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| Frage | Antworten |
| What are the main features of the genetic code? | •Each amino acid is coded for by a sequence of 3 nucleotide bases (codon) on mRNA •The code is degenerate meaning most amino acids have more than one codon •Three codons do not code for an amino acid, they are stop codons and mark the end of the polypeptide chain •The code is non-overlapping i.e. 123456 would be read as 123, 456 not 123, 234, 345 etc •It is universal ie the same codon codes for same amino acids in all species |
| Explain how the structure of an mRNA molecule differs from DNA nucleotide | •Pentose sugar ribose not deoxyribose •Phosphate group in both •1 of 4 bases: Uracil replaces thymine, cytosine, guanine and adenine |
| Give two ways in which the structure of a molecule of tRNA differs from the structure of a molecule of mRNA. | -tRNA ‘clover leaf’ shape; (allow reference to loop / folded structure) -tRNA standard length; -tRNA has an amino acid binding site; -tRNA has anticodon available / three exposed bases; -tRNA has hydrogen bonds (between base pairs) |
| Draw the structure of DNA | |
| Draw the structure of mrna | |
| Draw the structure of mrna and dna and label the differences | |
| draw the structure of trna | |
| What is the role of RNA polymerase in transcription? | join/attach nucleotides, to form a strand/along backbone/ phosphodiester bonds |
| Describe the stages in transcription | •This is the formation of pre mRNA •DNA helicase acts on specific region of DNA breaking hydrogen bonds between complimentary bases •The two strands separate and DNA molecule uncoils exposing nucleotide bases in that region •One of the strands is used as a template to make the mRNA copy •RNA polymerase moves along the template strand causing complimentary nucleotides to align from a pool in the nucleus •Guanine binds with Cytosine, Adenine with Uracil – Hydrogen bonds reform •Specific base pairing means mRNA strands ends up being a complementary copy of the DNA template strand •The RNA polymerase moves along the DNA, separating the strand and assembling the mRNA strand •As the RNA polymerase moves along the DNA strand rejoins behind it |
| Describe what happens during splicing | •Splicing removes Non-coding introns and the functioning exons fuse together •The exons can join in a number of different combinations meaning a single gene can code for up to a dozen proteins depending on the order they combine •Splicing occurs in the nucleus so the mRNA is now small enough to pass out the nuclear |
| Describe what happens during translation | •A ribosome becomes attached to the starting codon (AUG) at one end of the mRNA •The tRNA with the complimentary anticodon sequence (UAC) moves to the ribosome and pairs up with the sequence on the mRNA. The tRNA carries a SPECIFIC amino acid •A tRNA with a complimentary anticodon pairs up with the next mRNA codon •The ribosome moves along the mRNA, bringing together 2 tRNA molecules at any time •By means of an enzyme and ATP, the two amino acids are joined by a peptide bond •The first tRNA molecule moves away - leaving the AA behind • Synthesis continues until a stop codon is reached |
| Describe the processes by which the sequence of subunits in DNA determines the mRNA and protein products in a cell. | -section of DNA unwinds / uncoils; RNA nucleotides align; -complementary base pairing / example of pairing; -mRNA polymerase (joins nucleotides); -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 |
| Explain how the structure of DNA is related to its function. | -sugar - phosphate backbone gives strength; -(coiling gives) compact shape; sequence of bases allows information to be stored; -long molecule stores large amount of information; -information can be replicated / complementary base pairing; -(double helix protects) weak hydrogen bonds / double helix makes molecule stable prevents code being corrupted; -chains held together by weak hydrogen bonds; -chains can split for replication / transcription |
| What is a gene mutation? | A change in base/nucleotide; |
| How can mutations affect protein structure? | •Nonsense mutation – when a base changes and codon is now stop codon •Polypeptide synthesis will be terminated early and the final protein will almost certainly be dysfunctional •Mis-sense mutation where a base change results in a different amino acid being coded for •Difference will depend on role of amino acid in final tertiary structure •May have no effect but if it was e.g. located in active site of an enzyme, effect would be serious as the substrate would no longer be complimentary |
| What is a silent mutation and how does it affect protein structure? | •Silent mutation – as the genetic code in degenerate, a single substitution of the third base of the codon may code for the same amino acid – hence no change |
| Starting from the base sequence of DNA, describe how a polypeptide is produced. | -DNA uncoils; strands separate; -formation of mRNA; -complementary base pairing/RNA nucleotides pair with DNA nucleotides; -RNA polymerase joins nucleotides together/forms mRNA; -mRNA moves to ribosomes/rough ER; tRNA brings amino acids; -anticodons on tRNA pair with mRNA codon; -amino acids joined together by peptide bonds; -ribosome moves along to next codon |
