Protein Synthesis Transcription and Translation

Protein Synthesis: Transcription Transcription is divided into 3 processes: –Initiation, Elongation and Termination Initiation –RNA polymerase binds to the promoter region of a gene (DNA segment) –Promoter  sequence of DNA upstream and adjacent (just before) to a gene that contains a string of adenines and thymines recognized by RNA polymerase (‘TATA box’) –RNA polymerase opens up the double helix

Protein Synthesis: Transcription Elongation –RNA polymerase builds the single-stranded mRNA in the 5’ to 3’ direction without a primer (different from DNA) –The promoter region does not get transcribed –One DNA strand is used as a template for mRNA synthesis (‘template strand’; ‘anti-sense’), while the other is not (‘coding strand’; ‘sense’) Since mRNA is built in the 5’ to 3’ direction, the DNA strand that it is transcribed from is in the 3’-5’ direction (template or anti-sense strand)

Protein Synthesis: Transcription Termination –RNA polymerase recognizes the end of the gene at the terminator sequence –The newly made mRNA disassociates from the DNA template strand –RNA polymerase is free to bind to the promoter region of another gene See figure 2, page 243

Protein Synthesis: Transcription Posttranscriptional Modifications –The mRNA made up to this point is known as the ‘primary transcript’ since it needs to undergo modifications before leaving the nucleus –A 5’ cap (‘hat’) consisting of 7-methyl guanosine protects the mRNA from digestion as it exits the nucleus –~200 adenine nucleotides are added to the 3’ end (‘poly-A tail’) to protect it from degradation

Protein Synthesis: Transcription Posttranscriptional Modifications-cont’d –Eukaryotic DNA contains coding (‘exons’) and noncoding (‘introns’) regions  therefore the mRNA will have sections that do not code for protein Spliceosomes cut out the introns and join the exons together to arrive at a continuous coding piece of mRNA for protein  ‘mRNA transcript’ See figure 4, page 244

Protein Synthesis: Translation The Ribosome –Made up of a large (60S) and a small (40S) subunit (see figure 1, page 250) –The ribosome moves along the mRNA in the 5’ to 3’ direction –A new amino acid is added to the polypeptide chain every time a codon (3 bases) is read

Protein Synthesis: Translation Transfer RNA (tRNA) –A small, single-stranded nucleic acid that resembles a cloverleaf –Delivers the correct amino acids to the polypeptide- building site –Its anticodon (3 bases) recognizes the codon of the mRNA (see figure 3, page 251) They are complementary to one another: Ex. mRNA: UAU, tRNA: AUA –The 3’ end has the amino acid attachment site –There are at least 20 tRNAs since each only carries one specific amino acid –The tRNA is charged when it is carrying an amino acid

Protein Synthesis: Translation Translation is divided into 3 processes: –Initiation, Elongation and Termination Initiation Both ribosomal subunits bind to mRNA when it enters the cytoplasm by recognizing its 5’ cap See figure 2, page 250

Protein Synthesis: Translation Elongation –First codon recognized is the start codon AUG, which codes for the amino acid methionine Ensures correct reading position by the ribosome –The ribosome has 2 sites for tRNA A (Acceptor) site and P (Peptide) site

Protein Synthesis: Translation Elongation-cont’d: Steps –tRNA with methionine enters the P site –A second tRNA enters the A site with an amino acid. A peptide bond is formed between methionine and the second amino acid –The ribosome translocates (moves) one codon over and the first tRNA is released (uncharged) –The second tRNA is now in the P site and a third tRNA brings an amino acid to the A site –Sequence continues until termination occurs

Protein Synthesis: Translation Termination –Elongation continues until a stop codon is reached (UAG, UGA, UAA) for which there are no corresponding tRNAs –The release factor protein aids in the release of the polypeptide chain The two ribosome subunits disassemble and release the mRNA and newly formed polypeptide –The polypeptide enters the endoplasmic reticulum to be folded and modified –See figure 4, page 252