International A Level Biology / Gene technology
Microarrays and bioinformatics
Edexcel International A Level Biology (YBI11), chapter 25: Gene technology, specification point 8.20, 8.21. Free lesson.
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A microarray shows which genes are active by detecting labelled cDNA that binds to DNA probes on a chip. Bioinformatics uses computers to store and analyse biological data, including microarray results.
How a microarray is built
A microarray, or DNA chip, is a glass slide covered in thousands of tiny spots. Each spot tests one known gene. It holds short, single-stranded DNA probes with a known sequence. Matching cDNA can bind to a probe by complementary base pairing. This binding is called hybridisation. The probes are fixed to the chip, so they do not wash off. Each spot has a different probe, so its position tells you which gene is being tested. A fluorescent tag attached to cDNA glows under a laser. A spot glows when labelled cDNA has hybridised with its probe.
How the array reveals active genes
A cell makes mRNA from genes it is using. Different cells use different genes, which is called differential gene expression. The mRNA therefore records which genes are being transcribed. It is copied into cDNA because cDNA is more stable and can pair with the probes. A glowing spot means that gene was active and its cDNA bound to the matching probe. A brighter spot means more mRNA was present, so the gene was more active. A dark spot means no cDNA bound. The gene was not being transcribed in that sample.
How to compare two tissues
You can compare two samples on one array by tagging their cDNA with different colours. For example, healthy-cell cDNA can be tagged green and cancer-cell cDNA red. The colour at each spot shows which sample made the mRNA for that gene. Green means the gene was active in healthy cells only. Red means it was active in cancer cells only. Yellow is red and green light together. The gene was active in both tissues at a similar level. Dark means neither sample made mRNA for that gene.
What bioinformatics does
Bioinformatics is the use of computers to store, organise and analyse biological data. Give both parts of the definition: biological data is stored in databases, then analysed with computer software. Gene sequences are stored in databases. Software can find a gene or compare its sequence across species. Whole genomes contain all the DNA of an organism. Comparing differences between genomes can show how closely related species are. Protein data includes amino acid sequences. Software can predict a protein's 3D shape, which can give clues about its function. For microarrays, a computer…
Finding genes linked to psoriasis
To find genes that are more active in skin cells from people with psoriasis, compare samples from many people with psoriasis and many without it. The computer can then identify genes with brighter spots in the psoriasis group.