| Explain how exposure to a mutagenic agent may result in an inactive enzyme being produced by a cell. | mutation; change in the sequence of ----km -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 |
| Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another. | -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 |
| What are missense, nonsense and silent mutations? What happens when there are substitution, deletion and insertation mutations and what is a frameshift? | |
| Explain how mutation of a gene can result in a strain lacking a particular enzyme. | -mutation results in incorrect sequence of bases/nucleotides in DNA/frame shift of nucleotides; -incorrect codons/base triplets on mRNA; -so incorrect amino acids brought to ribosome/incorrect tRNA bring amino acids; -wrong sequence of amino acids changes tertiary structure or active site (of enzyme)/no longer functions as enzyme/no or different enzyme formed/protein non-functional |
| This fungus is haploid. Suggest why mutant genes can be identified more easily in haploid fungi than in diploid ones. | -haploid fungi have one set of chromosomes, diploid have two sets of chromosomes; -in haploid fungi all alleles are expressed; -in diploid fungi recessive alleles often hidden; -only expressed in homozygous individuals |
| Explain how a gene mutation may result in a different version of tyrosinase. | -change in code / base sequence; -detail e.g. substitution / addition / deletion; -of base(s); -different amino acid(s) inserted into protein /polypeptide; -role of tRNA |
| The faulty tyrosinase does not produce melanin. Suggest an explanation for this. | protein has different shape; tyrosine no longer fits into active site |
| Explain how the gene mutation results in failure to produce the enzyme phenylalanine hydroxylase. | -change in base sequence in mRNA / different mRNA codons; -different tRNA molecules pair with mRNA; -with different amino acids / change in primary structure; (reject produces different amino acids) -change in tertiary structure of protein; -change in shape of active site; |
| A deletion mutation occurs in gene 1. Describe how a deletion mutation alters the structure of a gene. | -removal of one or more bases/nucleotide; -frameshift/(from point of mutation) base sequence change |
| Explain how genes control cell division and how gene mutation can lead to the formation of oncogenes | •Proto-oncogenes stimulate cell division •In a normal cell, growth factors attach to a receptor protein on the cell-surface membrane and, via relay proteins in the cytoplasm, switch on genes necessary for DNA replication •A gene mutation can cause them to become oncogenes |
| How do oncogenes affect cell division? | affect cell division by: •Permanently activating the receptor on the cell membrane so cell division is switched on even in the absence of growth factors •Code for a growth factor that is produced in excessive amounts – excessive cell division •The result is that the cell divides too rapidly and a tumour or cancer develops |
| What are tumour suppressor genes and what happens if they mutate? | Tumour suppressor genes inhibit cell division, preventing formation of tumours •If it mutates it becomes inactive •Mutant cells formed are normally structurally and functionally different to normal cells •Most of them die but any that survive clone and form tumours •These may be harmless (benign) or harmful (malignant) |
| What happens when both proto-oncogenes and tumour suppressor genes are mutated? (use a car brake analogy to explain it) | |
| How is mutation rate increase? | - by exposure to mutagenic agents |
| How does the degenerate code affect mutations? | Due to the degenerate code, not all mutations result in the chance to the AA sequence of the encoded PP |
| Explain the roles of mRNA and molecule X (tRNA) in producing a particular type of protein. | Different tRNA/X for each amino acid / TRNA joins to one -type of amino acid; -mRNA determines which tRNA/X binds (to ribosome) / reference to codon-anticodon pairing/complementary base pairing; -Sequence of bases (in MRNA) determines sequence of amino acids / sequence of codons/triplets determines sequence of amino acids |
| Describe the similarities and differences between: the structures of RNA and DNA | Alike -both have phosphate/phosphoric acid/PO4; -bases/named bases/accept letters; nucleotides; -pentose sugar; Different -DNA deoxyribose; -DNA thymine; -DNA double stranded; -DNA larger/longer; -DNA one form RNA 3 types |
| Describe the similarities and differences between: the processes of replication and transcription. | Alike -H bonds break/DNA unwinds/DNA unzips; -between (complementary) bases; -complementary nucleotides/bases added/DNA acts as template; -same, correctly named, enzymes e.g. polymerase; Different -uracil/thymine used; all copied or only section copied respectively; -one strand used transcription, two in replication; -DNA/mRNA produced; -enzymes that are different, correctly named |
| Suggest why only about 1% of the genetic information is transcribed into functional RNA sequences in most mammalian cells. | -Only some genes transcribed; -only one strand transcribed; -different protein/enzyme required by different cells; -intron ref/some DNA does not code/non sense codes/junk DNA; -stutter sequences/repeat DNA |
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