Sickle cell anaemia is caused by a single nucleotide substitution in the HBB gene (haemoglobin beta-chain gene) located on chromosome 11. At the DNA level (coding strand): codon 6 changes from GAG → GTG. At the mRNA level: codon 6 changes from GAG (Glutamic acid) → GUG (Valine). This single base change (A to T in DNA, A to U in mRNA) results in replacement of glutamic acid (negatively charged, hydrophilic) with valine (nonpolar, hydrophobic) at position 6 of the 146-amino-acid beta-globin chain. This single amino acid change causes profound structural and functional changes in haemoglobin, leading to the entire spectrum of sickle cell disease manifestations.

The genetic code is the relationship between mRNA codons (triplets) and amino acids. Important codons: Start codon: AUG = Methionine (in eukaryotes); fMet in prokaryotes. Stop codons: UAA (ochre), UAG (amber), UGA (opal) — these do not code for amino acids. Key disease-related codons: GAG = Glutamic acid (normal beta-globin position 6). GUG = Valine (sickle cell mutation — codes for Val). CAG = Glutamine. GAA = Glutamic acid (another Glu codon). CAG→TAG creates a stop codon = nonsense mutation (protein truncation). The genetic code is degenerate: multiple codons for same amino acid (GAA and GAG both = Glu). Reading the genetic code: AUG GUG UAU = Met-Val-Tyr (reading 5' to 3' on mRNA).

Point mutations are changes in a single base pair. By molecular type: Transition: purine → purine (A↔G) or pyrimidine → pyrimidine (C↔T/U). The sickle cell mutation is a transition: A→T (DNA) / A→U (mRNA) in codon 6. Transversion: purine → pyrimidine or vice versa (A/G ↔ C/T). By effect on protein: Synonymous/silent mutation: codon changed but codes for same amino acid (genetic code degeneracy). No protein change. Missense mutation: codon changed → different amino acid. May be: Conservative (similar properties: Glu→Asp, both acidic). Non-conservative (very different properties: Glu→Val in sickle cell). Nonsense mutation: codon changed → stop codon → premature termination → truncated protein. Readthrough mutation: stop codon → amino acid codon → elongated protein. Frameshift mutations (not point mutations): insertions or deletions of bases (not multiples of 3) → shift reading frame → completely altered downstream sequence.

HbA (normal haemoglobin): beta-chain position 6 = Glutamic acid (Glu, E). Glu has charged (negative) side chain (-CH2-COOH at physiological pH). This charge keeps haemoglobin monomers apart → soluble → normal round RBC shape. HbS (sickle haemoglobin): beta-chain position 6 = Valine (Val, V). Val has non-polar hydrophobic side chain (-CH(CH3)2, isopropyl). In deoxygenated HbS: the Val at position 6 on one HbS molecule binds to a complementary hydrophobic pocket on an adjacent deoxyHbS molecule. Progressive stacking of HbS molecules → long, rigid fibres (polymers) inside the RBC → RBC distorts into sickle (crescent) shape. In oxygenated HbS: conformational change → Val6 pocket hidden → no polymerisation → cells appear normal. Deoxygenation (exercise, altitude, infection) → sickle crisis.

Sickle cell anaemia: autosomal recessive. HbB gene on chromosome 11. HbA/HbA: normal (no disease, no carrier). HbA/HbS: sickle cell TRAIT (carrier, usually asymptomatic, protected against malaria). HbS/HbS: sickle cell ANAEMIA (full disease). Cross of two carriers (HbA/HbS × HbA/HbS): 25% HbAA (normal), 50% HbAS (carrier), 25% HbSS (disease). Balanced polymorphism: in malaria-endemic regions of Africa, HbS allele maintained at high frequency (10-40%) because heterozygotes (HbAS) have survival advantage against Plasmodium falciparum malaria. In non-malaria regions: selection against HbSS without corresponding benefit → HbS frequency declining slowly. This is one of the best examples of natural selection operating in human populations.

HbA: normal adult (α2β2). HbA2: α2δ2. About 2-3% of adult haemoglobin. HbF: foetal haemoglobin (α2γ2). Higher O2 affinity than HbA (facilitates O2 transfer from mother to foetus). Predominant before birth. Replaced by HbA after birth. HbS: sickle cell (β Glu6Val). HbC: β Glu6Lys — another beta-chain mutation at same position. Less severe than HbSS. HbSC disease: compound heterozygote (one HbS + one HbC allele). Intermediate severity. HbE: β Glu26Lys — common in Southeast Asia. HbH: β4 tetramers in alpha-thalassaemia (excess beta chains). HbBarts: γ4 tetramers in severe alpha-thalassaemia. Methemoglobin: haem iron oxidised Fe3+ → cannot bind O2. Each variant has distinct electrophoretic mobility and disease manifestations.

Current treatments: Hydroxyurea (HU): increases HbF production by reactivating gamma-globin gene. HbF does not polymerise with HbS. HbF dilutes HbS → reduces sickling → fewer crises. FDA-approved 1998. Most effective drug treatment. Blood transfusions: correct anaemia, dilute HbS. Used in acute crises and stroke prevention. Bone marrow stem cell transplant: only cure currently. Requires matched donor (sibling). Risks of transplant vs severity of disease must be weighed. Gene therapy (latest, 2023-24): Two approved gene therapies: Casgevy (CRISPR-Cas9 based, CTX001): reactivates foetal gamma-globin gene → HbF produced → sickling prevented. Lyfgenia (lentiviral vector, bb2121): introduces anti-sickling beta-globin variant. First gene-editing treatment approved by FDA (December 2023). Potentially curative without matched donor.

The genetic code has important properties: Universal: same codons code for same amino acids in virtually all living organisms (exceptions: mitochondria use slightly different code). This universality supports the common origin of all life. Degenerate (redundant): 64 codons for only 20 amino acids + 3 stop codons. So multiple codons for same amino acid. Example: Glu = GAA or GAG. Degeneracy mainly at 3rd position (wobble). Non-overlapping: each nucleotide read only once (except very few exceptions). Comma-less: no punctuation between codons (except at start AUG and stop codons). Unambiguous: each codon codes for only one amino acid (not ambiguous). Triplet: 3 nucleotides per codon (64 combinations > 20 amino acids). Near-universal: exceptions include mitochondria (UGA = Trp in some; AUA = Met; CUN = Thr in yeast mito), some ciliates (UAA, UAG = amino acids).