AQA A-Level Biology: DNA, RNA and Chromosomes
A clear revision guide to DNA, RNA and chromosomes for AQA A-Level Biology: how DNA differs between eukaryotes and prokaryotes, the structure of mRNA and tRNA, and chromosomes.
Topic 4 is about how genetic information is stored, copied and used. This guide sets the foundations: the different forms DNA takes, how it is packaged into chromosomes, and the several kinds of RNA that carry its message. The basic structure of DNA, the double helix and complementary base pairing, is covered in the Topic 1 guide; here we build on it.
DNA in eukaryotes and prokaryotes
The individual nucleotides of DNA are identical wherever you find them: a deoxyribose sugar, a phosphate group and one of the four bases (adenine, thymine, cytosine, guanine). Adjacent nucleotides are always joined by phosphodiester bonds, and the two strands are held together by hydrogen bonds between complementary bases. What differs between organisms is how the DNA is arranged.
In eukaryotic cells the DNA is long, linear, and wound around histone proteins. In prokaryotic cells it is short, circular and not associated with proteins. Eukaryotic DNA also contains non-coding sections called introns, which prokaryotic DNA lacks.
There is a neat exception inside eukaryotic cells. The mitochondria and chloroplasts contain their own DNA, and it looks like prokaryotic DNA: short, circular and not associated with histones.
| Feature | Eukaryotic DNA | Prokaryotic DNA |
|---|---|---|
| Length | Long | Short |
| Shape | Linear | Circular |
| Histone proteins | Associated with histones | Not associated with proteins |
| Introns | Present | Absent |
Chromosomes
A chromosome is a molecule of long, linear DNA together with the histone proteins it is wound around, found in the nucleus of a eukaryotic cell. The histones matter because a molecule of DNA is far too long to fit inside the nucleus unwound; coiling it tightly around histones packages it into a compact chromosome. A molecule of DNA also cannot leave the nucleus at all, because it is too large to pass through the nuclear pores.
RNA
RNA is a single-stranded polymer of nucleotides. Each RNA nucleotide contains the sugar ribose, a phosphate group, and one of four bases: adenine, guanine, cytosine and uracil (which takes the place of thymine). There are three functional types, mRNA, tRNA and rRNA, and two of them are worth looking at in detail.
Messenger RNA (mRNA) is a long, single-stranded molecule made in the nucleus by transcription. It carries a copy of the genetic code out to the ribosomes, where it acts as a template for translation. The sequence of bases along the mRNA, which is set by the DNA, determines the sequence of amino acids in the polypeptide made. mRNA is chemically unstable and breaks down after a short time, which makes sense for a temporary message.
Transfer RNA (tRNA) is a small molecule of about 80 nucleotides, folded into a clover-leaf shape. At one end is a site where a specific amino acid attaches. At the other end is the anticodon, a sequence of three bases that pairs with the complementary codon on the mRNA. tRNA is the adaptor that brings the right amino acid to the right place during translation.
| Feature | mRNA | tRNA |
|---|---|---|
| Shape | Linear | Folded clover-leaf |
| Length | Long, variable | Short, fixed (about 80 nucleotides) |
| Base pairing within molecule | None | Hydrogen bonds between paired bases |
| Key feature | Carries codons | Has an anticodon and an amino acid site |
Diploid and haploid cells
Body cells usually carry two complete sets of chromosomes and are described as diploid (written 2n). In humans that is 23 pairs, 46 chromosomes in total. Gametes carry a single set and are haploid (written n), so a human gamete has 23 chromosomes.
The two chromosomes of a pair are homologous. They are the same length and carry the same genes at the same positions (loci), but they may carry different versions (alleles) of those genes. This pairing becomes important in meiosis, covered in the meiosis guide.