The complete set of genetic material (DNA) in a cell of an organism.

What is PCR used for?

To amplify a specific DNA sequence, making millions of copies from a very small sample.

What is the temperature and purpose of the denaturation step in PCR?

About 95 °C; it breaks the hydrogen bonds to separate the DNA strands.

What happens during the annealing step of PCR?

At about 55 °C primers bind to complementary sequences on the single strands.

What happens during the extension step of PCR?

At about 72 °C Taq polymerase adds nucleotides to the primers to make new strands.

Why is Taq polymerase used in PCR?

It is heat-stable, so it is not denatured at 95 °C and works through many cycles.

What is the role of primers in PCR?

They are short complementary sequences that bind to the template and give DNA polymerase a starting end.

How can gene sequencing predict a protein's amino acid sequence?

The base sequence is converted to mRNA codons and the genetic code gives the amino acids.

What is a short tandem repeat (STR)?

A short DNA sequence repeated a variable number of times, which differs between individuals.

What does gel electrophoresis do in DNA profiling?

It separates DNA fragments by size; smaller fragments travel further towards the positive electrode.

In a paternity test, how are the father's bands identified?

They are the child's bands that are not found in the mother's profile.

Why can a DNA profile match not prove guilt with certainty?

Unrelated people could in theory share a profile; the chance is very small but not zero.

Using gene sequencing and PCREdexcel A-Level Biology B: Flashcards

Card 1 of 130 of 13 known

Question

What is the genome?

Tap or press Space to reveal

Tap card or press Space to flip

See all 13 cards

What is the genome?: The complete set of genetic material (DNA) in a cell of an organism.
What is PCR used for?: To amplify a specific DNA sequence, making millions of copies from a very small sample.
What is the temperature and purpose of the denaturation step in PCR?: About 95 °C; it breaks the hydrogen bonds to separate the DNA strands.
What happens during the annealing step of PCR?: At about 55 °C primers bind to complementary sequences on the single strands.
What happens during the extension step of PCR?: At about 72 °C Taq polymerase adds nucleotides to the primers to make new strands.
Why is Taq polymerase used in PCR?: It is heat-stable, so it is not denatured at 95 °C and works through many cycles.
What is the role of primers in PCR?: They are short complementary sequences that bind to the template and give DNA polymerase a starting end.
How many copies after 10 PCR cycles from one molecule?: $2^{10}$ = 1024 copies.
How can gene sequencing predict a protein's amino acid sequence?: The base sequence is converted to mRNA codons and the genetic code gives the amino acids.
What is a short tandem repeat (STR)?: A short DNA sequence repeated a variable number of times, which differs between individuals.
What does gel electrophoresis do in DNA profiling?: It separates DNA fragments by size; smaller fragments travel further towards the positive electrode.
In a paternity test, how are the father's bands identified?: They are the child's bands that are not found in the mother's profile.
Why can a DNA profile match not prove guilt with certainty?: Unrelated people could in theory share a profile; the chance is very small but not zero.

Exam questions on Using gene sequencing and PCR

A forensic laboratory receives a swab from a crime scene that contains only a few cells' worth of DNA. Technicians mix the DNA with primers, free nucleotides, a heat-stable DNA polymerase and buffer, and place the tubes in a thermal cycler that repeatedly heats and cools the mixture.
Explain why primers are needed in the reaction mixture.2 marks
A baby has symptoms that suggest sickle cell disease. Doctors amplify and sequence part of the baby's gene for the beta chain of haemoglobin. In the healthy allele the template-strand triplet is CTC, which codes for glutamic acid. In the baby's allele the same triplet reads CAC.
Explain how the sequence of the baby's gene allows doctors to link the baby's symptoms to a genetically determined condition.2 marks
Police recover a bloodstain at a burglary that contains a very small amount of DNA. A forensic scientist amplifies several short tandem repeat (STR) regions of this DNA and produces a DNA profile from the sample. She compares it with DNA profiles from three suspects.
Explain why the DNA is amplified by PCR before the profile is produced.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).