Biology: Genetics: DNA Replication and Repair Worksheet

Question 1

Explain what it means that DNA replication is semiconservative.

Accepted Answer

DNA replication is semiconservative because each new DNA molecule contains one original parent strand and one newly made strand. The original strands serve as templates for building the new complementary strands.

Question 2

A DNA template strand has the sequence 3'-TAC GGA CTT AAG-5'. Write the complementary DNA strand in the 5' to 3' direction.

Accepted Answer

The complementary DNA strand is 5'-ATG CCT GAA TTC-3'. Adenine pairs with thymine, and cytosine pairs with guanine.

Question 3

Describe the role of helicase during DNA replication.

Accepted Answer

Helicase unwinds and separates the two DNA strands by breaking the hydrogen bonds between paired bases. This creates a replication fork where other enzymes can copy the exposed template strands.

Question 4

Why is primase necessary for DNA replication?

Accepted Answer

Primase is necessary because DNA polymerase cannot start a new DNA strand by itself. Primase makes short RNA primers that give DNA polymerase a starting point with a free 3' end.

Question 5

Compare the leading strand and the lagging strand during DNA replication.

Accepted Answer

The leading strand is synthesized continuously toward the replication fork. The lagging strand is synthesized discontinuously away from the fork in short Okazaki fragments that must later be joined together.

Question 6

DNA polymerase can add new nucleotides only to the 3' end of a growing DNA strand. Explain how this affects the direction of DNA synthesis.

Accepted Answer

DNA synthesis always occurs in the 5' to 3' direction because each new nucleotide is added to the 3' end of the growing strand. This requirement causes one strand to be copied continuously and the other to be copied in fragments.

Question 7

What is the function of DNA ligase during replication?

Accepted Answer

DNA ligase seals gaps in the sugar-phosphate backbone of DNA. On the lagging strand, it joins Okazaki fragments into one continuous DNA strand.

Question 8

A student says, "DNA replication is accurate only because base-pairing rules prevent all mistakes." Evaluate this statement.

Accepted Answer

The statement is incomplete. Base-pairing rules help make replication accurate, but mistakes can still occur. DNA polymerase proofreading and DNA repair systems also correct many errors.

Question 9

Describe how proofreading by DNA polymerase helps reduce mutations.

Accepted Answer

During proofreading, DNA polymerase checks newly added nucleotides and removes many incorrectly paired bases. It then replaces the wrong nucleotide with the correct one, reducing the chance that the error becomes a mutation.

Question 10

Ultraviolet light can cause thymine dimers, where two nearby thymine bases bond to each other. Explain why thymine dimers can be harmful to a cell.

Accepted Answer

Thymine dimers distort the shape of DNA and can block DNA replication or transcription. If they are not repaired, they can lead to incorrect base pairing and mutations.

Question 11

Summarize the basic steps of nucleotide excision repair.

Accepted Answer

In nucleotide excision repair, enzymes recognize a damaged section of DNA, cut out a short segment containing the damage, and use the undamaged strand as a template to fill the gap. DNA ligase then seals the repaired strand.

Question 12

How does mismatch repair differ from nucleotide excision repair?

Accepted Answer

Mismatch repair fixes base-pairing errors that remain after replication, such as a G paired with T. Nucleotide excision repair removes bulky DNA damage, such as thymine dimers, by cutting out a short damaged section.

Question 13

Explain why telomeres are important in eukaryotic chromosomes.

Accepted Answer

Telomeres are repeated DNA sequences at the ends of eukaryotic chromosomes. They protect important genes from being lost during DNA replication and help prevent chromosome ends from being mistaken for broken DNA.

Question 14

In many human body cells, telomeres shorten after repeated cell divisions. Explain one possible effect of telomere shortening.

Accepted Answer

Telomere shortening can limit how many times a cell can divide. When telomeres become too short, the cell may stop dividing or enter cell death pathways, which helps prevent damaged cells from continuing to divide.

Question 15

A mutation occurs in a gene that codes for a DNA repair enzyme. Predict how this could affect the cell over time.

Accepted Answer

If a DNA repair enzyme does not work properly, DNA damage and replication errors may accumulate over time. This can increase the mutation rate and may raise the risk of abnormal cell function or cancer.

Biology: Genetics: DNA Replication and Repair

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