Question 1

Which experiment provided direct evidence that DNA is the genetic material, and what was the key result that supported this conclusion?

Accepted Answer

The Hershey-Chase experiment, which showed that only DNA entered the bacterial cell during phage infection.

Question 2

Erwin Chargaff discovered that in DNA, the amount of adenine is always equal to the amount of thymine, and the amount of guanine is always equal to the amount of cytosine. What is the structural basis for this observation?

Accepted Answer

Adenine pairs with thymine and guanine pairs with cytosine via specific hydrogen bonds in the double helix.

Question 3

Given the DNA sequence 5'-ATCGTACG-3', what is the complementary strand, including the correct 5' and 3' ends?

Accepted Answer

3'-TAGCATGC-5'

Question 4

Why does RNA hydrolyze more rapidly than DNA in pure water under alkaline conditions?

Accepted Answer

RNA contains a 2'-hydroxyl group that can act as a nucleophile, promoting cleavage of the phosphodiester bond.

Question 5

Which of the following best explains why DNA melting temperature ($$T_m) $$increases with salt concentration?

Accepted Answer

Positive ions in solution shield the negative charges on the phosphate backbone, reducing repulsion and stabilizing the helix.

Question 6

What is the main reason that non-Watson-Crick base pairs are found in folded RNAs but not in DNA?

Accepted Answer

RNA can fold into complex three-dimensional structures that allow non-canonical base pairing, while DNA is constrained to a regular double helix.

Question 7

Which of the following is a consequence of methylation of guanine to O6-methylguanine in DNA?

Accepted Answer

O6-methylguanine pairs with thymine instead of cytosine, leading to mutations during replication.

Question 8

Why are dideoxynucleoside triphosphates (ddNTPs) used in Sanger sequencing to terminate DNA synthesis?

Accepted Answer

ddNTPs lack a 3'-hydroxyl group, preventing formation of the next phosphodiester bond.

Question 9

During PCR, what is the main function of raising the temperature to 94°C in the first step of each cycle?

Accepted Answer

To denature the double-stranded DNA, separating it into single strands.

Question 10

If the annealing temperature in a PCR reaction is set too low, what is the most likely outcome?

Accepted Answer

Primers may bind non-specifically, resulting in amplification of unintended DNA sequences.

Question 11

Which of the following best describes the B-form of DNA?

Accepted Answer

It is a right-handed helix with about 10 base pairs per turn and wide major grooves.

Question 12

What is the effect of doubling the concentration of DNA on the melting temperature ($$T_m) of a $$DNA solution?

Accepted Answer

$$T_m$$ increases because the equilibrium shifts toward more double-stranded DNA formation.

Question 13

Why is a denaturant required to estimate the molecular weight of single-stranded RNA by gel electrophoresis, but not for double-stranded DNA?

Accepted Answer

Single-stranded RNA can form secondary structures that affect migration, while double-stranded DNA remains linear.

Question 14

Which of the following molecular interactions is NOT a major contributor to the stability of the DNA double helix?

Accepted Answer

Covalent bonds between base pairs.

Question 15

What is the main reason DNA annealing (and melting) is considered cooperative?

Accepted Answer

The formation or disruption of one base pair increases the likelihood of neighboring base pairs forming or melting.

Question 16

If you rapidly cool a concentrated solution of denatured DNA, what is the most likely outcome?

Accepted Answer

DNA strands may anneal incorrectly, forming mismatched or incomplete duplexes.

Question 17

Which of the following is true about the sugar pucker in double-stranded DNA and RNA?

Accepted Answer

DNA typically has C2'-endo sugar pucker, while RNA has C3'-endo.

Question 18

How many times would you expect to find the dinucleotide AG in a random sequence of 3 billion base pairs (human genome)?

Accepted Answer

$$\frac{3,000,000,000}{16}$$

Question 19

What is the main purpose of using a thermostable DNA polymerase in PCR?

Accepted Answer

It can withstand repeated cycles of high temperature required for DNA denaturation.

Biochem chapter 8 practice problems

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