The Code Connects Sequences

A gene is a length of DNA whose base sequence can determine the amino acid sequence in a polypeptide. The amino acid sequence is the primary structure of a protein. The genetic code is the set of rules that connects the order of bases in a gene to the order of amino acids in the polypeptide made from that gene.

The key word is sequence. DNA is a sequence of bases, not a sequence of amino acids. A polypeptide is a sequence of amino acids. The genetic code is the bridge between those two different kinds of sequence.

The code is read in groups of three bases. A group of three bases is called a triplet. When the triplet is on mRNA it is usually called a codon. Keep the wording precise: a DNA gene has a base sequence; mRNA has codons; the polypeptide has amino acids.

[DIAGRAM: gene_triplet_code_to_primary_structure: Lesson 034: The nature of the genetic code - diagram 01; asset_slug: 034_m02_1_3_the_nature_of_the_genetic_code__diagram_01; recommended_method: drawn_biology; description: Deterministic 16:9 grey-line schematic showing a DNA coding-strand example grouped into non-overlapping triplets, corresponding mRNA codons, resulting amino-acid sequence as primary structure, plus compact panels for triplet/non-overlapping, degenerate and universal properties.]

The diagram is not asking you to memorise a codon table. It shows the logic: if the base sequence is grouped into triplets in a fixed order, the order of triplets determines the order of amino acids, so the gene determines the primary structure of the polypeptide.

Triplet And Non-Overlapping

There are four DNA bases: adenine, thymine, cytosine and guanine. A one-base code could only make four different instructions. A two-base code could make 4 x 4 = 16 different instructions. That is not enough for the 20 amino acids commonly found in proteins.

A triplet code gives:

That is enough possible triplets to specify the amino acids used in proteins. You do not need to memorise every codon in a codon table here. You do need the triplet idea: three bases code for one amino acid.

The code is also non-overlapping. This means each base is used in only one triplet as the sequence is read. For example, this base sequence:

A T G C C A G A A

is read as:

ATG | CCA | GAA

It is not read by sliding along one base at a time as ATG, then TGC, then GCC, and so on. That sliding pattern would reuse bases and would be overlapping, which is not the property being tested.

Now try the same grouping without the scaffold.

The Degenerate Code

The genetic code is degenerate. In biology this does not mean damaged or worse. It means that more than one triplet can code for the same amino acid.

This follows naturally from the triplet calculation. There are 64 possible triplets but only 20 commonly used amino acids. Some amino acids must therefore have more than one triplet that can specify them.

Be careful with the direction of the idea. Degenerate does not mean that one triplet randomly chooses several amino acids. In a given context, a triplet has a specific meaning. Degenerate means the reverse: several different triplets may lead to the same amino acid.

This idea explains why a change in a base sequence may or may not alter a protein. For this lesson, you do not need to classify mutation types. The point is simply that a changed triplet can sometimes still specify the same amino acid because the code is degenerate.

The Universal Code

The genetic code is universal. This means the same triplet or codon specifies the same amino acid in almost all organisms. The same code is used in bacteria, plants, animals and fungi.

The scientific caution is "almost all". A few rare exceptions exist, such as small differences in some mitochondrial genetic codes. For this course, the required property is still universal, so an exam answer should use that word and explain the main idea: the code is shared across organisms.

This matters because genes are portable biological information. If the code is the same in different organisms, then the base sequence of a gene can still specify the same amino acid sequence when it is interpreted in another organism. You do not need the details of genetic engineering for this row, but universality explains why the idea is biologically powerful.

Answer Precision

This lesson is short, but the wording must be sharp. Mark schemes often reward the exact link from bases to amino acids and then to primary structure.

A strong answer to "Describe the nature of the genetic code" could say:

"The genetic code is triplet, so three bases code for one amino acid. It is non-overlapping, so each base is read once as part of one triplet. It is degenerate because more than one triplet can code for the same amino acid, and it is universal because the same triplet codes for the same amino acid in almost all organisms. Therefore the sequence of bases in a gene determines the sequence of amino acids in the polypeptide, which is its primary structure."

Answer Precision Summary

Explain It Back

Use this as a self-explanation check after the section above. It is for diagnosing what you can already explain, not for learning new material from scratch.