Written for this site from the course textbook and class slides, with page references for each statement. The chapter is on pages 602–610 of the course textbook. Image credits are on slide 19.
Replication produces copies with identical base sequences. It happens in the S phase, before a cell divides. It is needed for reproduction, so that offspring inherit base sequences, and for growth and tissue replacement in multicellular organisms, so that every new cell has a full set of genes.
Watch out: replication and cell division are separate events. DNA is copied first, and the cell divides later.
The two strands separate and each acts as a template. Each new molecule has one old strand and one new strand, so replication is semi-conservative.
Adenine pairs only with thymine, and cytosine only with guanine. Hydrogen bonds form only between complementary bases, so the template decides which nucleotide joins next. This is why copying is so accurate.
| Enzyme | Role |
|---|---|
| Helicase | Unwinds the double helix and separates the strands, by breaking hydrogen bonds between bases |
| DNA polymerase | Links free nucleotides into a new strand, using the old strand as a template, by forming covalent bonds |
The polymerase chain reaction copies a chosen section of DNA. Each cycle doubles the DNA, so 30 cycles give over a billion copies.
| Step | Temperature | What happens |
|---|---|---|
| Denaturation | About 95°C | Strands separate |
| Annealing | About 40°C | Primers, which are short single strands of DNA, attach to mark the section |
| Elongation | About 72°C | Taq polymerase builds new strands from the primers |
Taq polymerase comes from a bacterium living in hot springs, so it is not denatured at 95°C.
In gel electrophoresis, DNA is placed in wells and an electric current is applied. Negatively charged phosphate groups make DNA move towards the positive electrode. The gel acts as a sieve, so short fragments travel furthest. A DNA ladder of known lengths allows comparison.
Watch out: charge makes DNA move, but length is what separates the fragments.
Some non-coding regions contain short tandem repeats. The number of repeats varies between people, so fragment lengths vary too. Many loci are compared, as a match at all of them is very unlikely by chance.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all five statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Paper strips or sets of coloured beads for bases, two colours of backbone
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
Meselson and Stahl used heavy nitrogen in 1958 to separate three models of replication, supporting the semi-conservative one.
The design made each hypothesis predict a different result. Consider whether a simple, decisive design is a kind of beauty, and whether beauty matters.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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