CHAPTER 14.docx

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21 Apr 2012
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14.1 THE CONNECTION BETWEEN DNA, RNA, AND PROTEINS
Beadle & Tatum ‘s experiment
- Lead to the one gene one-polypeptide hypothesis
- Their subject was neutospora (a type of bread mould)
o Two varites wild type and mutant (by exposing to x-rays)
o Each group was grown in minimal growth medium , mutant did not grow unless supplied
with additional nutrients (ex. amino acid arginine)
- Arg mutants they hypothesized had a deficiency in an ezyme needed for the synthesis of
argenine
Why is it a triplet code?
- Scientists knew that there were 20 amino acids
... if 1 base pair coded for 1 a.a. = 4 a.a.’s
…. If 2 base pairs coded for one a.a. = 4^2 = 16 amino acids
… so smallest possible number is 3 (4^3 = 64)
- Nirenberg & Leder (synthesising all possible codon’s and combining them in a mixture of tRNAs)
and Khorana (synthesising long and repeatative mRNA’s and mixing with ribosomes)
experiment’s helped plot the genetic code
- Sense codons they specify a certain amino acid (61 of 64 codons are sense)
o AUG codes for methionine, it is the start/ initiator codon
- Nonsense/ termination codons have no corresponding amino acid
o UAA, UAG, UGA
- Genetic code is degenerate (more than one codon for more amino acids)
- It is also commaless … must be read in the correct manner to make sense
14.2 TRANSCRIPTION: DNA-DIRECTED RNA SYNTHESIS
- Transcription is not replication
o mRNA is single stranded
o RNA polymerase (not DNA polymerase) catalyses rxns
RNA polymerase doesn’t need a primer either
o Only the part coding for gene takes part in transcription, not entire genome
o At a given point in time only one DNA strand acts as template
- Protein coding and non-protein (tRNA, mRNA, snRNA, rRNA) regions exist
- Promoter
- Transcription unit the part b/w promoter and terminator that is transcribed as mRNA
- the new RNA molecule temporarily winds with the template strand of DNA into a hybrid RNA-
DNA double helix
- template strand read 3’-5’
o 5’ end of RNA is made first
- (IN EUKS)
o They have a TATA box sequence right before the promoter that forms complex with
transcription factors , together they attract the RNA polymerase II and DNA begins to
unwind
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14.3 PROCESSING OF mRNAs IN EUKARYOTES
Post-transcription of EUKS
- Precursor (pre-) mRNA is created
- Modifications of pre-mRNA and mRNA ends
o 5’ cap is added to 5’ end of pre-mRNA by a capping enzyme
It is added after transcription
It has a reversed guanine nucleotide, so that the OH- group faces the beginning
rather than the end of the molecule
Attached to the rest of the chain by 3 phosphate groups
The cap stays even in mature mRNA , during translation is functions as initial
attachment site for ribosomes
o No terminator sequence at the end of the gene
At the 3’ end is a sequence that is to be transcribed … proteins bind to this
polyadenylation signal and cleave the pre-mRNA
This signals the RNA polymerase to stop transcription and poly(A) polymerase
adds a chain of adenine nucleotides to the 3’ pre-mRNA end
This poly(A) tail enables the mature mRNA to be translated efficiently
and protects the RNA from attack by RNA-digesting enzymes in the
cytoplasm
- Introns and exons are also present. No one knows why….
Process of removing introns and retaining exons
- Process: mRNA splicing , it occurs in a spliceosome
o Pre-mRNA and snurps (small ribonucleprotein particles) form a complex called
spliceosome
Snurps aka snRNP’s (not snRNA’s, different acronym) bind to the intron by
recognising the boundary (of intron/ exon) and loop the intron out. This brings
exons closer together…active spliceosome has formed
Splicesome cleaves the intron at its 5’ end, so that it bonds with itself near the
3’
The spliceosome cleaves the 3’ end of the intron, releasing it. The complex
disassembles
o Enzymes degrade the intron
o The catalytic activity of snRNP’s lies in the snRNA not the proteins
So snRNA acts as a ribozyme
o The intron-exon cut is perfectly exact …. Otherwise translation would produce gibberish
Significant of introns?
- Alternative splicing
o One pre-mRNA can produce several mature mRNA (different peptides)
Ex. alpha-tropomyosin
o Helps us understand how 25,000 genes can produce SO many proteins
- Exon shuffling
o … just what is sounds like
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