Protein Synthesis

AP BiologyAP Biology

Ch. 17Ch. 17

“One Gene-One Polypeptide” Theory George Beadle and Edward Tatum George Beadle and Edward Tatum (late 40’s to (late 40’s to

early 50’s) early 50’s) used X-rays to induce mutations in used X-rays to induce mutations in Neurospora Neurospora

crassa crassa (bread mold)(bread mold) AuxotrophsAuxotrophs—nutritional mutants unable to —nutritional mutants unable to

synthesize certain amino acid and vitamins. synthesize certain amino acid and vitamins. They traced the defect to the enzymes involved They traced the defect to the enzymes involved in their synthesis.in their synthesis.

Conclusion: One gene encodes one enzyme.Conclusion: One gene encodes one enzyme. Gene products encode both protein and RNAGene products encode both protein and RNA

Central Dogma of Molecular Biology In eukaryotic cells, a message (mRNA) In eukaryotic cells, a message (mRNA)

carries the genetic information (DNA) from carries the genetic information (DNA) from the nucleus to the cytoplasm the nucleus to the cytoplasm Transcription Transcription

An adapter (tRNA) translates the nucleic An adapter (tRNA) translates the nucleic acid (mRNA) into amino acid (protein) acid (mRNA) into amino acid (protein)

Translation Translation Information flow is unidirectionalInformation flow is unidirectional

GCTGCTAACGTCAGCTAGCTCGTAGCGCTAGCGCTTGCGTAGCTAAAGTCGAGCTCGCTTGCGTAGCTAAAGTCGAGCTGCTGCTAACGTCAGCTAGCTCGTAG

RNA Proteins

Structure of RNA Single strand of nucleotides [instead of 2 Single strand of nucleotides [instead of 2

strands in DNA]strands in DNA] Nucleotides contain 5-carbon sugar ribose Nucleotides contain 5-carbon sugar ribose

[instead of deoxyribose in DNA][instead of deoxyribose in DNA] 4 different nitrogenous bases4 different nitrogenous bases

Adenine (A)Adenine (A)Uracil (U) [instead of thymine in DNA]Uracil (U) [instead of thymine in DNA]Cytosine (C)Cytosine (C)Guanine (G)Guanine (G)

Forms of RNA 3 Main Types of RNA3 Main Types of RNA

mRNA (messenger RNA) brings DNA message mRNA (messenger RNA) brings DNA message out of nucleus to the cytoplasmout of nucleus to the cytoplasm

Each 3 bases on mRNA is a “codon”Each 3 bases on mRNA is a “codon” tRNA (transfer RNA) –The anticodon that tRNA (transfer RNA) –The anticodon that

matches with the codon from mRNA to determine matches with the codon from mRNA to determine which amino acid joins the protein chainwhich amino acid joins the protein chain

rRNA (ribosomal RNA) – make up the ribosomesrRNA (ribosomal RNA) – make up the ribosomes—RNA that lines up tRNA molecules with —RNA that lines up tRNA molecules with mRNA moleculesmRNA molecules

Transcription: Synthesis and Processing of RNA

RNA PolymeraseRNA Polymerase is an enzyme that is an enzyme that synthesizes RNA using one strand of the synthesizes RNA using one strand of the DNA as a templateDNA as a template Separates 2 strands of DNA and links Separates 2 strands of DNA and links

RNA nucleotides as they base-pair along RNA nucleotides as they base-pair along the DNA template in an overall is 5’ to 3’ the DNA template in an overall is 5’ to 3’ directiondirection

Transcription: Initiation RNA polymerase RNA polymerase

attaches to promotor attaches to promotor sequence (TATA box) sequence (TATA box) of DNA sequenceof DNA sequence ProkaryotesProkaryotes——

enzyme binds enzyme binds directly to sequencedirectly to sequence

EukaryotesEukaryotes——enzyme binds after enzyme binds after transcription factors transcription factors bind to the sitebind to the site

Transcription: Elongation RNA polymeraseRNA polymerase

Synthesizes in 5’Synthesizes in 5’ 3’ direction, anti-3’ direction, anti-parallel to DNA parallel to DNA templatetemplate

