Part 1: Nucleotides

DNA and RNA are both nucleic acids. They are polymers made from repeating monomers called nucleotides. Every nucleotide has three components:

• a pentose sugar
• a phosphate group
• a nitrogen-containing organic base

The sugar is what immediately separates DNA and RNA nucleotides.

• DNA nucleotides contain deoxyribose.
• RNA nucleotides contain ribose.

The bases also differ slightly.

• DNA uses adenine, thymine, cytosine, and guanine.
• RNA uses adenine, uracil, cytosine, and guanine.

So the two clean structural differences you should always be able to state are deoxyribose vs ribose and thymine vs uracil.

Part 2: From nucleotides to polynucleotides

Nucleotides join together by condensation reactions. A phosphodiester bond forms between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide. Repeating that reaction produces a long polynucleotide chain.

This gives nucleic acids a sugar-phosphate backbone with the bases projecting from it. The backbone gives structure, while the sequence of bases carries information.

The textbook notes also emphasise the 3 prime and 5 prime carbon atoms of the sugar. The phosphate is attached to the 5 prime carbon and the hydroxyl group is on the 3 prime carbon. This matters because nucleic acids are built in the 5 prime to 3 prime direction.

Part 3: The structure of RNA

RNA is a relatively short, single polynucleotide chain. Its sugar is ribose and its four bases are adenine, uracil, cytosine, and guanine.

For AQA, you should connect structure to role without overcomplicating it:

• mRNA carries genetic information from DNA to ribosomes.
• rRNA combines with proteins to make ribosomes.
• tRNA is involved in protein synthesis.

That is why the specification says ribosomes are formed from RNA and proteins.

RNA is generally less stable than DNA, and many cellular RNA molecules are short-lived. This suits RNA's role in transferring and using information rather than storing it long term.

Part 4: The structure of DNA

DNA is made of two very long polynucleotide strands twisted into a double helix. Each strand has a sugar-phosphate backbone, and the bases point inwards where they pair specifically.

The base pairing rules are:

• adenine pairs with thymine
• guanine pairs with cytosine

These are complementary base pairs. The two strands are therefore complementary because the sequence on one strand determines the sequence on the other.

The strands are also antiparallel. One runs in the 5 prime to 3 prime direction and the other runs in the 3 prime to 5 prime direction.

Hydrogen bonds hold the base pairs together:

• adenine and thymine form two hydrogen bonds
• guanine and cytosine form three hydrogen bonds

That extra hydrogen bond is why DNA with a higher proportion of G-C pairs is more stable.

A useful consequence of complementary pairing is that in any DNA molecule:

• amount of A = T
• amount of G = C

But the overall ratio of A + T to G + C varies between species.

Part 5: Why DNA is a stable information store

DNA is well adapted for storing genetic information.

• The sugar-phosphate backbone forms the outside of the molecule and helps protect the bases inside.
• Large numbers of hydrogen bonds hold the two strands together.
• Base stacking interactions add further stability.
• DNA is an extremely long molecule, so it can store a huge amount of information.

The key idea is that the information is in the sequence of bases, not just in the fact that there are four kinds of base. A very long chain built from four possible bases can still produce an enormous number of different sequences.

That is why DNA can carry hereditary information from cell to cell and from generation to generation.

Part 6: Why scientists once doubted DNA

Scientists did not always accept that DNA was the genetic material. DNA looked chemically simple because it contains only four different bases. Proteins seemed more likely because they are built from twenty different amino acids and so appeared much more varied.

The important exam point is this: scientists originally underestimated DNA because they focused on the small number of building blocks, rather than on the enormous number of possible base sequences in a very long molecule.

Feynman check

A language can make thousands of words from a small alphabet. DNA works in the same way. It does not need dozens of building blocks if the order of four bases can vary along millions or billions of positions.