✅ The same every time
The sugar and the phosphate group. Every nucleotide has one of each.
Reproduction » DNA Structure
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.1.5
- That DNA is a polymer made from four different nucleotides
- What a nucleotide is made of, and the four bases A, C, G and T
- How the sugar and phosphate strands are built
- How a sequence of three bases codes for an amino acid
- Higher tier: how the bases pair up between the two strands
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Unlock This CourseDNA is built from nucleotides
You already know that DNA is a very long polymer. A polymer is made of smaller units joined together. So what are the units in DNA?
They are called nucleotides. DNA is a polymer made from four different nucleotides. The DNA polymer is made up of repeating nucleotide units, joined one after another along each strand.
Think of a very long train. Each carriage is one nucleotide. The carriages are not all identical, but every carriage is built to the same basic plan.
Key terms:
- Nucleotide: one repeating unit of DNA, made of a sugar, a phosphate group and a base.
- Base: the part of a nucleotide that makes it different from the other nucleotides. DNA has four: A, C, G and T.
What is inside a nucleotide?
Every nucleotide has three parts:
- a common sugar,
- a phosphate group, and
- one of four different bases, attached to the sugar.
The sugar and the phosphate group are the same in every nucleotide. That is why the spec calls the sugar "common". Only the base changes. The four bases are called A, C, G and T.
🔄 Different each time
The base. It is A, C, G or T, and it is what makes the four nucleotides different.
So there are four nucleotides, one for each base. Each one has its own base attached to the same sugar and phosphate group.
The structure of a DNA strand
Each long strand of DNA is made of alternating sugar and phosphate sections. The pattern goes sugar, phosphate, sugar, phosphate, and so on, all the way along. This forms the backbone of the strand.
Attached to each sugar is one of the four bases. The bases stick out from the backbone, like the rungs of a ladder reaching towards the other strand.
Picture it like this
Backbone: sugar - phosphate - sugar - phosphate - sugar - phosphate.
Each sugar has one base hanging from it: A, C, G or T.
One sugar, one phosphate and one base together make one nucleotide.
You may be given a diagram of DNA in the exam. You will not be asked to draw it from memory, but you must be able to read it. Look for the alternating sugar and phosphate sections, then find the bases attached to the sugars.
How the order of bases makes a protein
A gene is a section of DNA that codes for a protein. Here is how the code works.
The bases are read in groups. A sequence of three bases is the code for one particular amino acid. For example, three bases in a row code for one amino acid, and the next three bases code for the next amino acid.
The order of the bases controls the order in which amino acids are assembled to make a particular protein. Change the order of the bases and you change the order of the amino acids. This is why different genes make different proteins.
Worked example
A short stretch of one strand reads: A A G C T T G G C.
Step 1: split it into groups of three bases: AAG, CTT, GGC.
Step 2: each group codes for one amino acid, so this stretch codes for 3 amino acids.
Step 3: the order AAG, then CTT, then GGC controls the order in which the 3 amino acids are joined.
A useful rule: the number of amino acids is the number of bases divided by 3. A stretch of 12 bases codes for 4 amino acids. A stretch of 30 bases codes for 10.
Higher tier only: complementary strands
Higher tier only
DNA has two strands. The bases on one strand are linked to the bases on the other strand. The pairing is always the same. In the complementary strands:
C is always linked to G on the opposite strand, and T is always linked to A.
So if you know the order of bases on one strand, you can work out the order on the other strand.
Example: one strand reads A T C G G A. The complementary strand reads T A G C C T.
Check each pair: A with T, T with A, C with G, G with C, G with C, A with T.
Common mistakes
Do not say that the bases make up the backbone. The backbone is the alternating sugar and phosphate sections. The bases are attached to the sugars.
Do not say that DNA has more than four bases. It has four: A, C, G and T.
Do not say that one base codes for one amino acid. It takes a sequence of three bases.
Higher tier: do not pair A with C or G with T. C goes with G, and T goes with A.
Exam-style question
(a) DNA is a polymer made from nucleotides. Describe the structure of one nucleotide. [2 marks]
(b) Name the four bases found in DNA. [2 marks]
(c) A section of a gene has 18 bases. How many amino acids does it code for? [1 mark]
(d) Higher tier: one strand of DNA has the base order G G T A C. Give the base order on the complementary strand. [2 marks]
Model answer
(a) A nucleotide has a common sugar and a phosphate group (1) with one of four different bases attached to the sugar (1).
(b) A, C, G and T (2 marks if all four are correct, 1 mark for two or three correct).
(c) 6 amino acids (1), because 18 divided by 3 is 6.
(d) C C A T G (2 marks if all five are correct, 1 mark for three or four correct). G pairs with C, and T pairs with A.
Exam tip
In part (c), the answer is 6, not 18. Always divide the number of bases by 3, because three bases make one amino acid.