Unit 5 Questions Genetics, DNA and other fluff

Question 1

Cells contain suppressor genes, which code for proteins that control cell division and growth. Describe what is meant by a mutation, and explain how a mutation in suppressor genes might lead to the development of a malignant tumour. (6)

Answer 1

Mutation of suppressor gene – up to 4 marks 1. Mutation is a change in the DNA / sense strand; 2. Base sequence altered / e.g.; 3. Suppressor gene produces wrong instructions / has different code; 4. (Therefore) different amino acid sequence; 5. Different protein structure / non-functional protein; Malignant tumour – up to 2 marks 6. Cell division by mitosis; 7. Tumour cells growth abnormal / continuous / uncontrolled / rapid; 8. Tumour cells spread / invade other tissues / form secondary tumours / metastasis; 9. Via blood / lymph system;

Question 2

Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another

Answer 2

Deletion causes frame shift / alters base sequence (from point of mutation); changes many amino acids / sequence of amino acids (from this point); substitution alters one codon / triplet; one amino acid altered / code degenerate / same amino acid coded for

Question 2

A gene is obtained from mRNA (using reverse transcriptase), rather than DNA. Suggest why.

Answer 2

Idea that mRNA is present in large amounts in cell making the protein /mRNA has been edited / does not contain introns / mRNA codes for single protein; Difficulty of finding one gene among all the genes in the nucleus / large amounts of mRNA coding for insulin will be present in insulin producing cells / idea that mRNA will be ‘edited’

Question 3

What is recombinant DNA?

Answer 3

contains genes/nucleotides/sections of DNA/artificial DNA from two species/2 types of organisms

Question 4

Describe one way in which the structure of the DNA of a gene may be changed as a result of a mutation?

Answer 4

Addition / deletion / substitution; Of a nucleotide / base Substitutions: mis-sense (codon changed so codes for new amino acid), non-sense (codon changed to a stop codon), silent (change in 3rd base of a codon and still results in the same amino acid due to degenerate genetic code) Addition/deletion: frame shift is caused, alters the base sequence from the point of mutation and can change many amino acids

Question 4

What are primers?

Answer 4

(short) length of DNA; single stranded; (reject reference to RNA) with specific base sequence/complementary base sequence; indicates where replication starts/stops annealing

Question 5

Explain how the use of a gene probe could enable the presence of a mutant allele of the cystic fibrosis gene to be detected.

Answer 5

Probe will attach (to mutant allele); attaches to one DNA strand; as a result of complementary base pairing; radioactivity detected on film/X-ray / by autoradiography (if mutant allele present);

Question 6

Why are primers needed?

Answer 6

To mark beginning and/or ends of the part of DNA needed / for attachment of enzymes or nucleotides / initiator / keeps strands apart; Attaches to / complementary to start of the gene / end of fragment; Replication of base sequence from here

Question 6

What is a DNA probe?

Answer 6

Piece of DNA; Single stranded; Complementary to/binds to known base sequence/gene

Question 7

Describe how the structure of DNA allows it to carry out its function

Answer 7

Sugar – phosphate backbone gives strength; Coiling gives compact shape; Sequence of bases allows information to be stored; Long molecule / coiling stores large amount of information; Complementary base pairing enables information to be replicated /transcribed; Double helix protects weak hydrogen bonds / double helix makes molecule stable; Many hydrogen bonds together give molecule stability; Prevents code being corrupted; Hydrogen bonding allows chains to split for replication / transcription OR molecule unzips easily for replication / transcription.

Question 8

Describe the molecular structure of DNA and explain how a sequence of DNA is replicated. (9)

Answer 8

composition of a nucleotide, 4 bases named; sugar-phosphate ‘backbone’; two (polynucleotide) strands; specific base-pairing; example e.g. A–T / C–G; hydrogen bonding; ‘uncoiling’ / ‘unzipping’; semi-conservative replication; DNA polymerase; new complementary strands form / identical DNA molecule produced; DNA inserted into plasmids; which are self-replicating;

Question 9

Compare tRNA to mRNA

Answer 9

1) tRNA Clover shaped Standard length Has an amino acid binding site anticodon tRNA has H bonds between complementary base pairs Limited number of types (64) 2) mRNA Linear Variable length (depends on the length of gene) Many different types (depends on the gene) No H-bonding No base pairs

Question 9

Explain the importance of marker genes

Answer 9

Allows transformed bacteria to be separated from non-transformed; Further detail e.g. transformed bacteria survive when antibiotic applied to medium

Question 10

Explain how exposure to a mutagenic agent may result in an inactive enzyme being produced by a cell.

