gene mutation and molecular medicine
CHAPTER 15 KEY POINTS
15.1 Mutations Are Heritable Changes in DNA
Point mutations can be transitions, transversions, or single base-pair insertions or deletions.
Point mutations in codons cause silent, missense, nonsense, loss-of-stop, and frame-shift mutations.
Point mutations in noncoding regions can have effects through changes in expression or splicing.
Large chromosomal rearrangements include deletions, duplications, inversions, and translocations.
Mutations are more likely in the presence of mutagens and can occur at hotspots.
Questions
For each of the following point mutations in a protein-coding gene, indicate whether the mutation is a transition or transversion and whether it is a silent, missense, nonsense, or loss-of-stop mutation.
Mutation 1: UCA → ACA
Mutation 2: UCC → UCU
Mutation 3: UGA → UGG
Mutation 4: UGG → UGA
Mutation 5: GGC → GGG
A cell culture is started from an organism possessing only two different chromosomes. Chromosome 1 contains eight regions, A—B—C—D—E—F—G—H, and chromosome 2 contains three regions, X—Y—Z. Over time, cells arise that carry different chromosomal rearrangements. Identify the type of chromosomal rearrangement for the following three cells: Cell 1: A—B—C—F—E—D—G—H and X—Y—Z; cell 2: X—D—E—F—G—H and A—B—C—Y—Z; cell 3: A—B—C—F—G—H and X—Y—Z.
15.2 Mutations in Humans Can Lead to Diseases
Many genetic diseases are caused by point mutations that alter only a single amino acid and render a protein non-functional.
Some genetic diseases involve large deletions or chromosomal abnormalities.
Multifactorial diseases arise from complex interactions between multiple genes and the environment.
Questions
Explain why some missense mutations cause genetic disease whereas others do not.
Genotyping of breast cancer tumours in women indicates that some women have two mutated alleles of the BRCA1 gene, even though there is no history of hereditary breast cancer in their families. How does the observation relate to BRCA1 being a cause of higher rates of breast cancer in some families?
In expanding triplet repeat diseases, the incidence of the genetic disorder can increase over generations within a family. Why might this be the case?
15.3 Mutations Can Be Detected and Analysed
Restriction enzymes are used to cut DNA into small fragments.
Highly variable loci consisting of short tandem repeats (STRs) are used for DNA fingerprinting.
A linked marker allows a disease-causing gene to identified.
Questions
PCR is used to amplify a 10,000-base-pair linear sequence of DNA from an individual. When digested with EcoRI, the PCR product gives two DNA molecules: one 3,000 and one 7,000 base pairs in length.
How many restriction sites are on the original amplified DNA molecule?
The same PCR is performed on a sample from another individual and the 10,000-base-pair sequence subjected to EcoRI digestion After digestion, a single 10,000-base-pair molecule is obtained. How many restriction sites are on the original DNA molecule that was amplified from this individual?
The same PCR is performed on a sample from a third individual, and upon EcoRI digestion three different-sized pieces of DNA are obtained: 10,000, 7,000, and 3,000 base pairs. Given your answers to a and b, explain this result.
15.4 Genetic Screening Is Used to Detect Diseases
Genetic screening for disorders can involve examination of a phenotype.
Genetic screening for disorders can involve examination of a genotype using DNA testing.
Questions
What are the advantages of phenotypic versus genotypic screening for individuals who are heterozygous for a genetic disease?
Genotypic screening sometimes involves hybridisation of an individual’s DNA to allele-specific DNA sequences. DNA corresponding to both mutant and wild-type DNA is used. Why use both, rather than use just the mutant DNA sequence?
15.5 Genetic Diseases Can Be Treated
Some genetic disease can be treated by restricting a substrate, inhibiting an enzyme, or supplying a missing protein.
The goal of gene therapy is to alter an individual’s genotype in those cells where the expression of the mutant allele is causing problems.
Questions
Type I diabetes is caused by an inability to make insulin. How might such a disorder be treated? Would the same strategy work for type II diabetes (insulin insensitive)?
More genetic disorders of the bone marrow have been successfully treated using gene therapy than have disorders of other organs such as the lungs or liver. Why might that be the case?
TERMS TO KNOW
chromosomal rearrangement: large changes in the sequence of DNA caused by breakage and rejoining of DNA molecules. includes deletions, duplications, inversions and translocations
deletion: a mutation resulting from the loss of continuous segment of a gene or chromosome. such mutations almost never revert to wild type
DNA fingerprinting: the identification of individuals based on their unique pattern of allele sequences, commonly short tandem repeats and single nucleotide polymorphisms
DNA testing: in human genetics, the determination of genotype by analysis of DNA sequence
duplication: a mutation in which a segment of a chromosome is duplicated, often by the attachment of a segment lost from its homolog
frame shift mutation: the addition or deletion of a single or two adjacent nucleotides in a gene’s sequence. results in the misreading of mRNA during translation and the production of a nonfunctional protein
gene therapy: treatment of a genetic disease by providing patiences with cells containing functioning alleles of the genes that are nonfunctional in their bodies
genetic screening: a technique for identifying genes involved in a biological process of interest. involves creating a large collection of randomly mutated organisms and identifying those individuals that are likely to have a defect in the pathway of interest. the mutated gene(s) in those individuals can then be isolated for further study
inversion: a rare 180-degree reversal of DNA sequence within a segment of a gene or chromosome
loss-of-stop: a mutation that results in a change from a stop (nonsense) codon to a sense codon, causing additional amino acids to be added to the end o the protein
missense: a change in a gene’s sequence that changes the amino acid at that site in the encoded protein
multifactorial: referring to the interaction of many genes and proteins with one or more factors in the environment. for example, cancer is a disease with multifactorial causes
mutagen: any agent that increases the mutation rate
mutation: a change in the genetic material not caused by recombination
nonsense: any of the three mRNA codons that signal the end of protein translation at the ribosome: UAG, UGA, UAA. also called stop codon
point mutations: a mutation that results from the gain, loss, or substitution of a single nucleotide
restriction enzyme: any of a type of enzyme that cleaves double-stranded DNA at specific sites; extensively used in recombinant DNA technology. also called a restriction endonuclease
short tandem repeats (STRs): short, moderately repetitive sequences of DNA. the number of copies of an STR at a particular location varies between individuals and is inherited
silent: a change in a gene’s sequence that has no effect on the amino acid sequence of a protein either because it does not change the amino acid specified by the corresponding codon
transitions: in genetics, a base pair substation such that a purine base is converted to the other purine and its matching pyrimidine is converted to the other pyrimidine
translocation: in genetics, a rare mutational event that moves a portion of a chromosome to a new location, generally on a non homologous chromosome
transversion: in genetics, a base pair substitution such that a purine base is converted to a pyrimidine and its matching pyrimidine is converted to a purine