SBI 4U: Metablic Processes TRANSLATION The reading of the mRNA transcript and the creation of the polypeptide chain. RIbosomes: Composed of a small subunit (40S) and a large subunit (60S) INITIATION: Ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA The subunits act as a clamp (80S) around the mRNA http://www.mtmi.vu.lt/pfk/funkc_dariniai/nanostructures/images/0901whitesides_ribo.jpg http://www.modares.ac.ir/elearning/Dalimi/Proto/Lectures/week2/ribosome_1%5B1%5D.gif Section 1.3

SBI 4U: Metablic Processes GENETIC CODE There are 20 amino acids found in proteins, only 4 bases in mRNA (U C A G) Codons: sequences of three bases used to code for an a.a. 43=64 possible codons (some amino acids have more than one codon) Ex. UUU UUC, UCU, UCC all code for phenylalanine (a.a.) This redundancy helps to reduce errors AUG: start codon (Methionine) used 99% of the time UAA, UAG, UGA: stop codons The mRNA transcript is read in sets of 3 nucleotides (one codon) to determine which a.a. is next. Section 1.3

http://anthro. palomar http://anthro.palomar.edu/biobasis/images/protein_synthesis_at_ribosomes.gif

ELONGATION of the POLYPEPTIDE CHAIN SBI 4U: Metablic Processes ELONGATION of the POLYPEPTIDE CHAIN AUG is the first codon read for every protein (start codon) Ensures the correct reading frame (phase in which the mRNA is read) is used by the ribosome The large subunit of the ribosome has two sites for tRNA A (acceptor) site P (peptide) site tRNA carrying the methionine enters the P site. The next tRNA carrying the required a.a. enters the A site. At the catalytic site (b/w A and P) a peptide bond forms between the two amino acids. The ribosome moves over one codon. The second a.acid moves to the P site, a tRNA brings the next a.acid to the A site and it is added to the chain. The first tRNA is released to be recycled for use with another amino acid. The process repeats with the polypeptide chain trailing in the cytoplasm. Section 1.3

http://sun.menloschool.org/~birchler/cells/animals/ribosome/synthesis.GIF

SBI 4U: Metablic Processes tRNA tRNA delivers amino acids to the ribosome Looks like a cloverleaf Has an anticodon – sequence of three bases that recognize the mRNA codon (complementary to) ex. The GCC codon specifies the a.a. alanine The anticodon on the tRNA carrying alanine would be CGG Opposite arm carries the amino acid binding site Aminoacyl t-RNA synthases add the amino acids to tRNA at the binding site, the tRNA is now a aminoacyl-tRNA Section 1.3

SBI 4U: Metablic Processes TERMINATION Ribosome will reach a stop codon (UGA, UAG, UAA) No tRNA exists for a stop codon A release-factor protein helps the two ribosome subunits to fall off the mRNA and the polypeptide chain is released. MODIFICATIONS Some amino acids may be glycosylated (sugar added) or phosphorylated (phosphate added) or altered in another way. Enzymes may cleave (cut) the chain at specific places. Section 1.3

SBI 4U: Metablic Processes Wobble Hypothesis In codons the third base may differ between 2 codons that code for the same amino acid (UAU and UAC both code for tyrosine). If the tRNA's anticodon is AUA it can still bind to UAC. This flexibility allows for the correct amino acid to be added to the polypeptide chain despite errors in the gene sequence. The proposal that tRNA can recognize more than one codon by unusual base pairing is known as the “wobble hypothesis”. Section 1.3