DNA and RNA

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Biochemistry › DNA and RNA

Questions 1 - 10
1

The backbone of a strand of DNA is comprised of which of these?

Sugars and phosphates

Sugars and nucleotides

Nucleotides and phosphates

Sugars only

Nucleotides only

Explanation

The backbone of DNA is made up of alternating phosphate groups and sugar groups, linked together via phosphodiester bonds. The nitrogenous bases jut off of the backbone and form bonds with nitrogenous bases on other strands of DNA to become double stranded. A nucleotide consists of a sugar, nitrogenous base, and one or more phosphate groups.

2

Which of the following statements about B DNA are incorrect?

B DNA has a wide and deep major groove and a narrow and shallow minor groove

B DNA is the most commonly found double helical structure

B DNA is right handed with 10 base pairs per turn

B DNA has a diameter of

All of these are correct

Explanation

B DNA has a wide and deep major groove and a narrow and deep minor groove. All other statements regarding B DNA are true.

3

Two strands of DNA were isolated from a solution, named DNA-1 and DNA-2. DNA-2 was found to have a higher melting point (temperature at which the two strands break apart). What can we conclude about the two strands' nucleotide composition?

DNA-2 has a higher guanine and cytosine concentration

DNA-1 has a higher guanine and cytosine concentration

DNA-2 has a higher adenine and thymine concentration

DNA-1 has a higher adenine and thymine concentration

Explanation

Recall that guanine and cytosine form three hydrogen bonds to one another in DNA, while adenine and thymine only form two hydrogen bonds to one another. This means that DNA strands with higher concentrations of guanine and cytosine will be more stable, and thus require greater energy to break apart. In this case, since DNA-2 required more energy (higher temperature) to denature, it has a higher concentration of guanine and cytosine.

4

The role of small nuclear RNA (snRNA) is to __________.

participate in the splicing of RNA exons

bind to complementary mRNA molecules to inhibit translation

carry amino acids to the ribosome for translation

bind to mRNA to facilitate its degradation

function as the template for protein synthesis

Explanation

The function of snRNA is to participate in the splicing of RNA exons. Micro RNA binds to complementary mRNA to inhibit translation. Small interfering RNA binds to mRNA to facilitate its degradation. mRNA functions as a template for protein synthesis. Transfer RNA (tRNA) carries amino acids to the ribosome during translation.

5

A histone is mutated so all lysine residues are changed to glutamate. What effect will this mutation have?

The histone will have a reduced binding affinity for DNA.

The histone will have an increased binding affinity for DNA.

The histone will be tagged with ubiquitin.

The histone will introduce more negative supercoils in the DNA.

Explanation

DNA is negatively charged because of the sugar-phosphate backbone. Histones are rich in lysine residues because they are positively charged and are ideal for DNA to wrap around. If the lysines became glutamates, the histone would overall have a negative charge, causing the DNA to bind weakly to each histone. The function of introducing negative supercoils would be reduced, and ubiquitination would not be affected.

6

What is the role of microRNA?

Binds to complementary mRNA strand to inhibit translation

Participates in the splicing of RNA exons

Binds to mRNA to facilitate its degradation

Carries amino acids to the ribosome during translation

Functions as a template for protein synthesis

Explanation

The function of snRNA is to participate in the splicing of RNA exons. Micro RNA binds to complementary mRNA to inhibit translation. Small interfering RNA binds to mRNA to facilitate its degradation. mRNA functions as a template for protein synthesis. Transfer RNA (tRNA) carries amino acids to the ribosome during translation.

7

1. chromosome

2. solenoid

3. nucleosome

4. radial loop

5. base pair

6. nucleotide

Place the above structures in order from smallest to largest.

Explanation

The correct order is base pair, nucleotide, nucleosome ("bead on a string"), solenoid, radial loop, chromosome. A solenoid is created by the packing of DNA with multiple nucleosomes, and a radial loop is compacted even further in chromatin.

8

In eukaryotes, the TATA box and/or Hogness box found on DNA are __________.

promoter sites

primer sites

DNA replication origin sites

transcription stop sites

degradation sites

Explanation

The TATA box and/or the Hogness box are regions on DNA that function as promoter sites. RNA polymerase scans DNA for these regions and when it sees one, it recognizes that it should begin transcription of the following gene.

9

Which of the following statements about RNA is false?

Hairpin turn structures are composed of two RNA molecules in a parallel configuration

A hydroxyl group on the 2’ of riboses makes RNA phosphodiester bonds fragile

RNA forms intramolecular double helices

mRNA has a half-life shorter than the cell division cycle

None of the other answers is false

Explanation

The presence of an group on the 2’ carbon if ribose does indeed make any phosphodiester bonds at this site subject to hydrolysis. RNA can sometimes form double-helices, such as in tRNA. mRNA is constantly being degraded in the cytoplasm, and so it has a very short half-life relative to the life of the cell. RNA’s hairpin turn structures are composed of only one molecule which has doubled back on itself, rather than two separate molecules.

10

Primase is needed for which of the following?

Suppling and RNA primer with a free 3' end

Supplying an RNA primer with a free 5' end

Suppling a DNA primer with a free 3' end

Supplying a DNA primer with a free 5' end

Reliving supercoiling stress on DNA

Explanation

Primase is an enzyme used in the replication of DNA. DNA polymerase synthesizes DNA in a 5' to 3' direction, so it needs a free 3' end to begin its replication from. The primase constructs an RNA substrate with a free 3' end, that binds complementary to the unwound and single-stranded DNA template that is about to be replicated.

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