Sec 5.1 / 5.2

One Gene – One PolypeptideHypothesisearly 20th century – Archibald Garrod

physician that noticed that some metabolic errors were found in numerous members in a family

alkaptonuria – metabolic disorder where tyrosine is not properly broken down

hypothesized that these people had a defective enzyme that usually breaks down tyrosine. He also hypothesized that this enzyme was under the control of a single gene.

1941 – Beadle & Tatum

first hypothesized that genes and enzymes are somehow related

Beadle & Tatum ExptUsed X-rays to cause bread mold (yeast) to mutate

Wild type (Normal): can survive on minimal medium (agar, inorganic salts, glucose and biotin)

can synthesize all other molecules they need from these simple molecules (e.g. amino acids and nutrients)

Mutant: could not survive on minimal medium could not synthesize amino acids and nutrients they need because

of deficit in specific enzymes

examining arginine synthesis

precursor ornithine citrulline arginineE1 E2 E3

Beadle & Tatum Conclusionsone mutation corresponded to a change in a single

enzyme

As researchers learned more about proteins they made minor revisions to the “one gene-one-enzyme” hypothesis

Not all proteins are enzymes (e.g. keratin – structural protein)

Some proteins are made up of more than one type of polypeptide, each controlled by a different gene (e.g. Hemoglobin – α and β)

More accurately: “one-gene-one-polypeptide”

DNA

mRNA

protein

COMPONENTS LOCATION PROCESS

nucleus transcription

cytoplasm translation

Eukaryotes vs. Prokaryotes

Figure 17.3b

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

mRNA

DNA

Pre-mRNA

Polypeptide

Ribosome

Nuclearenvelope

TRANSLATION

TRANSCRIPTION DNA

mRNA

Ribosome

Polypeptide

Transcription

Figure 17.7

PromoterTranscription unit

RNA polymerase

Start point

53

35

35

53

53

35

53

35

5

5

Rewound

RNA

RNA

transcript

3

3

Completed RNA transcript

Unwound

DNA

RNA

transcript

Template strand of DNA

DNA

1 Initiation. After RNA polymerase binds to

the promoter, the DNA strands unwind, and

the polymerase initiates RNA synthesis at the

start point on the template strand.

2 Elongation. The polymerase moves downstream, unwinding the

DNA and elongating the RNA transcript 5 3 . In the wake of

transcription, the DNA strands re-form a double helix.

3 Termination. Eventually, the RNA

transcript is released, and the

polymerase detaches from the DNA.

Figure 17.4

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

mRNA

Protein

TRANSLATION

Amino acid

A C C A A A C C G A G T

U G G U U U G G C U C A

Trp Phe Gly Ser

Codon

3 5

35

A codon in messenger RNA Is either translated into an amino acid or serves as a

translational stop signal

Figure 17.5

Second mRNA baseU C A G

U

C

A

G

UUUUUCUUAUUG

CUUCUCCUACUG

AUUAUCAUAAUG

GUUGUCGUAGUG

Met orstart

Phe

Leu

Leu

lle

Val

UCUUCCUCAUCG

CCUCCCCCACCG

ACUACCACAACG

GCUGCCGCAGCG

Ser

Pro

Thr

Ala

UAUUAC

UGUUGC

Tyr Cys

CAUCACCAACAG

CGUCGCCGACGG

AAUAACAAAAAG

AGUAGCAGAAGG

GAUGACGAAGAG

GGUGGCGGAGGG

UGGUAAUAG Stop

Stop UGA StopTrp

His

Gln

Asn

Lys

Asp

Arg

Ser

Arg

Gly

U

CA

GUCAG

UCAG

UCAG

Fir

st m

RN

A b

ase

(5

end

)

Th

ird

mR

NA

bas

e (3

en

d)

Glu

The following is the sequence of a bases on the template strand of DNA in the transcription unit

3’ – GGATCAGGTCCAGGCAATTTAGCATGCCCC – 5’

a) Transcribe this sequence into mRNA

b) List the order of amino acids

ClassworkSection 5.1 (pg. 236) #1-2,4-6Section 5.2 (pg. 241) #1-3,5,6, 7,