Answer 10

Change in the sequence of nucleotides/bases/addition/deletion/ substitution; changed order of amino acids/different protein/different tertiary structure; inactive enzyme if shape of active site is changed/enzyme-substrate complex does not form;

Question 12

A gene mutation may cause no change in the structure of the protein coded for. Explain why.

Answer 12

Degenerate code / clear description; (New triplet) codes for same amino acid;

Question 13

What is a gene mutation?

Answer 13

Change in base/nucleotide

Question 14

Explain how modified plasmids are made by genetic engineering and how the use of markers enables bacteria containing these plasmids to be detected

Answer 14

isolate wanted gene/DNA from another organism/mRNA from cell/organism; using restriction endonuclease/restriction enzyme/reverse transcriptase to get DNA; produce sticky ends; use ligase to join wanted 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 wanted gene; bacteria/colonies expressing the marker gene have the wanted gene as well

Question 14

Explain how Electrophoresis works

Answer 14

Separates DNA fragments of different sizes. DNA samples are placed into wells cut in an aragose gel, covered in a buffered ionic solution Current is passed through the gel and the negative charged DNA moves toward the positive electrode. DNA fragments diffuse through the pores in the gel, and the larger pieces face a greater resistance to movement than smaller pieces and so will travel a shorter distance

Question 15

What are restriction enzymes

Answer 15

These are enzymes that cut DNA at specific sites, recognition sequences (palindromic sequences) GAATTC, CTTAAG Some restriction enzymes cut straight across both chains, forming blunt ends, Most enzymes make a staggered cut in the two strands, forming sticky ends. The products are called restriction fragments

Question 16

The polymerase chain reaction (PCR) can be used to produce large quantities of DNA. Describe how the PCR is carried out. (6)

Answer 16

1. DNA heated to 90 to 95°C; 2. Strands separate; 3. Cooled / to temperature below 70°C 4. Primers bind; 5. Nucleotides attach; 6. By complementary base pairing; 7. Temperature 70 - 75°C; 8. DNA polymerase joins nucleotides together; 9. Cycle repeated;

Question 17

Benefits of GMO’s

Answer 17

• Medicines and drugs can be produced safely in large quantities from microbes rather than from slaughtered animals. These medicines benefit humans and can spare animal suffering as well. • Agricultural productivity can be improved while using less pesticides or fertilisers, so helping the environment. GM crops can grow on previously unsuitable soil or in previously unsuitable climates. • GM crops can improve the nutrition and health of millions of people by improving the nutritional quality of their staple crops.

Question 19

Sometimes errors occur during the copying of a sequence of bases. Explain why some errors have less severe consequences than others.

Answer 19

changes in base sequence will not necessarily affect amino acid coded for/most amino acids have more than one code; these codes differ only in the third base; so changes in the third base are likely to cause no change in the amino acid sequence/ protein; changes in first/second base result in an incorrect amino acid in the sequence/ formation of the incorrect protein/example of mutation causing this type of change; change in amino acid present may have no effect on functioning of protein/some amino acids more important in tertiary structure than others

Question 20

Describe transcription

Answer 20

Section of DNA unwinds / uncoils; DNA separates, h-bonds break RNA nucleotides align; complementary base pairing / example of pairing; U replaces T mRNA polymerase (joins nucleotides); mRNA is modified, introns are removed

Question 21

Transformation is the process of trying to introduce the recombinant plasmid into the bacteria. Explain how you could identify bacteria containing the plasmid

Answer 21

replica plating; use of agar plate containing ampicillin/no tetracycline and agar plate containing tetracycline; in bacteria with human DNA tetracycline gene no longer functional/ not resistant to tetracycline; bacteria with human DNA grow on plate with ampicillin/no tetracycline but are killed by tetracycline; bacteria with no extra DNA in plasmid not killed,

Question 22

Describe and explain how gene expression is controlled (pre-transcription)

Answer 22

Transcription depends on the activation of transcription factors Usually by hormones (like oestrogen) The transcription factors attach to the promoter region of the DNA forming a transcription factor initiation complex This complex is recognised by RNA polymerase which binds and begins transcription

Question 23

Disadvantages of Gene therapy

Answer 23

Disadvantages Success of gene uptake Success of gene expression Short lived (if cells die and are replaced and it is not copied) Multiple treatments and thus cost./inconvenience) Vectors can cause primary immune response and then repeated treatment causes a secondary immune response dangerous. If allele is up taken in wrong place, within a gene, like a tumour suppressor gene and causes cancer) Multiple gene disorders difficult to treat Cost eugenics

