3.4.2 - DNA & PROTEIN SYNTHESIS

Question 1

mRNA is used during translation to form polypeptides. describe how mRNA is produced in the nucleus of a cell (6)

Answer 1

1. DNA helicase
2. unzips the DNA helix by breaking hydrogen bonds between the base pairs
3. only one DNA strand acts as a template
4. free activated RNA nucleotides are attracted to exposed bases
5. bind by complementary base pairing
6. RNA polymerase joins RNA nucleotides together, by creating phosphodiester bonds
7. pre-mRNA is spliced to remove introns

Question 2

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

Answer 2

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

Question 3

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 3

1. mRNA has fewer nucleotides/mRNA has no introns/pre-mRNA has more nucleotides/pre-mRNA has exons and introns
2. because of splicing

Question 4

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

Answer 4

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

Question 5

describe how mRNA is produced from an exposed template strand of DNA.

do not include DNA helicase or splicing in your answer (3)

Answer 5

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

Question 6

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

Answer 6

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 the adenine on DNA/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/introns are removed to form mRNA

Question 7

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

Answer 7

1. mRNA attaches to ribosomes
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 is released after amino acid joined to polypeptide
7. the ribosome moves along the mRNA to form the polypeptide

Question 8

what is the proteome of a cell? (1)

Answer 8

the proteome is the full range/number of different proteins the genome/DNA is able to code for

OR

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

Question 9

give two structural differences between tRNA and mRNA (2)

Answer 9

any two:

1. mRNA does not have hydrogen bonding, but tRNA does/mRNA is linear but tRNA is cloverleaf shape
2. mRNA does not have an amino acid binding site, but tRNA does
3. mRNA has more nucleotides
4. different mRNAs have different lengths, all tRNA are the same length
5. mRNA has codons, tRNA has anticodon

Question 10

starting with the mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

do not include description of transcription and splicing in your answer (5)

Answer 10

1. mRNA associates with a ribosome/ribosome attaches to mRNA
2. ribosome moves to find the start codon (AUG)
3. tRNA brings/carries a specific amino acid
4. anticodon on tRNA is complementary to the codon on mRNA
5. ribosome moves along to the next codon
6. this process is repeated and amino acids join by peptide bonds to form a polypeptide

Question 11

the genetic code is described as degenerate. what is meant by this? (1)

Answer 11

more than on codon codes for a single amino acid

Question 12

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

Answer 12

1. the DNA strands separate by breaking the hydrogen bonds between complementary bases
2. only one of the strands (template strand) is used to make the mRNA
3. complementary base pairing: A + U, T + A, C + G and G + C
4. RNA nucleotides joined by RNA polymerase
5. pre-mRNA formed
6. splicing/introns removed to form mRNA

Question 13

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

Answer 13

1. rRNA/RNA
2. protein/peptide/polypeptide/amino acid

Question 14

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

Answer 14

1. mRNA binds to ribosome
2. idea of two codons/binding sites
3. allows tRNA with anticodons to bind
4. catalyses the formation of peptide bonds between amino acids that are held by the tRNA
5. moves along the mRNA to the next codon

Question 15

Below it shows the base sequence of part of a pre-mRNA molecule from a eukaryotic cell.

ACGCAUUAU

what is the base sequence of the DNA strand from which this pre-mRNA was transcribed (1)

Answer 15

TGCGTAATA

Question 16

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

Answer 16

1. there are introns in the pre-mRNA
2. these will be splices/removal of these sections

Question 17

not all mutations result in a change to the amino acid sequence of the encoded polypeptide. explain why (1)

Answer 17

1. genetic code is degenerate
   OR
2. mutation has occurred in an intron

Question 18

what is the role of RNA polymerase in transcription? (1)

Answer 18

to join mRNA nucleotides together by creating phosphodiester bonds to form an mRNA strand

Question 19

name the organelle involved in translation (1)

Answer 19

ribosome

Question 20

diamond-blackfan anaemia is an inherited condition caused by one of several gene mutations. the mutations can affect the function of the proteins that make up ribosomes.

explain what effect could this have on protein synthesis? (3)

Answer 20

1. affects the function of ribosomes
2. prevents ribosomes from translating mRNA into amino acids.
3. this prevents protein synthesis.

Question 21

an error occurs during transcription that accidentally inserts a stop signal into the middle of an mRNA sequence.

explain what affect this could this have on the protein that is eventually produced? (6)

Answer 21

1. results in a shorter amino acid sequence being produced
2. this changes the primary structure of the protein
3. and therefore the tertiary 3D structure of the protein
4. this can affect the function of the protein
5. this is because translation of an mRNA sequence only continues until a stop codon is reached.
6. any codons after the stop codon will not be translated into amino acids.

Question 22

explain how tRNA molecules pair up with mRNA during protein synthesis (2)

Answer 22

1. a tRNA molecule with an anticodon that is complementary to a codon on the mRNA attaches itself to the mRNA by complementary base pairing
2. a second tRNA attaches itself to the next codon on the mRNA in the same way, this continues until a stop codon is reached.

Question 23

what role does ATP play in translation? (2)

Answer 23

1. provides the energy needed for the bond between an amino acid and a tRNA molecule to form, allowing the tRNA to carry the amino acid to the ribosome.

Question 24

give a detailed description of tRNA’s role in translation (4)

Answer 24

1. tRNA molecules carry amino acids to the ribosome
2. a tRNA molecule with an anticodon that is complementary to the first codon of the mRNA attaches itself to the mRNA by complementary base pairing
3. a second tRNA molecule attaches itself to the next codon on the mRNA in the same way and the two amino acids are joined by a peptide bond
4. the first tRNA molecule moves away, leaving its amino acid behind, this process continues to produce a polypeptide chain.