Section 4Lesson 7– Genetic

Fingerprinting (DNA profiling)

What is DNA fingerprinting?

VNTRs (Variable Number Tandem Repeats)

All human share about 99.99% of their DNA. In order to create a DNA fingerprint that is unique to each person scientists had to identify areas in the genome that are highly variable.

There are regions in the genome called minisatellite regions that contain a variable number of repeated bases. These areas are called VNTRs.

Steps in DNA Profiling

1. PCR is used to amplify the DNA.2. Restriction enzymes are used to cut the section of

DNA containing the VNTRs into sections. It is important that the enzymes chosen do not cut the DNA WITHIN the VNTRs. The fragments created with be different lengths for different people depending on the length of their variable repeat.

3. Gel electrophoresis is used to separate the fragments by size.

4. Blotting – the DNA is denatured using a strong alkali (this makes it single stranded). It is transferred to a nylon membrane where it is then hybridised with a radioactive probe. The radioactive probe allows the pattern to be transferred to photographic paper where comparative analysis can be done.

ReliabilityThe more probes that are used for a profile, the more reliable the results will be in matching profiles. This is because there are more bands produced.

DNA Profiling in Action!

Catching a Killer

STRs and Modern Uses

Nowadays instead of VNTRs, scientists use STRs (short tandem repeats) and PCR to construct a DNA profile. STRs are also sometimes called microsatellites (not to me confused with minisatellites!)

DNAprofi ling:

Single-locusprobe

M 1 2 3 4 F

maternalbands

paternalbands

children

In this example a single restriction enzyme has cut the DNA at a specific point ( single locus ) This is known as a single locus probe

Who is the Daddy?

In this example a variety of restriction enzymes have cut the DNA at many specific points generating a large number of fragments.

DNAprofiling:Multi-locusprobe

To see a variety of forensics casesClick here

Who is the Baddy?

Advice from SQA on essay writing

PCR and DNA profiling•Sequencing determines the order of nucleotides in a section of DNA. •PCR and electrophoresis procedures are used in both processes.• In profiling, PCR is used to amplify or increase the number of copies of the DNA sample, which is then digested into fragments that will be unique to each individual because of VNTRs. • Fragments are separated by electrophoresis and the gel is blotted and probed to reveal the profile.•The sequencing process also uses PCR. As the unknown DNA base sequence is copied, the chain terminates randomly as a modified nucleotide (dideoxynucleotide triphosphate, ddATP etc) is attached to the growing chain. • After lots of cycles in the PCR machine, there should be large numbers of every fragment size, each terminated with the modified, dyed base. • Electrophoresis is again used to separate the DNA chains by size order, but this time the base sequence is of interest and is read off from the colour sequence.Note - ligase is not needed in PCR. The PCR process is not identical to DNA replication; there is no replication fork and no lagging strand fragments to be joined. DNA is melted to give two independent strands that are both copied from the primers in the correct direction.

Your Tasks

1.Make a flow chart to describe the stages in DNA profiling. Use page 86 in your monograph to help you.

2.Read pages 90 – 92 in your monograph and make notes on transgenic plants for next Monday’s lesson.

3.Check that your glossary is up to date with red terms and that you are up to date with Scholar activities – don’t leave this until the last minute!!!!