Biology: Genetics: DNA Replication and Repair
How cells copy DNA accurately and fix damage
Biology: Genetics: DNA Replication and Repair
How cells copy DNA accurately and fix damage
Biology - Grade 9-12
- 1
Explain what it means that DNA replication is semiconservative.
Think about what happens to the two strands of the original double helix.
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. - 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.
Match each base with its complement, then keep the new strand antiparallel to the template.
The complementary DNA strand is 5'-ATG CCT GAA TTC-3'. Adenine pairs with thymine, and cytosine pairs with guanine. - 3
Describe the role of helicase during DNA replication.
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. - 4
Why is primase necessary for DNA replication?
Focus on what DNA polymerase needs before it can add nucleotides.
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. - 5
Compare the leading strand and the lagging strand during DNA replication.
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. - 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.
Use the terms 5' to 3' and 3' end in your 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. - 7
What is the function of DNA ligase during replication?
DNA ligase seals gaps in the sugar-phosphate backbone of DNA. On the lagging strand, it joins Okazaki fragments into one continuous DNA strand. - 8
A student says, "DNA replication is accurate only because base-pairing rules prevent all mistakes." Evaluate this statement.
Mention at least two reasons replication is accurate.
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. - 9
Describe how proofreading by DNA polymerase helps reduce mutations.
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. - 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.
Think about how a change in DNA shape could affect enzymes that read or copy DNA.
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. - 11
Summarize the basic steps of nucleotide excision repair.
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. - 12
How does mismatch repair differ from nucleotide excision repair?
Compare the type of problem each repair system fixes.
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. - 13
Explain why telomeres are important in eukaryotic chromosomes.
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. - 14
In many human body cells, telomeres shorten after repeated cell divisions. Explain one possible effect of telomere shortening.
Connect telomere length to the ability of a cell to keep dividing.
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. - 15
A mutation occurs in a gene that codes for a DNA repair enzyme. Predict how this could affect the cell over time.
Focus on what happens when errors are not corrected.
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.