You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
This lesson covers the structure of genes and the nature of the genetic code as required by the Edexcel A-Level Biology specification (9BI0, Topic 7). You need to understand how genes are organised within DNA, the triplet nature of the genetic code, and its key properties.
A gene is a sequence of nucleotides on a DNA molecule that codes for a functional polypeptide (or a functional RNA molecule such as tRNA or rRNA). Genes represent only a small proportion of the total DNA in a eukaryotic cell — in humans, protein-coding sequences account for roughly 1.5% of the genome.
Each gene occupies a specific position on a chromosome called its locus (plural: loci). The two copies of a gene at the same locus on homologous chromosomes are called alleles. Alleles may be identical (homozygous) or different (heterozygous).
Exam Tip: Be precise with definitions. A gene codes for a polypeptide, not necessarily a whole protein — many functional proteins are composed of multiple polypeptide chains coded by different genes (e.g. haemoglobin has two alpha and two beta globin chains).
DNA is a double-stranded polynucleotide with an antiparallel arrangement. Each nucleotide consists of:
| Component | Details |
|---|---|
| Phosphate group | Links to the 5' carbon of deoxyribose |
| Deoxyribose sugar | A five-carbon (pentose) sugar |
| Nitrogenous base | Adenine (A), Thymine (T), Guanine (G) or Cytosine (C) |
The two strands are held together by hydrogen bonds between complementary base pairs:
The strands run in opposite directions — one strand runs 5' to 3' and the other 3' to 5'. This antiparallel arrangement is critical for replication and transcription.
The following diagram summarises the key events at the replication fork during DNA replication:
graph TD
A["Double-Stranded DNA"] -->|"Helicase unwinds"| B["Replication Fork"]
B --> C["Leading Strand<br/>(continuous, 5'→3')"]
B --> D["Lagging Strand<br/>(Okazaki fragments, 3'→5')"]
C --> E["DNA Polymerase III"]
D --> E
E -->|"Proofreading"| F["Two Identical<br/>DNA Molecules"]
The genetic code is a triplet code — each sequence of three consecutive nucleotide bases on the coding (sense) strand of DNA specifies one amino acid. These triplets are called codons when referring to mRNA.
With four bases and three positions per codon, there are 4 × 4 × 4 = 64 possible codons. Since there are only 20 amino acids commonly used in proteins, the code is said to be degenerate (redundant) — most amino acids are coded for by more than one codon.
| Property | Meaning |
|---|---|
| Triplet | Three bases code for one amino acid |
| Degenerate | Most amino acids have more than one codon |
| Non-overlapping | Each base is read only once as part of one triplet |
| Universal | The same codons code for the same amino acids in almost all organisms |
| Comma-free | Codons are read sequentially with no gaps or punctuation between them |
Exam Tip: The universality of the genetic code is strong evidence for a common evolutionary origin of all life. However, there are rare exceptions — for example, mitochondria and some protists use slightly different codes.
In eukaryotic genes, the coding sequence is not continuous. Genes contain:
The proportion of intronic DNA varies enormously between genes. Some genes have no introns at all, while others (such as the dystrophin gene) have dozens of large introns that make up over 99% of the gene's length.
Introns allow alternative splicing — different combinations of exons can be joined together to produce different mRNA molecules from the same gene. This means a single gene can code for multiple different polypeptides, greatly increasing the coding potential of the genome.
For example, the Drosophila DSCAM gene can produce over 38,000 different mRNA variants through alternative splicing.
DNA is double-stranded, but only one strand is used as a template during transcription:
| Strand | Also known as | Role |
|---|---|---|
| Template strand | Antisense strand, non-coding strand | Read 3' → 5' by RNA polymerase |
| Coding strand | Sense strand, non-template strand | Has the same base sequence as the mRNA (with T instead of U) |
The mRNA produced during transcription has the same base sequence as the coding strand, except that thymine (T) is replaced by uracil (U).
Exam Tip: When asked to write the mRNA sequence from a DNA strand, first check which strand you have been given. If it is the template strand, write the complementary sequence replacing T with U. If it is the coding strand, simply replace T with U.
Translation of mRNA into a polypeptide begins at a start codon and ends at a stop codon:
| Codon type | Codon(s) | Function |
|---|---|---|
| Start codon | AUG | Signals the start of translation; codes for methionine |
| Stop codons | UAA, UAG, UGA | Signal the end of translation; do not code for any amino acid |
The sequence of codons from the start codon to the stop codon is called the open reading frame (ORF). The reading frame is established by the position of the start codon — if the frame shifts by even one base, completely different amino acids will be specified.
A large proportion of eukaryotic DNA does not code for polypeptides. This includes:
Historically, non-coding DNA was dismissed as "junk DNA", but research has revealed that much of it has important regulatory and structural functions.
The Human Genome Project (HGP) was an international collaborative project completed in 2003 that determined the complete nucleotide sequence of the human genome. Key findings include:
The HGP has had profound implications for medicine, forensics and our understanding of evolution.
| Concept | Key Detail |
|---|---|
| Gene | Sequence of nucleotides coding for a polypeptide or functional RNA |
| Locus | Specific position of a gene on a chromosome |
| Triplet code | Three bases = one amino acid |
| Degenerate | Multiple codons for most amino acids |
| Universal | Same code in (almost) all organisms |
| Exons | Coding sequences |
| Introns | Non-coding sequences removed during splicing |
| Start codon | AUG (methionine) |
| Stop codons | UAA, UAG, UGA |
Exam Tip: Questions on the genetic code frequently ask you to explain why it is described as degenerate, non-overlapping or universal. Always give a clear definition and then an example or explanation of the biological significance.