transcription and translation

Question 1

describe two differences between the structure of a tRNA molecule and
the structure of an mRNA molecule. (2)

Answer 1

1. tRNA is ‘clover leaf shape’, mRNA is linear;
2. tRNA has hydrogen bonds, mRNA does not;
3. tRNA has an amino acid binding site, mRNA does not;
4. tRNA has anticodon, mRNA has codon;

Question 2

in a eukaryotic cell, the structure of the mRNA used in translation is
different from the structure of the pre-mRNA produced by transcription.
describe and explain a difference in the structure of these mRNA
molecules. (2)

Answer 2

1. mRNA few nucleotides
   OR
   pre-mRNA more nucleotides
   OR
   mRNA has no introns/has (only) exons
   OR
   pre-mRNA has (exons and) introns;
2. (because of) splicing;

Question 3

describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation. (3)

Answer 3

1. tRNA brings specific amino acid (to ribosome);
2. anticodon (on tRNA) binds to codon (on mRNA);
3. amino acids join by condensation reaction (using ATP)
   amino acids join to form a peptide bond (using ATP);

Question 4

describe how mRNA is produced from an exposed template strand of DNA.
do not include DNA helicase or splicing in your answer. (3)

Answer 4

1. (free RNA) nucleotides form complementary base pairs;
2. phosphodiester bonds form;
3. by (action of) RNA polymerase;

Question 5

describe how mRNA is formed by transcription in eukaryotes. (5)

Answer 5

1. hydrogen bonds (between DNA bases) break;
2. (only) one DNA strand acts as a template;
3. (free) RNA nucleotides align by complementary base pairing;
4. (in RNA) uracil base pairs with adenine (on DNA)
   OR
   (in RNA) uracil is used in place of thymine;
5. RNA polymerase joins (adjacent RNA) nucleotides;
6. (by) phosphodiester bonds (between adjacent nucleotides);
7. pre-mRNA is spliced (to form mRNA)
   OR
   introns are removed (to form mRNA);

Question 6

describe how a polypeptide is formed by translation of mRNA. (6)

Answer 6

1. (mRNA attaches) to ribosomes
   OR
   (mRNA attaches) to rough endoplasmic reticulum;
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. amino acids join by peptide bonds;
5. (amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. the ribosomes move along the mRNA to form the polypeptide;

Question 7

what is the proteome of a cell? (1)

Answer 7

(the proteome is the full) range of/number of different proteins that a cell is able to produce (at a given time);

Question 8

starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.
do not include descriptions of transcription and splicing in your answer. (5)

Answer 8

1. mRNA associates with a ribosome/ribosome attaches to mRNA;
2. ribosome moves to/find start codon/AUG;
3. tRNA brings/carries (appropriate/specific) amino acid;
4. anticodon (on tRNA complementary) to codon (on mRNA);
5. ribosome moves along to next codon;
   OR
   ribosome ‘fits’ around two codons/can fit two tRNAs;
6. (process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);

Question 9

the genetic code is described as degenerate.
what is meant by this? give an example to illustrate your answer. (2)

Answer 9

1. more than one codon codes for a single amino acid;
2. e.g. Val;

Question 10

describe how mRNA is produced in a plant cell. (5)

Answer 10

1. the DNA strands separate by breaking the H bonds;
   OR
   H bonds broken between (complementary) (DNA) bases;
2. (only) one of the strands/template strand is used (to male mRNA/is transcribed);
3. complementary base pairing so A->U, T->A, C->G, G->C;
4. (RNA) nucleotides joined by RNA polymerase;
5. pre-mRNA formed;
6. splicing/introns removed to form mRNA;

Question 11

give the two types of molecule from which a ribosome is made. (1)

Answer 11

one of RNA and one of protein(s)/polypeptide(s)/amino acid(s)/peptide(s)/ribosomal protein;

Question 12

describe the role of ribosome in the production of a polypeptide. do not include transcription in your answer. (3)

Answer 12

1. mRNA binds to ribosome;
2. idea of two codons/binding sites;
3. (allows) tRNA with anticodons to bind/associate;
4. (catalyses) formation of peptide bond between amino acids (held by tRNA molecules);
5. moves along (mRNA to the next codon)/ translocation described;

Question 13

in a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA.
explain why. (2)

Answer 13

1. introns(in pre-mRNA);
2. removal of sections of (pre-mRNA)/splicing;