Biochemistry : DNA Replication

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Dna Replication

Which of the following are the organic reactants used in DNA polymerization?

Possible Answers:

NTPs

dNTPs

Amino acids

NMPs

dNMPs

Correct answer:

dNTPs

Explanation:

The monomers from which DNA is polymerized are deoxyribonucleoside triphosphates (dNTPs). When DNA is in its polymerized form, the monomers are deoxyribonucleoside monophospates (dNMPs). This means that each nucleotide that is layed down by DNA polymerase must first have two of its phosphates hydrolyzed (beta and gamma). It is this hydrolysis that drives the nonspontaneous reaction of DNA polymerization.

Example Question #2 : Dna Replication

Which statement is true of prokaryotic DNA replication?

Possible Answers:

It occurs on free ribosomes in the cytoplasm

It involves replication of linear chromosomes

It involves multiple origins of replication

It takes less time to complete than eukaryotic replication

It occurs in the nucleus

Correct answer:

It takes less time to complete than eukaryotic replication

Explanation:

Prokaryotic DNA replication occurs in the cytoplasm, since these cells lack nuclei. Prokaryotic genomes are comprised of a single circular chromosome, with one origin of replication. Translation is the process of protein synthesis, which occurs on ribosomes free in the cytosol (or on ribosomes embedded in the rough endoplasmic reticulum in eukaryotes).

The only true statement is that prokaryotic DNA replication is faster than eukaryotic DNA replication.

Example Question #3 : Nucleic Acid Synthesis

Which of the following are true?

Possible Answers:

DNA polymerases require a primer for DNA synthesis

DNA polymerase III is also called primase

RNA polymerases require a primer

RNA polymerases take on a major role in DNA repair

The RNA polymerase that synthesizes rRNA also synthesizes mRNA

Correct answer:

DNA polymerases require a primer for DNA synthesis

Explanation:

Primase is actually an RNA polymerase, not a DNA polymerase. Primase creates an RNA primer which is used to replicate short-stranded DNA. Primers serve as the starting point for DNA synthesis, so RNA polymerases wouldn’t require them. There are a number of means by which DNA is repaired including direct repair, excision repair, and homologous recombination. DNA repair, however, does not involve the use of RNA polymerases. All rRNA (except 5S rRNA) is synthesized by RNA polymerase I (or to be specific, the polymerase creates a pre-RNA which matures into rRNA), while the precursors of mRNA are synthesized by RNA polymerase II. Primers are short, complementary RNA sequences that serve as the starting point for DNA synthesis; the DNA polymerase begins replication at the primer’s 3’ end, and uses the opposite strand as a template. Without the primer, the DNA polymerase would not have an existing strand of nucleotides onto which it could attach new nucleotides.

Example Question #4 : Nucleic Acid Synthesis

Suppose that a molecule of DNA has an  ratio of 2:1. Once this DNA molecule replicates, what will the new  ratio be?

Possible Answers:

Correct answer:

Explanation:

To answer this question, it's important to understand that DNA replicates in a semi-conservative fashion. This means that the two complementary strands of DNA split apart, and a new complementary strand is added to each of the parent strands. Thus, each daughter DNA molecule will be composed of one parent strand, and one newly synthesized strand. Since we know that adenine base pairs with thymine, and guanine base pairs with cytosine, the ratio of  is expected to remain the same, provided no mutations occur.

Example Question #3 : Dna Replication

DNA replication involves breaking which types of bonds that allow the enzyme helicase, to unwind the strands?

Possible Answers:

Non-covalent bonds

Covalent bonds

Peptide bonds

Hydrogen bonds

Van der Waals interactions

Correct answer:

Hydrogen bonds

Explanation:

DNA bases are joined together by hydrogen bonds. Adenine and thymine are bound by two hydrogen bonds, and cytosine and guanine are bound by three hydrogen bonds. These bases make up the "rungs" of the twisted ladder that is DNA. Therefore, if the bases need to be separated, hydrogen bonds must be broken to separate the two strands. Also, remember that these bases need to be rejoined, so the bonds between them need not be as strong as covalent bonds since they are continuously broken and reformed.

