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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)

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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

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+Alternative splicing

A single gene can code for more than one kind of polypeptideDepends on what segments are

treated as exons during RNA splicing

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+mRNA splicing

http://www.hhmi.org/biointeractive/mrna-splicing

http://www.youtube.com/watch?v=aVgwr0QpYNE