Nucleic acids 7.1 HL

The basics of DNA structure and DNA replication were covered in the SL topic so this topic looks at some extra details including, the 3' and 5' ends of the two antiparallel strands and further details about the enzymes responsible for DNA replication and some of the DNA regions which don't code for proteins but that regulate gene expression, form introns, telomeres or genes for tRNA. To understand how modern sequencing machines work a clear understanding of PCR and gel electrophoresis is also required.

Key concepts

Learn and test your biological vocabulary for 7.1 Nucleic acids using these flashcards.


These slides summarise the essential understanding and skills in this topic. 
They contain short explanations in text and images - good revision for all students.

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Summary list for topic 7.3 Translation

DNA structure

  • Part of DNA supercoiling are structures called Nucleosomes.
  • DNA structure gives a clue to the mechanism of DNA replication.
  • Non-coding regions of DNA have other important functions, limited to regulators of gene expression, introns, telomeres and genes for tRNAs.

DNA replication (in prokaryotes only)

  • DNA polymerase enzymes can only add nucleotides to the 3’ end of a primer.
  • Continuous DNA replication occurs on the leading strand and discontinuous on the lagging strand.
  • A complex group of enzymes do DNA replication including; helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III.

Crossing over

  • DNA replication makes a second chromatid in each chromosome in interphase before meiosis.
  • Crossing over exchanges pieces of DNA between non-sister homologous chromatids and forms new combinations of alleles on the chromosomes formed in meiosis.

Skills (can you ...)

  • See that Rosalind Franklin’s and Maurice Wilkins’ X-ray diffraction work gave evidence for helix and two strands in DNA structure.
  • Awareness that the Sanger method of base sequencing uses nucleotides containing dideoxyribonucleic acid (DNA with deoxyribose missing 2 oxygen molecules) to stop DNA replication at a specific base which allows sequencing using fluorescent markers and computers. (Sanger chain termination. Video here).
  • Awareness that in DNA profiling Tandem repeats are used as these vary greatly from person to person.
  • Ability to analyse of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.(This is a nice graphic).
  • Analyse of molecular visualisations of the association between protein and DNA in a nucleosome.


Exam style questions

How to prepare to answer a difficult IB question about DNA replication.
From your revision notes create between 8 and 10 quick questions.
Test yourself.  Can you remember the answers?

1. What does helicase do?
2. Which proteins keep the DNA strands separate?
3. What is the role of the enzyme primase?
4. How is DNA polymerase III different from primase?
5. Which direction does DNA polymerase III along the template stand?
6. What is the leading strand?
7. What is the lagging strand?
8. Why are okazaki fragments produced on the lagging strand?
9. What is the role of the enzyme DNA polymerase I ?
10. What is the role of DNA ligase enzyme?

Model answers

Extra exam question

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Test yourself

This quiz contains multiple choice questions covering the understandings and skills in this topic.


Just for fun

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