Topic 8E: DNA probes, Medical Applications and Genetic Fingerprinting

Question 1

Describe how scientists could use a radioactively labelled DNA probe to show that the cells of a plant contain a certain gene. [4]

Answer 1

1. Extract DNA and add restriction endonuclease;
2. Separate fragments using electrophoresis;
3. (Treat DNA to) form single strands;
4. The probe will bind to/hybridise/base pair with the gene;
5. Use auto radiation (to show he bound probe)

Question 2

To study the role of the the gene SUT1 in a plant, scientists reduced the expression of this gene. When the gene is transcribed, the mRNA produced is called ‘sense’ SUT1 mRNA. The scientists genetically modified plants by inserting an extra gene so that this also allowed the production of ‘antisense’ SUT1 mRNA.

Suggest how the production of ‘antisense’ SUT1 mRNA would reduce the expression of theSUT1 gene. [4]

Answer 2

1. Antisense mRNA is complementary to sense mRNA;
2. Antisense mRNA would bind/base pair to (sense) mRNA;
3. Ribosomes would not be able to bind;
4. Preventing/less transcription (of mRNA)

Question 3

Describe how a DNA sample from a patient could be screened for all known harmful mutations of a gene. [4]

Answer 3

1. Use PCR to amplify (DNA/sample);
2. Cut (DNA) using restriction endonuclease;
3. Separate (DNA fragments) using electrophoresis;
4. Add (labelled) DNA probes that bind (by DNA hybridisation);
5. (Mutations) identified by fluorescence/radioactivity
   OR
   Compare positions/bands (to known) DNA sample with (all harmful) mutations

Question 4

Give two features of DNA molecules which enable them to be separated by gel electrophoresis. [2]

Answer 4

1. Number of nucleotides/repeats/bases
   OR
   Length/mass;
2. (Negative) charge

Question 5

Describe how short tandem repeats could be removed from a sample of DNA. [2]

Answer 5

1. Restriction endonucleases;
2. (Cut DNA) at specific base sequence

Question 6

Genetic fingerprinting using STRs requires amplification of the STRs using the polymerase chain reaction. The short base sequence either side of a specific STR are known.

Explain the importance of knowing these base sequences in PCR. [2]

Answer 6

1. (For) primers;
2. (To produce) a complementary base sequence
   OR
   (Primers provide starting sequence) for DNA/taq polymerase
   OR
   (Primers) sop (original) DNA strands re-joining

Question 7

Suggest how you could determine the size of different DNA fragments that have been separated by gel electrophoresis. [2]

Answer 7

1. (Separate) DNA fragements of known sizes/lengths (simultaneously alongside unknown fragments);
2. Compare position/distance/bands with unknown fragments

Question 8

Wall geckos show phenotypic diversity.

Suggest two factors that have resulted in this phenotypic diversity. [2]

Answer 8

1. Mutations;
2. Environment
   OR
   (Natural) selection;
3. Epigenetics;
4. Crossing over;
5. Independent segregation (of homologous chromosomes)
6. Random fertilisation

Question 9

Dark geckos live mainly on the dark trunks of olive trees and are diurnal (active during the day). These diurnal geckos can change skin colour when occupying different surfaces during the day.

The ability of these geckos to change skin colour is advantageous.

Explain why. [2]

Answer 9

1. Provides camouflage;
2. (So) not seen by predators
   OF
   Catch (more) prey

Question 10

Explain how comparison of mitochondrial genes could indicate that the nocturnal geckos formed a distinct genetic group from the diurnal geckos.

In your answer, explain how new technologies enable the comparison of genes to be completed rapidly. [3]

Answer 10

1. Compare DNA base/nucleotide sequence
   OR
   Compare banding/position of DNA fragments;
2. A dusting (group) will have different alleles/DNA/banding (from other groups)
3. DNA sequencing is automated/computerised
   OR
   Genetic/DNA fingerprinting is automated/computerised
   OR
   PCR amplified DNA/genes
   OR
   Genetic fingerprinting/electrophoresis separates fragments/genes/allels
   OR
   Use of DNA proves/hybridisation to identify genes/alleles

Question 11

Scientists used the mark-release-recapture method to estimate the size of the population of geckos in a habitat.

Describe and explain two precautions requires to ensure that the estimate of the size of the population was valid.

Do **not*v include sample size as one of the required precautions.

In your answer, include the formula to estimate the size of the population. [3]

Answer 11

1. Marking not toxic so does not affect survival
   OT
   Marking not visible to predators
   OR
   Marking does not wash/rub off so recaptured (geckos) incentivised;
2. Time/delay after release so (geckos) spread (in the population)
   OR
   Time/delay before recapture so (geckos) spread (in the population)
3. (Population =) (number in) first sample x (number in) second sample divided by (number) marked in second sample/number recaptured

Question 12

HindIII is an enzyme that cuts DNA into smaller fragments.

What general name is given to enzymes such as HindIII? [1]

Answer 12

1. Restriction endonuclease

Question 13

Sticky ends are useful in genetic engineering. Explain how. [2]

Answer 13

1. Joining two pieces of DNA;
2. By complementary binding/base-pairing

Question 14

A circular DNA molecule was cut by a restriction endonuclease at two separate sites on the DNA. How many DNA fragments result from this? [1]

Answer 14

1. 2

Question 15

Plasmids are often used as vectors in genetic engineering.

What is the role of a vector? [1]

Answer 15

1. Transfer/carry genes from one organism to another / into bacteria/cells

Question 16

Describe the role of restriction endonucleases in the formation of plasmids that contain donor DNA. [2]

Answer 16

1. Cut open plasmid;
2. Cut donor DNA, to remove gene;
3. Cut donor DNA and plasmid with the same enzyme / enzyme that cuts at the same base sequence;
4. Sticky ends with, sing strand/bases exposed;
5. Association/attachment/pairing of complementary strand

Question 17

Describe the role of DNA Ligase in the production of plasmids containing donor DNA. [1]

Answer 17

1. Annealing/splicing/backbones joined/phosphodiester bonds

Question 18

What is a DNA probe? [2]

Answer 18

1. (Short) Single strand of DNA;
2. Bases complementary (with DNA/allele/gene)

Question 19

Describe how DNA is broken down into smaller fragments. [2]

Answer 19

1. Restriction endonuclease;
2. (Cuts DNA at specific) base sequence
   OR
   (Breaks) phosphodiester bonds
   OR
   (Cuts DNA) at recognition/restriction site

Question 20

The DNA on the nylon membrane is treated to form single strands. Explain why. [1]

Answer 20

1. (So DNA) probe binds/attaches/anneals

Question 21

What is meant by a genome? [1]

Answer 21

1. (All) the DNA in a cell/organism

Question 22

Suggest how single-stranded cDNA could prevent the transcription of a gene. [1]

Answer 22

1. Binds to gene/DNA/mRNA
   OR
   Binds to transcription factor gene/DNA
   OR
   Binds to promoter

Question 23

Describe the roles of two named types of enzymes used to insert DNA fragments into plasmids. [2]

Answer 23

1. Restriction (endonuclease) to cut plasmid/vector;
2. Ligase joins gene/DNA to plasmid/vector

Question 24

Suggest two features of different protein that enable them to be separated by gel electrophoresis. [2]

Answer 24

1. Mass/number of amino acids/polypeptides;
2. Charge;
3. R groups (differ)

Question 25

Suggest and explain **one** reason why the age at which a mutation is expressed can vary. [2]

Answer 25

1. Epigenetics / environment; 2. Methylation (of genes) OR Acetylation (of histones)