nucleic acids exam questions

Question 1

steps involved in transcription
exam q

Answer 1

• binding of RNA polymerase to target gene
• separation of DNA strands
• binding of free floating RNA nucleotides to template strand
• formation of phosphodiester bonds between free floating nucleotides
• RNA polymerase reaches a stop codon
• mRNA removed from template strand
• DNA strands join together
• mRNA leaves nucleus

Question 2

how ribosomes produce a polypeptide (translation)
exam q

Answer 2

• mRNA binds to ribosome on binding site
• tRNA has anticodons complementary to mRNA codons that bind together
• each tRNA molecule is bound to specific amino acid
• ribosome moves along mRNA to next codon
• new tRNA molecule comes in binding to mRNA. peptide bond forms between 2 amino acids
• repeated until stop codon reached, polypeptide released

Question 3

mRNA vs tRNA structure
exam q

Answer 3

• mRNA has more nucleotides than tRNA
• mRNA is a straight molecule, tRNA is folded
• mRNA has no paired bases or hydrogen bonds, tRNA has paired bases

Question 4

stages of translation
exam q

Answer 4

• mRNA binds to ribosome
• 1 molecule of tRNA binds to 1st codon
• 1st molecule of tRNA brings corresponding amino acid
• 2nd molecule of tRNA binds to 2nd codon, brings amino acid
• peptide bond forms between amino acids
• ribosome moves along mRNA, new codon enters ribosome
• new amino acids brought in and added until stop codon
• polypeptide chain released from ribosome

Question 5

describe transcription (3)

Answer 5

• DNA helicase unzips DNA helix, hydrogen bonds between complementary base pairs break
• 1 DNA strand acts as template, exposing unpair bases
• RNA polymerase adds complementary nucleotides, forming phosphodiester bonds between

Question 6

how are different proteins produced from a single gene? (3)

Answer 6

• splicing (cutting pre-mRNA) removes introns, joins exons
• more than 1 arrangement of mRNA / exons can be made
• each protein has a unique amino acid sequence, with proteins folding differently

Question 7

why is the genetic code degenerate? (1)

Answer 7

some amino acids coded for by more than 1 mRNA codon

Question 8

how does a change in base sequence produce a non-functional polypeptide? (2)

Answer 8

• primary structure changed
• affects: protein shape/folding/tertiary structure
• affecting function

Question 9

describe the process of translation (4)

Answer 9

• tRNA brings amino acid to ribosome
• each tRNA has specific amino acid
• anticodon matches to mRNA codon / complementary base pairing
• 2nd amino acid brought to ribosome
• 2 binding sites = 2 tRNA molecules attach to mRNA
• peptide bond forms between amino acids
• ribosome moves along mRNA until stop codon