AQA A-Level Biology: Nucleic Acids
A clear revision guide to nucleic acids for AQA A-Level Biology: nucleotide structure, the DNA double helix, complementary base pairing, how DNA and RNA differ, and semi-conservative replication.
Nucleic acids carry the information that tells a cell how to build its proteins. This guide covers the nucleotide they are built from, the structure of DNA and of RNA, and how DNA is copied. The theme is that DNA's structure is not incidental: every feature of it exists because of something the molecule has to do.
Nucleotides
The monomer of both DNA and RNA is the nucleotide, and it has three parts joined together:
- a pentose sugar, meaning a sugar with five carbons;
- a phosphate group;
- a nitrogen-containing organic base.
Nucleotides join by condensation reactions, linking the phosphate of one to the sugar of the next and releasing water. The bond formed is a phosphodiester bond, and the chain of alternating sugars and phosphates it builds is the sugar-phosphate backbone. The bases stick out sideways from it.
DNA structure
DNA is two polynucleotide strands wound into a double helix. Four features of it are examined, and each has a reason:
- The two strands are held together by hydrogen bonds between their bases. Each bond is weak, which lets the strands be separated when the DNA is copied or read, but there are so many of them along the molecule that it is stable overall.
- The pairing is specific. This is complementary base pairing: adenine always pairs with thymine by two hydrogen bonds, and cytosine always pairs with guanine by three. It follows that one strand carries all the information needed to rebuild the other.
- The two strands run in opposite directions, described as antiparallel.
- The sugar is deoxyribose, and the backbone of sugar and phosphate is on the outside, protecting the bases within it.
Taken together, DNA is stable, it is very long so it holds a great deal of information, and the order of its bases acts as a code. Those are the properties a molecule needs if its job is to store information for the life of an organism and pass it on unchanged.
RNA structure
RNA is a single polynucleotide strand, and it is much shorter than DNA. Its sugar is ribose rather than deoxyribose, and the base uracil takes the place of thymine.
Its job is to carry information rather than store it. Messenger RNA (mRNA) carries a copy of the code from the nucleus to the ribosome, and transfer RNA (tRNA) brings the matching amino acids to be joined. Being short-lived and single-stranded suits a molecule that is made, used and broken down again.
| Feature | DNA | RNA |
|---|---|---|
| Strands | Two, in a double helix | One |
| Sugar | Deoxyribose | Ribose |
| Bases | Adenine, thymine, cytosine, guanine | Adenine, uracil, cytosine, guanine |
| Length | Very long | Much shorter |
| Role | Stores genetic information | Transfers information for protein synthesis |
Semi-conservative replication
Before a cell divides, its DNA is copied. The process is called semi-conservative replication, and it happens in four stages:
- DNA helicase breaks the hydrogen bonds between the bases, so the double helix unwinds and the two strands separate. Each one now acts as a template.
- Free DNA nucleotides in the nucleus line up against the exposed bases by complementary base pairing, adenine against thymine and cytosine against guanine.
- DNA polymerase catalyses condensation reactions between the nucleotides that have lined up, forming phosphodiester bonds to join them into a new backbone.
- The hydrogen bonds re-form and each new molecule winds into a double helix.
The result is two DNA molecules, and each one contains one original strand and one new strand. That is what semi-conservative means, and it is the phrase the mark scheme wants: half of each new molecule is conserved from the old one.
Two details are often asked for. Because the strands are antiparallel, DNA polymerase can only work along a template strand in one direction, so the two new strands are built in opposite directions. And because base pairing is complementary, the copy is faithful: an error only arises if the wrong nucleotide is joined, which is where mutations come from.
How this fits together
The structure of DNA and the way it is copied are the foundation for everything about genetic information. How that information is read and turned into a protein is covered in protein synthesis, and how DNA is packaged into chromosomes in DNA, RNA and chromosomes.
One nucleotide has a job of its own, quite separate from carrying information, and that is the subject of the ATP, water and inorganic ions guide.