Year 13: 8. The control of gene expression

Question 1

Explain what is meant by the terms totipotent and pluripotent.

Answer 1

• totipotent cells can give rise to a complete human/all cell types;
• pluripotent can only give some cell types;

Question 2

Explain how cells produced from stem cells can have the same genes yet be of different types.

Answer 2

• {not all / different} genes are switched {on / off} /active / activated ;
• correct and appropriate reference to factors /mechanisms for gene switching; e.g. reference to promoters / transcription factors

Question 3

Describe the mechanism by which a signal protein causes the synthesis of mRNA. [5]

Answer 3

• signal protein {binds to / joins to / interacts with / activates}
• receptor on surface membrane;
• messenger molecule moves from cytoplasm and enters nucleus;
• {produces / activates} transcription factor;
• binds to promoter region;
• RNA polymerase transcribes target gene;

Question 4

Explain how oestrogen enables RNA polymerase to transcribe its target gene. [5]

Answer 4

• Oestrogen diffuses through the cell membrane;
• attaches to ERα receptor;
• ERα receptor changes shape;
• ERα receptor leaves protein complex which inhibited it’s action;
• oestrogen receptor binds to promoter region;
• enables RNA polymerase to transcribe target gene.

Question 5

Compare the structure of dsRNA and DNA. [4]

Answer 5

• Similarities; 2 max
• Polynucleotides/polymer of nucleotides;
• Contain Adenine, Guanine, Cytosine;
• Have pentose sugar/5 carbon sugar;
• Double stranded/hydrogen bonds/base pairs.
• Differences; 2 max
• dsRNA contains uracil, DNA contains thymine;
• dsRNA contains ribose DNA contains Deoxyribose;
• dsRNA is Shorter than DNA; fewer base pairs in length;

Question 6

Explain how the methylation of tumour suppressor genes can lead to cancer. [3]

Answer 6

• Methylation prevents transcription of gene;
• Protein not produced that prevents cell division / causes cell death / apoptosis;
• No control of mitosis.

Question 7

Describe how alterations to tumour suppressor genes can lead to the development of tumours [4]

Answer 7

• (Increased) methylation (of tumour suppressor genes);
• Mutation (in tumour suppressor genes);
• Tumour suppressor genes are not transcribed/expressed OR Amino acid sequence/primary/ tertiary structure altered;
• (Results in) rapid/uncontrollable cell division;

Question 8

Describe what is meant by a malignant tumour.

Answer 8

• mass of undifferentiated / unspecialised / totipotent cells;
• uncontrolled cell division;
  OR
• metastasis / (cells break off and) form new tumours
• spread to other parts of body;

Question 9

Describe how altered DNA base sequence may lead to cancer [5]

Answer 9

• (DNA altered by) mutation;
• (mutation) changes base sequence;
• of gene controlling cell growth / oncogene / that monitors cell division;
• of tumour suppressor gene;
• change protein structure / non-functional protein / protein not formed;
• (tumour suppressor genes) produce proteins that inhibit cell division;
• mitosis;
• uncontrolled / rapid / abnormal (cell division);
• malignant tumour;

Question 10

Describe how alterations to tumour suppressor genes can lead to the development of tumours. [3]

Answer 10

• (Increased) methylation (of tumour suppressor genes); OR Mutation (in tumour suppressor genes);
• Tumour suppressor genes are not transcribed / expressed OR Amino acid sequence / primary structure altered;
• (Results in) rapid/uncontrollable cell division;

Question 11

Define epigenetics

Answer 11

• Heritable phenotype changes (gene function) that do not involve alterations in the DNA sequence/mutation.

Question 12

Name 3 types of single point gene mutation

Answer 12

Substitution

Addition

Deletion

Question 13

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why.

Answer 13

1. Triplets code for same amino acid / DNA is degenerate
2. Occurs in introns /non-coding sequence;

Question 14

Define ‘gene mutation’ and explain how a gene mutation can have:
* no effect on an individual
* a positive effect on an individual.

[5]

Answer 14

(Definition of gene mutation)
1. Change in the base/nucleotide (sequence of chromosomes/DNA);
2. Results in the formation of new allele;
(Has no effect because)
3. Genetic code is degenerate (so amino acid sequence may not change); OR
Mutation is in an intron (so amino acid sequence may not change);
Accept description of ‘degenerate’, eg some amino acids have more than one triplet/codon.
4. Does change amino acid but no effect on tertiary structure;
5. (New allele) is recessive so does not influence phenotype;
(Has positive effect because)
6. Results in change in polypeptide that positively changes the properties (of the protein) OR Results in change in polypeptide that positively changes a named protein; For ‘polypeptide’ accept ‘amino acid sequence’ or ‘protein’.
7. May result in increased reproductive success OR May result in increased survival (chances);

Question 15

Give an example of an UNIPOTENT stem cell.

Answer 15

Cardiomyocyte

Question 16

How can a somatic (Specialised body cell) be converted into an iPS cell (induced pluripotent stem cell)

Answer 16

• Provide the appropriate Transcription factors
• Demethylate the DNA
• Acetylate histones

Question 17

Adding Methyl groups can silence genes by binding to …….

Answer 17

DNA promoter regions

Question 18

How does RNAi affects protein synthesis?

Answer 18

• Single stranded RNAi binds to complementary bases of ‘target’ mRNA (within the cytoplasm)
• Leads to destruction/hydrolysis of (target) mRNA
• Less translation (by ribosomes) of mRNA

Question 19

Describe how DNA is replicated in a cell. [5]

Answer 19

• DNA strands separate / hydrogen bonds broken;
• Parent strand acts as a template / copied / semi-conservative replication;
• Nucleotides line up by complementary base pairing; (Adenine & Thymine etc)
• Role of DNA polymerase: joins adjacent nucleotides on the developing strand via condensation and formation of phosphodiester bond;
• 5’ to 3’ direction
• Each new DNA molecule has 1 template and 1 new strand
• Formed by semi-conservative replication.

