Q: Which of the following equations correctly represents the probability of forming the codon 'AAC' when synthesizing a mixed heteropolymer mRNA with a 1A:5C ratio?

$$ (1/6)^2$ \times (5/6) = 2.3\% $$

Question 1

Which DNA repair mechanism is responsible for repairing thymine dimers caused by UV radiation, and what is the key enzyme involved in this process?

Accepted Answer

Photoactivation repair; photoreactivation enzyme (PRE)

Question 2

A patient with hereditary colon cancer is found to have a defect in a DNA repair pathway. Which repair system is most likely affected, and what is the sequence of enzymatic activities in this pathway?

Accepted Answer

Mismatch repair; endonuclease → exonuclease → DNA polymerase → DNA ligase

Question 3

Which of the following best describes the mechanism of post-replication repair as depicted in the first image?

Accepted Answer

A gap opposite the lesion is filled by recombination with the undamaged parental strand, followed by DNA synthesis and ligation

Question 4

Which DNA repair system is defective in individuals with Xeroderma Pigmentosum, and what is the consequence of this defect?

Accepted Answer

Nucleotide excision repair; increased susceptibility to skin cancer due to inability to remove bulky DNA lesions

Question 5

Which of the following equations correctly represents the probability of forming the codon 'AAC' when synthesizing a mixed heteropolymer mRNA with a 1A:5C ratio?

Accepted Answer

$$ (1/6)^2$ 	imes (5/6) = 2.3\% $$

Question 6

Which of the following statements about the genetic code is FALSE?

Accepted Answer

The code is ambiguous, so one codon can specify multiple amino acids

Question 7

A mutation in the first base of a codon often results in a change to an amino acid with similar chemical properties. What feature of the genetic code explains this phenomenon?

Accepted Answer

The code is ordered, with chemically similar amino acids sharing the same middle base in their codons

Question 8

Which of the following best describes the difference between DNA transposons and retrotransposons?

Accepted Answer

DNA transposons move via a cut-and-paste mechanism without an RNA intermediate, while retrotransposons move via a copy-and-paste mechanism using an RNA intermediate

Question 9

In the context of the lac operon, what is the effect of a mutation in the operator sequence (O^C) and why can't this mutation be complemented by adding a wild-type operator elsewhere in the genome?

Accepted Answer

The repressor cannot bind to the operator, leading to constitutive expression; the operator is a cis-acting element and must be adjacent to the structural genes to function

Question 10

Which of the following best explains the role of the Shine-Dalgarno sequence in bacterial translation initiation?

Accepted Answer

It base pairs with the 16S rRNA of the small ribosomal subunit to align the start codon with the P site

Question 11

A patient is found to have X-ray hypersensitivity and a predisposition to hereditary breast and ovarian cancers. Which DNA repair pathway is most likely defective?

Accepted Answer

Double-strand break repair

Question 12

Which of the following is a key difference between prokaryotic and eukaryotic translation initiation?

Accepted Answer

Prokaryotes use a Shine-Dalgarno sequence, while eukaryotes use a 5' cap and Kozak sequence for ribosome binding

Question 13

Which of the following best describes the function of the enzyme aminoacyl-tRNA synthetase?

Accepted Answer

It covalently attaches the correct amino acid to its corresponding tRNA

Question 14

Which of the following is a real-world example of a disease caused by active transposition of LINEs or SINEs in the human genome?

Accepted Answer

Hemophilia A caused by insertion of a LINE element into the factor VIII gene

Question 15

Which of the following best describes the process of attenuation in the regulation of the trp operon?

Accepted Answer

A leader sequence in the mRNA forms alternative secondary structures that regulate transcription termination based on tryptophan availability

Question 16

Which of the following is a function of small noncoding RNAs (sRNAs) in bacteria?

Accepted Answer

They regulate the translation of other genes by base pairing with target mRNAs

Question 17

Which of the following best describes the role of the CRISPR/Cas system in prokaryotes?

Accepted Answer

It provides adaptive immunity by destroying the DNA of invading phages based on prior exposure

DNA Repair Systems, Transposable Elements, and the Genetic Code: Study Notes for Genetics Students

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