What is the connection between genes (DNA) & phenotype?

IOW, how, exactly, do genes determine who we are?

Proteins, Proteins, Proteins!• Many are structural in nature, but most traits are

determined by enzymes (or lack thereof!)• Transcription & translation link genes to proteins

DNA RNA protein

transcription translation

One gene, one-enzyme hypothesis

• Proposed by Garod, 1909

• Confirmed by Beadle & Tatum, 1930– Worked w/ neurospora

(bread mold) mutants

– Modified to one gene, one polypeptide

Overview

The Triplet Code• 3 bases on DNA specify a particular amino acid

• A gene contains enough triplet codes to specify a particular polypeptide chain,along with regulatory DNA

The genetic code• Refers specifically to mRNA

codons– Complementary to DNA triplets

• Universal

• Degenerate

• Wobble

DNA- CCA/GGT/TTT/CGA

RNA- GGU/CCA/AAA/GCU

Aminos - gly/pro/lys/ala

DNA- CCA/GGT/TTT/CGA

m RNA- GGU/CCA/AAA/GCU

tRNA- CCA/GGU/UUU/CGA

Aminos - gly/ pro/ lys/ alaAminos - gly/pro/lys/ala

Transcription-mRNA synthesis• 3 stages-initiation, elongation, termination• Catalyzed by RNA polymerase(synthesizes 5’ 3’)• DNA is organized into Transcription units (genes)

– Initiation sequence, triplet codes for polypeptide, termination• Per gene, only one strand of DNA is transcribed(template, anti-sense)

– Which strand may vary from gene to gene

initiation

1. RNA pol binds to promoter

• Promoter includes binding site &

initiation site

2. RNA pol unzips DNA & begins

Putting Complementary bases in place

elongation

New strand is built in 3’ to 5’ directionUracil instead of thymineUses nucleoside triphosphates

termination

•Terminator sequence (DNA)Signals end of polypeptide•RNA pol dissociates, DNA zips up, mRNA released

Eukaryotes vs prokaryotes

• In nucleus

• Promoter includes the TATA box (for transcription factors)

• Transcription units only contain info for1 polypeptide chain

• mRNA must be processed before translation

• In cytoplasm• No TATA box or

transcription factors• Transcription units may

contain info for several different polypeptide chains, but have 1 set of regulators

• No mRNA processing• Translation can take place

simultaneously w/ transcription

RNA processing

• Methylguanosine “cap” added to 5’end

– Protection,Ribosomal attachment

• Poly-A tail added to 3’ end

– Protection,Transport to cytoplasm

• INTRONS (intervening sequences) cut out

• EXONS left & spliced together

– Spliceosomes-proteins (enzymes) & RNA

• Alternative RNA processing

RNAi (interference)

• Post-transcriptional gene silencing

• Short Double-stranded RNA (siRNA) triggers degradation of homologous mRNA– May be protective

(proofreading)

• research

The Anti-Codon

• Complementary to codons

• Found on tRNA– “cloverleaf” shape– Attachment site for

amino acid– Anti codon at opposite

end

• 1 tRNA per amino acid– Aminoacyl tRNA

synthetase– Inosine (U,C, or A)