DNA II

Question 1

translation

Answer 1

• the process whereby the sequence of bases in mRNA is converted into a sequence of amino acids in a polypeptide chain

Question 2

transcription

Answer 2

• synthesis of an RNA molecule with a base sequence complementary to a section of a DNA

Question 3

processing of pre-mRNA

Answer 3

the pre-mRNA undergoes RNA splicing whereby the introns are excised and exons flanking the introns are spliced to form a mature mrna

Question 4

structure of tRNA

Answer 4

• single stranded polyribonucleotide of about eighty RNA nucleotides
• folds back upon itself to form a clover-leaf shape with three loops and a stem
• structure held together by hydrogen bonds between complementary bases of different regions
• presence of anticodon loop - a triplet nucleotide sequence ( anticodon) that complementary base pairs with a particular codon on the mRNA
• presence of CCA stem at 3’ end - where tRNA attaches covalently to its specific amino acid coded for by the anticodon of tRNA

Question 5

role of tRNA

Answer 5

• to act as an intermediate molecule between the codon of mRNA and amino acid sequence of the polypeptide chain
• to carry the correct amino acid from the cytoplasm to the polypeptide chain being synthesised at the ribosome

Question 6

why transcription and translation are able to occur simultaneously

Answer 6

-RNA polymerase involve in transcription (nucleus) and ribosomes involve in translation the cytoplasm
- no post transcriptional modification are carried out after transcription and hence translation can begin

Question 7

features of genetic code

Answer 7

-code is a triplet code. each amino acid in a polypeptide is coded for by a triplet
-genetic code is universal, the same triplet of nucleotides codes for the same amino acid in all organisms
-the genetic code is degenerate, more than one codon can code for the same amino acid

Question 8

initiation ( transcription)

Answer 8

• RNA polymerase recognizes and binds to the core promoter, forming a transcription initiation complex together with general transcription factors
  -RNA unwinds and unzips the DNA molecule , breaking the hydrogen bonds between the complementary bases, transcribing DNA template

Question 9

elongation ( transcription)

Answer 9

• RNA polymerase moves along DNA in the 3’ to 5’ direction
  -Free RNA nucleotides align the template strand via complementary bases
• RNA polymerase joins the RNA nucleotides together by catalysing a formation of a phosphodiester bond between RNA nucleotides
• RNA strand synthesised in the 5’ to 3’ direction
  -RNA strand peels away

Question 10

termination (transcription)

Answer 10

RNA polymerase reaches the terminator
RNA polymerase dissociates from the tRNA molecule to form new RNA strand

Question 11

amino acid activation ( translation)

Answer 11

• a specific amino acid is attached to the 3’ end of a specific tRNA forming an aminoacyl-tRNA. this reaction is catalysed by a specific aminoacyl-tRNA synthetase.
  the active site of each type of aminoacyl-tRNA synthetase fits only a specific combination of amino acid and the anticodon on the tRNA

Question 12

initiation ( translation)

Answer 12

• initiation factors mediate the formation of the translation initiation complex
  -initiator tRNA carrying the amino acid methionine (Met) with anticodon UAC, binds to the small ribosomal subunit
• small ribosomal subunit recognizes and binds to the 5’ end of the mRNA and moves in the 5’ to 3’ direction along mRNA until it reaches the first AUG codon that will serve as the start codon
  -anticodon UAC of the initiator tRNA complementary base pairs with start codon AUG on the mRNA
  -anticodon UAC of initiator tRNA, small ribosomal subunit and mRNA is followed by attachment of the large ribosomal subunit, completing the transcription initiation complex
• initiator tRNA fits into the P site of the large ribosomal unit and the vacant A site ready for the next aminoacyl- tRNA

Question 13

elongation ( translation)

Answer 13

• anticodon of the next incoming aminoacyl-tRNA carrying its specific amino acid, undergoes complementary base pairing and forms hydrogen bonds with the mRNA codon in the A site of the ribosome
  -peptide bond is formed between amino end of the amino acid in the A site and the carboxyl end of the polypeptide in the P site catalysed by peptidyl transferase
• amino end of the amino acid translocates one codon downstream along mRNA in a 5’ to 3’ direction
  -tRNA in the A site moves to the P site , tRNA in the P site move to the E site

Question 14

termination ( translation)

Answer 14

• elongation continues until a stop codon, UAA, UAG or UGA reaches the A site

Question 15

role of mRNA

Answer 15

• acts as a template for polypeptide synthesis
  -ribosomes use genetic information coded in the mRNAs to synthesise polypeptides

Question 16

roles of rRNA

Answer 16

• combines with proteins in the nucleolus to form small and large ribosomal subunits which will combine to form ribosomes
• rRNA in small ribosomal subunit complementary base pair to the mRNA during translation
  -rRNA in the large ribosomal subunit complementary base pair to the tRNA in P site and A site
• ribozyme, peptidyl transferase in large ribosomal subunit catalyses the formation of peptide bonds between adjacent amino acids