7.1.1

Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines.

2

Major and minor grooves, direction of the “twist”, alternative B and Z forms, and details of the dimensions are not required.

7.1.2

Outline the structure of nucleosomes.

2

Limit this to the fact that a nucleosome consists of DNA wrapped around eight histone proteins and held together by another histone protein.

7.1.3

State that nucleosomes help to supercoil chromosomes and help to regulate transcription.

1

7.1.4

Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.

2

Highly repetitive sequences (satellite DNA) constitutes 5–45% of the genome. The sequences are typically between 5 and 300 base pairs per repeat, and may be duplicated as many as 105 times per genome.

TOK: Highly repetitive sequences were once classified as “junk DNA”, showing a degree of confidence that it had no role. This addresses the question: To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?

7.1.5

State that eukaryotic genes can contain exons and introns.

1

7.2.1

State that DNA replication occurs in a direction.

1

The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesized.

7.2.2

Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates.

3

The explanation of Okazaki fragments in relation to the direction of DNA polymerase III action is required. DNA polymerase III adds nucleotides in the direction. DNA polymerase I excises the RNA primers and replaces them with DNA.

7.2.3

State that DNA replication is initiated at many points in eukaryotic chromosomes.

1

7.3.1

State that transcription is carried out in a direction.

1

The 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that is already synthesized.

7.3.2

Distinguish between the sense and antisense strands of DNA.

2

The sense strand (coding strand) has the same base sequence as mRNA with uracil instead of thymine. The antisense (template) strand is transcribed.

7.3.3

Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.

3

The following details are not required: there is more than one type of RNA polymerase; features of the promoter region; the need for transcription protein factors for RNA polymerase binding; TATA boxes (and other repetitive sequences); and the exact sequence of the bases that act as terminators.

7.3.4

State that eukaryotic RNA needs the removal of introns to form mature mRNA.

1

Further details of the process of post-transcriptional modification of RNA are not required.

7.4.1

Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy.

3

Each amino acid has a specific tRNA-activating enzyme (the name aminoacyl-tRNA synthetase is not required). The shape of tRNA and CCA at the 3’ end should be included.

7.4.2

Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites.

2

7.4.3

State that translation consists of initiation, elongation, translocation and termination.

1

7.4.4

State that translation occurs in a direction.

1

During translation, the ribosome moves along the mRNA towards the 3’ end. The start codon is nearer to the 5’ end.

7.4.5

Draw and label a diagram showing the structure of a peptide bond between two amino acids.

1

7.4.6

Explain the process of translation, including ribosomes, polysomes, start codons and stop codons.

3

Use of methionine for initiation, details of the T factor and recall of actual stop codons are not required.

7.4.7

State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.

1

7.5.1

Explain the four levels of protein structure, indicating the significance of each level.

3

Quaternary structure may involve the binding of a prosthetic group to form a conjugated protein.

Aim 7: Simulation exercises showing three-dimensional molecular models of proteins are available.

7.5.2

Outline the difference between fibrous and globular proteins, with reference to two examples of each protein type.

2

7.5.3

Explain the significance of polar and non-polar amino acids.

3

Limit this to controlling the position of proteins in membranes, creating hydrophilic channels through membranes, and the specificity of active sites in enzymes.

7.5.4

State four functions of proteins, giving a named example of each.

1

Membrane proteins should not be included.

7.6.1

State that metabolic pathways consist of chains and cycles of enzyme-catalysed reactions.

1

7.6.2

Describe the induced-fit model.

2

This is an extension of the lock-and-key model. Its importance in accounting for the ability of some enzymes to bind to several substrates should be mentioned.

TOK: Scientific truths are often pragmatic. We accept them as true because they give us predictive power, that is, they work. The German scientist Emil Fischer introduced the lock-and-key model for enzymes and their substrates in 1890. It was not until 1958 that Daniel Koshland in the United States suggested that the binding of the substrate to the active site caused a conformational change, hence the induced-fit model. This is an example of one model or theory, accepted for many years, being superseded by another that offers a fuller explanation of a process.

7.6.3

Explain that enzymes lower the activation energy of the chemical reactions that they catalyse.

3

Only exothermic reactions should be considered. Specific energy values do not need to be recalled.

7.6.4

Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

3

Competitive inhibition is the situation when an inhibiting molecule that is structurally similar to the substrate molecule binds to the active site, preventing substrate binding.

Limit non-competitive inhibition to an inhibitor binding to an enzyme (not to its active site) that causes a conformational change in its active site, resulting in a decrease in activity.

Reversible inhibition, as compared to irreversible inhibition, is not required.

7.6.5

Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites.

3