Question

1 multiple choice 1 point
what dna repair mechanism is primarily responsible for replacing thymine dimers?
nucleotide excision repair
dna ligase
homologous recombination
3 exonuclease activity
2 multiple choice 1 point
which of the following is found in eukaryotic genes but not prokaryotic genes?
repressors
introns
operons
3 multiple choice 1 point
the nitrogenous base thymine is what type of base?
purine
benzene
pyrimidine
monoamine
4 multiple choice 1 point
which of the following accurately shows dna base pairing and hydrogen bond number?
adenine-uracil, 2 hydrogen bonds
adenine-thymine, 3 hydrogen bonds
cytosine-guanine, 2 hydrogen bonds
adenine-thymine, 2 hydrogen bonds

Explanation:

1. Nucleotide excision repair is the mechanism that removes bulky lesions like thymine dimers. DNA ligase seals nicks. Homologous recombination is for double - strand breaks. 3' exonuclease activity is for proof - reading.
2. Introns are non - coding regions in eukaryotic genes. Prokaryotic genes usually lack them. Repressors are in both (though different mechanisms). Operons are in prokaryotes.
3. Thymine is a pyrimidine. Purines are adenine and guanine. Benzene is not a nitrogenous base in DNA. Monoamine is not relevant here.
4. In DNA, adenine pairs with thymine via 2 hydrogen bonds. Cytosine pairs with guanine via 3 hydrogen bonds. Uracil is in RNA.

Answer:

1. nucleotide excision repair
2. introns
3. pyrimidine
4. Adenine−Thymine, 2 hydrogen bonds