GCSE Biology revision

DNA structure (biology only)

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AQA 4.1.1.1

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AQA student objectives

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ALL 🎯

Identify and describe What is DNA Made Of?, The Double Helix Structure and The Triplet Code in the context of dNA structure (biology only).

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Explain the relationships between What is DNA Made Of?, The Double Helix Structure and The Triplet Code and how they contribute to dNA structure (biology only).

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Apply and analyse knowledge of What is DNA Made Of?, The Double Helix Structure and The Triplet Code to interpret unfamiliar information about dNA structure (biology only).

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Key knowledge

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What is DNA Made Of?

DNA is a polymer made up of repeating units called nucleotides, which are the monomers of DNA.
Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group, and a nitrogenous base.
The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
Nucleotides are joined together via phosphate bonds between the deoxyribose sugars, forming a long polynucleotide chain.

1. What is DNA Made Of?AQA

Notes

DNA is a polymer made up of repeating units called nucleotides, which are the monomers of DNA.
Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group, and a nitrogenous base.
The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
Nucleotides are joined together via phosphate bonds between the deoxyribose sugars, forming a long polynucleotide chain.

The Double Helix Structure

DNA has a double helix structure with a sugar-phosphate backbone on the outside and bases on the inside.
Purines (adenine and guanine) always bind with pyrimidines (thymine and cytosine) through complementary base pairing.
Adenine always pairs with thymine (A–T) and cytosine always pairs with guanine (C–G); any other pairing causes severe physiological abnormalities.
The bonds between complementary base pairs are the stronger bonds that hold the two strands of DNA together.

2. The Double Helix StructureAQA

Notes

DNA has a double helix structure with a sugar-phosphate backbone on the outside and bases on the inside.
Purines (adenine and guanine) always bind with pyrimidines (thymine and cytosine) through complementary base pairing.
Adenine always pairs with thymine (A–T) and cytosine always pairs with guanine (C–G); any other pairing causes severe physiological abnormalities.
The bonds between complementary base pairs are the stronger bonds that hold the two strands of DNA together.

The Triplet Code

Each amino acid is coded for by a sequence of three bases called a triplet code or codon.
The triplet code is universal, meaning all organisms use the same codons to code for the same amino acids.
The code is degenerate, meaning more than one codon can code for the same amino acid, offering some protection against the effects of mutation.
The code is non-overlapping, meaning it is read in consecutive groups of three bases, so any deletion or insertion can cause a frameshift mutation that alters the entire protein produced.

3. The Triplet CodeAQA

Notes

Each amino acid is coded for by a sequence of three bases called a triplet code or codon.
The triplet code is universal, meaning all organisms use the same codons to code for the same amino acids.
The code is degenerate, meaning more than one codon can code for the same amino acid, offering some protection against the effects of mutation.
The code is non-overlapping, meaning it is read in consecutive groups of three bases, so any deletion or insertion can cause a frameshift mutation that alters the entire protein produced.

Transcription: Copying the DNA

Protein synthesis occurs in two stages: transcription and translation.
Transcription takes place in the nucleus, where a complementary strand of messenger RNA (mRNA) is synthesised from the DNA template.
In mRNA, uracil (U) replaces thymine (T) as the complementary base to adenine, so the mRNA sequence is complementary (not identical) to the DNA template strand.
The mRNA molecule then leaves the nucleus and travels to the ribosomes in the cytoplasm, keeping the DNA safely within the nucleus.

4. Transcription: Copying the DNAAQA

Notes

Protein synthesis occurs in two stages: transcription and translation.
Transcription takes place in the nucleus, where a complementary strand of messenger RNA (mRNA) is synthesised from the DNA template.
In mRNA, uracil (U) replaces thymine (T) as the complementary base to adenine, so the mRNA sequence is complementary (not identical) to the DNA template strand.
The mRNA molecule then leaves the nucleus and travels to the ribosomes in the cytoplasm, keeping the DNA safely within the nucleus.

Translation: Building the Protein

Translation occurs at the ribosomes in the cytoplasm, where the mRNA sequence is used to assemble a polypeptide chain.
Transfer RNA (tRNA) molecules carry specific amino acids and have anticodon sequences that are complementary to the codons on the mRNA.
The ribosome moves along the mRNA, and tRNA molecules bring the correct amino acids in sequence, which are then joined together to form a polypeptide.
The order of bases in the DNA ultimately determines the order of amino acids in the polypeptide, and therefore the structure and function of the protein.

5. Translation: Building the ProteinAQA

Notes

Translation occurs at the ribosomes in the cytoplasm, where the mRNA sequence is used to assemble a polypeptide chain.
Transfer RNA (tRNA) molecules carry specific amino acids and have anticodon sequences that are complementary to the codons on the mRNA.
The ribosome moves along the mRNA, and tRNA molecules bring the correct amino acids in sequence, which are then joined together to form a polypeptide.
The order of bases in the DNA ultimately determines the order of amino acids in the polypeptide, and therefore the structure and function of the protein.

