Genome projects and sequencingAQA A-Level Biology: Revision notes
Section 1
Genome projects
The genome is the complete set of genes (DNA) in a cell. Sequencing projects have read the genomes of a wide range of organisms, including humans. Knowing the base sequence of all of an organism's DNA allows comparisons between species and individuals.
Section 2
From genome to proteome
The proteome is the full range of proteins that a cell is able to produce. In simpler organisms, such as bacteria, most of the DNA codes for protein, so the base sequence of the genes lets us determine the amino acid sequences of the proteins.
One application is identifying potential antigens for vaccine production: surface proteins of the pathogen are found from its genome.
Section 3
Complex organisms
In more complex organisms the genome cannot easily be translated into the proteome because of:
- non-coding DNA, which does not code for amino acids, so coding sequences are hard to identify
- regulatory genes, which switch other genes on or off, so different cells make different proteins from the same genome
Compare organisms using the percentage of DNA that codes for protein.
Section 4
Developing sequencing methods
Sequencing methods are continuously updated and have become automated. Modern machines read DNA much faster and more cheaply than the methods used for the first human genome, and computers analyse the data. This allows many more genomes to be sequenced.
Section 5
Worked example: comparing genomes
Human genome: 3.2 billion base pairs, 20,000 genes. E. coli: 4.6 million base pairs, 4,300 genes.
Size ratio: 3.2 × 10⁹ ÷ 4.6 × 10⁶ ≈ 700. Gene ratio: 20,000 ÷ 4,300 ≈ 4.7. The human genome is about 700 times larger but has only about 4.7 times as many genes, because most human DNA is non-coding.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Genome projects and sequencing
- Researchers sequenced the genome of a disease-causing bacterium. It has a single circular chromosome of 2.2 million base pairs and about 2,000 genes. The team wants to identify proteins on the surface of the bacterium that could be used as antigens in a vaccine.Explain how knowing the genome of the bacterium allows potential antigens for a vaccine to be identified.2 marks
- The human genome contains about 3.2 billion base pairs. Only about 1.5% of this DNA codes for proteins, and there are about 20,000 protein-coding genes. A human cell can produce many more different proteins than there are genes, and different cell types produce very different sets of proteins from the same genome.Explain why only part of the human genome can be used to predict the proteins produced by a cell.2 marks
- During an outbreak of a new bacterial disease, scientists used automated sequencing machines to read the whole genome of the bacterium in two days. The first human genome took 13 years to sequence, but sequencing methods have been continuously updated and now cost far less.Suggest three benefits of automated sequencing methods compared with the methods used in the first human genome project.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).