Unwinds about 20 Unwinds about 20 base pairs as it reads base pairs as it reads and synthesizes and synthesizes complementary complementary RNARNA

Transcription: Termination RNA polymerase terminates transcription RNA polymerase terminates transcription

at special DNA sequences, terminatorat special DNA sequences, terminator ProkaryoteProkaryote—stops right at the end of —stops right at the end of

termination signaltermination signal EukaryoteEukaryote—continues past termination —continues past termination

signal, to a poly(A) tail (AAUAAA) in signal, to a poly(A) tail (AAUAAA) in the pre-mRNAthe pre-mRNA

Transcription

The code on DNA tells how mRNA is put together.

Example: DNAACCGTAACG

mRNAUGGCAUUGC

Each set of 3 bases is called a triplet or codon

(in mRNA)

UGG CAU UGC

RNA Splicing Pre-mRNA never Pre-mRNA never

leaves the nucleusleaves the nucleus Introns—non-Introns—non-

coding regionscoding regions Exons—coding Exons—coding

regionsregions snRNPs join with snRNPs join with

other proteins to form other proteins to form spliceosome, which spliceosome, which releases intronsreleases introns

Translation Genetic information in Genetic information in

mRNA is read by mRNA is read by codonscodons

64 different codons64 different codons Genetic code is Genetic code is

redundant—more than redundant—more than one codon codes for one codon codes for many of the many of the 20 amino 20 amino acidsacids

AUGAUG—universal start —universal start codoncodon

UAA, UAG, UGAUAA, UAG, UGA——stop codonsstop codons

Structure of tRNA

Aminoacyl-tRNA Formation

Structure of a Ribosome

Translation: Initiation Small ribosomal Small ribosomal

subunit binds to start subunit binds to start codon (AUG) on codon (AUG) on mRNAmRNA

Initiator tRNA (UAC) Initiator tRNA (UAC) base-pairs with mRNAbase-pairs with mRNA

Large ribosomal unit Large ribosomal unit binds so initiator tRNA binds so initiator tRNA is in the P siteis in the P site

GTP provides energy GTP provides energy for intiation processfor intiation process

Translation: Elongation

Translation: Termination

Review entire process

Coupled Transcription

and Translation in Bacteria

Point Mutations

Substitution Mutations

Missense mutation: altered codon still codes Missense mutation: altered codon still codes for an amino acid, although maybe not the for an amino acid, although maybe not the right oneright one

Nonsense mutation: altered codon is a stop Nonsense mutation: altered codon is a stop codon and translation is terminated codon and translation is terminated prematurelyprematurely Leads to nonfunctional proteinsLeads to nonfunctional proteins

Insertions and Deletions

Frameshift mutation: addition or loss of one Frameshift mutation: addition or loss of one or more nucleotide pairs in a gene shifts the or more nucleotide pairs in a gene shifts the reading frame for translation and incorrect reading frame for translation and incorrect protein is madeprotein is made

The end

Ribosome

5’ 3’U U C U G G A U G U G G U U C U G G

Reading direction

mRNAAU C

Met

Incoming tRNA charged with amino acid

AUC

Met

CCA

Trp

Ribosome

5’ 3’U U C U G G A U G U G G U U C U G G

Reading direction

mRNACCA

Trp

UCA

Outgoing “empty” tRNA

Met

A GA

Phe

Ribosome

5’ 3’U U C U G G A U G U G G U U C U G G

Reading direction

mRNA

CCA

Trp

Met

A GA

Phe

UCA

CCA

Trp

Ribosome

5’ 3’U U C U G G A U G U G G U U C U G G

Reading direction

mRNA

CCA

Trp

Phe

CCA

Trp

AGA

UCA

Met

Amino acid

Incoming tRNA charged with amino acid

Ribosome

5’ 3’U U C U G G A U G U G G U U C U G G

Reading direction

mRNAA CC

Trp

Anticodon

A GA

Phe

A CC

Trp

A A G

Phe

Outgoing “empty” tRNA

CU A

UCA

Met