Protein Synthesis: notes and practice questions
- This topic covers the processes of transcription and translation, which together synthesize proteins from a DNA template.
- Transcription involves RNA polymerase synthesizing messenger RNA (mRNA) from a DNA template, with adenine (A) on DNA pairing with uracil (U) on RNA.
- DNA templates remain stable and unchanged during transcription, ensuring genetic information is conserved and gene expression can be switched on or off.
- Translation synthesizes polypeptides from mRNA, involving mRNA, ribosomes, and transfer RNA (tRNA).
- The genetic code is a triplet code, degenerate, and universal, allowing amino acid sequences to be deduced from mRNA codons.
- Ribosomes move along mRNA, linking amino acids by peptide bonds to form a growing polypeptide chain.
- Gene mutations, such as substitutions, insertions, or deletions, can alter protein structure.
How it is examined
Codon-table deduction is a reliable 1 to 2 mark item and students lose it by reading the template strand instead of the mRNA, or by reading the table in the wrong order. Transcription and translation outline items are marked point by point in sequence. At HL, post-transcriptional modification is asked as explain why mature mRNA differs from the primary transcript, wanting introns removed, exons spliced, cap and tail added.
- D1.2.1 Transcription as the synthesis of RNA using a DNA template, including the roles of RNA polymerase.
- D1.2.2 Role of hydrogen bonding and complementary base pairing in transcription, including the pairing of adenine (A) on the DNA template strand with uracil (U) on the RNA strand.
- D1.2.3 Stability of DNA templates. Single DNA strands can be used as a template without the DNA base sequence changing, and in somatic cells that do not divide such sequences must be conserved for the life of the cell.
- D1.2.4 Transcription as a process required for the expression of genes, limited to understanding that not all genes are expressed at any given time and that transcription, being the first stage, is a key point at which expression can be switched on and off.
- D1.2.12 Directionality of transcription and translation, and what 5' to 3' transcription and 5' to 3' translation mean.
- D1.2.13 Initiation of transcription at the promoter, considering transcription factors that bind to the promoter.
- D1.2.14 Non-coding sequences in DNA that do not code for polypeptides, limited to regulators of gene expression, introns, telomeres, and genes for rRNAs and tRNAs in eukaryotes.
- D1.2.15 Post-transcriptional modification in eukaryotic cells, including removal of introns and splicing of exons to form mature mRNA, and addition of 5' caps and 3' polyA tails to stabilize transcripts.
Guiding questions
- How does a cell produce a sequence of amino acids from a sequence of DNA bases?
- How is the reliability of protein synthesis ensured?
Linking questions
- How does the diversity of proteins produced contribute to the functioning of a cell?
- What biological processes depend on hydrogen bonding?
Practice questions
8 questions · 2 easy · 4 medium · 2 hardQuestion 1
EasyPaper 1A · calculator1 markWhich events occur during translation?
I. Formation of peptide bonds between amino acids
II. Binding of transfer RNA anticodons to complementary codons
III. Unwinding of the DNA double helix
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Remember that translation is the second step of gene expression, occurring at the ribosome, where mRNA is used to build a protein. Think about whether DNA is directly involved in this step.
Question 2
MediumPaper 1A · calculator1 markWhich of the following events occur during transcription in eukaryotes?
I. RNA polymerase binds to a promoter region on the DNA.
II. The entire gene, including introns and exons, is transcribed into pre-mRNA.
III. The new RNA strand is synthesized in a 3' to 5' direction.
A. I only
B. I and II only
C. II and III only
D. I, II and III
Review the key steps of transcription. Consider where the process starts, what the initial product is, and the direction in which new nucleic acids are always synthesized.
Question 3
HardPaper 2 · calculator15 marksProteins are highly diverse macromolecules that perform a wide range of functions in living organisms, including acting as biological catalysts.
Outline how the sequence of amino acids in a polypeptide is determined and how this leads to the vast diversity of proteins.
Enzymes are globular proteins that act as biological catalysts. Outline the effects of temperature and pH on the rate of enzyme-catalyzed reactions.
Explain the mechanism of enzyme action and how metabolic pathways can be controlled by feedback inhibition, using the synthesis of isoleucine as an example.
Think about what codes for the amino acid sequence during translation and how many different amino acids exist to build these chains.
