An Act in 3 PartsAn Act in 3 Parts

Part 3 - TranslationPart 3 - Translation

The CastThe CastThe RibosomeThe Ribosome

In Eukaryotes, it In Eukaryotes, it consists of two consists of two subunits (40S subunits (40S and 60S)and 60S)

Role: binds to Role: binds to the mRNA and the mRNA and “reads” it“reads” it

Figure 1 p. 250

The CastThe Cast Transfer RNAs (tRNA)Transfer RNAs (tRNA)

A small, single A small, single stranded RNA, stranded RNA, cloverleaf shapecloverleaf shape

Role: Recognizes Role: Recognizes ‘codons’ on mRNA ‘codons’ on mRNA with with complimentary complimentary ‘anticodon’ and ‘anticodon’ and brings the brings the appropriate amino appropriate amino acid acid

Figure 3 p. 251

Each tRNA carries only one specific Each tRNA carries only one specific amino acidamino acid

Therefore 20, tRNAs are requiredTherefore 20, tRNAs are requiredBut, there are 64 possible But, there are 64 possible

combinations of nucleotide tripletscombinations of nucleotide tripletsThus, there is some redundancy. Thus, there is some redundancy.

Some amino acids are coded for by Some amino acids are coded for by more than one triplet sequencemore than one triplet sequence

Why are the nucleotides (bases) Why are the nucleotides (bases) read in groups of 3? read in groups of 3?

There are 4 nitrogenous bases…There are 4 nitrogenous bases…How many combinations would be possible How many combinations would be possible

if one nucleotide coded for an a.a?if one nucleotide coded for an a.a?_ or _ or 4411 = 4 combinations = 4 combinations

How many combinations would be possible How many combinations would be possible if two nucleotides coded for an a.a?if two nucleotides coded for an a.a?

_ x _ or _ x _ or 4422 = 16 combinations = 16 combinationsHow many combinations would be possible How many combinations would be possible

if three nucleotides for an a.a?if three nucleotides for an a.a?_ X _ X _ or _ X _ X _ or 4433 = 64 combinations = 64 combinations

Figure 7 p. 240

Step 1: InitiationStep 1: Initiation Small rRNA subunit recognizes 5’ cap and binds Small rRNA subunit recognizes 5’ cap and binds

to mRNAto mRNA Small rRNA positions itself at the start codon AUG Small rRNA positions itself at the start codon AUG

to ensure proper reading frameto ensure proper reading frame tRna with anticodon UAC binds to the rRNA and tRna with anticodon UAC binds to the rRNA and

mRNA complexmRNA complex Now the large subunit clamps onto this whole Now the large subunit clamps onto this whole

complex so that mRNA is sandwiched between complex so that mRNA is sandwiched between the small and large rRNA subunitsthe small and large rRNA subunits

Step 2: ElongationStep 2: ElongationRibosome contains two sites for the Ribosome contains two sites for the

tRNA (A site – amino site and P site – tRNA (A site – amino site and P site – peptide site)peptide site)

The next mRNA codon is exposed in The next mRNA codon is exposed in the A site.the A site.

Figure 4a p. 252

The next tRNA carrying the required The next tRNA carrying the required a.a enters the A (aminoacyl) site, and a.a enters the A (aminoacyl) site, and a peptide bond forms between a peptide bond forms between adjacent a.a.adjacent a.a.

NOTE: Ribosomes read mRNA in the 5’ to 3’ direction adding a new amino acid for each codon read

The ribosome shifts over one codon.The ribosome shifts over one codon.Second a.a moves into the P (peptide) Second a.a moves into the P (peptide)

site while the third a.a enters the A site while the third a.a enters the A site.site.

This process continues and the tRNAs This process continues and the tRNAs that are released can be recycled that are released can be recycled (pick up amino acids again)(pick up amino acids again)

Step 3: TerminationStep 3: TerminationRibosome eventually reaches a stop Ribosome eventually reaches a stop

codoncodonThe three stop codons are UGA, UAG The three stop codons are UGA, UAG

and UAAand UAAThese codons do not code for an amino These codons do not code for an amino

acid and therefore there is no acid and therefore there is no corresponding tRNAcorresponding tRNA

Release factorRelease factor (a protein) aids in the (a protein) aids in the release of the polypeptide chain from release of the polypeptide chain from the ribosomethe ribosome

The two subunits of The two subunits of the ribosome fall off the ribosome fall off the mRNA and the mRNA and translation ceasestranslation ceases

The polypeptide is The polypeptide is then modified and then modified and folded into the folded into the protein and targeted protein and targeted to the area of the cell to the area of the cell where it is required.where it is required.

EncoreEncoreRead pp. 237 – 241Read pp. 237 – 241Read pp.250 – 253Read pp.250 – 253Questions p. 254 #1a-c, 2, 3, 4, 6, 7Questions p. 254 #1a-c, 2, 3, 4, 6, 7