Lesson 3 of 6 / Molecular genetics
From a gene to a polypeptide
How does DNA information affect what a cell can do?
I can connect the order of DNA bases to a specific polypeptide and its possible function.
I can define a gene as a nucleotide sequence, polypeptide code and inheritance unit.Syllabus
Syllabus K325, Topic 12(d). Define a gene as a nucleotide sequence, polypeptide code and inheritance unit.
I can connect DNA information to specific polypeptide synthesis.Syllabus
Syllabus K325, Topic 12(e). Connect DNA information to specific polypeptide synthesis.
About 5 min
Make a guess. You are not marked.
A gene "codes for" a protein. What does the gene actually provide?
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The order in which amino acids are joined.
The base sequence of the gene sets the amino acid sequence. That sequence decides how the protein folds and works.
The key idea
The sequence in a gene carries information used to assemble a particular amino acid sequence. The polypeptide can fold into a protein with a specific role.
A polypeptide is a chain of amino acids. A protein consists of one or more folded polypeptides. This lesson follows the information relationship without the detailed stages of protein synthesis.
Explore the idea
Follow information to function
The DNA sequence supplies information. Amino acids supply the material for the polypeptide, whose folding contributes to protein function.
Original conceptual model. The coloured circles are illustrative amino acids, not an actual coded sequence. Not every DNA change alters a polypeptide or its function.
Explanation
The order of bases in a gene carries the genetic information. Cells use that information to assemble amino acids in a particular order, producing a specific polypeptide. Details of the intermediate transcription and translation stages are not needed in this syllabus.
A polypeptide folds, and one or more polypeptides can form a functional protein. The arrangement of amino acids influences the folded structure. For an enzyme, the structure includes the active site that enables a suitable substrate to bind.
Proteins have many roles, including forming cell structures, transporting substances and catalysing reactions. A gene can therefore affect a characteristic through the function of its protein product. The connection is a causal chain, not a claim that a whole visible trait is always controlled by only one gene.
A change in the DNA sequence can change the amino acid sequence, potentially changing protein shape or function. Not every change has that effect; the sequence change and its context matter. The main idea is that genetic information specifies a product, while amino acids supply the material for building it.
Step by step
- 1
Locate the information
It is in the order of DNA bases within the gene.
- 2
Name the product
The information is used to synthesise a particular polypeptide from amino acids.
- 3
Connect structure to function
The amino acid sequence helps determine folding and the protein's role.
Worked example
An enzyme with a changed active site
A DNA change alters the amino acid sequence of an enzyme, changing its active-site shape. Explain why its reaction rate may decrease.
One way to explain it
The altered amino acid sequence changes the enzyme's folded structure. Its active site may no longer be complementary to the substrate, so fewer productive enzyme-substrate complexes form and the reaction rate can decrease.
Why this answer works
- Connect DNA information to amino acid sequence.
- Connect sequence to protein structure.
- Connect active-site shape to enzyme function.
Watch out for this
A student says: "The gene leaves the chromosome and turns into a protein." What is wrong with this?
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The DNA sequence supplies information for synthesis. The protein is built from amino acids; it is not made by converting DNA material into protein.
Check your understanding
Which chain best explains how a gene can affect enzyme activity?
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DNA base sequence specifies amino acid sequence, which affects protein shape and function.
This connects inherited information to the structure and activity of the protein product.
Exam-style questions
Check your understanding
A gene has 900 bases in the coding sequence, read in groups of three. How many amino acids can it code for at most?
Show the answer
300
Three bases code for one amino acid, so 900 / 3 = 300.
Quick recall
How does DNA information affect what a cell can do?