Nucleotides & DNA vs RNA

The big idea: DNA and RNA are long chains called nucleic acids.

They are built from many small repeating units called nucleotides — the monomer (single building block) of every nucleic acid.

Every nucleotide has the same three parts: a phosphate group, a pentose sugar, and a nitrogenous base.

Picture one nucleotide: Think of three pieces joined in a row:

phosphate — sugar — base

The phosphate and sugar make the backbone; the base sticks out to the side and is the part that codes information.

DNA and RNA are both made of nucleotides, so they share the same three parts. They differ in three specific ways: the sugar, one of the bases, and the number of strands.

Nucleotide

The monomer of a nucleic acid — a phosphate group, a pentose sugar and a nitrogenous base joined together.

Pentose sugar

A sugar with five carbon atoms; deoxyribose in DNA, ribose in RNA.

Nitrogenous base

The information-carrying part of a nucleotide: adenine (A), cytosine (C), guanine (G), and either thymine (T, in DNA) or uracil (U, in RNA).

Strand

A single chain of nucleotides. DNA has two strands; RNA has one.

The three differences to memorise: 1. Sugar: DNA has deoxyribose, RNA has ribose.

2. One base: DNA uses thymine (T), RNA uses uracil (U) in its place.

3. Strands: DNA is double-stranded, RNA is single-stranded.

The phosphate, and the bases A, C and G, are the same in both.

A memory hook: DNA has Deoxyribose and Double strands. RNA swaps Thymine for Uracil — 'U' is the new letter that only appears in RNA.

Phosphorus — needed for more than just DNA: Each nucleotide's phosphate group contains the element phosphorus, so building DNA and RNA needs a supply of phosphorus.

But DNA and RNA are not the only living molecules that contain phosphorus. Two more important biochemicals also need it: ATP (the cell's energy-carrying molecule) and phospholipids (the molecules that build cell membranes).

This is why phosphorus is an essential raw material for life — a plant or cell with too little phosphorus cannot make enough DNA, RNA, ATP or phospholipids.

Biochemical

A chemical compound made by, and used inside, living organisms (for example a nucleic acid, ATP or a phospholipid).

DNA and RNA (nucleic acids)

Their nucleotides each carry a phosphate group, so building them requires phosphorus.

ATP

The molecule cells use to carry and release usable energy. The 'P' stands for phosphate — ATP contains three phosphate groups, so it needs phosphorus.

Phospholipid

The main molecule of cell membranes. Its 'head' is a phosphate group, so it too contains phosphorus.

If asked to list phosphorus-containing biochemicals: Any two of these score the marks: DNA, RNA, ATP, or a phospholipid.

A safe pair to remember is ATP and phospholipids (or DNA and ATP) — all of them contain a phosphate group, which is the part that holds the phosphorus.

How this is tested: On Paper 2 a 3-mark Draw question can ask you to draw and label a single DNA or RNA nucleotide — show the phosphate, sugar and base joined together.

Distinguish / contrast questions ask for the structural differences between DNA and RNA (sugar, one base, strands).

On Paper 1A you may have to identify which sugars and bases belong to DNA, RNA or both.

✓ The diagram you should draw: Three labelled shapes joined like this:

phosphate — sugar (deoxyribose) — base

Check yours shows the sugar in the middle, joined on one side to the phosphate and on the other side to the base, with all three labelled. For an RNA nucleotide the only changes are: sugar = ribose, and the base can be uracil.