Describe what happens to enzyme activity as temperature and pH increase and decrease, and explain why these changes occur at a molecular level.
Explain how enzymes lower activation energy and use the induced-fit model. Then describe how the end product of a pathway can inhibit an earlier step, specifically mentioning isoleucine and threonine.
Question 4
EasyPaper 1A · calculator1 markThe table shows the genetic code.
| 1st base in codon | 2nd base: U | 2nd base: C | 2nd base: A | 2nd base: G | 3rd base in codon |
|---|---|---|---|---|---|
| U | Phe | Ser | Tyr | Cys | U |
| U | Phe | Ser | Tyr | Cys | C |
| U | Leu | Ser | STOP | STOP | A |
| U | Leu | Ser | STOP | Trp | G |
| C | Leu | Pro | His | Arg | U |
| C | Leu | Pro | His | Arg | C |
| C | Leu | Pro | Gln | Arg | A |
| C | Leu | Pro | Gln | Arg | G |
| A | Ile | Thr | Asn | Ser | U |
| A | Ile | Thr | Asn | Ser | C |
| A | Ile | Thr | Lys | Arg | A |
| A | Met | Thr | Lys | Arg | G |
| G | Val | Ala | Asp | Gly | U |
| G | Val | Ala | Asp | Gly | C |
| G | Val | Ala | Glu | Gly | A |
| G | Val | Ala | Glu | Gly | G |
Which mRNA could code for the sequence Ala-Asn-Ser-Tyr?
A. CGA AAC UCG UAU
B. GCA AAU UGG UAU
C. GCA AAC UCG UAU
D. GCC AAC AGC UAA
Use the table to find the mRNA codon for each amino acid in the sequence one by one. Pay close attention to the row for the first base and the column for the second base.
Question 5
MediumPaper 1A · calculator1 markWhich of the following are features of the genetic code?
I. It is degenerate, meaning multiple codons can specify the same amino acid.
II. It is universal, meaning it is nearly the same in all organisms.
III. It is read in overlapping triplets of bases.
A. I only
B. I and II only
C. II and III only
D. I, II and III
Review the fundamental properties of the genetic code. What does 'degenerate' mean? Is the code the same across different species? How does the ribosome move along the mRNA to read the codons?
Question 6
HardPaper 2 · calculator15 marksEukaryotic cells are characterized by the presence of membrane-bound organelles, which compartmentalize various biochemical processes.
(a) Compare and contrast the structure of chloroplasts and mitochondria.
(b) Describe the advantages of compartmentalization in eukaryotic cells.
(c) Explain the role of compartmentalization in the synthesis, modification, and transport of proteins destined for secretion.
Think about the membranes, genetic material, and internal structures of both organelles. Remember to provide both similarities and differences.
Consider how having separate 'rooms' in a cell might help with efficiency, optimal conditions, and protecting the cell from its own enzymes.
Trace the pathway of a protein from the DNA code in the nucleus to its release outside the cell. Mention the specific organelles and vesicles involved.
Question 7
MediumPaper 1A · calculator1 markWhat is a consequence of the genetic code being degenerate?
A. A single codon can code for more than one type of amino acid.
B. A base substitution mutation may not alter the polypeptide produced.
C. The same codons are translated into the same amino acids in almost all organisms.
D. A single tRNA molecule can bind to multiple different amino acids.
Recall the definition of 'degenerate' in the context of the genetic code. Does one codon code for many amino acids, or do many codons code for one amino acid?
Question 8
MediumPaper 1A · calculator1 markThe table shows four different mRNA sequences and their corresponding amino acid sequences.
Which sequence provides evidence for the degeneracy of the genetic code?
| mRNA sequence | Amino acids | |
|---|---|---|
| A. | UAU UGU UAU | Tyr Cys Tyr |
| B. | UAU UAC UGU | Tyr Tyr Cys |
| C. | UAU UGG UCG | Tyr Trp Ser |
| D. | UAU CAU CGU | Tyr His Arg |
Recall the definition of 'degeneracy' in the genetic code: it means that multiple different codons can code for the same amino acid.
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Where marks are lost
- Command terms are consistently under-read. Students answer describe when the question said explain, so they give an account with no reasons and cap at half marks. The reverse also happens on outline, where a student writes an essay for a 2-mark summary and runs out of time.
- compare and contrast answers give only similarities, or only differences.
