concept 17.4: translation is the RNA- directed synthesis of a polypeptide: a closer look

translation

genetic information going from mRNA to protein

what happens to mRNA in prokaryotes

immediately after production it binds to ribosomes in the cytoplasm

what happens to mRNA in eukaryotes

mRNA undergoes processing before leaving the nucleus and binding to ribosomes in the cytoplasm

what does tRNA do

transfers amino acids to the growing polypeptides in a ribosome, each tRNA carries a specific amino acid on the 3’ end of the tRNA molecule

what is the anticodon

a set of three nitrogenous bases that will base pair with the nucleotides (codons) found on the mRNA strand

what carries the anticodons

tRNA has an anticodon that base pairs with a complementary codon on the mRNA strand

what does a tRNA molecule looks like

clover leaf

what are the two steps involved in translation

a correct match between tRNA and an amino acid and then a correct match between the tRNA anticodon and the mRNA codon

what does the enzyme aminoacyl- tRNA synthase do

matches the tRNA up to the amino acid

what does wobble mean

when flexible pairing of a codon allows some tRNAs to bind to more than one codon, also the reason why some codons are redundant

what facilitates the coupling of tRNA and mRNA codons

ribosomes

what is the difference between eukaryotic ribosomes and bacterial ribosomes

eukaryotic ribosomes are larger and have different molecular composition

what makes up ribosomes

rRNA (ribosomal RNA) and proteins

what are the three binding sites for tRNA called on ribosomes

the p site, the a site, and the e site (EPA)

the p site

holds the tRNA that carries the growing polypeptide chain

what binds at the p site

peptidyl- tRNA

the a site

holds the tRNA that carries the next amino acid to be added to the chain

what binds at the a site

aminoacyl- tRNA

the e site

the EXIT SITE, where discharged tRNAs leave the ribosome

what are the three stages of translation

first- initiation, second- elongation, third- termination

what occurs during initiation

the mRNA and tRNA are brought together at a subunit and the small subunit together reaches the start codon- AUG- and the translation starts

what is used in to form the translation initiation complex

energy in the form of GTP (ATP but with guanine instead of adenosine)

what occurs during elongation

amino acids are added one by one to the c-terminus of the growing polypeptide chain and each addition involves proteins called elongation factors

what are the three steps in adding amino acids to the polypeptide chain

codon recognition- GTP is used

peptide bond formation- rRNA of large ribosomal units catalyze the peptide bond formation

translocation-  "#1" tRNA moves to E site, "#2" tRNA moves to the P site and mRNA moves towards the 3' end and mRNA moves towards the 3' end

what occurs in termination

when a stop codon (UAA, UAG, or UGA) on the mRNA reaches the A site of the ribosome and the A site accepts a protein called the release factor and the polypeptide is released from the tRNA

what do all three stages of translation require

protein factors

how much GTP is used in the process of translation

1 molecule of GTP used for codon recognition and 1 used for translocation

what is a polyribosome

multiple ribosomes can translate a single mRNA simultaneously

what do polyribosomes enable

they allow a cell to make copies of a polypeptide quickly

what separates transcription and translation in eukaryotes

the nuclear envelope

what separates transcription and translation in prokaryotes

nothing, they can even being translation while transcription is occurring

what must happen after the polypeptid chain is made

they need to be altered or targeted to specific sites in the cell

what happens to a polypeptide chain while it is being synthesized

is begins to coil and fold spontaneous to form a protein with a specific shape- 3d with secondary and tertiary structure

what does a chaperone protein do

chaperonin helps the polypeptide fold correctly

what determines the primary structure of a protein

a gene

what does primary structure of a protein determine

it determines shape

where are free ribosomes and what do they make

they are located in the cytosol and make proteins that function in the cytosol

where are bound ribosomes and what do they make

they are attached to the ER and make proteins for the endomembrane system and proteins that are to be secreted from the cell

where does translation always begin

it always beings in the cytosol on the free ribosomes

signal peptide

a sequence of 20 amino acids near the leading end of the polypeptide being produced during translation

signal- recognition particle

a particle that binds to the signal peptide that will escort the ribsome to a receptor protein build into the ER membrane

what happens to a signal peptide after the ribosome attaches to the ER

the signal peptide is removed by an enzyme

where does polypeptide synthesis end

in the cytosol unless the polypeptide signals the ribosome to attach to the ER