How does the base sequence of DNA carry the code for making proteins?
The double-stranded structure of DNA with complementary base pairing, the role of the base sequence as the genetic code for the order of amino acids, and how mRNA carries a complementary copy from the nucleus to the ribosome for protein synthesis.
An SQA National 5 Biology answer on DNA and the production of proteins, covering the double-stranded structure of DNA, complementary base pairing, the base sequence as the genetic code, and how messenger RNA carries a copy from the nucleus to the ribosome for protein synthesis.
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What this dot point is asking
The SQA wants you to describe the structure of DNA, state the complementary base-pairing rule, explain that the base sequence is the genetic code that sets the order of amino acids in a protein, and describe how messenger RNA (mRNA) carries a copy of that code from the nucleus to the ribosome, where the protein is made.
The structure of DNA
Because pairing is complementary, the sequence on one strand tells you the sequence on the other. If one strand reads A-T-G-C, the partner strand must read T-A-C-G. This rule lets DNA be copied accurately before a cell divides.
DNA is organised into chromosomes inside the nucleus. A gene is a section of DNA that codes for one protein.
The base sequence is the genetic code
Each protein has its own amino acid order, and that order is set by the base sequence of its gene. Change the base sequence and you can change the protein, which is why the code must be copied accurately.
Why a copy is needed: mRNA
The DNA stays inside the nucleus, but proteins are built at ribosomes in the cytoplasm. The code therefore has to be copied and carried out.
So the flow is: gene in DNA gives a complementary mRNA copy, mRNA travels to a ribosome, and the ribosome reads the mRNA to join amino acids in the correct order, building the protein.
Building the protein
At the ribosome, amino acids are joined together in a specific order to form the protein. There are only about twenty different amino acids, but the order in which they are joined, set by the mRNA, gives a huge variety of proteins, each with its own shape and function.
Examples in context
Example 1. One gene, one protein. The gene for the hormone insulin has a fixed base sequence. Its mRNA copy is read at the ribosome, joining amino acids in the exact order that gives a working insulin molecule. If the base sequence were different, the amino acid order, and so the protein, would change. This is why genetic engineering can move the insulin gene into bacteria and still get correct insulin (covered in the genetic engineering key area).
Example 2. Sickle cell and a single base change. A change to just one base in the gene for haemoglobin changes one amino acid in the protein. That single change alters the shape of the haemoglobin and causes sickle cell disease. This shows how directly the base sequence controls the amino acid order and the final protein.
Try this
Q1. A DNA strand reads C-G-A-T. Write its complementary strand. [1 mark]
- Cue. Apply C-G and A-T pairing: G-C-T-A.
Q2. State the site of protein synthesis in the cell. [1 mark]
- Cue. The ribosome.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksDescribe the structure of DNA, including the names of the bases and the base-pairing rule.Show worked answer →
A 3-mark describe answer should cover the shape, the bases, and the pairing rule.
DNA is a double-stranded molecule twisted into a double helix. The two strands are held together by complementary base pairs.
There are four bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
The base-pairing rule is that adenine always pairs with thymine (A with T) and cytosine always pairs with guanine (C with G).
Markers reward (1) the double-stranded or double-helix structure, (2) naming the four bases, and (3) the correct A-T and C-G pairing rule.
SQA N5 style3 marksExplain the role of mRNA in the production of a protein.Show worked answer →
The question is about the messenger, so trace it from nucleus to ribosome.
The genetic code is held in the DNA in the nucleus, but DNA cannot leave the nucleus. A copy is needed.
Messenger RNA (mRNA) is made as a complementary copy of the base sequence of a gene. It is a single strand.
The mRNA then carries this copy out of the nucleus to a ribosome, which is the site of protein synthesis, where the code is used to join amino acids in the correct order.
Markers reward (1) mRNA being a complementary copy of the DNA, (2) mRNA carrying it from the nucleus to the ribosome, and (3) the ribosome being where the protein is made.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)