Biology · What Happens in Cells
Nucleotides and Protein Synthesis
A closer look at DNA shows how it stores information. This lesson is for separate Biology. It describes the four nucleotides and complementary base pairing, and, for Higher tier students, how the order of bases in a gene decides the order of amino acids in a protein.
Learning Objectives
- 1Describe DNA as being made from four different nucleotides, each with a common sugar and phosphate group and one of four bases.
- 2Describe complementary base pairing: A with T, and G with C.
- 3(Higher tier) Recall a simple description of protein synthesis: the gene is copied to mRNA in the nucleus, and tRNA brings amino acids to the ribosome in the cytoplasm.
- 4(Higher tier) Explain simply how the structure of DNA affects the proteins made, using the triplet code.
Retrieval practice
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1
What is a polymer?
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A large molecule made of many smaller units joined together.
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2
What is a gene?
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A section of DNA that codes for a protein.
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3
Which part of a cell is the site of protein synthesis?
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The ribosomes.
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4
What are proteins made of?
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Amino acids.
Four letters, one code
A written message is made from a small alphabet. DNA does the same with only four chemical letters, called bases. The order in which the four bases are arranged along a gene is the code for a protein. To see how this works you need to know how the DNA polymer is built from its units, the nucleotides, and which bases pair with which.
Four different nucleotides
DNA is a polymer of nucleotides. There are four different nucleotides, and they differ only in their base.
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Three parts
Each nucleotide has a phosphate group, a sugar, and one base.
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The same in all four
The sugar and the phosphate group are the same in every nucleotide. They join to make the backbone of the strand.
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Four bases
The base can be adenine (A), thymine (T), guanine (G) or cytosine (C).
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A strand
A strand of DNA is a long chain of nucleotides, with the sugars and phosphate groups forming the backbone and the bases sticking out sideways.
Complementary base pairing
The two strands of DNA are joined by pairs of bases.
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A with T
Adenine always pairs with thymine.
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G with C
Guanine always pairs with cytosine.
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Complementary
Because each base only pairs with one other, the bases on one strand decide the bases on the other. The strands are called complementary.
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Weak bonds
The pairs are held together by weak bonds, so the strands can be separated and then copied.
Nucleotides and base pairs
A and T always pair, and G and C always pair.
Writing the complementary strand
One strand of DNA has the base sequence A T G C C T. Write the sequence of bases on the complementary strand.
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- 1 Pair A with T A pairs with T
- 2 Pair T with A T pairs with A
- 3 Pair G with C G pairs with C
- 4 Pair C with G C pairs with G
- 5 Read the whole strand \(A\,T\,G\,C\,C\,T\) pairs with \(T\,A\,C\,G\,G\,A\)
AnswerThe complementary strand is T A C G G A.
From gene to protein (Higher tier)
DNA stays in the nucleus, so a copy of the gene is made and taken to the ribosomes, where proteins are made.
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Unzipping
The part of the DNA that is the gene unzips, which means the two strands separate.
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Transcription
One strand is used as a template to make a copy of the gene, called messenger RNA (mRNA), in the nucleus. The bases are copied by complementary base pairing.
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To the cytoplasm
The mRNA leaves the nucleus and travels to a ribosome in the cytoplasm.
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Translation
At the ribosome, the order of bases in the mRNA is read. Molecules of transfer RNA (tRNA) carry amino acids to the ribosome, and the amino acids are joined in the order the mRNA specifies.
Making a protein (Higher tier)
Transcription makes mRNA in the nucleus, and translation makes the protein at the ribosome.
The triplet code (Higher tier)
The order of bases in a gene decides the order of amino acids in the protein.
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Triplets
A group of three bases is called a triplet. Each triplet codes for one amino acid.
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Order
The order of the triplets decides the order of the amino acids. A gene with 300 bases codes for 100 amino acids.
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The protein
The order of amino acids decides how the chain folds into a particular shape, and the shape decides what the protein does.
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A change
If the order of bases changes, the amino acids may change, and the protein may fold into a different shape and no longer work.
How many amino acids?
A section of a gene has 96 bases. Each triplet of bases codes for one amino acid. Calculate the number of amino acids in the protein that the section codes for.
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- 1 Each amino acid needs 3 bases One triplet codes for one amino acid
- 2 Divide the bases by 3 \(96 \div 3\)
- 3 Give the answer \(= 32\) amino acids
AnswerThe section codes for 32 amino acids.
DNA and mRNA (Higher tier)
DNA
- Two strands in a double helix
- Stays in the nucleus
- Contains the whole gene and many genes
- Is not used to make the protein directly
mRNA
- A single strand
- Leaves the nucleus
- A copy of one gene
- Read by the ribosome to make the protein
Case study
One wrong base, one wrong protein
Sickle cell anaemia is caused by a change to a single base in the gene for one of the proteins in haemoglobin. The change makes one amino acid different, and the protein folds into a different shape. The red blood cells that contain it can become bent into a sickle shape, so they carry oxygen less well and can block small blood vessels. This shows that a small change in the order of bases can change the protein.
The big idea
DNA is a polymer of four nucleotides, whose bases pair A with T and G with C, and for Higher tier the order of the bases in a gene decides the order of amino acids in the protein.
Nucleotides and proteins
Nucleotides and proteins
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Nucleotide
- phosphate group
- sugar and one base
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Four bases
- A
- T
- G and C
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Base pairs
- A with T
- G with C
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Protein synthesis
- transcription to mRNA
- translation at the ribosome (Higher tier)
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Triplet code
- three bases code for one amino acid (Higher tier)
Summary and exam focus
- DNA is made from four nucleotides, each with a phosphate group, a sugar and one base.
- The bases are A, T, G and C.
- A pairs with T, and G pairs with C: this is complementary base pairing.
- (Higher tier) The gene is copied to mRNA in the nucleus, and the mRNA is read at a ribosome.
- (Higher tier) tRNA brings amino acids to the ribosome, where they are joined in order.
- (Higher tier) Three bases, a triplet, code for one amino acid.
Exam focus
Describe the structure of a nucleotide and explain what is meant by complementary base pairing. (4 marks) (4 marks)
Name all three parts of a nucleotide: a phosphate group, a sugar and a base. Then say that A pairs with T and G pairs with C, so one strand decides the order of bases on the other.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Nucleotide
- A unit of DNA made of a phosphate group, a sugar and one base.
- Base
- One of the four chemicals A, T, G and C in DNA.
- Complementary base pairing
- The rule that A pairs with T and G pairs with C.
- mRNA
- A copy of a gene, made in the nucleus, that carries the code to the ribosome (Higher tier).
- Transcription
- Making a copy of a gene as mRNA (Higher tier).
- Translation
- Reading the mRNA at the ribosome to join amino acids in order (Higher tier).
- tRNA
- The molecule that carries an amino acid to the ribosome (Higher tier).
- Triplet
- A group of three bases that codes for one amino acid (Higher tier).
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