Genomes and Chromosomes
This topic covers the fundamental organisation of an organism's genetic material. It defines the 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.