8.4.4Unit 1: Lifestyle, Transport, Genes and Health
Identification of Active Genes — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 29 of 29 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Purpose of a Microarray
Purpose of a Microarray
Active gene
- Active gene A gene that is being expressed, so that mRNA is made from it and then translated into a polypeptide.
Scale
- Scale A single microarray can test thousands of genes at once, since it can carry 10 000 or more spots per square centimetre.
Medicine
- Medicine It can reveal harmful mutations or the level of expression of genes linked with disease.
Other uses
- Other uses It is also used in biotechnology to find useful genes, and in forensic work to compare samples of DNA.
Part 2 of 5Structure of a Microarray
Support
- Support A small slide of glass, plastic or silicon carries a grid of tiny spots.
Gene spots
- Gene spots Each spot holds many copies of one DNA probe, so every spot tests for a different gene.
DNA probe
- DNA probe A short single-stranded DNA sequence that binds only to a complementary sequence in the sample.
Label
- Label The sample, or probe, carries a fluorescent or radioactive label so that binding can be detected and located.
Reference sample
- Reference sample A control sample, for example from a person with a known mutation, is compared with the unknown sample.
Part 3 of 5Method
1
mRNA is collected from the reference and from the test tissue, because only active genes are being transcribed into mRNA.
2
Reverse transcriptase converts the mRNA into complementary DNA, which is then labelled with a fluorescent tag.
3
Reference cDNA is labelled green and test cDNA is labelled red, and the two are mixed together.
4
The mixture is added to the microarray, where cDNA hybridises with complementary probes, and unbound DNA is washed away.
5
The array is scanned under ultraviolet light, and a computer records the colour of each spot and analyses the pattern.
Part 4 of 5Reading the Results
Reading the Results
Yellow spot
- Yellow spot Both samples hybridised equally, so the gene is expressed to the same extent in the reference and test tissue.
Green spot
- Green spot Mainly the reference cDNA bound, so the gene is more active in the reference than in the test sample.
Red spot
- Red spot Mainly the test cDNA bound, so the gene is more active in the test sample than in the reference.
No colour
- No colour Neither sample bound to the probe, so the gene is not being expressed in either tissue.
Cancer example
- Cancer example High expression of oestrogen receptor genes suggests that a drug blocking oestrogen receptors could be an effective treatment.
Part 5 of 5Exam Checklist & Tips
8.4.4 – Key Points for the Exam
- Microarray A slide carrying many DNA probes, used to test which genes are active in a sample.
- DNA probe A single-stranded DNA sequence with a label, which binds to a complementary sequence.
- Hybridisation The pairing of complementary single-stranded DNA by hydrogen bonds between bases.
- cDNA DNA made from mRNA by reverse transcriptase.
- Active gene A gene that is being transcribed and translated.
Common Mistakes
- Using DNA instead of mRNA to find active genes;
- mRNA is used because it only exists where a gene is being transcribed.
- Thinking yellow means no expression;
- yellow shows equal expression in both samples, while no colour shows no expression.
- Forgetting to wash off unbound DNA, which would give false signals on the array.
- Saying the probe is double-stranded; probes are single-stranded so they can bind complementary sequences.
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