Example Question #6 : Nucleic Acid Synthesis

Which of the following is true regarding DNA replication?

I. Upon completion of DNA replication, the parent strands are re-hybridized

II. Epigenetic changes can change the rate of DNA replication

III. There are two daughter strands produced for every parent strand

Possible Answers:

I and III

I and II

III only

None of these

Correct answer:

None of these

Explanation:

DNA replication is the process of producing a duplicate copy of a DNA strand. DNA double helix is first unwound by breaking the hydrogen bonds between nitrogenous bases, giving two parent strands. Next, these unwound DNA strands are utilized as a template strand (parent strand) to create a daughter strand that is identical to the parent strand. After completion of the replication, the parent strand and daughter strand hybridize (hydrogen bonds re-form between bases) and form a double helix. Note that the original parent strands never re-hybridize.

Epigenetic changes refer to alterations in DNA molecules or histones. These alterations can enhance or suppress transcription of DNA to RNA. DNA replication is unaffected by epigenetic changes.

As mentioned, each parent strand produces an identical, daughter strand that ultimately re-hybridizes with the parent strand (forms double helix structure); therefore, each parent strand only produces one daughter strand.

Example Question #1 : Dna Replication

One of the first steps in DNA replication is the unwinding of the double helix. This is accomplished by an enzyme called DNA helicase. What atom will not be involved in a bond broken by DNA helicase?

Possible Answers:

Nitrogen

Oxygen

Fluorine

All of these are involved in the bond

Correct answer:

Fluorine

Explanation:

Unwinding of the double helix involves breaking the hydrogen bonds between nitrogenous bases from adjacent DNA molecules. Recall that hydrogen bonds occur between a hydrogen atom and either a nitrogen, oxygen, or fluorine atom. The nitrogenous bases found in DNA and RNA molecules do not contain any fluorine atoms; therefore, fluorine (although it is involved in hydrogen bonds in other molecules) is not involved in hydrogen bonding between nitrogenous bases.

Example Question #8 : Nucleic Acid Synthesis

DNA replication occurs between __________ phase and the __________ phase of the cell cycle.

Possible Answers:

mitosis . . . G0

G2 . . . mitosis

G1 . . . G2

G0 . . . G1

Correct answer:

G1 . . . G2

Explanation:

Cell cycle has four main phases: G1, S, G2 phases, and mitosis. DNA replication occurs during the S phase of the cell cycle, which occurs between G1 and G2 phase; therefore, DNA replication occurs between G1 and G2 phases.

Note that there is a cell checkpoint before the beginning of S phase (at the end of G1 phase) to ensure that the DNA molecules in the cell’s nucleus are prepared and stable for DNA replication. If it fails this checkpoint, the cell stays in the G1 phase until the DNA is ready for replication.

Example Question #9 : Nucleic Acid Synthesis

What type of bonds hold the two DNA strands together?

Possible Answers:

Hydrogen

Peptide bonds

Disulfide bonds

Ionic bonds

van der Waals

Correct answer:

Hydrogen

Explanation:

Hydrogen bonds are found between two strands of DNA (between nitrogenous bases). Peptide bonds and disulfide bonds are found in proteins. Ionic bonds are not found in between DNA strands, and van der Waals interactions are too weak to hold two DNA strands together.

Example Question #10 : Nucleic Acid Synthesis

Which of the following describes the primary function of primase?

Possible Answers:

Excises the primer that was used to initialize DNA synthesis 

Creates a free 5' hydroxyl group on the end of the DNA strand which allows it to be elongated

Primes for transcription by creating promoter regions in the DNA strand

Primes translation by creating a promoter region on mRNA

Allows for DNA synthesis by creating a starting point with a free 3' hydroxyl group

Correct answer:

Allows for DNA synthesis by creating a starting point with a free 3' hydroxyl group

Explanation:

Primase acts to initialize DNA synthesis. DNA synthesis requires a free 3' hydroxyl group bound to the template strand to begin. This is accomplished by having a temporary strand of RNA called the primer attached to the template DNA - it is created by primase.  It must be the 3' hydroxyl end that is free because DNA synthesis occurs in the 5' to 3' direction, and that 3' hydroxyl group is the substrate for DNA polymerase.

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