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Chapter 3

HMB265H1 Chapter Notes - Chapter 3: Missense Mutation, Nonsense Mutation, Silent Mutation

Human Biology
Course Code
Maria Papaconstantinou

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Lecture 3 Reading
Chapter 2 (emphasis on pages 22-23); Chapter 7 (emphasis on pages 215-219);
Chapter 8 (emphasis on pages 236-239); Chapter 15 (emphasis on pages 491-500)
Genes encode Proteins
Pea shape gene encodes an enzyme known as SBE(starch branching enzyme )
Catalyze conversion of unbranched starch to branched starch (more starch
made )
Dominant allele of the gene causes the formation of SBE1 enzyme
Cystic fibrosis
Protein cystic fibrosis transmembrane conductance regulator (CFTR) controls
the flow of chloride ion into and out of the cell
Normal allele → produces CFTR that regulates ion exchange → good
Patient: abnormal protein cannot be inserted in membrane --: lack CL
Gene expression in c elegans involved trans-splicing and polycistronic transcripts
C elegans transcribes some groups of adjacent genes as one long polycistronic
primary transcripts
Processed by trans-splicing into mature mRNA
The effect of mutations on gene expression and
gene function
Mutations in a gene's coding sequence may alter the gene product
Silent mutation: do not alter protein’s primary structure
Missense mutation replace one amino acid with another
Nonsense mutation shorten a polypeptide by replacing a codon with a stop signal
Frameshift mutations result in a change in reading frame downstream of the addition
or deletion
Mutations outside the coding region can also disrupt gene expression
Silent mutations
Change a codon into a mutant codon that specifies the same amino acid
Mostly change the 3rd codon
No effect on phenotypes
Missense mutations
Specifies a different amino acid
Conservative: the amino acids are kind of similar (little/no effect on function)
non conservative: more noticeable consequence
Nonsense mutation
Changes a aa-codon into a stop codon
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Result in smaller protein
Unable to function
Frameshift mutations
Results from insertion or deletion of nucleotides within the coding sequence
Destroy protein function
Mutations outside the coding sequence can also alter gene expression
E,g, changes in promoter/terminator
Changes in splice acceptor
These Mutations can lower the affinity of mRNA for the ribosome → less protein
Most mutations that affect gene expression reduce gene
Loss-of-function mutation: any mutation inside or outside a coding region that
reduces or abolishes protein activity
Recessive loss-of-function alleles
Null (amorphic): loss-of -function alleles that completely block the function of protein
E.g. in an Aa heterozygote, the A would generate functional protein, the null a
will not
If the amount of protein produced by A is above the threshold amount
sufficient to fulfill the normal requirement, the phenotype of the Aa will
be wild type
AA make more than twice as much
Hypomorphic mutation: a loss-of-function mutation that produces either much less
protein or a protein with weak but detectable function
Incomplete dominance
When a phenotype varies continuously with levels of protein function
E.g. red flower + white flower = pink flower
Co- dominance → some red petals and some white
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