High School Biology : Understanding Replication Proteins

Study concepts, example questions & explanations for High School Biology

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Example Questions

Example Question #1 : Understanding Replication Proteins

Which of the following replication proteins is used to unwind the DNA double helix?

Possible Answers:

Primase

DNA polymerase

Helicase

DNA ligase

Correct answer:

Helicase

Explanation:

DNA helicase unwinds the double helix, separating the two strands so they may be replicated by DNA polymerase.

Primase adds an RNA primer to help initiate DNA replication. DNA ligase is responsible for joining Okazaki fragments on the lagging strand during replication.

Example Question #1 : Understanding Replication Proteins

Which of the following is true about DNA replication?

Possible Answers:

Helicase unwinds the DNA double helix

RNA polymerase proofreads the DNA daughter strand

DNA polymerase seals Okazaki fragments into one long string

DNA polymerase II splits the double helix into two separate sides

DNA ligase adds new nucleotides to each strand

Correct answer:

Helicase unwinds the DNA double helix

Explanation:

DNA replication is the process of copying the parent DNA helix into two identical daughter helices. The process is semi-conservative, which means that one parent strand is passed down to each daughter strand. The process begins when helicase unwinds the double helix and separates the two strands to create the replication fork. Topoisomerase helps this process by relieving rotational strain on the helix when it is being unwound. DNA polymerase adds new nucleotides to the daughter strand, synthesizing the new DNA strand.

During replication there is a leading strand, which occurs when replication occurs from 5' to 3' and moves towards the replication fork, and a lagging strand, when replication occurs away from the replication fork. Replication occurs in short segments on the lagging strand, known as Okazaki fragments. The protein DNA ligase is responsible for finally fusing these fragments together after they are made by DNA polymerase.

Example Question #2 : Dna, Rna, And Proteins

During DNA replication, an enzyme called DNA helicase "unzips" the molecule of double-stranded DNA. What is the most likely mechanism of DNA helicase?

Possible Answers:

DNA helicase breaks down the hydrogen bonds between purines and purines

DNA helicase breaks down the covalent bonds between the purines and pyrimidines

DNA helicase breaks down the covalent bonds between pyrimidines and pyrimidines

DNA helicase breaks down the hydrogen bonds between the purines and pyrimidines

Correct answer:

DNA helicase breaks down the hydrogen bonds between the purines and pyrimidines

Explanation:

The question states that DNA helicase "unzips" the two strands of DNA; therefore, this enzyme must be breaking down the bonds between base pairs.

The bonds between base pairs are called hydrogen bonds, which is a noncovalent bond. This means that the DNA helicase is breaking down the hydrogen bonds between base pairs in order to separate the two strands. In DNA, there are two kinds of base pairs: purines and pyrimidines. Recall that adenine and guanine are classified as purines whereas thymine and cytosine are classified as pyrimidines; therefore, a base pairing in DNA always occurs between a purine and a pyrimidine. This means that the DNA helicase is breaking down the hydrogen bonds between purines and pyrimidines.

Example Question #2 : Dna

Of the following DNA replication proteins, which one links the Okazaki fragments of the lagging strand?

Possible Answers:

Helicase

Single-strand binding proteins (SSB)

DNA polymerase I

DNA polymerase III

DNA ligase

Correct answer:

DNA ligase

Explanation:

DNA ligase is the protein responsible for linking, or ligating, Okazaki fragments together in order to form a single complete DNA strand. This action only necessary on the lagging strand; the leading strand can be made continuously by DNA polymerase since it is able to read away from the replication fork in the 3'-to5' direction. Since the DNA polymerase on the lagging strand must read toward the replication form, it cannot by synthesized continuously.

Example Question #2 : Understanding Replication Proteins

DNA is naturally found as a double-helix, but for it to replicated it must first be unwound so that DNA replication proteins can access the two strands. The double-helix structure of DNA is very stable, and after being unwound for DNA replication to occur, the two strands can easily return to the double-helix structure. If the strands re-anneal, proteins necessary for DNA replication cannot enter and begin the process of replication.

Which of the following pairs of DNA replication proteins is responsible for unwinding the DNA double-helix and maintaining the separation of the DNA strands?

Possible Answers:

Helicase and single-strand binding protein (SSB)

DNA polymerase and DNA ligase

Single-strand binding protein (SSB) and helicase

DNA ligase and helicase

DNA polymerase and helicase

Correct answer:

Helicase and single-strand binding protein (SSB)

Explanation:

Helicase is the protein resposible for unwinding the DNA double-helix. Single-strand binding proteins attach to the freshly unwound strands of DNA and ensure that the strands do not re-anneal. Helicase creates the replication fork opening, allowing replication proteins to enter and bind; single-strand binding proteins keep the replication fork open as proteins enter.

Example Question #4 : Dna, Rna, And Proteins

Which of the following causes a DNA fragment to be formed in the 5' to 3' direction?

Possible Answers:

Polarity causes DNA polymerase III can only add nucleotides to the 3' end of a DNA strand.

Hydrogen bonds prevent DNA polymerase III from adding nucleotides to a DNA strand

DNA polymerase III can add nucleotides to any random point on a DNA strand regardless of polarity

None of the choices

Polarity causes DNA polymerase III can only add nucleotides to the 5' end of a DNA strand

Correct answer:

Polarity causes DNA polymerase III can only add nucleotides to the 3' end of a DNA strand.

Explanation:

A DNA fragment will be formed in the 5' to 3' direction because of the polarity of the DNA molecule. Adding nucleotides to the 3' end allows DNA polymerase to use the phosphate molecules as "fuel," and add a new nucleotide to the DNA strand.

Example Question #5 : Dna, Rna, And Proteins

Which protein is responsible for the removal of the RNA primer from the 5' end of a lagging strand of DNA, and replacing it with DNA nucleotides?

Possible Answers:

Primase

DNA Polymerase III

None of the choices

Topoisomerase

DNA Polymerase I

Correct answer:

DNA Polymerase I

Explanation:

DNA Polymerase I removes the primer from the 5' end of a lagging strand, and replaces it with DNA nucleotides. This allows DNA synthesis to begin on the lagging strand. 

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