14.4: Translation is the RNA-directed synthesis of a polypeptide: A Closer Look

Question 1

transfer RNA(tRNA)

Answer 1

translator for a series of codons along an mRNA molecule, brings amino acids to the ribosomes which add each amino acid brought to it by a tRNA to the growing end of a polypeptide chain from the cytoplasmic pool of amino acids

Question 2

How is the 3d shape of tRNA made ?

Answer 2

tRNA is a short strand of RNA that folds on itself due to hydrogen-bonding in nucleotide base pairs and creates the 3D shape

Question 3

What does the protruding 3’ end of the tRNA show?

Answer 3

The protruding 3’ end acts as the attachment site for an amino acid

Question 4

anticodon

Answer 4

loop from the other end of the , triplet that base-pairs to a specific mRNA codon, written in 3’ → 5’ direction antiparallel to the codons

Question 5

How is the genetic message translated from mRNA?

Answer 5

the genetic message is translated as tRNAs position each amino acid in the order prescribed

Question 6

How is tRNA made and used ?

Answer 6

tRNA like mRNA is made in the nucleus and transfers to the cytoplasm, used repeatedly, and picks up certain amino acids after leaving the ribosomes

Question 7

2 instances of molecular recognition for accurate translation

Answer 7

1. tRNA binds to an mRNA codon and specifies a particular amino acid must carry that amino acid to the ribosome

aminoacyl-tRNA synthetases: correct matching up of tRNA and amino acid carried out by this family of related enzymes, 20 different types of this. Catalyzes covalent bond of the amino acid to its tRNA through hydrolysis and ATP, tRNA then called charged tRNA

1. The paring of the tRNA anticodon with the appropriate mRNA codon, there are only 45 anticodons and 61 codons, they can only pair cause the 3rd nucleotide paring is relaxed
   wobble: flexible base pairing, 3rd nucleotide

Question 8

Ribosomes

Answer 8

facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis, made of proteins and rRNAs(most abundant RNA), must be assembled, 3 units in bacteria and 4 in eukaryotes, made of s30 and 50s, eukaryotes slightly larger

Question 9

The sites of tRNA

Answer 9

P site: holds the tRNA carrying the growing polypeptide chain (controlled by rRNAs)

A site: holds the tRNA carrying the next amino acid to be added to the chain (controlled by rRNAs)

E site: discharged tRNA leaves from this site

• polypeptides pass through the exit tunnel and are released through it in the large subunit

Question 10

initiation of translation

Answer 10

• brings together an mRNA, a tRNA bearing the first amino acid of the polypeptide, and 2 subunits of a ribosome
  1. A small ribosomal subunit binds to both the mRNA and a specific initiator tRNA, carries the amino acid methionine(N-term). In bacteria, the small subunit can bind these 2 in either order; it binds the mRNA at a specific RNA sequence. In eukaryotes, the small subunit, with the initiator tRNA already bound, binds to the 5’ cap of the mRNA and then moves downstream along the mRNA until it reaches the AUG start codon, where the initiator tRNA hydrogen-bonds.

proteins called initiation factors and GTP is used to bring the components together

Question 11

translation initiation complex

Answer 11

all attached components of the ribosome,

Question 12

elongation of translation

Answer 12

amino acids added one by one to the previous amino acid at the C-terminus of the growing chain

• Energy from GTP is needed in order to shift and attach tRNA, energy expenditure steps, GTP hydrolyzation needed to maintain accuracy to provide for the translocation step, used for attaching tRNA transferring from the A site (carries the chain of polypeptides)
• 5’→ 3’ direction on the mRNA

Question 13

release factor

Answer 13

binds to the stop codon in the A site, water is used to release the polypeptide and split the GTP, multiple mRNAs can work, when finished they degrade

Question 14

3 things that happens to the amino acid after translation

Answer 14

• Protein folds
• Certain enzymes may be modified by the attachment of any of the 4 macromolecules
• Polypeptide chains can be modified in various ways

Question 15

free ribosomes

Answer 15

not bound

Question 16

bound ribosomes

Answer 16

bound to the rough ER or the nuclear envelope, make proteins of the endomembrane system

Question 17

signal peptide

Answer 17

polypeptides of proteins destined for the endomembrane system or for secretion are marked by this

Question 18

SRP

Answer 18

recognizes the signal peptide as it emerges from the ribosomes

Question 19

Steps of proteins to the ER

Answer 19

1. Polypeptide synthesis begins on a free ribosome in the cytosol
2. SRP binds to the signal peptide, halting synthesis momentarily
3. SRP binds to a receptor protein in the ER membrane, part of a protein complex that forms a pore
4. The SRP leaves, and polypeptide synthesis resumes, with simultaneous translation across the membrane
5. The signal peptide is cleft by an enzyme in the receptor protein complex
6. The rest of the completed polypeptide leaves the ribosome and folds into its final conformation

Question 20

polyribosomes

Answer 20

multiple ribosomes translate a single mRNA can be bound or free of protein is not needed, the mRNA is degraded

• Bacteria and eukaryotes also increase the number of copies of a polypeptide by transcribing multiple mRNAs from the same gene
• Bacteria do not have separate areas due to no membrane so it is a bit different