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Transcription and Translation.notebook February 21, 2012
Transcription and TranslationRecall the central dogma:
Gene expression is a two step process: First the genetic code must be transcribed from DNA to RNA, then the code must be translated into a polypeptide chain.
Transcription and Translation.notebook February 21, 2012
TRANSCRIPTIONTo transcribe means to make a copy or transfer information from one form to another. Genetic Transcription involves making a copy of a small portion of the genome.
Transcription happens in three stages:
Initiation
Elongation
Termination
Sound familiar? These are the same three stages of replication!
Locating the correct spot on the DNA to start copying
Copying the correct number of nucleotides form the DNA to the mRNA (messenger RNA)
Signals the copying process to end at the correct place, ensuring all the information has been copied
And final processing before the mRNA leaves the nucleus.
Transcription and Translation.notebook February 21, 2012
Getting Started: InitiationPromoter Sequences These nucleotide sequences ensure that the correct SENSE STRAND is selected for copying (as opposed to the antisense strand), that the transcription begins at the correct nucleotide, and that it proceed in the correct direction.
They provide a binding site for RNA Polymerase, the enzyme that catalyses mRNA sysnthesis.
TATAbox
The TATA box is a region rich in T and A that helps RNA polymerase find the correct binding site.
Transcription and Translation.notebook February 21, 2012
Keep it Going: ElongationJust as in DNA replication, free nucleotides are added to create a new molecule, only this time it is RNA instead of DNA.
RNA polymerase only works in the 5' to 3' direction like DNA polymerase, bit since only one strand is being made there are no Okazaki fragments.
RNA POLYMERASE-FAST!-opens DNA as it goes, detaches when done-several enzymes can work in succession, one after another-not as accurate because cannot repair errors (but since only one polypeptide will be made, errors have less impact)
Transcription and Translation.notebook February 21, 2012
Ending it: TerminationTerminator Sequences
No, not bad movie clips from the 80s....
Highly specific sequences signal RNA polymerase to dissociate from the DNA molecule. The RNA polymerase is free to catalyze the production of more mRNA molecules.
The transcribed mRNA molecule is called precursor mRNA, because it will be processed before heading off to the cytoplasm.http://www.youtube.com/watch?v=Jqx4Y0OjWW4&feature=related
http://www.youtube.com/watch?v=tzUVM0KOcE&feature=related
Transcription and Translation.notebook February 21, 2012
PremRNAA few things happen to the molecule before it is ready to go make protein:
pre-mRNA Cap and TailThe molecule gets a 5' cap a special G nucleotide and a polyA tail, a long series of A nucleotides attached to the 3' end. These protect the mRNA from enzymes in the cytoplasm.
mRNA SplicingA molecule called a splicesome splices out the introns of noncoding information in the molecule, joining the exons together and shortening the mRNA molecule.
Transcription and Translation.notebook February 21, 2012
Ready to make protein: TRANSLATION
Now the code carried in the codons of the mRNA molecule is ready to be translated into the sequence of amino acids that will make the protein expressed by the gene.
Translation happens in three stages:hmmm...this look familiar!
Initiation
Elongation
Termination
When a mature mRNA reaches the cytoplasm, a specific binding sequence allow it to associate with a ribosome.
Once the mRNA and ribosome are correctly set, the cycle of reading the codons and bonding the amino acids together begins.
A STOP codon prevents another amino acid from being attached and causes a release factor to shut off the ribosome's machinery.
Transcription and Translation.notebook February 21, 2012
The players:tRNATransfer RNA molecules link each mRNA codon to a specific amino acid. They are coded for in DNA like mRNA but fold into a characteristic clover leaf shape because of base pairing between nucleotides. The 3' end of the
stand has an amino acid attachment site. When it is carrying an amino acid it is known as an aa-tRNA.
At the end of one lobe their is an anti-codon, the sequence complementary to the codon of the mRNA molecule.
Activating Enzymes link the tRNA to the correct amino acid and correct mRNA codon at these sites.
Transcription and Translation.notebook February 21, 2012
RibosomesThese small organelles provide a site for the synthesis of protein by bringing together the mRNA, and the aatRNA and the activation enzymes required to drive synthesis forward.
The ribosome is made up of two subunits that fit together to make an active ribosome. These units are composed of rRNA (ribosomal RNA) and protein.
P Siteholds one
aatRNA and the aa chain
A Siteholds
incoming aatRNAE site
releases "used" tRNA
mRNA
Transcription and Translation.notebook February 21, 2012
Let's get going: InitiationSTART CODON: AUGThe AUG codon binds to the mRNA and the smaller sub unit of the ribosome. It has the amino acid Methionine, and allows the larger subunit to come in and complete the ribosome complex.
The leader sequences in the mRNA interact with the rRNA to ensure that the AUG sequence is the start codon and not a coincidental sequence somewhere in the middle of the mRNA.
Transcription and Translation.notebook February 21, 2012
Making Meat: ElongationA three step cycle is repeated to elongate the amino acid chain:1 Pairing. The mRNA codon in the A site base pairs with the anticodon of an incoming aatRNA2 Bonding. Enzymes catalyze a peptide bond to link the incoming aa to the aa in the P site.
3 Translocation. The ribosome shifts along the mRNA to the next codon. The used tRNA moves to the E site and is released.http://www.youtube.com/watch?v=1PSwhTGFMxs&feature=related
Transcription and Translation.notebook February 21, 2012
Transcription and Translation.notebook February 21, 2012
...and FINALLY: TerminationSTOP CODONS
Elongation continues until a stop codon on the mRNA enters the A site. No tRNA matches this codon.The aa chain and the mRNA are both stuck there until a releasing factor comes along and cleaves the aa chain from the last tRNA.
The ribosome sub units come apart, ready to reattach to a new mRNA and facilitate more protein synthesis.
http://www.youtube.com/watch?v=zb6r1MMTkc&feature=related
Transcription and Translation.notebook February 21, 2012
http://www.youtube.com/watch?v=D3fOXt4MrOM&feature=fvsr
http://www.youtube.com/watch?v=983lhh20rGY&feature=related
A nice summary note can be found at:http://classes.midlandstech.edu/carterp/Courses/bio101/chap14/chap14.htm
Videos:
