4.1 The double helix and replication
DNA is a long chain of nucleotides, each made of a sugar (deoxyribose), a phosphate and one of four bases (adenine, thymine, guanine, cytosine). The two strands are held together by hydrogen bonds between bases: A always pairs with T (2 hydrogen bonds) and G with C (3 hydrogen bonds). So in any DNA the amount of A equals T and G equals C (Chargaff's rule). In 1953 James Watson and Francis Crick proposed the double helix, drawing also on Rosalind Franklin's X-ray images.
In replication, the enzyme helicase unzips the two strands and each acts as a template for a new strand; DNA polymerase adds nucleotides according to base pairing. Each new DNA therefore carries one old and one new strand: replication is semi-conservative. Because DNA polymerase works only in the 5'→3' direction, one strand is made continuously and the other in short pieces (Okazaki fragments).
FormulaA = T, G = C · %A + %G = %T + %C = 50%
4.2 Gene expression: transcription and translation
In transcription, RNA polymerase reads the template strand of DNA and makes a complementary mRNA; RNA has uracil (U) instead of thymine. In eukaryotes the mRNA is processed before leaving the nucleus: introns, the non-coding parts of the gene, are cut out and the exons are joined.
Translation happens at the ribosome. The mRNA is read in three-base codons; a tRNA carrying the matching anticodon brings the right amino acid. Of the 64 codons, 61 specify amino acids and 3 (UAA, UAG, UGA) are stop codons. The start codon is AUG, which codes for methionine. Several codons can specify the same amino acid, which is why the genetic code is called "degenerate".
4.3 Mutations
A permanent change in the DNA sequence is a mutation. Swapping one base for another (a point mutation) may be silent (same amino acid), change the meaning (a different amino acid) or create an early stop codon. In sickle-cell anaemia, a single change in the β-globin gene (GAG → GTG) turns a glutamic acid in the protein into valine and distorts red blood cells.
Inserting or deleting one or two bases shifts the reading frame (a frameshift): every codon after that point changes and the protein usually doesn't work. Most mutations are neutral or harmful, but rarely one is helpful — and this variety is the raw material of evolution.
Key terms
- Codon
- A three-base sequence on mRNA that specifies an amino acid or a stop.
- Intron
- A non-coding part of a gene that is cut out of the mRNA.
- Semi-conservative replication
- Each new DNA keeping one old and one new strand.
- Frameshift
- An insertion or deletion that changes the reading frame.
End-of-unit problems
Solve each one yourself first, then open the solution.
In a double-stranded DNA molecule, 30% of the bases are adenine. Find the percentages of guanine, thymine and cytosine. Is this DNA or one of the same length with 20% adenine more resistant to high temperature?
Show the solution
A = T = 30%, 60% in total. The remaining 40% is shared equally: G = C = 20%. A DNA with 20% adenine has G + C = 60%. G–C pairs have three hydrogen bonds, so the second DNA, richer in G–C, is more heat-resistant.
A DNA template strand reads 3'-TAC GGA CTT ATT-5'. Write the mRNA and the resulting peptide. (AUG = Met, CCU = Pro, GAA = Glu, UAA = stop)
Show the solution
Base pairing (T→A, A→U, G→C, C→G): mRNA 5'-AUG CCU GAA UAA-3'. Translation: Met – Pro – Glu, then it stops at UAA. The peptide has 3 amino acids.
In the Meselson–Stahl experiment, if ¹⁵N bacteria divide for 3 generations in ¹⁴N medium, what percentage of the DNA molecules is intermediate and what percentage light?
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The two heavy strands of the original molecule are never lost; after 3 generations they are found in only 2 of 2³ = 8 molecules. Those 2 are intermediate (25%) and the other 6 are light (75%).
Base 10 in the 300-base coding region of a gene is deleted. How would the effect on the protein differ if bases 10, 11 and 12 were deleted together?
Show the solution
A single deletion causes a frameshift: every codon from the 4th onwards changes, an early stop codon probably appears and the protein doesn't work. Deleting three bases keeps the reading frame; the protein loses just one amino acid (and at most one neighbouring codon changes) and often still partly works.
🦉 An easier explanation: Professor Owl's lesson: Heredity and DNA