Question 1

Which enzyme catalyzes the elongation of a DNA strand in the 5' → 3' direction?

Accepted Answer

DNA polymerase III is the only enzyme among the given options that catalyzes the elongation of a DNA strand in the 5' → 3' direction. Primase synthesizes RNA primers, DNA ligase joins two DNA strands by catalyzing the formation of phosphodiester bonds, topoisomerase relieves the torsional stress of DNA strands during replication, and helicase unwinds the double-stranded DNA molecule at the replication fork.

Question 2

Polytene chromosomes of Drosophila salivary glands each consist of multiple identical DNA strands that are aligned in parallel arrays. How could these arise?

Accepted Answer

Polytene chromosomes are formed through repeated rounds of DNA replication without cell division, resulting in parallel arrays of identical DNA strands.

Question 3

At a specific area of a chromosome, the sequence of nucleotides below is present where the chain opens to form a replication fork:
3' C C T A G G C T G C A A T C C 5'
An RNA primer is formed starting at the underlined T (T)of the template. Which of the following represents the primer sequence?

Accepted Answer

RNA primers are synthesized using the DNA template strand, and they are complementary and antiparallel to the template. The correct sequence must replace thymine (T) with uracil (U) because RNA contains uracil instead of thymine. Therefore, the correct sequence starting from the underlined T and going in the 5' to 3' direction (opposite to the 3' to 5' direction of the template strand) is 5' A C G U U A G G 3'.

Question 4

Suppose you are provided with an actively dividing culture of E. coli bacteria to which radioactive thymine has been added. What would happen if a cell replicates once in the presence of this radioactive base?

Accepted Answer

During DNA replication, each parent strand separates and serves as a template for a new complementary daughter strand. When radioactive thymine is present, it will be incorporated into the DNA molecule during replication. Therefore, both daughter strands will contain radioactive thymine and the DNA in both daughter cells will be radioactive.

Question 5

The enzyme telomerase solves the problem of replication at the ends of linear chromosomes by which method?

Accepted Answer

Telomerase adds numerous short DNA sequences composed of TTAGGG to the 3' end of the new DNA strand during replication, which creates a hairpin turn that serves as a buffer to prevent the loss of important genetic information. This is how the enzyme solves the problem of replication at the ends of linear chromosomes.

Question 6

In E. coli, there is a mutation in a gene called dnaB that alters the helicase that normally acts at the origin. Which of the following would you expect as a result of this mutation?

Accepted Answer

DnaB is responsible for unwinding the double-stranded DNA at the replication origin, creating a replication fork. Therefore, a mutation in the dnaB gene would prevent the formation of a replication fork and halt the replication process.

Question 7

The DNA of telomeres has been found to be highly conserved throughout the evolution of eukaryotes. What does this most probably reflect?

Accepted Answer

The high conservation of telomere DNA across eukaryotes suggests that the critical functions they perform, such as protecting chromosome ends and aiding in chromosome replication, must be maintained for cellular and organismal viability. This conservation implies that any significant mutations could be detrimental, emphasizing the importance of their role in genome stability.

Question 8

Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

Accepted Answer

In DNA, the amount of cytosine (C) is equal to the amount of guanine (G), and the amount of adenine (A) is equal to the amount of thymine (T). If cytosine makes up 42%, then guanine also makes up 42%. This leaves 16% for adenine and thymine combined, so thymine would be 8%.

Question 9

In his transformation experiments, what did Griffith observe?

Accepted Answer

Griffith observed that mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form. This is known as transformation and was a significant discovery in the field of genetics.

Question 10

Which of the following can be determined directly from X-ray diffraction photographs of crystallized DNA?

Accepted Answer

X-ray diffraction photographs can reveal the diameter of the helix of the DNA molecule, which is one of the key features of its structure.

Question 11

How do we describe transformation in bacteria?

Accepted Answer

Transformation in bacteria refers to the process of assimilation of external DNA into a cell, which can result in genetic changes and/or acquisition of new traits. The other choices (A, B, C, and D) do not accurately describe transformation in bacteria.

Question 12

Eukaryotic telomeres replicate differently than the rest of the chromosome. This is a consequence of which of the following?

Accepted Answer

Eukaryotic telomeres replicate differently due to gaps left at the 5' end of the lagging strand after the removal of RNA primers, which cannot be filled by DNA polymerase, leading to progressive shortening of chromosomes.

Question 13

An Okazaki fragment has which of the following arrangements?

Accepted Answer

Okazaki fragments are short sequences of DNA nucleotides which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication. They start with a short RNA primer at the 5' end, which is later replaced by DNA nucleotides extending to the 3' end. This arrangement is correctly described in option C.

Question 14

In trying to determine whether DNA or protein is the genetic material, Hershey and Chase made use of which of the following facts?

Accepted Answer

Hershey and Chase used radioactive isotopes of phosphorus and sulfur to label the DNA and protein of bacteriophages. Since DNA contains phosphorus, while proteins do not, they were able to differentiate between the two and determine that DNA is the genetic material.

Question 15

Which of the following investigators was/were responsible for the following discovery?
In DNA from any species, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.

Accepted Answer

This is known as Chargaff's rule, which was discovered by Erwin Chargaff.

Question 16

Replication in prokaryotes differs from replication in eukaryotes for which of the following reasons?

Accepted Answer

Prokaryotic chromosomes have a single origin of replication, whereas eukaryotic chromosomes have multiple origins of replication. This difference in origin number affects the overall speed and process of DNA replication in each type of organism.

Question 17

It became apparent to Watson and Crick after completion of their model that the DNA molecule could carry a vast amount of hereditary information in which of the following?

Accepted Answer

The sequence of bases (adenine, guanine, cytosine, and thymine) in DNA is responsible for carrying hereditary information.

Question 18

In an analysis of the nucleotide composition of DNA, which of the following will be found?

Accepted Answer

The nucleotide composition of DNA follows Chargaff's rule, where A is always equal to T and C is always equal to G. Therefore, A + C = G + T.

Question 19

After mixing a heat-killed, phosphorescent strain of bacteria with a living nonphosphorescent strain, you discover that some of the living cells are now phosphorescent. Which observations would provide the best evidence that the ability to fluoresce is a heritable trait?

Accepted Answer

The fact that descendants of the living cells are also phosphorescent indicates that the ability to fluoresce is a heritable trait, passed down from generation to generation. Option A and E suggest that DNA and protein from the heat-killed strain may have been transferred to the living strain, but they do not necessarily indicate that the ability to fluoresce is a heritable trait. Option B suggests protein transfer, which is not how heritable traits are typically passed on. Option C's brightness level is not a clear indication of whether the trait is heritable or not.

Question 20

What is meant by the description "antiparallel" regarding the strands that make up DNA?

Accepted Answer

Antiparallel refers to the orientation of the two strands of a DNA molecule running in opposite directions, with the 5' end of one strand paired with the 3' end of the other strand. This is essential for the process of replication and transcription in DNA.

Quiz 16: The Molecular Basis of Inheritance