Microbiology Exam 3

Spontaneous mutations typically arise during DNA replication due to errors in base pairing. DNA polymerase III plays a crucial role in replicating DNA accurately by proofreading mismatches. This enzyme's exonuclease activity allows it to detect and correct errors, minimizing the occurrence of mutations.

Mutagens are substances that can cause changes in DNA sequences by reacting chemically with DNA, leading to mutations.

The correct types of chemical mutagens are UV light, Base analogs, Intercalating agents, and transposons. Acids, Phosphates, Proteins, and Hormones are not categorized as chemical mutagens.

Base analogs and tautomers are molecules that resemble normal nucleic acid bases but have modified structures that can lead to errors in DNA replication and gene expression.

Intercalating agents are flat molecules that disrupt DNA replication by slipping between base pairs. They bind into the major groove of DNA, not to the phosphate backbone, sugar-phosphate backbone, or hydrogen bonds. Ethidium bromide is a well-known example of an intercalating agent used in DNA staining.

Thymine dimers are lesions formed by the covalent bonding of adjacent thymine residues in the DNA strand, ultimately impeding normal DNA replication and transcription processes by disrupting base pairing and H bonds required for synthesis.

Point mutations are changes in a single nucleotide base in the DNA sequence. The correct answer provides three common types of point mutations that can occur. The incorrect answers offer different scenarios that do not align with the definition of point mutations.

A missense mutation is a type of point mutation where a single nucleotide change results in a codon that specifies a different amino acid. This can lead to changes in the way a protein folds and functions.

A nonsense mutation changes a normal codon into a stop codon, prematurely ending the translation process and resulting in a shortened or nonfunctional protein.

Back mutations or reversions refer to mutations that reverse the effect of a previous mutation. The three different kinds are back mutations (reverting to the original sequence), second-site mutations (occur at a different site in the gene and restore function), and suppressor mutations (occur at a different site and suppress the effect of the initial mutation).

A back mutation refers to a mutation that corrects the original mutation, essentially reverting the gene back to its original state. This is also known as a reversion. Mutations from the wild-type version are considered forward mutations, not back mutations.

A second-site mutation specifically refers to a mutation that occurs elsewhere in the same gene to counteract the effects of the original mutation and restore the wild-type phenotype.

The Ames test is specifically designed to measure the mutagenic potential of a test agent by culturing Salmonella on minimal medium and comparing reversion frequencies with and without exposure to the agent.

Base excision repair is a process involving specific enzymes recognizing and repairing damaged DNA bases. It does not involve DNA helicase unwinding the DNA strands, RNA polymerase synthesizing RNA, or histone proteins repairing damaged DNA.

Base excision repair is a cellular mechanism that corrects non-bulky lesions in DNA bases. It involves the removal of damaged bases by DNA glycosylases, followed by DNA polymerases filling in the correct bases. This process helps maintain the integrity of the genetic information in cells by repairing small, specific changes in DNA bases.

Nucleotide excision repair involves a specific process where repair endonucleses or Uvr ABC endonucleses cut out mutations, DNA polymerase (III) fills gaps, and DNA ligase joins fragments to ensure proper repair.

Mutations are changes in the genotype, the genetic makeup of an organism, and not in the phenotype (physical characteristics), alleles (different forms of a gene), or chromosomes (the structures that hold genes).

UV light causes adjacent thymine residues to dimerize, leading to mutations and potentially harmful effects on DNA.

A suppressor mutation is a mutation that occurs in a different gene and has the ability to suppress or restore the phenotype that was altered by the original mutation, essentially correcting the genetic defect.

Transposition is a type of rearrangement mutation where a segment of DNA is moved from one location to another within the genome. Substitution involves the replacement of one nucleotide with another, deletion is the removal of a segment of DNA, and insertion is the addition of extra nucleotides to the genome.

The Ames test is specifically designed to assess the mutagenic potential of chemical compounds.

The Ames test utilizes Salmonella typhimurium as it is a histidine auxotroph and easily reverted to prototroph, making it suitable for detecting mutagenic chemicals.

Histidine is necessary for DNA repair as it is an essential amino acid involved in the process.

In the Ames test, a chemical is considered to be a mutagen when it causes more mutations than occur spontaneously. This indicates the ability of the chemical to induce genetic mutations, which are an indicator of potential carcinogenicity.

Nucleotide excision repair identifies and removes a wide range of DNA damage, including thymine dimers and bulky mutations, thereby maintaining the integrity of the DNA. It does not replicate damaged DNA strands, prevent mutations, or enhance protein production.

The correct answer, photoreactivation, involves the use of light-activated DNA photolyase to directly repair thymine dimers. This process is specific to repairing these specific DNA lesions.