Protein Structure and Function

Once a polypeptide chain is formed, it folds into a unique three-dimensional shape determined by its amino acid sequence.
The specific shape of a protein is critical for its function; for example, enzymes have a uniquely shaped active site that must be complementary to their substrate.
The primary sequence of amino acids (determined by the DNA base sequence) dictates the final structure and therefore the function of every protein in the body.

6. Protein Structure and FunctionAQA

Notes

Once a polypeptide chain is formed, it folds into a unique three-dimensional shape determined by its amino acid sequence.
The specific shape of a protein is critical for its function; for example, enzymes have a uniquely shaped active site that must be complementary to their substrate.
The primary sequence of amino acids (determined by the DNA base sequence) dictates the final structure and therefore the function of every protein in the body.

Coding and Non-Coding DNA

Not all DNA codes for proteins; DNA is divided into coding regions, which directly determine the proteins produced, and non-coding regions.
Non-coding DNA acts as a regulatory or controlling sector, switching genes on and off to control which proteins are produced in different cell types.
Every cell in the body contains an identical copy of all the DNA, but specialised cells only express certain genes because non-coding DNA turns other genes off.
This gene regulation explains why liver cells can produce insulin whilst muscle cells produce different proteins such as ATP synthase, despite having the same DNA.

7. Coding and Non-Coding DNAAQA

Notes

Not all DNA codes for proteins; DNA is divided into coding regions, which directly determine the proteins produced, and non-coding regions.
Non-coding DNA acts as a regulatory or controlling sector, switching genes on and off to control which proteins are produced in different cell types.
Every cell in the body contains an identical copy of all the DNA, but specialised cells only express certain genes because non-coding DNA turns other genes off.
This gene regulation explains why liver cells can produce insulin whilst muscle cells produce different proteins such as ATP synthase, despite having the same DNA.

Mutations: Changes in the DNA Sequence

Mutations are random changes in the DNA base sequence that can alter the polypeptide produced and potentially result in non-functional proteins.
A deletion mutation occurs when a base is removed from the sequence, which can cause a frameshift mutation that changes every codon downstream.
An insertion mutation occurs when an extra base is added to the sequence, which can also cause a frameshift mutation.
A substitution mutation occurs when one base is replaced by another, which may change the codon and potentially code for a different amino acid.
Because the triplet code is degenerate, some substitution mutations may still code for the same amino acid, meaning the protein's function is unaffected.
Even small mutations in genes coding for enzymes can alter the shape of the active site, making the enzyme non-functional and causing significant problems in the body.

8. Mutations: Changes in the DNA SequenceAQA

Notes

Mutations are random changes in the DNA base sequence that can alter the polypeptide produced and potentially result in non-functional proteins.
A deletion mutation occurs when a base is removed from the sequence, which can cause a frameshift mutation that changes every codon downstream.
An insertion mutation occurs when an extra base is added to the sequence, which can also cause a frameshift mutation.
A substitution mutation occurs when one base is replaced by another, which may change the codon and potentially code for a different amino acid.
Because the triplet code is degenerate, some substitution mutations may still code for the same amino acid, meaning the protein's function is unaffected.
Even small mutations in genes coding for enzymes can alter the shape of the active site, making the enzyme non-functional and causing significant problems in the body.

Consequences of Mutations

Most mutations do not cause profound changes to proteins because the degenerate nature of the triplet code means the same amino acid may still be produced.
However, mutations in genes coding for key structural or functional proteins can have serious consequences, such as altering an enzyme's active site so it can no longer bind its substrate.
Mutations in DNA that codes for structural repair genes can disrupt the body's ability to maintain and repair tissues correctly.

9. Consequences of MutationsAQA

Notes

Most mutations do not cause profound changes to proteins because the degenerate nature of the triplet code means the same amino acid may still be produced.
However, mutations in genes coding for key structural or functional proteins can have serious consequences, such as altering an enzyme's active site so it can no longer bind its substrate.
Mutations in DNA that codes for structural repair genes can disrupt the body's ability to maintain and repair tissues correctly.

Key Terminology to Remember

Monomer: a single nucleotide; polymer: a long chain of nucleotides joined together to form DNA.
Transcription: the process of copying DNA into mRNA in the nucleus; translation: the process of assembling a polypeptide at the ribosome using mRNA and tRNA.
Complementary base pairing: A pairs with T (or U in RNA), and C pairs with G — this rule underpins both DNA structure and protein synthesis.
The three key features of the triplet code are that it is universal, degenerate, and non-overlapping.

10. Key Terminology to RememberAQA

Notes

Monomer: a single nucleotide; polymer: a long chain of nucleotides joined together to form DNA.
Transcription: the process of copying DNA into mRNA in the nucleus; translation: the process of assembling a polypeptide at the ribosome using mRNA and tRNA.
Complementary base pairing: A pairs with T (or U in RNA), and C pairs with G — this rule underpins both DNA structure and protein synthesis.
The three key features of the triplet code are that it is universal, degenerate, and non-overlapping.