Nucleic Acids and Molecular Biology Basics
Nucleic acids store and express genetic information. This domain tests nucleotide structure, base pairing, the DNA double helix, RNA types and the central dogma.
Core rules
- A nucleotide has a nitrogenous base, a pentose sugar and one or more phosphate groups.
- Purines are adenine and guanine; pyrimidines are cytosine, thymine and uracil.
- DNA contains deoxyribose and thymine; RNA contains ribose and uracil.
- DNA is double-stranded and antiparallel; A pairs with T by two hydrogen bonds, and G pairs with C by three.
- Chargaff's rules apply to double-stranded DNA: %A = %T and %G = %C.
- mRNA carries the message, tRNA brings amino acids, and rRNA forms part of ribosomes. The central dogma is DNA → RNA → protein.
Worked example
A double-stranded DNA sample contains 28% adenine. Find the percentages of the other bases. By base pairing, %T = %A = 28%. A + T = 56%, so G + C = 100 − 56 = 44%. Since %G = %C, each is 22%. The reasoning depends on complementary base pairing, not on equal amounts of all four bases.
Traps that cost marks
Uracil is in RNA, not DNA. Chargaff's equality rules apply to double-stranded DNA, not to single-stranded RNA. Do not say A pairs with U in DNA. Remember that DNA strands are antiparallel and written 5′→3′ by convention.