Biochemistry : Nucleic Acid Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1171 : Biochemistry

Which of the following enzymes relieves the tension that is created due to DNA coiling?

Possible Answers:

Topoisomerase

RNA polymerase

DNA polymerase

Helicase

Primase

Correct answer:

Topoisomerase

Explanation:

A double helix DNA structure can be coiled or even supercoiled. In order to relieve the tension that is inevitably formed by this coiling, topoisomerase acts upon the DNA to relieve the stress that has been created. Helicase unwinds DNA before replication. DNA polymerase elongates the DNA during replication. RNA polymerase makes RNA from a template strand of DNA. Primase creates a temporary primer to begin DNA synthesis/replication.

Example Question #33 : Anabolic Pathways And Synthesis

DNA replication is an important process that allows hereditary information to be passed to new cells. As such, there are a variety of enzymes that are crucial to this process. Helicase is one such enzyme. What is the function of helicase in DNA replication.

Possible Answers:

Holds the parent DNA strands in place during replication to prevent them from associating with one another

Stitches together the various daughter DNA fragments into a single strand

Lays down an RNA primer so that the synthesis of complementary daughter DNA can occur

Separates the two parental strands of DNA by breaking hydrogen bonds

Correct answer:

Separates the two parental strands of DNA by breaking hydrogen bonds

Explanation:

In DNA replication, the role of helicase is to unwind the strand by breaking the hydrogen bonds that hold that two strands together.

All of the other answer choices describe a role performed by a different DNA replication enzyme. Let's go ahead and review these.

  • Lays down an RNA primer so that the synthesis of complementary daughter DNA can occur

This enzyme is called Primase.

  • Stitches together the various daughter DNA fragments into a single strand

This enzyme is called DNA Ligase. It is able to join the okazaki fragments formed on the lagging strand, as well as any other areas where there is a break in the strand.

  • Holds the parent DNA strands in place during replication to prevent them from associating with one another

This enzyme is known as single-strand binding protein.

Example Question #11 : Nucleic Acid Synthesis

Which DNA polymerase excises RNA primer with a  exonuclease?

Possible Answers:

DNA polymerase I

DNA polymerase II

DNA polymerase is not associated with the exonuclease

DNA polymerase III

Correct answer:

DNA polymerase I

Explanation:

DNA polymerase I is in prokaryotes only. It degrades the RNA primer and fills in the gap with DNA. DNA polymerase III has  synthesis and proofreads with  exonuclease. This is also in prokaryotes only. It elongates the leading strand by adding deoxynucleotides to the 3’ end. It elongates the lagging strand until it reaches primer of preceding fragment.  exonuclease activity “proofreads” each added nucleotide. The function of DNA polymerase II is unknown.

Example Question #13 : Dna Replication

Given the DNA sequence, what is the correct synthesis?

DNA sequence: TCGGTCAAG

Possible Answers:

5' CTTGACCGA

3' GATTGACCT

5' TCCAGTTAG

5' AGCCAGTTC

Correct answer:

5' CTTGACCGA

Explanation:

Remember, it’s complementary and antiparallel. Therefore, when writing the complement of the DNA sequence, it’s 3’ to 5’, so you must change answer to be 5’ to 3’. 

Example Question #11 : Nucleic Acid Synthesis

In DNA replication, which is true regarding ribonucleotide reductase? 

Possible Answers:

Ribonucleotide reductase is inhibited by hydroxyurea

Ribonucleotide reductase converts ribonucleotide diphosphate to deoxyribonucleotide diphosphate

Deoxythymidine diphosphate is not a substrate for ribonucleotide reductase

All of these

Ribonucleotide reductase regulates the rate of DNA synthesis

Correct answer:

All of these

Explanation:

Ribonucleotide reductase regulates the rate of DNA synthesis and the total  DNA to cell mass ratio. The enzyme converts adenosine diphosphate (ADP), guanosine diphosphate (GDP), cytidine diphosphate (CDP), uridine diphosphate (UDP). The ribonucleotide thymidine diphosphate is not a substrate for this enzyme. Thymidine nucleotides are products of another enzyme: thymidylate kinase.

Example Question #12 : Nucleic Acid Synthesis

Which amino acids are required for the the synthesis of adenine and guanine (purines) from ribose?

I. Lysine

II. Aspartate

III. Glutamine

IV. Glycine

Possible Answers:

II and IV

I, II, III, and IV

II, III, and IV

II and III

I, II, and III

Correct answer:

II, III, and IV

Explanation:

Purines can be synthesized de novo from ribose phosphate. 5-phosphoribosylamine is converted to inosine monophosphate, which is an intermediary for adenine monophosphate and guanine monophosphate production. The reaction requires the presence of glycine, aspartate and glutamine, but not lysine.

Example Question #12 : Nucleic Acid Synthesis

Which of the following regarding phosphoribosyl pyrophosphate (PRPP) in nucleotide synthesis necessary for DNA replication is true? 

Possible Answers:

All of these

PRPP is formed from ribose-5-phosphate by PRPP synthase

Increased levels of PRPP can cause an increase in uric acid and produce gout

PRPP is the precursor of purines

PRPP is the precursor of pyrimidines

Correct answer:

All of these

Explanation:

Phosphoribosyl pyrophosphate (PRPP) is a precursor of both purines (adenine and guanine), as well as pyrimidines (cytosine, uracil, and thymine) in nucleotide synthesis. In certain enzyme deficiencies, levels of PRPP can increase leading indirectly to uric acid production and gout.

Example Question #13 : Nucleic Acid Synthesis

In DNA replication and repair what is the role of nucleases?

Possible Answers:

Nucleases can be exonucleases or endonucleases

Nucleases can remove mispaired nucleotides

All of these

Nucleases may remove nucleotides from the ends of DNA or from within the molecule of DNA

Nucleases hydrolyze phosphodiester bonds in DNA

Correct answer:

All of these

Explanation:

Nucleases are important in excision of  nucleotides from the DNA chain. Phosphodiester bonds between two nucleotides are broken and the DNA chain becomes fragmented. Site-specific nucleases can function as restriction enzymes cutting phosphodiester bonds at specific locations inside the DNA molecule. In DNA repair they remove nucleotides incorrectly placed during DNA synthesis.

Example Question #14 : Nucleic Acid Synthesis

In DNA replication, in bacteria, what is the role of the DnaA protein? 

Possible Answers:

In bacteria, DnaA replaces DNA with RNA

In bacteria, DnaA recognizes the base sequence at the origin of replication

In bacteria, DnaA protein synthesizes RNA primers

In bacteria, DnaA protein stabilizes unwound template strands

In bacteria, DnaA unwinds the DNA double helix

Correct answer:

In bacteria, DnaA recognizes the base sequence at the origin of replication

Explanation:

DnaA protein in bacteria binds to a base pair sequence at the origin of replication serving as an initiation factor for DNA synthesis. All the other answers are wrong as: helicase, not DnaA protein unwinds DNA; single-stranded DNA binding protein stabilizes separated DNA strands; primase synthesizes RNA primers; and DNA polymerase I replaces RNA with DNA.

Example Question #15 : Nucleic Acid Synthesis

Which of the following enzymes joins together the Okazaki fragments in the lagging strand during DNA replication?

Possible Answers:

DNA ligase

DNA topoisomerase

DNA polymerase

DNA primase

DNA gyrase

Correct answer:

DNA ligase

Explanation:

Because the lagging strand is created in various separate segments during DNA replication, after polymerization they must be joined together by an enzyme. The enzyme that is responsible for these connections is DNA ligase.

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