GENES & PROTEINS

Garrod: “inborn errors of metabolism” Linked genes & phenotypes Alkaptonuria – black urine, inherited, inability to

break down alkapton

Beadle & Tatum: “one gene – one enzyme” Experimented with Neurospora crassa 3 classes, all unable to synthesize arginine (a.a.)

Revision: “one gene – one polypeptide” not all gene products are enzymes

Each mutant unable to synthesize one of the necessary enzymes for one step in the arginine pathway

Transcription: DNA → RNA

Translation: mRNA → protein

mRNA: carries genetic “code” from DNA to ribosome

RNA processing: yields finished mRNA

Primary transcript: initial RNA transcript

Triplet code: genetic instructions as a series of 3 nucleotides

Template strand: DNA strand that is transcribed

Codons: mRNA base triplets

TERMINOLOGY

PROKARYOTIC CELL

•No processing•No nuclear membrane•Transcription & translation

occur simultaneously

Eukaryotic cell

CRACKING THE GENETIC CODE

•Nirenberg – 1st codon, UUU for phenylalanine

•Redundancy but no ambiguity

•Reading frame – group of 3 nucleotides, must be read in order

•mRNA nucleotides read from left to right (5’-3’) in groups of 3

•Message written w/o spaces but read as a series of non-overlapping 3 letter words (codons).

Summary: Genetic information is encoded as a sequence of non-overlapping base triplets or codons, each of which is translated into a specific amino acid

STAGES OF TRANSCRIPTION

INITIATION

ELONGATION

TERMINATION

INITIATION

•RNA polymerase bindsto promoter

•DNA strands unwind

•Polymerase initiates RNA synthesis at the start point

ELONGATION

•Polymerase moves downstream, unwinding the DNA & elongating the RNA transcript 5’→3’

•DNA strands reform double helix

TERMINATION

•Polymerase transcribes terminator sequence which signals end of transcription unit

•RNA transcript released and polymerase detaches

Transcription factors – mediate the binding of RNA polymerase and the initiation of transcription

Transcription initiation complex – assembly of transcription factors and RNA polymerase

TATA box – crucial promoterDNA sequence for forming the Initiation complex

PROCESSING

• List 4 components of mRNA processing

• Explain how tRNA is activated

RNA PROCESSING

mG cap

Poly A tail

RNA PROCESSING

Introns removed Exons spliced

1) Pre-mRNA combines with small nuclear ribonucleoproteins (snRNPs) and other proteins → spliceosome

2) Within spliceosome, snRNA base-pairswith nucleotides at ends of intron

3) RNA transcript cut to release intronand the exons are spliced

TERMINOLOGY

Ribozymes – RNA molecules that function as enzymes

Alternative RNA splicing – one gene, multiple products depending on which segments are treated as exons

Domains – independently folding part of a protein, discrete structural & functional regions; one domain active site, another attachment site

tRNA

•Small 75-85 nucleotides

•Anticodon

•Amino acid attachment site (CCA)

•Processed in cytoplasm

•Transcribed in nucleus