chapter 6: RNA structure & role, protein synthesis and mutations

Question 1

what is mRNA?
(messenger RNA)

Answer 1

• mRNA is a product of transcription of DNA, hence the base sequence is complementary to the template DNA sequence
• mRNA acts as a carrier molecule, carrying the genetic code transcribed from DNA and attach to the ribosomes where polypeptides are made via translation
• in eukaryotic cells, mature RNA needs to exit through the nuclear pores to the cytoplasm, where the ribosomes are located before translation can take place
• in prokaryotic cells, the DNA is not membrane-bond, hence, ribosomes attach to the mRNA as soon as it is made
• hence mRNA has a low Mr and is short so that they can be easily transported out of the nucleus through the nuclear pores
• each mRNA has a specific sequence of bases organised into triplets (codon)
• each codon codes for 1 amino acid
• the sequence of bases on the mature mRNA dictate the order, type and number of amino acids in a polypeptide chain (primary structure)
• Hence, mRNA is single stranded so that its bases are exposed for complementary base-pairing with anti-codon of the amino-acid carrying tRNA

Question 2

what is a tRNA?
(transfer RNA)

Answer 2

• it consists of a single strand which folds back upon itself at different parts of the same strand to form a 3D structure
• the folding back is due to complementary stretches of bases that can form hydrogen bonds with each other
• sections with sequences which are not complementary form loos
• the sequence CCA ( from 5’ to 3’) at the 3’ end is not base-paired and is the point of attachment for amino acid
• each amino acid is covalently joined (by an ester bond) to the correct tRNA by an enzyme, aminoacyl-tRNA synthetase
• there are three nucleotides called anticodon at the anticodon loop of the tRNA
• the anticodon is complementary to the codon on the mRNA and binds via complementary base pairing to its corresponding codon on the mRNA during translation

Question 3

what is the function of tRNA?

Answer 3

• tRNA acts as a trasnfer molecule to bring its designated amino acids present freely in the cytoplasm to the ribosomes
• the anticodon of the tRNA complementary base pairs with the corresponding codon on the mRNA and adds the amino acid to the growing polypeptide chain
• it then leaves the ribosome, free for another amino acid to be attached to its 3’ amino acid attachment site

each cell has at least 20 different types of tRNA, at

Question 4

what is rRNA?
(ribosomal RNA)

Answer 4

• it consists of a single strand which folds back upon itself at many different parts of the same strand to form a 3D structure
• the folding back is due to the presence of complementary stretches of bases that can form hydrogen bonds with each other
• sections with sequences which are not complementary, form loops

-rRNA assembles with ribosomal proteins to form the small and large ribosomal subunits
- the rRNA molecule in the large ribosomal subunit is ribozyme
- it is a peptidyl transferase which catalyses the formation of peptide bonds between amino acids during translation

Question 5

relate these structural features of ribosomes to its features

structural feature:
each ribosome has:
- 1 mRNA binding site at its small ribosomal subunit and
- 3 tRNA binding sites at its large ribosomal subunit:
> Exit site
> peptidyl-tRNA site
> aminoacyl-tRNA site

Answer 5

• this holds the tRNA and mRNA is close proximity for translation to take place
• **Exit site: **allows the discharged tRNA to leave the ribosome (not carrying amino acid)
  - Peptidyl-tRNA: site holds the tRNA carrying the growing polypeptide chain
• Aminoacyl-tRNA site : holds the tRNA carrying the next amino acid to be added to the chain

Question 6

relate these structural features of ribosomes to its features

structural feature:
contains ribozyme- an rRNA molecule with enzymatic function that is complexed to the large ribosomal subunit

Answer 6

• the ribozyme is a peptidyl transferase which catalyses the formation of peptide bons between adjacent amino acids

Question 7

relate these structural features of ribosomes to its features

structural feature:
bound to endoplasmic reticulum or nuclear envelope

Answer 7

• bound ribosomes synthesise proteins that would be transported through the endomembrane system
  these proteins may be:
• proteins that are found in vacuoules, lysosomes
• proteins to be embedded in cell membrane
• or secrete out of the cells

Question 7

relate these structural features of ribosomes to its features

structural feature:
they can exist as polysomes/polyribosomes- a cluster of ribosomes present on a strand of mRNA

Answer 7

• carry out translation of one mRNA to synthesise proteins at a faster rate
• since many ribosomes are translating one mRNA to form many of the same polypeptide chain at the same time

Question 8

relate these structural features of ribosomes to its features

structural feature:
exist as free ribosomes

Answer 8

• free ribosomes synthesise proteins that stay in the cytosol and function
• ## translation in all ribosomes begins on free ribosomes in the cytosol

Question 10

features of the genetic code

what is redundancy or degenracy of the genetic code?