DNA replication
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of DNA structure. You can test your knowledge on this below.
Draw a DNA nucleotide (2 marks).
Describe the structure of DNA (4 marks).
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DNA is a polymer of nucleotides
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Each nucleotide is made from a phosphate group, deoxyribose sugar and a nitrogenous base
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DNA is double stranded with hydrogen bonds between complementary base pairs
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Nucleotides are joined by phosphodiester bonds
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(DNA has a double helix structure)
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(The two strands in DNA are antiparallel)
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains DNA replication or read the full notes below. Once you've gone through the whole note, try out the practice questions!
During which stage of the cell cycle is DNA replicated?
DNA is replicated during interphase of the cell cycle.
Remember, there are three phases in the cell cycle:
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Interphase - DNA is replicated in the Synthesis (S) phase of interphase.
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Mitosis (nuclear division) - chromosomes are pulled to opposite poles of the cell and two new nuclei form
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Cytokinesis (cell division) - the cytoplasm and the cell membrane divide to form 2 genetically identical daughter cells.
DNA Replication
There are two key enzymes used in DNA replication:
1. DNA helicase
2. DNA polymerase
DNA Replication - the process explained
1. DNA helicase breaks hydrogen bonds between the complementary bases.
2. This separates the two strands.
3. Both strands then act as templates.
4. Free nucleotides line up with the template strands according to the base pair rule.
5. Adenine pairs with thymine and cytosine pairs with guanine.
6. DNA polymerase joins adjacent nucleotides together by making phosphodiester bonds in condensation reactions. This requires ATP.
7. DNA replication is described as semi-conservative replication. This is because the DNA molecules formed each contain 1 original strand and 1 new strand.
Key ideas in DNA replication
1. What is the job of DNA Helicase?
It breaks the hydrogen bonds between the bases to separate the two strands.
2. What is the job of DNA polymerase?
It joins adjacent nucleotides together by making phosphodiester bonds.
3. If an inhibitor of DNA polymerase were introduced into a cell, what would be the effect on cell division?
Cell division would stop as DNA replication cannot occur.
This is because DNA polymerase cannot join nucleotides together and the new DNA strands will not form.
4. Why does DNA polymerase move in opposite directions along the two strands?
The two strands in DNA are antiparallel (they run in opposite directions). DNA polymerase is an enzyme with a specific tertiary structure and a specifically shaped active site. It can only attach new nucleotides at the 3’ end of a DNA nucleotide. This means it can only move in the 5’ to 3’ direction.
Maths Skill
Scientists determined that a sample of DNA contained 18% adenine.
You can use this information to calculate the percentage of the other bases in the DNA.
For example, if the % of adenine is 18% we know that the % of thymine must also be 18%. This is because adenine always pairs with thymine due to complementary base pairing.
We also know that the % of all 4 bases in DNA must equal 100%
100% - (18 + 18) = 64%
This 64% must account for both cytosine and guanine.
These bases will be present in equal quantities as guanine always pairs with cytosine.
Therefore, cytosine must equal 32% and guanine must equal 32%
No answer provided.
Key Terms
- Cell cycle: A series of stages a cell will go through in order to divide and produce two new cells.
- DNA helicase: an enzyme used in DNA replication. It breaks the hydrogen bonds between complementary bases to separate the two strands.
- DNA polymerase: an enzymes used in DNA replication. It joins adjacent nucleotides together by making phosphodiester bonds in condensation reactions. This requires ATP.
- Condensation reactions: making a bond by removing water.
- Semi-conservative replication: The DNA molecules made in DNA replication will contain one of the original, parent strands and one newly synthesised strand.
- DNA nucleotide: the monomer of DNA.
No answer provided.
Exam Tip
Remember, DNA polymerase makes phosphodiester bonds between adjacent nucleotides. It does not make hydrogen bonds between complementary bases; this does not require an enzyme!
No answer provided.
Explain how DNA replicates (4 marks).
1. DNA helicase causes strands to separate / breaks H-bonds between complementary base pairs.
2. Both strands act as templates.
3. (Free) nucleotides attach.
4. Complementary / specific base pairing / AT and GC.
5. DNA polymerase joins nucleotides together (on new strand).
6. By making phosphodiester bonds.
7. Semi-conservative replication / new DNA molecules contain one old strand and one new strand.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!