Genetics Homework Chapter 9 (15)

Identify the mutagenic effects of deaminating agents, alkylating agents, and base analogs.

1. Deaminating agents change cytosine to uracil and adenine to hypoxanthine by converting an amino group to a keto group. 2. Alkylating agents add a methyl or ethyl group to the amino or keto groups of nucleotides, changing base-pair affinities. 3. Base analogs, such as 5-bromouracil and 2-amino purine, are incorporated as thymine and adenine, but base pair with guanine and cytosine respectively.

How may it lead to a mutation?

It allows hydrogen bonding of normally noncomplementary bases.

Which of these concepts that affect gene mutations in diploid organisms would apply to a haploid organism such as E. coli.

Mutations are a source of genetic variation and provide the raw material for natural selection. Mutations have a wide range of effects on organisms depending on the type of base-pair alteration, the location of the mutation within the chromosome, and the function of the affected gene product. Mutations comprise any change in the base-pair sequence of DNA. Mutations can occur spontaneously as a result of natural biological and chemical processes, or they can be induced by external factors, such as chemicals or radiation.

What is a spontaneous mutation?

Mutations that occur as a result of natural biological and/or chemical processes are considered spontaneous.

Identify the various types of DNA repair mechanisms known to counteract the effects of UV radiation.

1. Photoactivation repair is dependent on a photon-activated enzyme that cleaves thymine dimers. 2. Excision repair is the process by which an endonuclease clips out UV- induced dimers, DNA polymerase III fills in the gap, and DNA ligase rejoins the phosphodiester backbone. 3. Recombinational repair uses the corresponding region on the umdamaged parental strand of the same polarity. 4. SOS repair is a process in E. coli that induces error-prone DNA replication in an effort to fill gaps by inserting random nucleotides.

How do these defects create the phenotypes associated with the disorder?

by decreasing the rate of DNA mutation repair in skin cells

Two formal terms used to describe categories of mutational nucleotide substitutions in DNA are

transversions and transitions

Which of the following statements about xeroderma pigmentosum (XP) is FALSE?

Cells from XP patients are deficient in unscheduled DNA synthesis. Mutations in one of several genes can contribute to the XP phenotype. XP patients are extremely sensitive to UV radiation in sunlight. False: The XP phenotype is caused by defects in double-stranded break repair pathways. (The XP phenotype is caused by defects in nucleotide excision repair (NER) pathways.)

A 2-aminopurine is an alkylating agent.

False

Certain base analogs such as 5-bromouracil cause mutations by chemically altering nitrogenous bases in nonreplicating DNA.

False

Pyrimidine dimers are typically caused by the mutagen 2-aminopurine.

False

During mismatch repair, an exonuclease enzyme creates a nick in the backbone of the unmethylated DNA strand near the mismatch.

False An exonuclease enzyme breaks down nucleic acid molecules by breaking the phosphodiester bond at the 3′- or 5′-terminal nucleotides. An endonuclease is required for the breaking of internal phosphodiester bonds.

Most mutations in a diploid organism are recessive. Why?

In most cases, the amount of product from one gene of each pair is sufficient for production of a normal phenotype.

What is tautomeric shift?

Tautomeric shift is an intramolecular proton shift that changes the bonding structure of the molecule.

Why are spontaneous mutations rare?

They are relatively rare in comparison to induced mutations that are more directed to the physical or chemical properties of DNA.

What genetic defects result in the disorder xeroderma pigmentosum (XP) in humans?

defects in the DNA repair system

Explain why mutations in these regions often lead to severe disease, although they may not directly alter the coding regions of the gene.

mutations in upstream sequences may disrupt transcription factor and/or polymerase binding mutations in introns may affect RNA splicing mutations in introns may affect mRNA stability or translation