protein synthesis ap biology ch. 17 “one gene-one polypeptide” theory george beadle and edward...
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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