Lesson 1 of 6 / Molecular genetics
DNA, genes and chromosomes
How are these three terms connected?
I can explain that genes are DNA sequences carried on chromosomes and act as units of inheritance.
I can relate DNA, genes and chromosomes.Syllabus
Syllabus K325, Topic 12(a). Relate DNA, genes and chromosomes.
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.
About 4 min
Make a guess. You are not marked.
Which is the largest: a gene, a chromosome or a DNA base?
Show the answer
A chromosome, which holds many genes.
A chromosome is one long DNA molecule carrying many genes. Each gene is a section made of many bases.
The key idea
A chromosome contains a long DNA molecule carrying many genes. A gene is a sequence of nucleotides with instructions for a polypeptide and is a unit of inheritance.
DNA is the molecule that carries genetic information. A nucleotide is one of the smaller units joined to form DNA; the next lesson explores its structure.
Explore the idea
Find a gene within the DNA
A chromosome organises a long DNA molecule with associated proteins. It carries many genes, not only the highlighted region.
Original hierarchy model with changing scale. The condensed shape shows one unreplicated chromosome; chromosomes are not always visibly condensed. Gene regions are coloured for teaching, not naturally orange.
Explanation
DNA is a long molecule that stores genetic information in the order of its nucleotide bases. In a typical plant or animal nucleus, DNA is organised into chromosomes with associated proteins. A chromosome carries many genes along its length.
A gene is a particular sequence of nucleotides forming part of a DNA molecule. For this syllabus, each gene codes for one polypeptide. A polypeptide is a chain of amino acids that can form part or all of a protein.
A gene is also a unit of inheritance: genetic information can be passed from parent to offspring. The resulting proteins contribute to cell structures and activities, helping produce characteristics. A characteristic need not depend on only one gene or be unaffected by the environment.
Keep the levels distinct. DNA is the information-carrying molecule; a gene is a section with a particular instruction; a chromosome is the organised structure containing a long DNA molecule and many genes. Diagrams often enlarge a small region to make this relationship visible.
Step by step
- 1
Start with the chromosome
It carries a long DNA molecule together with associated proteins in a typical plant or animal nucleus.
- 2
Locate a gene on the DNA
A gene is a sequence within that molecule, not an extra chromosome or a protein.
- 3
State its informational role
It specifies a polypeptide and forms a unit of inherited information.
Worked example
Correcting a mixed-up label
A student labels a short section of DNA "a chromosome made of one gene". Explain the correction.
One way to explain it
A short functional sequence within a DNA molecule can represent a gene. A chromosome contains a long DNA molecule carrying many genes, so the short section should not be identified as the whole chromosome.
Why this answer works
- Identify the level being shown.
- Place a gene within a DNA molecule.
- Place many genes along a chromosome.
Watch out for this
A student says: "A gene is a small protein inside a chromosome." What is wrong with this?
Show the answer
A gene is a DNA nucleotide sequence. Its information is used to make a polypeptide; the gene and its protein product are different kinds of molecule.
Check your understanding
Which statement gives the correct relationship?
Show the answer
A gene is a section of DNA carried on a chromosome.
Keep the molecule, a particular sequence and the organised chromosome structure distinct.
Quick recall
How are these three terms connected?