Nucleic acids

MDCAT Biology · Biology · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

DNA and RNA are chains of nucleotides. Each nucleotide is a pentose sugar, a phosphate group and a nitrogenous base. DNA is a double helix: two antiparallel strands held by hydrogen bonds between the bases, with A always paired with T and G with C. RNA is usually single-stranded, has ribose instead of deoxyribose and uracil instead of thymine.

Before a cell divides, DNA copies itself by semi-conservative replication: each new molecule keeps one old strand and gains one new strand.

A gene is a length of DNA that codes for a polypeptide. In transcription the code is copied into mRNA. In translation, ribosomes read the mRNA three bases (one codon) at a time and tRNA brings the matching amino acids.

Rules to remember

Purines (two rings): adenine, guanine. Pyrimidines (one ring): cytosine, thymine, uracil. Nucleoside = sugar + base; nucleotide = sugar + base + phosphate
Base pairing: A=T with 2 hydrogen bonds, G≡C with 3. So A = T, G = C and (A + G) = (T + C)
Double helix: 10 base pairs per turn, 3.4 nm per turn, 0.34 nm between base pairs, 2 nm wide; nucleotides in one strand joined by phosphodiester bonds
Replication: helicase unwinds; primase makes an RNA primer; DNA polymerase III adds nucleotides 5′ → 3′; leading strand continuous, lagging strand in Okazaki fragments joined by DNA ligase
Transcription: RNA polymerase copies the template strand into mRNA; mRNA has the same sequence as the coding strand with U in place of T
Genetic code: 64 triplet codons; 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA); AUG is the start codon (methionine)
mRNA carries the code, tRNA carries amino acids and has the anticodon, rRNA builds ribosomes

Common mistake

Putting thymine into RNA, or pairing the mRNA with the wrong DNA strand. mRNA is complementary to the template strand and contains U wherever DNA would have T.

Worked example

A double-stranded DNA fragment has 100 base pairs, of which 40 are G–C pairs. How many hydrogen bonds hold the two strands together?

A–T pairs = 100 − 40 = 60.
Bonds in G–C pairs = 40 × 3 = 120.
Bonds in A–T pairs = 60 × 2 = 120.
Total = 120 + 120 = 240.

Answer: 240 hydrogen bonds.

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