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Biology
Grade 9-12
DNA Replication Step-by-Step Cheat Sheet
A printable reference covering DNA helicase, primase, DNA polymerase, leading and lagging strands, Okazaki fragments, and ligase for grades 9-12.
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DNA replication is the process cells use to copy their DNA before cell division. This cheat sheet helps students follow the process step by step, from unzipping the double helix to proofreading the new strands. Understanding replication is important because it explains how genetic information is passed accurately from one cell to the next.
Key Facts
- DNA replication is semi-conservative, meaning each new DNA molecule contains one original strand and one newly made strand.
- Helicase breaks the hydrogen bonds between base pairs and separates the two DNA strands at the replication fork.
- Primase builds a short RNA primer that gives DNA polymerase a starting point for adding DNA nucleotides.
- DNA polymerase adds nucleotides only to the 3' end, so new DNA is always built in the 5' to 3' direction.
- The leading strand is built continuously toward the replication fork because its template runs 3' to 5'.
- The lagging strand is built discontinuously away from the replication fork in short Okazaki fragments.
- DNA ligase joins Okazaki fragments by sealing gaps in the sugar-phosphate backbone.
- Base-pairing rules are A pairs with T and C pairs with G, which allows each original strand to guide formation of a complementary strand.
Vocabulary
- DNA replication
- The process of copying a DNA molecule so a cell can pass genetic information to daughter cells.
- Replication fork
- The Y-shaped region where the DNA double helix is opened and new strands are made.
- Helicase
- The enzyme that unwinds DNA by breaking hydrogen bonds between complementary bases.
- DNA polymerase
- The enzyme that adds DNA nucleotides to a growing strand and helps proofread for errors.
- Okazaki fragments
- Short pieces of DNA made on the lagging strand during replication.
- DNA ligase
- The enzyme that joins DNA fragments by sealing the sugar-phosphate backbone.
Common Mistakes to Avoid
- Building the new strand in the wrong direction is incorrect because DNA polymerase can add nucleotides only to the 3' end, so synthesis must occur 5' to 3'.
- Saying both strands are copied continuously is wrong because only the leading strand is continuous, while the lagging strand is made in Okazaki fragments.
- Forgetting the RNA primer is a mistake because DNA polymerase cannot start a new strand by itself and needs a primer to begin.
- Matching A with C or G with T is incorrect because DNA base-pairing rules are A with T and C with G.
- Confusing helicase with ligase is wrong because helicase separates DNA strands, while ligase connects DNA fragments.
Practice Questions
- 1 A template strand reads 3'-TAC GGA CTT-5'. What is the complementary DNA strand built from it?
- 2 If a DNA molecule has 24% adenine, what percent thymine does it have, and what percent cytosine does it have?
- 3 List the correct order of these enzymes during replication: DNA ligase, helicase, primase, DNA polymerase.
- 4 Explain why the lagging strand must be made in fragments instead of as one continuous strand.