Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 625–636 of the course textbook. Image credits are on slide 15.
Transcription
D1.2.12
Direction
pp. 625–626
RNA polymerase reads the template strand 3′ to 5′ and builds RNA 5′ to 3′, adding nucleotides to the free 3′ -OH. Ribosomes read mRNA 5′ to 3′, building the polypeptide from its amine end to its carboxyl end.
D1.2.13
Starting at the promoter
pp. 626–627
General transcription factors bind the promoter just before a gene; RNA polymerase then joins them. Activators bound to enhancers can loop round and raise the rate. Without the right factors, the gene is not transcribed. RNA polymerase itself is not a transcription factor.
D1.2.14
Non-coding DNA
pp. 627–628
Only about 1.5% of human DNA codes for polypeptides. Non-coding sequences include regulators of gene expression (promoters, enhancers), introns, telomeres that protect chromosome ends during replication, and genes for rRNA and tRNA, whose RNA is the final product.
Processing
D1.2.15
Post-transcriptional modification
pp. 628–629
In eukaryotes, introns are removed from pre-mRNA and exons spliced together; a 5′ cap and a 3′ poly-A tail are added, which protect the mRNA and help it leave the nucleus and bind ribosomes.
D1.2.16
Alternative splicing
pp. 629–630
Different combinations of exons can be kept, so one gene can give several versions of a protein. The troponin T gene is spliced differently in foetal and adult heart muscle.
Translation and after
D1.2.17
Starting translation
pp. 630–633
The small subunit binds the 5′ end of mRNA and moves to the start codon AUG; the initiator tRNA (methionine, anticodon UAC) pairs with it; the large subunit joins, with the initiator tRNA in the P site. A tRNA with the next amino acid enters the A site, a peptide bond forms, the ribosome moves one codon, and the empty tRNA leaves from the E site.
D1.2.18
Modifying polypeptides
pp. 633–635
Many polypeptides are modified before they work. Pre-proinsulin loses its signal sequence in the ER, giving proinsulin, which folds with three disulfide bonds; later the C-peptide is cut out, leaving insulin: A and B chains held by disulfide bonds.
D1.2.19
Proteasomes
pp. 635–636
Damaged or unneeded proteins are tagged with ubiquitin and broken down by proteasomes. The amino acids are reused, so a working proteome depends on constant breakdown as well as synthesis.
Videos
Select a video to play it here. Select it again to close it.
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.
A printable worksheet covering all eight 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.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 10 marks in total. Your teacher has the mark scheme.
Introns, exons and globin genes
10 marks
Eukaryotic genes contain exons, which are kept in mature mRNA, and introns, which are removed. The table shows data for five human genes.
Gene (protein)
Gene length / kb
Number of exons
Mature mRNA / kb
INS (insulin)
1.4
3
0.47
HBB (beta-globin)
1.6
3
0.63
PAH (phenylalanine hydroxylase)
79
13
2.4
COL1A1 (collagen)
18
51
4.8
DMD (dystrophin)
2200
79
14
Calculate the percentage of the HBB gene that ends up in its mature mRNA. [1]
Analyse the relationship between gene length and the number of exons. [2]
The coding sequence of PAH mRNA, from start codon to stop codon, is 1359 nucleotides. Determine the number of amino acids in phenylalanine hydroxylase. [1]
Haemoglobin is made of four globin chains, two of one type and two of another. The graph shows which globin chains are made before and after birth.
State which globin chains are made first after conception. [1]
Compare the expression of the gamma and beta genes. [3]
Deduce the chain composition of haemoglobin at 20 weeks after conception and at six months after birth. [2]
Practical
Modelling alternative splicing
Model25 minD1.2.15, D1.2.16
You will need: Paper strip printed with exons and introns, scissors, tape
Cut out the introns and join the exons
Add labels for the cap and tail
Make a second version that leaves out one exon
Discuss how one gene can code for more than one polypeptide
Safety: Take care with scissors.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Falsification
In 1977 Roberts and Sharp found that genes can be split. This overturned the simple one gene, one polypeptide picture.
Theory of knowledge
What happens when a simple rule meets its exceptions?
Consider when an exception should change a rule, and when it should only qualify it.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
D1.2.12The direction in which transcription and translation proceedp. 625
D1.2.13How transcription starts at the promoterp. 625
D1.2.14DNA sequences that do not code for polypeptidesp. 626
D1.2.15How RNA is modified after transcription in eukaryotesp. 627
D1.2.16Alternative splicing: how one gene can give several versions of a proteinp. 629
D1.2.17How translation startsp. 630
D1.2.18How polypeptides are modified to become functionalp. 635
