3.1 Essential ideas
3.1.5 Genetic modification and biotechnology
Biologists have developed techniques that allow for the artificial manipulation of DNA, cells and organisms.
Polymerase chain reaction
- The polymerase chain reaction (PCR) is a technology used to amplify small quantities of DNA into large samples that can be further analysed.
- Sequences of are targeted using primers that bind by complementary base pairing to initiate replication.
- Millions of copies of the targeted sequence can be made within hours.
- Gel electrophoresis is used to separate charged molecules (proteins or fragments of ) according to their size.
- The technique uses a gel, immersed in a conducting fluid onto which an electrical field is applied.
- fragments of varying lengths are loaded into wells on the gel at the negative electrode.
- fragments, being slightly negatively charged, move through the gel towards the positive electrode.
- Smaller fragments move faster and travel further down the gel than larger fragments.
- profiling involves comparing samples of .
- Every individual has a unique pattern of fragmentation (like a fingerprint), so this technique is useful for forensic investigations, and in cases of unknown paternity.
profile is made by:
- obtaining a sample of (i.e. from a crime scene)
- using PCR to amplify the amount of in the sample
- cutting the enzymes that target specific sequences (the resulting fragments will be different lengths) into fragments using restriction
- separating the fragments of gel electrophoresis. sample by
- When samples from different individuals are loaded on the same gel, banding patterns can be compared.
Theof two men (F1 and F2) who claim to be the father of a child is compared to the of the mother (M) and the child (C) in the profile below.
Source: Griffiths et al, Introduction to Genetic Analysis, 6th edition
- Print the profile. Label the negative electrode and the positive electrode.
- Circle the longest fragment on the image.
- Account for the bands in C by indicating whether the band is common between mother and child, or between father and child.
- Determine whether F1 or F2 is the father of C. Explain your choice.
(Scroll over image for answers.)
- Genetic modification, or genetic engineering, involves gene transfer between species.
- Genes may be transferred to bacterial plasmids, restriction endonucleases and DNA ligase.
- The bacterial plasmids are then used as vectors to deliver new genes into organisms. Other vectors might be viruses or a recombinant cell.
- Restriction endonucleases are enzymes that digest DNA strands at specific sequences, leaving behind single-stranded ‘sticky ends’.
- The desired gene is mixed with bacterial plasmids that have been cut with endonucleases. The sticky ends bind by complementary base pairing to the inserted gene. DNA ligase enzyme joins the DNA backbones to produce a recombinant plasmid.
- The recombinant is cloned in small populations and then on a larger scale to produce the desired gene product.
Figure 3.1.5f – Thermal cycler
Care must be taken to ensure that samples loaded into a PCR thermal cycler are not contaminated.
The use of DNA evidence in legal cases is well-established. What criteria are necessary for establishing the reliability of evidence?