1From DNA to ProteinChapt. 12DNA in NucleusRNA copyProtein in cytoplasmhttp://instruct1.cit.cornell.edu/courses/biomi290/MOVIES/CENTRALDOG.HTMLCentral Dogma• DNA carries the genetic code and transcribes an RNA copy of the code • The RNA copy is translated by ribosomesto make proteinDNA RNA ProteinTranscription Translation1 22Step 1: RNA SYNTHESIS-TRANSCRIPTIONThe process of converting the information contained in a DNA segment into proteins begins with the synthesis of mRNA molecules containing anywhere from several hundred to several thousand nucleotides, depending on the size of the protein to be made. Each of the 100,000 or so proteins in the human body is synthesized from a different mRNA that has been transcribed from a specific gene on DNA.Why do we need mRNA if DNA holds all the genetic instructions for the proteins the cell is supposed to produce?• DNA must be protected…If DNA is damaged in any way, then the coding sequence is changed and a mutationcould result which could greatly affect a cell or even the whole organism!Transcription• If a protein is required by a cell, that gene is activated (turned on)..• The gene makes an RNA copy of itself in the form of a messenger RNA molecule (mRNA)• Enzyme RNA polymerase runs along open DNA strand and synthesizes RNA complementary to the DNA3Transcription• Messenger RNA is synthesized in the cell nucleus by transcription of DNA, a process similar to DNA replication. As in replication, a small section of the DNA double helix unwinds, and the bases on the two strands are exposed. RNA nucleotides (ribonucleotides) line up in the proper order by hydrogen -bonding to their complementary bases on DNA, the nucleotides are joined together by a DNA dependent RNA polymerase enzyme, and a piece of mRNA is produced. • UNLIKE what happens in DNA replication where both strands are copied, only ONE of the two DNA strands is transcribed into mRNA (remember that RNA is a single-stranded molecule). The DNA strand that is transcribed is called the template strand and is a copy of the DNA informational strand!TranscriptionPolmRNADNA3’5’RNA Polymerase• Runs along DNA in a 5’ to 3’ direction (adding bases to the 3’ end) and forms mRNA• Until it hits a STOP signal, falls off and mRNA is released…..DNA reseals… http://www.ncc.gmu.edu/dna/mRNAanim.htm4How does the RNA pol know where to start reading a Gene?• The starting point of a gene is marked by a certain base sequence… called a promoter site. These sites are recognized by factors (termed "SIGMA“) which recognize the promoter sites and "tell" the RNA polymerase where to begin. The RNA polymerase then carries out the process of transcription.• Similarly, there are other base sequences at the end of a gene that signal a STOP to mRNA synthesis. A factor called "RHO" aids in terminating the process. The interaction of ‘rho’ with the RNA polymerase causes the enzyme to "fall off" the DNA template strand, thus stopping transcription.The Genetic CodeIf DNA is a long repeating length of… ACTGAATTGCCCTTCATGGTCATGGCTHow do you make a useful code???How can you make a useful code of this?5The Genetic Code• Based on 3 letter words• Every 3 nucleotide bases in DNA is a Code…• In RNA, the 3 complementary bases are a Codon… DNA CodeRNA CodonThe RNA CodonRemember: RNA travels to cytoplasm to make the protein• The Codon alphabet consists of 4 nucleotides….. AUGC (U replaces T in DNA)• The words are 3 letter combinations• How many possible combinations?• 43= 64 possible combinations• But, what ARE the words being spelled?The Genetic CodeEvery 3 nucleotides on mRNA ‘spell’ for one amino acidACA spells ThreonineCAC spells HistidineGUU spells ValineUUA spells LeucineText pg. 2246A Redundant Code…• The 20 Amino Acids can all be ‘spelled’ with just 20 codons• But, there are 64 possible codons• Several triplets account for the same AA…therefore, the code is redundant.• GUU, GUC, GUA, GUG all ‘spell’ ValineQuiz1) What would be the mRNA codon directing a cell to produce a protein beginning with the amino acids, Histidine and Serine? 2) What would be the Corresponding DNA code?How does the Gene end? Stop/Start Codons• Also, AUG spells START reading here• And, UAA spells STOP• Without a set Start point, the code would be nonsenseOWHYDIDTHERATDIELOVEISNOWHERE7The Genetic Code• The specific sequence of 3 nucleotide bases indicates how a protein is to be constructed• An mRNA sequence such as:UUU-UUG-GUA-CCCMeans that a protein of Amino Acids… Phenylalanine-Leucine-Valine-Prolineis to be made How to make that Protein?Step 2: Translation• mRNA is produced in nucleus by RNA polreading the DNA code• mRNA travels to cytoplasm where proteins are made….How?• In a process known as Protein Synthesis or Translation 2Translation Requires:• Message in the form of mRNA…• A Ribosome…..• Another type of RNA…called Transfer RNA (tRNA)• A pool of amino acids in cytoplasm Free Amino AcidsAA1AA2AA38tRNA…Single stranded RNA which folds into characteristic shapeJob is to carry AA from cytoplasm and drop them into place during protein synthesis20 different types of tRNA each carry one type of AA…Text pg. 226Two views of a tRNA moleculeAA1Translation Process:Initiation• Translation is the process of converting the mRNA codon sequences into an amino acid sequence. The initiator codon (AUG) codes for the amino acid N-formylmethionine (f-Met). No transcription occurs without the AUG codon. f-Met is always the first amino acid in a polypeptide chain, although frequently it is removed after translation. • After the initiation phase the message gets longer during the elongation phase…. Text pg 228Translation: Elongation• A Ribosome runs along mRNA reading codons, beginning at AUG (Start)• A tRNA carrying the corresponding AA drops into position and leaves AA off• New protein emerges from ribosome as a growing peptide chain9ElongationNew tRNAs bring their amino acids to the open binding site on the ribosome/mRNA complex, forming a peptide bond between the amino acids. The complex then shifts along the mRNA to the next triplet, opening the A site. The new tRNA enters at the A site.When the codon in the A site is a termination codon, a releasing factor binds to the site, stopping translation and releasing the ribosomal complex and mRNAText pg 228Elongation• Text pg 228,