Cell Bio: Translation

Define translation

• process by which the sequence of nucleotides in a messenger RNA molecule directs the incorporation of amino acids into protein

Define Genetic code

• the set of rules by which the nucleotide sequence of a gene through an intermediary mRNA molecule is translated into the amino acid sequence of a protein

What is a codon?

• a group of 3 consecutive nucleotides that specifies one amino acid

What is the reading frame?

• one of the three possible ways in which a set of successive nucleotide triplets can be translated to a protein

What is a start codon?

• a special nucleotide sequence that signals where translation should start and sets the reading frame

What does the start codon AUG code for?

• methionine

For all cells, the very first start codon is what?

• AUG - methionine

What is the first amino acid in translation?

• methionine

Which type of RNA matches amino acids to codons?

• tRNA

What does an anticodon do?

• base pairs to codon (anticodon is complementary to codon)

Which type of RNA are the codons and anticondons?

• codon = mRNA

• anticodon = tRNA

Can tRNA molecule hydrogen bond to itself forming double helices?

• yes

How many segments are apart of the tRNA molecule that hydrogen bond to form double helices?

4

What is the function form of tRNA?

• L shaped molecule

What are the parts of the tRNA molecule?

• acceptor arm

• anticodon

• T loop

• D loop

• Variable loop

• unusual bases

Acceptor arm

• 3’ end

• CCA

• where amino acid attaches

Anticodon

• recognizes the codon in mRNA

T Loop

• recognize and interact with ribosome

D loop

• structural stabilization and recognition by aminoacyl tRNA synthetases

Variable loop

• structure and recognition

• always different from one amino acid ot the next

The genetic code is degenerate, what does that mean?

• most amino acids have more than one codon

• many amino acids have multiple tRNAs

• If 4 codons (mRNA) then 4 anticodons (tRNAs)

What is the wobble position of tRNA?

• many tRNAs only require accurate base pairing for the first two nucleotides in a Condon. The third position tolerates mismatch, so it’s called wobble position

What does aminoacyl tRNA synthetase do?

• enzyme responsible for “charging” tRNA molecules with amino acids

Most organisms have different aminoacyl tRNA synthetase for every amino acid. How many aminoacyl tRNA synthetase do most organisms have?

• 20

Does aminoacyl tRNA require atp?

• yes

What does the “Charging” of a tRNA create?

• high energy bond used to catalyze the formation of a peptide bond during translation

which organelle is responsible for protein synthesis?

• ribosome

The recognition of the codon by the anticodon and the formation of a polypeptide require what?

• a ribosome

What do ribosomes do during translation?

• latch onto mRNA

• position the correct tRNA molecule

• covalently link amino acids

What are ribosomes made of?

• ribosomal rna (rRNA)

• ribosomal proteins

Which type of ribosomes make proteins?

• free ribosomes

• located in cytosol

What type of ribosomes makes bound proteins?

• bound ribosomes

• located in ER

What are the 2 subunits of the ribosome?

• small and large subunits

Large subunit

• catalyzes the addition of an amino acid to the growing protein

small subunit

• matches tRNA to the codon

Once the large and small subunits come together on mRNA, when is synthesis finished?

• when the small and large subunits separate

When do the large and small subunits come together?

• when its time to make a protein

• once protein is made, they separate

What are the different tRNA binding sites on the ribosome?

• E site (exit site)

• P site (peptidyl site)

• A site (aminoacyl site)

E site

• holds the empty tRNA that has already donated its amino acid

P site

• holds the tRNA that’s carrying the polypeptide

A site

• where charged tRNAs enter

mRNA binding site on ribosome

• holds the mRNA being translated

Where on the ribosome are the tRNA binding sites and the mRNA binding sites?

• tRNA binding sites on large subunit

• mRNA binding site on small subunit

what are the 3 stages of translation?

• initiation

• elongation

• termination

Where does translation imitation start?

• start codon

• AUG is start codon for all proteins

what does the start codon do?

• indicates where translation should begin and sets the reading frame for the RNA molecule

What does initiator tRNA do?

• tRNA that carries methionine which is always the first amino acid in a polypeptide

What are required to start translation in eukaryotes?

• translation initiation factors

Where do translation initiation factors bind?

• small ribosomal subunit along with initiator rRNA

What does the small ribosomal subunit with bound initiator tRNA and translation initiation factors do?

• moves along mRNA searching for first AUG

Once the first AUG is found, what happens?

• translation initiation factors dissociate

• large ribosomal subunit binds

What happens after large ribosomal subunit binds?

• charged tRNA binds to the second codon (A site)

• first peptide bond forms

What’s used in prokaryotic translation initiation?

• polycistronic mRNA: mRNA molecules that encode several proteins

• Ribosome binding sequence: sequences in polycistronic mRNA that tell ribosomes where to bind

• Formyl-methionine: modified methionine that’s always the first amino acid in bacterial proteins

What proteins are required for elongation to proceed?

• elongation factors

What are the 4 steps of elongation that’s repeated over and over?

1. charged tRNA enters the A-site

2. amino acid at the A-site is linked to the growing polypeptide chain which is held at the p site

3. the large ribosomal subunits shifts moving the spent tRNA to the E site

4. the spent tRNA is ejected

What do stop codons do during termination?

• tell the ribosome where to stop translation

• don’t code for amino acids

codon to stop codon ratio

one start codon, 3 stop codons

steps of termination

1. stop codon tells ribosome where to stop translation

2. Release factors bind at the stop codon altering the activity of the ribosome (bind at stop codon at A site)

3. ribosome adds a water molecule to the released polypeptide

4. ribosome dissociates

What are the three stop codons?

• UAG - amber

• UAA - ochre

• UGA - opal/umber

what is a poly ribosome?

• mRNA with multiple ribosomes

How do polyribosomes affect translation?

• increase efficiency of translation