Genomes and Chromosomes
An organism's genetic material is organised as a hierarchy, from the complete genetic blueprint (the genome) down to the structures that hold it (chromosomes) and the molecule itself (DNA).
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The genetic instructions for an organism are encoded in a molecule called DNA.
- Inside the nucleus of a eukaryotic cell, this DNA is tightly coiled and organised into structures called chromosomes.
- The genome is the term for the entire, complete set of an organism's DNA.
- Therefore, the genome is comprised of all the DNA found across all the chromosomes within a typical cell of that organism.
Definitions
- Genome
- The full set of genetic material (DNA) present in an organism.
- Chromosome
- A long, thread-like structure composed of tightly packaged DNA, found within the cell's nucleus.
- DNA
- Deoxyribonucleic acid, the molecule that carries the genetic instructions for life.
Worked example
A biologist is describing the genetic material of a fruit fly. Which statement correctly orders the components from largest and most inclusive to smallest and most fundamental?
- 1
Identify the three key terms:
Genome, Chromosome, DNA.
- 2
Recall the definitions.
The 'genome' refers to the entire collection of genetic material.
- 3
The genome is organised into individual 'chromosomes'.
Therefore, the genome is a more inclusive (larger) concept than a single chromosome.
- 4
Each 'chromosome' is made of a long, coiled molecule of 'DNA'.
Therefore, a chromosome is a larger structure than the DNA molecule it contains.
- 5
The correct order from most inclusive to most fundamental is Genome → Chromosome → DNA.
- 6
Check the options provided in the full question to find the one that matches this hierarchy.
Answer: The correct hierarchy is Genome, then Chromosome, then DNA.
Common mistakes
- ×Confusing the hierarchy. A frequent error is to state that a chromosome contains the entire genome. The genome is the total set of ALL chromosomes.
- ×Mixing up the composition. For instance, incorrectly stating 'the genome is made of chromosomes' is less precise than 'the genome consists of the DNA which is organised into chromosomes'.
No-calculator tips
- ✓Use an analogy to remember the scale: The Genome is the entire library, a Chromosome is a single book in that library, and DNA is the text written in the book.
- ✓Focus on keywords in questions like 'set', 'collection', 'structure', and 'molecule' to distinguish between the genome, a chromosome, and DNA.