Quiz 2: DNA Damage, Repair and Recombination

A silent mutation refers to a change in the DNA sequence that does not result in any observable change in the expression or functioning of a gene or gene product. This means that even though the DNA sequence has been altered, the resulting protein remains unchanged, and there is no impact on the organism's phenotype. Silent mutations often occur in the third position of a codon, where changes in the DNA sequence do not affect the amino acid that is encoded.

A transversion is a type of point mutation where a purine (adenine or guanine) is replaced by a pyrimidine (cytosine or thymine), or vice versa. This mutation can cause a change in the DNA sequence, leading to alterations in the corresponding protein sequence. Transversions are considered more rare than transitions, which involve the replacement of one purine with another or one pyrimidine with another.

A missense mutation refers to a change in a nucleotide sequence that results in the substitution of one amino acid with another in the encoded protein. This alteration in the codon sequence leads to the incorporation of a different amino acid during protein synthesis. Unlike silent mutations that do not affect the final protein product, missense mutations can have functional consequences, potentially altering the structure and function of the protein. Therefore, a missense mutation is the correct answer as it accurately describes the given scenario of a nucleotide sequence alteration leading to a change in the encoded amino acid.

A lesion refers to an alteration or damage to the normal chemical or physical structure of DNA. It can be caused by various factors such as environmental factors, radiation, or errors during DNA replication. Lesions can lead to mutations in the DNA sequence, which can have various effects on gene expression and protein function. Therefore, the given answer is "Lesion".

Frameshift mutations occur when the reading frame of a gene is disrupted, causing a shift in the way the genetic code is read. This can be caused by deletions, where one or more nucleotides are removed from the gene sequence, or insertions, where one or more nucleotides are added to the gene sequence. Transition mutations, which involve the substitution of one nucleotide for another, do not typically cause frameshift mutations. Therefore, the correct answer is II and III, as both deletions and insertions can result in frameshift mutations.

Mutation refers to permanent, heritable alterations in the base sequence of the DNA. This means that the DNA sequence is permanently changed, and this alteration can be passed on to future generations. Mutations can occur due to various factors such as errors during DNA replication or exposure to certain chemicals or radiation. These alterations in the DNA sequence can lead to changes in the protein produced, which can have significant effects on an organism's phenotype and evolution.

A nonsense mutation is a type of DNA mutation that results in the appearance of a stop codon in the resulting mRNA synthesis. This stop codon prematurely terminates the translation process, leading to the production of a truncated or non-functional protein. Nonsense mutations can have significant effects on protein structure and function, and are often associated with genetic disorders.

Mutations can arise through spontaneous processes in DNA replication, in meiotic recombination, and as a consequence of the damaging effects of physical or chemical agents on the DNA. This means that mutations can occur during the normal replication of DNA, during the process of meiotic recombination which shuffles genetic material during the formation of reproductive cells, and as a result of external factors such as exposure to radiation or certain chemicals that can cause damage to the DNA. Therefore, all three options I, II, and III are correct.

Cyclobutane pyrimidine dimers are formed when two adjacent pyrimidine bases in DNA are covalently bonded due to exposure to UV radiation. These dimers can be monomerized again, meaning the bond between the pyrimidine bases can be broken, by a process called photoreactivation. DNA photolyases are enzymes that are capable of performing this photoreactivation process in the presence of visible light. Therefore, DNA photolyases are the correct answer for this question.

A nonsense mutation refers to a genetic alteration in a nucleotide sequence that causes a triplet coding for an amino acid to be changed into a termination codon. This means that instead of coding for an amino acid, the mutated sequence signals the end of protein synthesis. This type of mutation often leads to the production of nonfunctional or truncated proteins, which can have significant effects on an organism's phenotype.

Transition is the correct answer because it refers to a specific type of point mutation where a purine (adenine or guanine) is replaced by another purine, or a pyrimidine (thymine or cytosine) is replaced by another pyrimidine. This type of mutation does not change the amino acid sequence of the resulting protein, making it a conservative mutation.

Transposons are small DNA sequences that have the ability to move to different positions within a cell's genome. They can "transpose" or jump from one location to another, making them capable of moving to virtually any position in the genome. This mobility allows transposons to potentially have significant effects on gene expression and genome evolution.

Transposons are genetic elements that can move from one location to another within a genome. They encode a specific enzyme called transposase, which catalyzes the process of insertion, allowing the transposon to be integrated into a new genomic location. The other options listed, such as DNA glycosylase, excisionase, DNA polymerase, and integrase, are not directly involved in the insertion process of transposons.

In base excision repair, the lesion is removed by DNA glycosylase. DNA glycosylase is an enzyme that recognizes and removes damaged or incorrect bases from the DNA molecule. It specifically recognizes and cleaves the glycosidic bond between the damaged base and the sugar-phosphate backbone, resulting in the removal of the damaged base. This repair process is important for maintaining the integrity of the DNA molecule and preventing the accumulation of mutations.

The correct answer is A and B. Methylmethane sulfonate (MMS) and ethylnitrosourea (ENU) are both examples of alkylating agents. Alkylating agents are compounds that can add alkyl groups to DNA, causing damage to the DNA structure and interfering with DNA replication and transcription. Nitrous acid is not an alkylating agent, as it does not add alkyl groups to DNA. Therefore, the correct answer is A and B.

RecBCD is the correct answer because it is an enzyme in Escherichia coli that plays a crucial role in repairing double-stranded DNA breaks through homologous recombination. This enzyme acts as a nuclease, meaning it can cleave DNA, and it is involved in the initial steps of DNA repair by generating single-stranded DNA ends that can be used for recombination. RNA polymerase is responsible for synthesizing RNA from a DNA template, DNA polymerase is involved in DNA replication, DNA ligase is responsible for joining DNA fragments, and DNA glycolase is involved in DNA repair by removing damaged bases.

In nucleotide excision repair, an endonuclease creates nicks on both sides of the lesion, removing the damaged portion. This creates a gap in the DNA strand. The gap is then filled by DNA polymerase, which adds new nucleotides to the strand, and DNA ligase, which seals the newly synthesized DNA to the existing strand by forming a phosphodiester bond. Therefore, both DNA polymerase and DNA ligase are involved in filling the gap and making the final phosphodiester bond.

5-Bromouracil is an analog of thymine that can react with deoxyribonucleic acid to produce a polymer with increased susceptibility to mutation. This means that when 5-Bromouracil is present in DNA, it can cause changes in the DNA sequence during replication, leading to mutations. Thymine is one of the four nucleotide bases found in DNA, and by being an analog of thymine, 5-Bromouracil can substitute thymine in DNA, disrupting the normal base pairing and increasing the likelihood of mutations.

The dispersed repetitive sequences found in higher eukaryotic DNA, such as LINES and SINES, are likely to spread through the genome by transposition. Transposition refers to the process by which these sequences can move from one location to another within the genome. This movement can occur through mechanisms such as "copy and paste" or "cut and paste," allowing the sequences to be duplicated and inserted into new genomic locations. Transposition is a common mechanism for the expansion and diversification of repetitive sequences in eukaryotic genomes.

The correct answer is recA because recA is a gene that plays a key role in general recombination, which is the exchange of homologous regions of DNA between two DNA molecules. However, the question asks for the exchange of nonhomologous regions of DNA, which is known as illegitimate recombination. Illegitimate recombination is independent of recA and instead involves other factors such as IS elements and retrotransposons.