+
Transcription
Protein Synthesis: Ch 17
+One Gene – One protein In 1902, Archibald Garrod described the inherited
disorder alkaptonuria as an "inborn error of metabolism." He proposed that a gene mutation causes a specific defect in the biochemical pathway for eliminating liquid wastes. The phenotype of the disease — dark urine — is a reflection of this error.
(from DNA from the Beginning)
+Beadle & TatumInvestigated bread moldFound that mutated bread mold could
not produce essential nutrients,and would die.
However, if the mutated mold was provided with a specific supplement, the mold could grow.
Each mutant was defective for a particular gene, so they concluded that the function of the gene was to determine the production of a particular enzyme.
+Central Dogma of Molecular Biology
How are genes connected to proteins?
+Triplet codeCodon –a triplet - made of 3 consecutive nucleotides – found on mRNA
each codon codes for a specific amino acid20 amino acids – 64 codons (3 are STOP codons)Codons are read in the 5 3 direction along mRNADetermined from triplet on DNA (complementary)
+
The reading frame is critical for codons:
i.e. “The fat cat ate the wee rat”Vs. “T hef atc ata tet hew eer at”
+TranscriptionmRNA is transcribed from DNA
Downstream = direction of transcription
+Steps for TranscriptionInitiation
RNA polymerase binds to promoterProkaryotes – direct bindingEukaryotes- transcription factors bind first, then RNA polymerase
The promoter is a specific sequence of DNA where the RNA polymerase attaches (includes a TATA box)
+Steps for TranscriptionElongation
RNA polymerase untwists DNA, read from 3’-5’, elongates mRNA from 5’-3’
RNA nucleotides have ribose instead of deoxyribose, and uracil instead of thymine
NTPs are used, so cleavage of two phosphates powers transcription
+Steps for TranscriptionTermination
When terminator sequence in DNA is reached, the transcribed terminator (in RNA) signals the endProkaryotes – stops at end of
termination signalEukaryotes –transcribes a
polyadenylation sequence that codes for the polyadenylation signal (AAUAAA)The pre mRNA is cut about 10-35
nucleotides past AAUAAA sequence
+Transcription videos
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html
http://www.dnalc.org/resources/3d/12-transcription-basic.html
+Eukaryotic – RNA processing
Eukaryotes modify pre-mRNA before it leaves the cytoplasm
Pre -mRNA – the immediate product of RNA polymerase in eukaryotes
+Alteration of mRNA endsGTP cap – A modified guanosine triphosphate
is added to the 5’ end (also known as a “methyl cap”, 7-
methylguanylate cap)Poly (A) tail – 50-250 adenine nucleotides
added to 3’ endFunctions for both: Helps mRNA leave nucleusProtects mRNA from degradationHelp ribosomes attach to 5’ end, once
mRNA in cytoplasm
+mRNA processing
http://www.youtube.com/watch?v=YjWuVrzvZYA
+RNA splicingPre mRNA has
Introns – intervening sequences Exons – expressed sequences
The introns are removed and the exons are spliced together
Spliceosome – does the cutting & splicingMade of snNRP- small nuclear
ribonucleoproteins + larger proteins
+
+Alternative splicing
A single gene can code for more than one kind of polypeptideDepends on what segments are
treated as exons during RNA splicing
+
+mRNA splicing
http://www.hhmi.org/biointeractive/mrna-splicing
http://www.youtube.com/watch?v=aVgwr0QpYNE