07 Nucleic acids HL

Topic 7: Nucleic acids (Higher level)

This page lists the understandings and skills expected for topic two. 
Detailed revison notes, activities and questions can be found on each of the sub-topic pages

  • 7.1 DNA structure and replication
  • 7.2 Transcription and gene expression
  • 7.3 Translation

7.1 DNA structure and replication

IB Biology: DNA structure - Extras for HL studentsDNA 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.


  • Say how Rosalind Franklin’s and Maurice Wilkins’ X-ray diffraction work gave insights into the structue of DNA.
  • Know that the Sanger method of base sequencing uses nucleotides containing dideoxyribonucleic acid (DNA with deoxyribose missing 2 oxygen molecules) that stops DNA replication at a specific base.
    It allows sequencing using fluorescent markers and computers. (Sanger chain termination. Video here)
  • Say that in DNA profiling Tandem repeats are used as these vary greatly from person to person.
  • Analyse results of the Hershey and Chase experiment providing evidence that DNA is the genetic material. .Graphic
  • Use of molecular visualization software to analyse the association between protein and DNA within a nucleosome

7.2 Transcription

  • The direction of Transcription is in a 5’ to 3’ direction as RNA polymerase adds the 5´ end (phosphate) of the free RNA nucleotide to the 3´ end of the growing mRNA molecule.
  • Transcription is partly regulated by Nucleosomes in eukaryotes.
  • Eukaryotes modify mRNA after transcription.
  • Splicing of mRNA increases the number of different proteins an organism can produce.
  • Gene expression is regulated by proteins that bind to specific base sequences in DNA. - eg. methylation
  • Gene expression is affected by the environment of a cell and of an organism.


  • Awareness that the promoter region is an example of non-coding DNA.
  • The skill to analyse changes in the DNA methylation patterns in connection with gene expression

7.3 Translation

Three stages of translation

  • Initiation is the assembly of the components (large and small ribosome subunits, mRNA and tRNA molecules) that carry out the process.
  • Synthesis of the polypeptide involves a repeated cycle on a ribosome where tRNA binds to the A (aminoacyl), P (peptidyl) and E (exit) sites in turn. Polypeptide molecule is produced.
  • (examples of start and stop codons not needed)
  • Termination of translation is followed by disassembly of the components.


  • Free ribosomes synthesize proteins for use primarily within the cell.
  • Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
  • Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.
  • The sequence and number of amino acids in the polypeptide is the primary structure.
  • The secondary structure is the formation of alpha helices and beta pleated sheets stabilised by hydrogen bonding.
  • The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups. (Polar and non-polar amino acids are relevant to the bonds formed between R groups.)
  • The quaternary structure exists in proteins with more than one polypeptide chain. and may involve the binding of a prosthetic group to form a conjugated protein.