mutations alter one of these bases. The result is that one of the 2 introns is not spliced out of the mRNA. The polypeptide translated from these mRNAs is very different from normal globin, resulting in severe anemia.
• Polyadenylation site mutations. The primary RNA transcript of a gene is cleaved at the poly-A addition site, and 100-200 A’s are added to the 3’ end of the RNA. If this site is altered, an abnormally long and unstable mRNA results. Several beta globin mutations alter this site: one example is AATAAA -> AACAAA. Moderate anemia was the result.
Chromosome Mutations
• May Involve:
– Changing the structure of a chromosome
– The loss or gain of part of a chromosome
Numerical Mutations
• Euploid – organisms with complete or normal sets of chromosomes Polypoid – organisms with extra sets of chromosomes
- triploid; a haploid sperm fertilises a diploid egg – three copies of each chromosome - most human polypoids die as embryos or foetuses, infant survives a few days with defects in nearly all organs.
Aneuploid – Cells missing or having an extra chromosome
- autosomal aneuploidy – mental retardation
- sex chromosome aneuploidy – less severe
- extra genetic material less dangerous than missing
material
- extra – trisomy (trisomy 21, 13, 18) – 47 chromosomes
- missing – monosomy (XO) – 45 chromosomes
- caused by non-disjunction – chromosome pairs fail to
separate at 1st or 2nd meiotic division
- 49 ways in which one chromosome can be missing or
extra – each of 22 autosomes and 5 abnormal types of sex
chromosome combination (YO, XO, XXX, XXY, XYY)
Trisomy 21 (3 separate chromosomes)
Monosomy 21 (Non-viable)
Trisomy 21 (2 copies of one parental chromosome)
Meiosis I Non-disjunction
Meiosis II Non-disjunction
Fertilisation
- only 9 types were seen in newborns, the others cease developing before birth – 50% spontaneous abortions (missing X, triploids or trisomy 16) - 9% spontaneous abortions are trisomy 13, 18, 21 and are the most common autosomal aneuploids seen in newborns - aneuploidy and polypoidy rises during mitosis - mosaics
HG pp 184
Structural Mutations • results from chromosome breakage; breakage and reunion in a different configuration Deletions - a missing chromosome segment (deficiency)
- in a diploid organism, makes part of the genome hypoploid - associated with phenotypic effect - eg. Cri du chat syndrome caused by deletion in the
short arm of chromsome 5 - severely impaired, mentally and physically, cat-
like crying
Duplication - an extra chromosome segment
- can attach to one of the chromosomes or exist as a new, separate chromosome (free duplication) - the organism is hyperploid for part of its genome - associated with phenotypic effect
- can be caused by viruses (mumps), drugs (anticancer) and radiation ( medical X-rays or UV)
Translocation - different chromosomes exchange parts or combine - can be caused by viruses (mumps), drugs (anticancer) and radiation ( medical X-rays or UV) - translocation carrier – no gene breakage during chromosome exchange - 3 types:- i) reciprocal
- 2 different chromosomes exchange parts - chromosome number remains - easily visualised by FISH - common – long arm of Ch 11 and 22 - incidence in general population :- 1: 500
ii) non-reciprocal - a piece of one chromosome breaks off and attaches to another chromosome - eg. tip of Ch 22 attaches to Ch 9 ( chronic myeloid leukemia)
iii) Robertsonian - named after the cytologist F.W. Robertson - breakage of 2 acrocentric chromosomes (13, 14, 15, 21 & 22) at or close to their centromeres,
with subsequent fusion of their long arms (centric fusion)
- functionally balanced rearrangement - loss of short arm has no clinical importance – contain genes for ribosomal RNA and can be found in other acrocentric chromosomes - overall incidence – 1:1000 - common fusion – 13q14q - Down’s syndrome - 10% risk for having Down’s syndrome baby if
13q21q or 14q21q - if 21q21q, all gametes either nullisomic or disomic – consequent pregnancies will end with spontaneous miscarriage or birth of a Down’s syndrome baby
Insertions - a segment of one chromosome becomes inserted into another chromosome
- balanced karyotype if moved into another chromosome - carriers have 50% risk of producing unbalanced gametes; either the deletion or the insertion
Inversion – 2-break rearrangement involving a single chromosome in which a segment is flipped around 180º (inverted), as a result the segment’s gene is reversed - 5 – 10% inversions cause health problems – interrupt important genes - inherited from the parents or arose from in a germ cell - 2 types of inversion i) pericentric inversion – involves centromere - produces 1 normal, 1 inverted chromosome and 2 abnormal chromosomes (have duplications and deletions) - have one centromere each
ii) paracentric inversion - does not include the centromere - produces 1 normal, 1 inverted & 2 very abnormal chromosomes (1 Ch with 2 centromeres (dicentric), extra and missing genes; the other without centromere (acentric) - Acentric chromosomes (chromosomal fragments) cannot undergo mitosis - Dicentric chromosomes are unstable - parental paracentric – will have normal baby
Isochromosome – occurs when centromere spilts at wrong plane - the centromere divided transversely rather than longitudinally - meiotic error - most common - X chromosomes; 15% of Turner syndrome cases
Ring chromosomes – telomeres are lost, generating “sticky” ends that tend to close up as a ring - exposure to radiation - autosome ring chromosomes gives serious effects - eg. Ch 22 causes cat eye syndrome:- have vertical pupils, heart and urinary tract anomalies, mentally retarded - unstable in mitosis
