D1.2Theme D — Continuity and change
Protein synthesis — IB Biology SL Revision Notes & Topical Questions
5 min read 13 parts Lesson 3 of 14 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D1.2 Protein synthesis — IB BIOLOGY SL notes: Describe transcription and translation, the genetic code, and how a base substitution changes a polypeptide.Protein synthesis — two stages, transcription and translationTranscription = synthesis of an mRNA copy of a gene in the nucleu…
- Enzymes compared
- Base pairing
- Coding strand
- Template strand
- Non-dividing cells
- Gene expression
Part 1 of 11Transcription
Protein synthesis
- Protein synthesis two stages, transcription and translation
Transcription
Transcription synthesis of an mRNA copy of a gene in the nucleus using a DNA template
1
RNA polymerase binds at the start of a gene.
2
The DNA unwinds and the hydrogen bonds between the bases break, exposing the gene.
3
Free RNA nucleotides pair with the exposed bases of the template strand.
4
RNA polymerase joins the sugar-phosphate groups of the nucleotides into the mRNA backbone, building it in the 5′ to 3′ direction.
5
The hydrogen bonds between mRNA and DNA break and the DNA double helix re-forms.
6
The mRNA leaves the nucleus through a nuclear pore.
Why mRNA
- DNA is too large to pass through the nuclear pores, so a mobile copy carries the message to the cytoplasm
Enzymes compared
- RNA polymerase is used in transcription and DNA polymerase in replication
Part 2 of 11Template and coding strands
Base pairing
- A pairs with U in RNA, C pairs with G, and the RNA is complementary to the template strand
- Only one DNA strand is transcribed for a gene.
- There is no thymine in RNA.
Coding strand
- Coding strand the DNA strand with the same base sequence as the mRNA, with T in place of U
Template strand
- Template strand the other DNA strand, which is the one transcribed
Reuse
- Reuse the DNA stays intact, so the gene can be transcribed again and again
Part 3 of 11DNA stability and gene expression
Stability of DNA
- Many hydrogen bonds hold paired bases together.
- Strong phosphodiester bonds run along each backbone.
- The sequence rarely changes on its own, so DNA is a reliable template over many cell generations.
Non-dividing cells
- Non-dividing cells neurones and some muscle cells keep the same DNA for decades while still transcribing it
Gene expression
- Gene expression humans have about 20 000 protein-coding genes but each cell type expresses only the genes it needs
- Heart muscle does not make insulin.
- A switched-on gene is transcribed and translated and a switched-off gene is silenced.
- Transcription is the first major point of control.
Part 4 of 11Ribosomes and RNA in translation
Translation
Translation assembly of a polypeptide at a ribosome using the mRNA code, in the cytoplasm after transcription
Ribosome
- Ribosome a large and small subunit made of rRNA and protein
- mRNA binds to the small subunit.
- Two tRNAs bind to the large subunit at one time.
- An area on the large subunit catalyses peptide bond formation.
- Free ribosomes make proteins for use inside the cell and ribosomes bound to rough ER make proteins for export or membranes.
| Feature | mRNA | tRNA | rRNA |
|---|---|---|---|
| Role | Carries the copy of the gene | Delivers amino acids | Forms the ribosome and catalyses bonds |
| Shape | Linear single strand | Folded clover-leaf | Folded and bound to proteins |
| Key bases | Codons | Anticodon | None read as code |
| Lifetime | Short, broken down after use | Reused many times | Long-lasting |
Part 5 of 11Triplets, codons and anticodons
Triplet
Triplet three DNA bases that code for one amino acid
Codon
Codon three mRNA bases complementary to the template triplet
Anticodon
Anticodon three tRNA bases complementary to a codon
- Each tRNA carries only the amino acid matching its anticodon.
| Feature | DNA triplet | mRNA codon | tRNA anticodon |
|---|---|---|---|
| Molecule | DNA template strand | mRNA | tRNA |
| Example for methionine | TAC | AUG | UAC |
| Relationship | The original code | Complementary to the template triplet | Complementary to the codon |
- Start and stop
- AUG is the start codon for methionine, and UAA, UAG and UGA are stop codons that add no amino acid
Part 6 of 11Features of the genetic code
Features of the genetic code
Triplet code
- Triplet code three bases per amino acid, which gives 43 = 64 codons for 20 amino acids
Degenerate
- 61 codons code for amino acids and 3 are stops, so most amino acids have several codons
- Some base changes therefore have no effect.
