Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 604–614 of the course textbook. Image credits are on slide 11.
DNA polymerase joins the 5′ phosphate of an incoming nucleotide to the free 3′ -OH at the end of the growing strand, using energy from the nucleotide’s extra phosphates. So new strands grow only 5′ to 3′, and the polymerase needs an existing 3′ end, a primer, to start.
Because the strands are antiparallel, one new strand, the leading strand, is made continuously towards the fork from a single primer. The other, the lagging strand, is made discontinuously away from the fork as short Okazaki fragments, each needing its own primer.
In prokaryotes, helicase separates the strands; DNA primase makes short RNA primers; DNA polymerase III extends them with DNA nucleotides; DNA polymerase I removes the RNA primers and replaces them with DNA; and DNA ligase joins the fragments into a continuous strand.
If DNA polymerase III adds a nucleotide with a mismatched base, it removes it from the 3′ end and adds the correct one. Proofreading cuts the error rate about a hundredfold, keeping mutations rare. Taq polymerase, used in PCR, cannot proofread.
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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 four 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: Long paper strips marked 5' and 3', short primer strips, cards for each enzyme
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
In the 1960s, Reiji and Tsuneko Okazaki found short DNA fragments, which explained how a strand can be copied against its direction.
A contradiction between models can lead to discovery. Consider why anomalies can be more useful than confirming results.
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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