From Gene to Protein: Historical Genetics Experiments and Transcription Insights

Chapter 17From Gene to Protein

You Will Be Able To: • Describe historical experiments in genetics • Compare and contrast RNA & DNA structures • and functions • Compare/contrast transcription to replication

You Will Be Able To: • Describe the details of transcription • Contrast prokaryotes’ transcription & • translation to eukaryotes • Describe how viruses infect cells • Explain how mutations occur and their • significance in genetics

enzymes break down amino acids phenylalanine & tyrosine into CO2 and water Garrod’s studies: An “inborn errorof metabolism”

Early evidence indicating most genes specify the structure of proteins: • 1908 Garrod’s work on “Inborn Errors of Metabolism” in early 1900s • Mutation in specific gene causes lack of enzyme 1926 James Sumner purifies urease and shows that enzymes are proteins

Beadle & Tatum’s experiments in 1940’s • Used common bread mold: Neurospora • Testing: do genes act as enzymes, or have a more complicated role in enzyme formation?

Beadle and Tatum’s experiments in the 1940’s • Neurospora mold was chosen because: Zapped spores w/ x-rays to create mutations.

Fig. 17-2a Do individual genes specify the enzymes that function in a biochemical pathway? EXPERIMENT Growth: Wild-type cells growing and dividing No growth: Mutant cells cannot grow and divide Minimal medium

Fig. 17-2b RESULTS Classes of Neurospora crassa Wild type Class III mutants Class I mutants Class II mutants Minimal medium (MM) (control) MM + ornithine Condition MM + citrulline MM + arginine (control)

Fig. 17-2c Conclusion: one gene  one protein CONCLUSION Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Wild type Precursor Precursor Precursor Precursor Gene A Enzyme A Enzyme A Enzyme A Enzyme A Ornithine Ornithine Ornithine Ornithine Gene B Enzyme B Enzyme B Enzyme B Enzyme B Citrulline Citrulline Citrulline Citrulline Gene C Enzyme C Enzyme C Enzyme C Enzyme C Arginine Arginine Arginine Arginine

Extra! Extra! Home-boy Makes Good! Linus Pauling 1949: hemoglobin mutation and sickle cell anemia Other researchers continued and refined the one-gene-one protein hypothesis • Linus Pauling works with • mutations on hemoglobin http://pr.caltech.edu/events/caltech_nobel/nobel_people/pauling.gif

Big question still unanswered: • How do genes code for proteins (or a polypeptide chain)?

Big question still unanswered: DNA is “blueprint” • RNA is “working copy” • Used at ribosome • Recycled when done using it

1. Transcription Information flow from DNA to protein: • RNA moleculecomplementary to • thetemplate DNA strandissynthesized

2. Translation Information flow from DNA to protein: • Polypeptide (amino acid chain) • specified bymessenger RNA (mRNA) • is formed

Strands unwind & base pairs stick to complementary bases on each strand. Then DNA polymerase DNA replication:

DNA base-pairing helps in DNA replication

Base pairing also used in transcription and translation

Structure of RNA similar to DNA: *Formed from nucleotide subunits *Like DNA, RNA forms side chains *Like DNA, RNA forms H bonds between complementary base pairs

Only difference btwnribose sugar & deoxyribose is: 2nd carbon (2’C) on Ribose has an extra Oxygen atom

RNA has a ribose, a base, & phosphate groups: 5΄carbon’s Phosphate covalently bonds to the 3΄ carbon’s Hydroxide group forms alternating sugar-phosphate backbone like DNA

Uracil instead of thymine Nucleotide structure of RNA Single strand, not double stranded Ribose sugar

RNA nucleotide has other functions: Does that look familiar?

RNA other functions: Can form AMP - a product of hydrolysis of ATP Cyclic AMP - 2ndary messenger for several hormones.

Incoming RNA nucleotides with 3 phosphatespair withcomplementary bases on DNA strand • RNA polymerase cleaves 2 phosphates • from each nucleotide: Transcription:

Covalently links remaining phosphate to 3΄end of RNA chain Makes a sugar–phosphate-sugar-phosphate side chain Transcription:

DNA “blueprint” copied onto mRNA “Copy” is not identical to DNA code but is Transcription

Synthesis of mRNA – similar enzyme to DNA polymerase: DNA-dependent RNA polymerase RNA polymerase begins transcription after recognizing

mRNA Splicing • In Eukaryotes, many nucleotide bases in DNA do not Code for “anything”. • Before mRNA leaves the Nucleus, “useless” Nucleotides are spliced out of mRNA

mRNA Splicing Cont... • Parts of DNA that contain decipherable codes are • Parts of DNA strand that do not contain decipherable codes are

mRNA Splicing • Most of eukaryotic DNA Molecule is made of • Introns exist between

Eukaryotic gene structure exons DNA introns promoter Typical eukaryotic gene: sequences of DNA called exons, code for a.a.’s of a protein (medium blue), intervening sequences, introns (dark blue), do not. Promoter determines where RNA polymerase will begin transcription.

RNA synthesis and processing in eukaryotes DNA transcription initial RNA transcript RNA splicing introns cut out and broken down completed mRNA to cytoplasm for translation RNA polymerase transcribes exons & introns,

RNA synthesis and processing in eukaryotes DNA transcription initial RNA transcript RNA splicing introns cut out and broken down completed mRNA to cytoplasm for translation Enzymes in nucleus

Splicing mRNA Introns remain behind in nucleus and Exons are released to leave nucleus and

Chapter 17From Gene to ProteinPART 2

Translation • Prokaryotes: mRNA is immediately translated without more processing • Eukaryotic cell: nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified thru RNA processing  finished mRNA

A primary transcript is initial RNA transcript from any gene • Cells are governed by a cellular chain of command:

Fig. 17-3a-2 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell

Fig. 17-3b-3 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide (b) Eukaryotic cell

Translation: • mRNA base triplets, codons, are read in • Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

Fig. 17-4 Gene 2 DNA molecule Gene 1 Gene 3 DNA template strand TRANSCRIPTION mRNA Codon TRANSLATION Protein Amino acid

Cracking the Code • Of the 64 codon triplets:

Cracking the Code • Genetic code is _____________________ but not ________________; no codon specifies more than • Codons must be read in correct reading frame (correct groupings) in order for specified polypeptide to be produced

Fig. 17-5 Second mRNA base First mRNA base (5 end of codon) Third mRNA base (3 end of codon)

Evolution of the Genetic Code • Genetic code is universal, • shared by simplest bacteria to most complex animals • Genes can be transcribed and translated after being transplanted from one species to another!

Fig. 17-6a (a) Tobacco plant expressing a firefly gene

Fig. 17-6b (b) Pig expressing a jellyfish gene

Scientists are developing artificial DNA using different base pairs&sugars

Three representations of a tRNA molecule

From Gene to Protein: Historical Genetics Experiments and Transcription Insights

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