Evidence for Evolution - PCR

what techniques can be used to process DNA to show evolutionary relationships

• PCR (polymerase chain reaction)- making many copies of the sample

• Restriction enzymes- cutting DNA into smaller lengths

• Gel Electrophoresis- separating the lengths of DNA to produce DNA profile

• Sanger’s method- determining sequence of nucleotide bases

Define PCR (Polymerase chain reaction)

Segments of DNA (a specific section) are artificially multiplied through a series of repeated cycles of duplication using an enzyme called DNA polymerase.

when is PCR useful and what is it used for?

useful when only small amounts of DNA are available for analysis. as from a single sample many copies can be produced. can be used to identify people and their relationship, but not specific genes.

what is studied when looking at results after PCR

only key markers as it is not practical to look at all of DNA’s differences.

define key markers

simple, repetitive sequences in the non-coding section of DNA on different chromosomes

other names for key markers

• short tandem repeats (STR)

• variable number tandem repeats (VNTR)

what determines the length of STR/VNTR

the number of repeats present

use of restriction enzymes in obtaining key markers

The DNA can be chopped up and the STR/VNTR isolated

what is gel electrophoresis

process which sorts pieces of DNA (prev chopped up by restriction enzymes) according to size

likelihood someone will share the same pattern of repeats when STR or VNTR are analysed

highly unlikely

The three steps in PCR

1.Denaturing (Denaturation): 2 strands of DNA separated by heating the DNA to 96 degrees celsius to break hydrogen bonds between strands. Seperates strands without disrupting each individual strand.

2.Annealing/Hybridisation with a primer: At approx 55-65 degrees celsius, short synthetic DNA fragments (primers) which are complementary to the target DNA sequence bind (anneal) to separated strands. Act as start points for replication of new DNA molecules.

3.Extension/Elongation/Synthesis using DNA polymerase: Done at approx 72 degrees celcius. Starting at the primer, the DNA polymerase (taq ploymerase) reads the DNA code and builds a complementary strand of DNA. Chain reaction occurs: Cycle repeated many times, resulting in compounding amplification.

what is thermocycling?

a process in which the PCR cycle is repeated 20-30 times. Each cycle takes 3-4 minutes. Takes 2-3 hours to produce abotu a billion copies of the original DNA

situations in which PCR could be used and information that would be gathered

• DNA profiling/DNA fingerprinting - comparing an individual’s DNA to a library of DNA from known individuals

• Early detection of infectious disease - foreign DNA from virsues/bacteria

• Tiny samples of DNA from fossils of extinct species - Information for phylogenetic studies/relatedness, migration information