DNA, RNA and chromosomes
Experiments showed that DNA is the genetic material: Griffith's transformation experiment (1928), Avery, MacLeod and McCarty's identification of DNA as the transforming principle (1944), and Hershey and Chase's work with bacteriophages (1952). In some viruses such as tobacco mosaic virus (TMV), RNA is the genetic material.
DNA structure: Watson and Crick proposed the double helix model in 1953. Each strand is a chain of nucleotides; each nucleotide has a deoxyribose sugar, a phosphate group and a nitrogenous base. Bases are purines (adenine A, guanine G) and pyrimidines (cytosine C, thymine T). The two strands are antiparallel and held by hydrogen bonds: A pairs with T (2 bonds) and G pairs with C (3 bonds). Hence, by Chargaff's rule, A = T and G = C.
Worked example: if 30% of the bases in a DNA sample are adenine, then T = 30%, and G + C = 40%, so G = 20% and C = 20%.
RNA has ribose sugar, uses uracil (U) instead of thymine and is usually single-stranded. Types: mRNA (carries the code), tRNA (brings amino acids), rRNA (part of ribosomes).
Chromosomes: in eukaryotes, DNA is wound around histone proteins to form nucleosomes; these coil into chromatin, which condenses into chromosomes during division. Each chromosome has a centromere; by its position chromosomes are metacentric (middle), sub-metacentric, acrocentric and telocentric (end). Genes are segments of DNA at fixed positions (loci) on chromosomes.
Key points
- DNA double helix: Watson and Crick, 1953.
- A–T (2 H-bonds), G–C (3 H-bonds); A = T, G = C.
- RNA: ribose + uracil; DNA: deoxyribose + thymine.
- DNA + histones → nucleosome → chromatin → chromosome.
Exam tips
Base-percentage questions are common: use A = T, G = C and total = 100%. Also learn which sugar and base belong to DNA vs RNA.
Common mistakes
- Pairing A with G or C with T.
- Writing thymine in RNA.
- Assuming DNA is the genetic material of every virus.