DNA Replication

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Biochemistry › DNA Replication

Questions 1 - 10
1

Which statement is true of prokaryotic DNA replication?

It takes less time to complete than eukaryotic replication

It involves multiple origins of replication

It occurs in the nucleus

It occurs on free ribosomes in the cytoplasm

It involves replication of linear chromosomes

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.

2

Given the DNA sequence, what is the correct synthesis?

DNA sequence: TCGGTCAAG

5' CTTGACCGA

5' AGCCAGTTC

3' GATTGACCT

5' TCCAGTTAG

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’.

3

What type of bonds hold the two DNA strands together?

Hydrogen

Disulfide bonds

van der Waals

Peptide bonds

Ionic bonds

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.

4

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

I. Lysine

II. Aspartate

III. Glutamine

IV. Glycine

II, III, and IV

II and IV

I, II, and III

I, II, III, and IV

II and III

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.

5

In DNA replication, which is true regarding ribonucleotide reductase?

All of these

Ribonucleotide reductase is inhibited by hydroxyurea

Ribonucleotide reductase regulates the rate of DNA synthesis

Ribonucleotide reductase converts ribonucleotide diphosphate to deoxyribonucleotide diphosphate

Deoxythymidine diphosphate is not a substrate for ribonucleotide reductase

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.

6

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

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.

7

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

None of these

III only

I and II

I and III

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.

8

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?

Fluorine

Nitrogen

Oxygen

All of these are involved in the bond

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.

9

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

DNA ligase

DNA primase

DNA topoisomerase

DNA polymerase

DNA gyrase

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.

10

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.

Separates the two parental strands of DNA by breaking hydrogen bonds

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

Stitches together the various daughter DNA fragments into a single strand

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

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.

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