Question 20

Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment. [4]

Answer 20

• (Requires DNA fragment) (Taq)** DNA polymerase**, (DNA) nucleotides and primers;
• Heat to 95 °C to break hydrogen bonds (and separate strands);
• Reduce temperature (40-65°C) so primers bind to DNA/strands;
• Increase temperature (70 to 75 °C), DNA polymerase joins nucleotides (and repeat method);

Question 21

Why is the DNA heat to 95°C during PCR?

Answer 21

• Produce single stranded DNA
• Breaks WEAK hydrogen bonds between strands

Question 22

Why do you add primers during PCR?

Answer 22

• Attaches to / complementary to start of the gene / end of fragment;
• Replication of base sequence from here;
• Prevents strands annealing

Question 23

Explain why ‘base-pairs’ is a suitable unit for measuring the length of a piece of DNA.

Answer 23

• DNA = 2 chains / joined by linking of 2 bases / A with T and G with C/ purine pairs with pyrimidine;
• Bases are a constant distance apart / nucleotides occupy constant distance/
• each base-pair is same length / sugar-phosphate is a constant

Question 24

Name one mutagenic agent.

Answer 24

• high energy radiation /ionising particles e.g. named particles/α, β, γ & X-rays;
• benzene;
• x rays/cosmic rays;
• uv (light);
• carcinogen / named carcinogen;
• mustard gas / phenols / tar (qualified);

Question 25

A deletion mutation occurs in gene 1.

Describe how a deletion mutation alters the structure of a gene.

Answer 25

• removal of one or more bases/nucleotide;
• frameshift/(from point of mutation) base sequence change;

Question 26

Describe the main stages in the copying, cutting and separation of the DNA. [5]

Answer 26

• heat DNA to 95°C / 90°C;
• strands separate;
• cool so that primers bind to DNA;
• add DNA polymerase/nucleotides;
• use of restriction enzymes to cut DNA at specific base sequence/ breaks phosphodiester bonds
• use of electric current and agar/gel;
• shorter fragments move further;

Question 27

Describe the polymerase chain reaction. [5]

Answer 27

• Heat DNA to break hydrogen bonds/separates strands;
• Add primers, Add nucleotides & DNA Polymerase
• Cool (to allow) binding of nucleotides/primers;
• Reheat to activate (DNA) polymerase;
• Repeat cycle many times;

Question 28

Describe a plasmid.

Answer 28

• circular DNA;
• separate from main bacterial DNA;
• contains only a few genes;

Question 29

Suggest one reason why DNA replication stops in the polymerase chain reaction.

Answer 29

• Limited number of primers/nucleotides; / Primers / nucleotides ‘used up’.
• DNA polymerase (eventually)denatures

Question 30

Suggest why the restriction enzyme has cut the human DNA in many places but has cut the plasmid DNA only once.

Answer 30

• enzymes only cut DNA at specific base sequence/recognition site/specific point;
• sequence of bases/recognition site/specific point (on which enzyme acts)
• occurs once in plasmid and many times in human DNA;
• (max 1 if no reference to base sequence or recognition site)

Question 31

Explain what is meant by a vector.

Answer 31

• Carrier of DNA/gene; (context of foreign DNA)
• Into cell/other organism/host;

Question 32

Explain how modified plasmids are made by genetic engineering and how the use of markers enable bacteria containing these plasmids to be detected. [6]

Answer 32

• isolate TARGET gene/DNA from another organism/mRNA from cell/organism;
• using restriction endonuclease/restriction enzyme/reverse transcriptase to
• produce sticky ends;
• use DNA ligase to join TARGET gene to plasmid also include marker gene (example of marker e.g. antibiotic resistance);
• add plasmid to bacteria to grow (colonies);
• (replica) plate onto medium where the marker gene is expressed;
• bacteria/colonies not killed have antibiotic resistance gene and (probably) the TARGET gene;
• bacteria/colonies expressing the marker gene have the TARGET gene as well;

Question 33

Describe how STRs could be removed from a sample of DNA.

Answer 33

1. Restriction endonucleases/enzymes;
2. (Cut DNA) at specific base sequences/pairs OR (Cut DNA) at recognition/restriction sites within the intergenic regions.

Question 34

What does VNTR stand for?

Answer 34

Variable Number of Tandem Repeats

Question 35

mRNA may be described as a polymer. Explain why.

Answer 35

• Made up of many (similar) molecules/monomers/nucleotides/units;

Question 36

What is a DNA probe?

Answer 36

• (Short) single strand of DNA;
• Bases complementary (with DNA/allele/gene);

Question 37

Name three techniques used by scientists to compare DNA sequences.

Answer 37

• Polymerase Chain Reaction
• DNA fingerprinting
• Gel electrophoresis

Question 38

What name is used for the non-coding sections of a gene?

Answer 38

• Introns

Question 39

Where are VNTR’s located?

Answer 39

• Intergenic regions of chromosomes (Between adjacent genes on the same chromosome)

Question 40

Explain why fragments of DNA from cancer cells may be present in blood plasma.

Answer 40

• cancer cells die / break open releasing DNA;

Question 41

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

Answer 41

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

Question 42

Describe how enzymes could be used to insert the GH gene into a plasmid.

Answer 42

• Restriction endonucleases/enzymes cuts plasmid; OR Restriction endonucleases/enzymes produces ‘sticky ends’;
• Reject restriction enzymes cuts the gene.*
• Ligase joins gene/DNA and plasmid OR Ligase joins ‘sticky ends’;