By the end of this lesson
- Define genome and gene.
- Relate DNA sequence to functional products.
- Distinguish genes from the whole genome.
BIG QUESTION
How can one molecule contain the instructions for an entire organism?
A genome is organised into many functional and regulatory DNA sequences rather than a single uninterrupted instruction.
The genome is the complete genetic material
An organism’s genomeAll the genetic material (DNA) of an organism. is its complete set of genetic material. In eukaryotes, most DNA is in nuclear chromosomes, with small amounts in mitochondria and, in plants, chloroplasts.
Different species have different genome sequences and organisations. Genome size alone does not measure biological complexity.
A gene is a functional DNA sequence
A geneA length of DNA on a chromosome that codes for a functional product, usually a protein. is a length of DNA that contributes to a functional product, commonly a protein or functional RNA. Protein-coding genes specify amino-acid sequences through an RNA intermediate.
Genes also require regulatory sequences that help determine where, when and how strongly they are expressed.
Not every DNA sequence is a protein-coding gene
Genomes contain protein-coding genes, RNA genes, regulatory DNA and other non-coding sequences. Non-coding does not automatically mean useless.
Most body cells contain the same genome but express different combinations of genes, producing specialised cell functions.
Levels of genetic information
The terms describe different scales.
| Term | Scale | Meaning |
|---|---|---|
| Genome | All genetic material. | Complete DNA complement of an organism/cell. |
| Chromosome | One long DNA molecule plus proteins. | Carries many genes. |
| Gene | Defined DNA sequence. | Produces a functional RNA or protein product. |
| Allele | Version of a gene. | Differs in DNA sequence. |
| Base | Single DNA unit component. | Contributes to encoded sequence. |
Language that earns precision
- genome
- Complete genetic material of an organism.
- gene
- DNA sequence that contributes to a functional RNA or protein product.
- gene expression
- Use of genetic information to produce a functional product.
- non-coding DNA
- DNA not translated into protein; it may have regulatory or other functions.
Common misconceptions
Claim: The genome is one gene controlling everything.
Correction: It contains many genes and non-coding sequences.
Claim: Every DNA sequence codes for a protein.
Correction: Many sequences are regulatory, structural or transcribed into functional RNA.
Claim: Different body cells contain completely different genes.
Correction: Most contain the same genome but express different genes.
Match the answer to the command word
Genome = complete genetic material; gene = a functional DNA sequence.
State scale as well as function.
Recall, apply, then justify
Distinguish a genome from a gene.
Hint
Complete set versus one functional sequence.
Suggested marking points
- A genome is the complete genetic material of an organism.
- A gene is a DNA sequence contributing to a functional product.
Explain how cells with the same genome can have different functions.
Hint
Expression → proteins → specialised function.
Suggested marking points
- Different cell types express different combinations of genes.
- They therefore produce different proteins.
- The proteins give cells different structures and functions.
Evaluate the statement “non-coding DNA has no function”.
Planning hint
Consider regulation, RNA and evidence.
What a strong design includes
- Non-coding means not translated into protein, not necessarily functionless.
- Some non-coding DNA regulates gene expression.
- Some is transcribed into functional RNA.
- Other sequences contribute to chromosome structure.
- Some sequences may have little or no selected function.
- Function must be supported by evidence rather than assumed from coding status.