7.3 ribosomes and tRNA

Question 1

ribosomes

Answer 1

made of protein and ribosomal RNA
- small (mRNA binding site) and large (tRNA binding sites: A site, P site, and E site) subunits
- can be found free floating in cytosol or bound to rough ER

Question 2

tRNA

Answer 2

fold into cloverleaf structure with 4 key regions:
- acceptor stem carries amino acid
- anticodon associates with mRNA codon
- T arm associates with ribosome
- D arm associates with tRNA activating enzyme

Question 3

tRNA activation

Answer 3

each tRNA mol binds with specific amino acid in cytoplasm in a reaction catalyzed by tRNA activating enzyme
- enzyme binds ATP to amino acid to form complex by high energy bond
- amino acid is then coupled to tRNA and the AMP Is released: tRNA mol is charged and ready for use

Question 4

translation

Answer 4

initiation: assembly of 3 components that carry out the process
- small ribosomal subunit binds to 5’ end of mRNA and moves along it until it reaches start codon
- appropriate tRNA mol bind to codon via its anticodon
- large ribosomal subunit aligns itself to tRNA mol at P site and forms complex with small subunit
elongation:
- second tRNA mol pairs with net codon in ribosomal A site
- amino acid in P site covalently attaches via peptide bond to amino acid in A site
translocation:
- ribosome moves long mRNA strand by one codon position (5’ to 3’ direction)
- deacylated tRNA moves into E site and is released, while tRNA carrying the peptide chain moves to P site
- cycle is repeated
termination:
- elongation and translocation continue in repeating cycle until ribosome reaches stop codon
- polypeptide is release and the ribosome disassembles back into its 2 independent subunits

Question 5

polysome

Answer 5

group of 2 or more ribosomes translating an mRNA sequence simultaneously

Question 6

primary protein structure

Answer 6

• formed by covalent peptide bonds between amine and carboxyl groups of adjacent amino acids

Question 7

secondary structure

Answer 7

• the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets
• provides the polypeptide chain with a level of mechanical stability

Question 8

tertiary structure

Answer 8

• the way the polypeptide chain coils and turns to form a complex molecular shape
• may be important for the function of the protein

Question 9

quaternary structure

Answer 9

• multiple polypeptides or prosthetic groups may interact to form a single, larger, biologically active protein
• prosthetic group is an inorganic compound involved in protein structure or function
• may be held together by a variety of bonds