Non-overlapping
- Non-overlapping each base belongs to only one codon
Universal
- Universal almost every organism uses the same code
- Genetic information can therefore be transferred between species.
- This is why genetic engineering is possible.
Part 7 of 11Steps of translation
1
The small ribosomal subunit binds to the mRNA.
2
The initiator tRNA pairs with the AUG start codon.
3
The large subunit joins to complete the ribosome.
4
A second tRNA pairs with the next codon.
5
A peptide bond forms by condensation between the two amino acids.
6
The ribosome moves along one codon, the empty tRNA leaves, and the process repeats.
7
At a stop codon the polypeptide is released and folds.
- Energy
- peptide bond formation is anabolic and needs ATP supplied by the mitochondria
| Feature | Transcription | Translation |
|---|---|---|
| Location in eukaryotes | Nucleus | Cytoplasm on free or bound ribosomes |
| Molecule read | DNA template strand | mRNA codons |
| Product | Single-stranded RNA | Polypeptide |
| Monomers joined | RNA nucleotides | Amino acids |
| Bond made | Phosphodiester | Peptide |
| Main catalyst | RNA polymerase | Ribosome (rRNA) |
Part 8 of 11Deducing an amino acid sequence
1From the coding strand
- From the coding strand the mRNA matches it with U in place of T
2From the template strand
- From the template strand write the complementary bases, with U opposite A
1
Write the template strand using A-T and C-G pairing, if starting from the coding strand.
2
Write the mRNA codons complementary to the template.
3
Read each codon, not the DNA, in the codon table.
4
List the amino acids in order.
- Anticodons
- complementary to the codons, so the codon CAG pairs with the anticodon GUC
Part 9 of 11Mutations and polypeptides
Mutations and polypeptides
Gene mutation
Gene mutation a change in the sequence of bases in DNA, which may produce a new allele
- Mutations occur randomly, often as copying errors during replication in S phase.
- Most are harmful or neutral and some are beneficial.
- Mutations in body cells are not inherited, and those in gametes are inherited.
Point mutation (base substitution)
- Point mutation (base substitution) one base is altered, which may change one amino acid
Silent change
- Silent change the new codon codes for the same amino acid because the code is degenerate
Part 10 of 11Sickle cell disease
Cause
- Cause a point mutation in the beta-globin gene of haemoglobin
- The coding strand changes from GAG to GTG, and the template triplet becomes CAC.
- The mRNA codon changes from GAG to GUG.
- Valine replaces glutamic acid at position 6 of the polypeptide.
Alleles
- HbA is the usual allele and HbS carries the substitution
Effects
- Haemoglobin S makes the red blood cells sickle-shaped at low oxygen.
- They carry less oxygen and block capillaries.
- People suffer acute pain, fatigue and anaemia.
Malaria link
- HbS is more frequent where malaria is common because carriers show some resistance to the parasite
Part 11 of 11More from the specification
More from the specification
Compartmentalised synthesis
- In eukaryotic cells, transcription occurs in the nucleus, after which mRNA traverses nuclear envelope pores for translation on cytosolic ribosomes.
Transcription enzyme
- RNA polymerase unzips DNA and matches complementary RNA nucleotides against the antisense strand.
- (incorporating uracil instead of thymine)
Translation and codons
- Triplet codons on mRNA base-pair with tRNA anticodons via temporary hydrogen bonding, assembling amino acids in specified order.
Polypeptide chain maturation
- Nascent polypeptide chains fold spontaneously into secondary and tertiary protein structures determined by primary amino acid sequences.
Nuclear envelope
- The double membrane around the nucleus, with pores.
Key terms
| Term | Meaning |
|---|---|
| Transcription | The synthesis of mRNA from a DNA template strand by RNA polymerase. |
| Translation | The assembly of a polypeptide at a ribosome using the mRNA code. |
| Codon | A sequence of three mRNA bases that codes for one amino acid or a stop signal. |
| Anticodon | A sequence of three tRNA bases complementary to a codon. |
| Degenerate code | A genetic code in which several codons specify the same amino acid. |
| Template strand | The DNA strand that is transcribed. |
| Point mutation | A change in a single base of a DNA sequence. |
| RNA polymerase | The enzyme that builds mRNA on a DNA template. |
Avoid these mistakes
1
Confusing the template and coding strands — the mRNA is complementary to the template and matches the coding strand
2
Reading the codon table with DNA bases — it is read with mRNA codons
3
Using thymine in RNA — RNA uses uracil in place of thymine
4
Saying one amino acid has only one codon — the code is degenerate
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