Question 25

Name mutagens/factors that increase frequency of mutations

Answer 25

High energy ionized particles/X-rays/ultraviolet light/high energy radiation/uranium/plutonium/gamma rays/tobacco tar/ caffeine/pesticides/mustard gas/base analogues/free radicals; (reject radiation) High energy radiation / X rays / ultraviolet light / gamma rays; high energy particles / alpha particles / beta particles; named chemical mutagens e.g. benzene / caffeine / pesticide / mustard gas / tobacco tar / free radicals

Question 26

Describe how the technique of genetic fingerprinting is carried (6)

Answer 26

1. DNA is cut; 2. Using restriction enzyme; 3. Electrophoresis; 4. Separates according to length/mass/size; 5. DNA made single-stranded; 6. Transfer to membrane/ Southern blotting; 7. Apply probe; 8. Radioactive/ single stranded/ detected on film/ fluorescent; 9. Reference to tandem repeats/VNTRs/minisatellites; 10. Pattern unique to every individual;

Question 27

Compare DNA to RNA

Answer 27

Similarities: Contain phosphate Made up of nucleotides Contains organic bases (A, C and G) (not T as it is replaced by U in RNA) Pentose sugar Differences RNA single stranded RNA has non-coding strands (introns) removed Ribose sugar in RNA deoxyribose in DNA U in ribose replaces the T 3 types of RNA, only one DNA Smaller than DNA

Question 28

Describe and explain how gene expression is controlled (post-transcription)

Answer 28

RNA interference (RNAi) prevents translation of the mRNA siRNA is double stranded and shorter than RNA It is split by an endonuclease into a passenger and guide strand The guide strand is loaded into the RISC complex which then attaches to the complementary section of bases on the mRNA and cleaves the mRNA

Question 29

Explain why the genetic code is described as non-overlapping and degenerate

Answer 29

Each base is part of only one codon. Some amino acids are coded for by more than one codon/ base sequence

Question 31

Describe how altered DNA may lead to cancer

Answer 31

1 (DNA altered by) mutation; 2 (mutation) changes base sequence; 3 of gene controlling cell growth / oncogene / that monitors cell division; 4 of tumour suppressor gene; 5 change protein structure / non-functional protein / protein not formed; 6 (tumour suppressor genes) produce proteins that inhibit cell division; 7 mitosis; 8 uncontrolled / rapid / abnormal (cell division); 9 malignant tumour;

Question 32

Risks of GMO’s

Answer 32

• Risks to the modified organism. Genetic modification of an organism may have unforeseen genetic effects on that organism and its offspring. These genetic effects could include metabolic diseases or cancer. •Transfer to other organisms. Genes transferred into GMOs could be transferred again into other organisms, by natural accidents. These natural accidents could include horizontal gene transmission in bacteria, cross-species pollination in plants, and viral transfer. This could result in a weed being resistant to a herbicide, or a pathogenic bacterium being resistant to an antibiotic. To avoid transfer via crosspollination, genes can now be inserted into chloroplast DNA, which is not found in pollen. • Risks to the ecosystems. A GMO may have an unforeseen effect on its food web, affecting other organisms. • Risk to biodiversity. GMOs may continue to reduce the genetic biodiversity already occurring due to selective breeding. • Risks to human societies. For example if GM bananas could be grown in temperate countries, that would be disastrous for the economies of those Caribbean countries who rely on banana exports.

Question 33

Genetic fingerprint process:

Answer 33

Extract the DNA: using a solvent like chloroform or phenol Amplify the DNA: using PCR and primers designed to copy the desired VNTR sequence Digestion of DNA: using restriction enzymes Separation: using gel electrophoresis Develop image using either photography-or southern blot.

Question 35

What is Gene therapy?

Answer 35

Transfer of genetic material into cells or tissue to cure diseases, aims to replace defective genes with healthy genes. Most likely to treat single gene disorders (cystic fibrosis, SCID, Huntington’s, muscular dystrophy)

Question 36

Describe translation

Answer 36

Translation: mRNA moves into cytoplasm / through nuclear pore / to ribosome; tRNA carries specific amino acid; mRNA read in codons / triplets; anticodon of tRNA matches codon of mRNA; ATP used in activation / joining amino acids; amino acids join by peptide bonds; tRNA used repeatedly; sequence of bases / codons determines sequence of amino acids;