1. An Act in 3 Parts Part 3 - Translation

2. The Cast The Ribosome In Eukaryotes, it consists of two subunits (40S and 60S) Role: binds to the mRNA and “reads” it Figure 1 p. 250

3. The Cast Transfer RNAs (tRNA) Transfer RNAs (tRNA) A small, single stranded RNA, cloverleaf shape A small, single stranded RNA, cloverleaf shape Role: Recognizes ‘codons’ on mRNA with complimentary ‘anticodon’ and brings the appropriate amino acid Role: Recognizes ‘codons’ on mRNA with complimentary ‘anticodon’ and brings the appropriate amino acid Figure 3 p. 251

4. Each tRNA carries only one specific amino acid Each tRNA carries only one specific amino acid Therefore 20, tRNAs are required Therefore 20, tRNAs are required But, there are 64 possible combinations of nucleotide triplets But, there are 64 possible combinations of nucleotide triplets Thus, there is some redundancy. Some amino acids are coded for by more than one triplet sequence Thus, there is some redundancy. Some amino acids are coded for by more than one triplet sequence

5. Why are the nucleotides (bases) read in groups of 3? There are 4 nitrogenous bases… How many combinations would be possible if one nucleotide coded for an a.a? How many combinations would be possible if one nucleotide coded for an a.a? _ or 4 1 = 4 combinations How many combinations would be possible if two nucleotides coded for an a.a? How many combinations would be possible if two nucleotides coded for an a.a? _ x _ or 4 2 = 16 combinations How many combinations would be possible if three nucleotides for an a.a? How many combinations would be possible if three nucleotides for an a.a? _ X _ X _ or 4 3 = 64 combinations

6. Figure 7 p. 240

7. Step 1: Initiation Small rRNA subunit recognizes 5’ cap and binds to mRNA Small rRNA subunit recognizes 5’ cap and binds to mRNA Small rRNA positions itself at the start codon AUG to ensure proper reading frame Small rRNA positions itself at the start codon AUG to ensure proper reading frame tRna with anticodon UAC binds to the rRNA and mRNA complex tRna with anticodon UAC binds to the rRNA and mRNA complex Now the large subunit clamps onto this whole complex so that mRNA is sandwiched between the small and large rRNA subunits Now the large subunit clamps onto this whole complex so that mRNA is sandwiched between the small and large rRNA subunits

8. Step 2: Elongation Ribosome contains two sites for the tRNA (A site – amino site and P site – peptide site) Ribosome contains two sites for the tRNA (A site – amino site and P site – peptide site) The next mRNA codon is exposed in the A site. The next mRNA codon is exposed in the A site. Figure 4a p. 252

9. The next tRNA carrying the required a.a enters the A (aminoacyl) site, and a peptide bond forms between adjacent a.a. The next tRNA carrying the required a.a enters the A (aminoacyl) site, and a peptide bond forms between adjacent a.a. NOTE: Ribosomes read mRNA in the 5’ to 3’ direction adding a new amino acid for each codon read

10. The ribosome shifts over one codon. The ribosome shifts over one codon. Second a.a moves into the P (peptide) site while the third a.a enters the A site. Second a.a moves into the P (peptide) site while the third a.a enters the A site. This process continues and the tRNAs that are released can be recycled (pick up amino acids again) This process continues and the tRNAs that are released can be recycled (pick up amino acids again)

11. Step 3: Termination Ribosome eventually reaches a stop codon Ribosome eventually reaches a stop codon The three stop codons are UGA, UAG and UAA The three stop codons are UGA, UAG and UAA These codons do not code for an amino acid and therefore there is no corresponding tRNA These codons do not code for an amino acid and therefore there is no corresponding tRNA Release factor (a protein) aids in the release of the polypeptide chain from the ribosome Release factor (a protein) aids in the release of the polypeptide chain from the ribosome

12. The two subunits of the ribosome fall off the mRNA and translation ceases The two subunits of the ribosome fall off the mRNA and translation ceases The polypeptide is then modified and folded into the protein and targeted to the area of the cell where it is required. The polypeptide is then modified and folded into the protein and targeted to the area of the cell where it is required.

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