DNA replication is the process cells use to copy their genetic information before cell division. It matters because each new cell must receive a complete and accurate set of DNA instructions. The two strands of DNA separate, and each original strand serves as a template for building a new complementary strand.
This produces two DNA molecules that carry the same genetic code as the original.
Key Facts
- DNA replication is semiconservative: each new DNA molecule contains one original strand and one newly made strand.
- Base-pairing rules guide copying: A pairs with T, and C pairs with G.
- DNA polymerase adds nucleotides only to the 3' end, so new DNA is made in the 5' to 3' direction.
- Helicase unwinds and separates the DNA strands at the replication fork.
- The leading strand is copied continuously, while the lagging strand is copied in short Okazaki fragments.
- Ligase seals gaps between Okazaki fragments by forming phosphodiester bonds in the sugar-phosphate backbone.
Vocabulary
- Replication fork
- The Y-shaped region where the parental DNA strands are separated and copied.
- Helicase
- An enzyme that unwinds DNA by breaking hydrogen bonds between paired bases.
- DNA polymerase
- An enzyme that builds new DNA by adding complementary nucleotides to a growing strand.
- Okazaki fragment
- A short piece of newly made DNA formed on the lagging strand during replication.
- Semiconservative replication
- A copying method in which each daughter DNA molecule has one old strand and one new strand.
Common Mistakes to Avoid
- Saying both strands are copied continuously is wrong because DNA polymerase can only build in the 5' to 3' direction, so one strand must be made in fragments.
- Forgetting the role of primase is wrong because DNA polymerase cannot start a new DNA strand without an RNA primer.
- Thinking helicase builds new DNA is wrong because helicase only separates the parental strands, while DNA polymerase adds nucleotides.
- Mixing up base pairs is wrong because adenine pairs only with thymine in DNA, and cytosine pairs only with guanine.
Practice Questions
- 1 A DNA template strand has the sequence 3'-T A C G G A-5'. What complementary DNA strand will DNA polymerase build, including its direction?
- 2 A lagging strand region is copied as 8 Okazaki fragments. If each fragment needs one RNA primer, how many primers are needed for that region?
- 3 Explain why the leading strand and lagging strand are copied differently even though they are made by the same type of DNA polymerase.