From Genes to Proteins
Transcription in prokaryotes Simple process No nucleus; ribosomes lie close to the DNA in cytoplasm 1 st part of the mRNA attaches directly to ribosomes The rest is being transcribed Assembly of aa into proteins begins right away
Transcription in eukaryotes Transcription MUCH MORE complex!!!! DNA wound around protein spools Before transcription happens… Spools unwound Enzyme cannot begin process-able to attach to start signal region(replication fork) Once new mRNA is transcribed from DNA template mRNA breaks into pieces. Why?
Eukaryotic transcription mRNA reassembles to form finished mRNA Portions of the original is left out. Finished strand is shorter Why does the original mRNA molecule break up into pieces? What is discarded when the pieces are spliced back together?
What is happening?
What next? During mRNA processing Introns are cut out and exons are spliced together Events occur in the cell Introns: intervening sets of nucleotides (junk) Exons: code for aa, because they are expressed Once mRNA is processed, where do it go? What happens to the mRNA molecule after processing is complete?
Translation Removing introns from mRNA is known as mRNA processing. During Translation-processed mRNA attaches to free ribosomes or attached ribosomes on RER Polypeptide synthesis begins mRNA carries genetic info in sequence of codons Particular order of of aa
Protein synthesis To manufacture protein requires deciphering the info and assembling in an particular order Synthesis of a polypeptide chain (small protein) from info carried by mRNA is called TRANSLATION!!!!! How is mRNA “language” translated into protein “language”
20 amino acids/64 possible Combinations Job bringing aa to ribosomes is performed by tRNA 2 parts[ anticodon & aa] Anticodon-a set of 3 bases that is specific to the type of aa the tRNA carries. Each have a different anticodon Binds ONLY to the complementary mRNA codon
Codons & anticodons joining
mRNA codons amino acids
Joining codons & anticodons Depends on proper translation Ribosome attaches to 1 st codon on mRNA strand tRNA approaches carrying aa If tRNA recognizes the 1st mRNA codon, 2 join tRNA w/aa moves along mRNA strand, exposing the next mRNA codon Again, proper tRNA w/its aa joins mRNA strand
Forming a polypeptide Once 1 st two aa are in place, 1 st tRNA is then released Leaving it free to pick up another aa Process continues-ribosomes moves along mRNA Polypeptide chain grows Aa added to a chain at 15/s One of 3 codons stop the translation process No tRNA to match up Translation stops polypeptide complete Breaks away from the assembly line
summary DNA stores instructions for polypeptide synthesis The DNA instructions transcribed mRNA, carrying info from the nucleus into cytoplasm Ribosomes & tRNA interact w/mRNA to translate the info & build polypeptides
proteins They are made to order Synthesized when needed, in quantity needed Only certain genes are expressed at one time Kinds of proteins synthesized by a cell differentiate it from other kinds of cell in an organism Protein in a muscle cell is different in nerve cell. Get it?