4.2.4Additional lessons
Protein Structure and Mutations — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 105 of 142
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.
Part 1 of 4Levels of Protein Structure
A protein's function depends on its shape, and its shape depends on the sequence of amino acids.
Primary
- Primary The sequence of amino acids in a polypeptide, held together by peptide bonds and set by the gene.
Secondary
- Secondary Regular folding such as alpha helices and beta sheets, stabilised by hydrogen bonds along the backbone.
Tertiary
- Tertiary The 3D shape of a polypeptide, held by bonds between R groups such as hydrogen bonds, ionic bonds and disulfide bridges.
Quaternary
- Quaternary Two or more polypeptides combined in one protein, such as the four chains in haemoglobin.
Part 2 of 4Point Mutations
Gene mutations alter the base sequence, and this may change the amino acid sequence of a protein.
Point Mutations
Definition
- Definition A gene mutation is a change in the sequence of bases in DNA and may produce a new allele.
Randomness
- Randomness Mutations arise at random, for example as copying errors during DNA replication in S phase.
Inheritance
- Inheritance Mutations in gametes can be inherited; mutations in body cells are not passed to offspring.
Effects
- Effects Most mutations are harmful or neutral, but a few are beneficial and may spread by natural selection.
Part 3 of 4Sickle Cell Anaemia
A single base substitution is enough to cause a serious inherited disease.
1
A point mutation in the beta-globin gene changes the coding triplet GAG to GTG.
2
The template strand changes from CTC to CAC, so the mRNA codon changes from GAG to GUG.
3
During translation, valine replaces glutamic acid at position 6 of the polypeptide.
4
The altered haemoglobin, HbS, can stick together in low oxygen, distorting red blood cells into a sickle shape.
5
Sickled cells block small vessels and carry less oxygen, causing the symptoms of the disease.
6
People with two copies of the HbS allele have the disease; carriers with one copy usually have milder effects.
| Level | Change |
|---|---|
| DNA | GAG → GTG |
| mRNA | GAG → GUG |
| Amino acid | Glu → Val |
| Protein | HbA → HbS |
Part 4 of 4Exam Checklist & Tips
4.2.4 – Key Points for the Exam
- Primary structure The sequence of amino acids in a polypeptide.
- Tertiary structure The overall 3D folding of a single polypeptide.
- Point mutation A change to a single base in DNA.
- Allele A different form of a gene, which may arise by mutation.
- Haemoglobin The oxygen-carrying protein of red blood cells, made of four polypeptides.
- Substitution The replacement of one base by another in a DNA sequence.
Common Mistakes
- Saying all mutations are harmful; many have no effect and some are beneficial.
- Stating that the whole protein changes after a single substitution; only one amino acid is altered.
- Confusing the sequence of amino acids with the folded shape.
- Believing mutations in body cells are passed on to children.
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