Recombinant DNA, microarrays and bioinformatics

What you’ll learn

• Explain the principles of recombinant DNA and gene transfer.
• Interpret gene-expression comparisons.
• Evaluate applications and risks using biological evidence.

Understand the idea

Recombinant DNA joins genetic material from different sources. At the conceptual level, a restriction endonuclease cuts DNA at a recognised sequence and DNA ligase seals the sugar–phosphate backbone between compatible fragments. A suitable expression system must include control sequences recognised by the host. For a human protein expressed in bacteria, an intron-free coding sequence is needed because bacteria do not carry out human pre-mRNA splicing. A gene being present does not guarantee correct protein production or processing.

DNA can be introduced into cells by biological carriers or physical methods; the appropriate method depends on the organism and purpose. Modified microorganisms, plants or animals can produce useful proteins, including medicines. Production must be followed by testing, purification and quality control. This lesson concerns the biological principles rather than a practical genetic-engineering protocol or a sequence design.

A DNA microarray contains known probes. Labelled complementary DNA made from a sample’s messenger RNA binds to matching probes, so signal intensity can compare transcript abundance across many genes when normalisation and controls are appropriate. In a two-colour comparison, red and green identify relative expression according to the stated labelling key; yellow can indicate similar expression in both. Transcript abundance is not automatically identical to active protein abundance. Bioinformatics stores, compares and analyses sequence or expression data to identify patterns and generate testable hypotheses.

Benefits of genetic modification may include reliable medicine supply or reduced crop losses; possible risks and trade-offs include gene flow, ecological effects, animal welfare, ownership and unequal access. Evaluate the particular organism, trait and environment, comparing alternatives and the quality of evidence. Neither “all modification is safe” nor “all modification is harmful” follows from the method alone.

Key formulae

$$\text{expression fold change}=\frac{\text{normalised expression in treatment}}{\text{normalised expression in control}}$$

Worked example

In a two-colour microarray, treated-sample cDNA is labelled red and control-sample cDNA green. A probe appears strongly red. Interpret the result and give one limitation.

1. The probe binds cDNA with a complementary sequence representing a particular transcript.
2. A stronger red contribution indicates greater transcript abundance in the treated sample after appropriate normalisation.
3. Protein abundance or activity may differ because translation, processing and degradation also regulate function.

Answer: The result supports increased expression of that transcript in the treated sample; it does not alone establish increased active protein or a causal role.