Overview: The Flow of Genetic Information The information content of DNA
-
Upload
estevan-gutierrez -
Category
Documents
-
view
24 -
download
0
description
Transcript of Overview: The Flow of Genetic Information The information content of DNA
Dr. Sharma Jan,09
Overview:
The Flow of Genetic Information The information content of DNA
Is in the form of specific sequences of nucleotides along the DNA strands
Dr. Sharma Jan,09
The DNA inherited by an organismLeads to specific traits by dictating the synthesis of proteins
The process by which DNA directs protein synthesis, gene expression
Includes two stages, called transcription and translation
Dr. Sharma Jan,09
The ribosomeThe ribosomeIs part of the cellular machinery for Is part of the cellular machinery for translation, polypeptide synthesistranslation, polypeptide synthesis
Dr. Sharma Jan,09
Genes specify proteins via transcription and translation
Dr. Sharma Jan,09
TranscriptionTranscriptionIs the synthesis of RNA under the direction Is the synthesis of RNA under the direction of DNAof DNAProduces messenger RNA (mRNA)Produces messenger RNA (mRNA)
TranslationTranslationIs the actual synthesis of a polypeptide, Is the actual synthesis of a polypeptide, which occurs under the direction of mRNAwhich occurs under the direction of mRNAOccurs on ribosomesOccurs on ribosomes
Dr. Sharma Jan,09
prokaryotesprokaryotesTranscription and translation occur togetherTranscription and translation occur together
Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.
(a)
TRANSLATION
TRANSCRIPTIONDNA
mRNA
Ribosome
Polypeptide
Dr. Sharma Jan,09
EukaryotesEukaryotesRNA transcripts are modified before RNA transcripts are modified before becoming true mRNAbecoming true mRNA
Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclearenvelope
Dr. Sharma Jan,09
Cells are governed by a cellular chain of command
DNA RNA protein
Dr. Sharma Jan,09
Genetic informationIs encoded as a sequence of non overlapping base triplets, or codons
Dr. Sharma Jan,09
The gene determines the The gene determines the sequence of bases along the sequence of bases along the length of an mRNA moleculelength of an mRNA molecule
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
Dr. Sharma Jan,09
A codon in messenger RNAA codon in messenger RNAIs either translated into an amino acid or Is either translated into an amino acid or serves as a translational stop signalserves as a translational stop signal
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
Dr. Sharma Jan,09
Codons must be read in the correct reading frame
For the specified polypeptide to be produced
Dr. Sharma Jan,09
The genetic code The genetic code
is nearly universalis nearly universal
Non-overlappNon-overlappDegenerate-Degenerate-- more than one codon specifies/ codes for a particular amino acid
--Shared by organisms from the Shared by organisms from the simplest bacteria to the most simplest bacteria to the most complex animalcomplex animal
Dr. Sharma Jan,09
Molecular Components of Transcription
RNA synthesisIs catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotidesFollows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine
Dr. Sharma Jan,09
Synhesis of an RNA Synhesis of an RNA TranscriptTranscript
InitiationElongationTermination
PromoterTranscription unit
RNA polymerase
Start point
53
35
35
53
53
35
53
35
5
5
Rewound
DNA
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.
2Elongation. 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.
Dr. Sharma Jan,09
Elongation
RNApolymerase
Non-templatestrand of DNA
RNA nucleotides
3 end
C A E G C A A
U
T A G G T TA
AC
G
U
AT
CA
T C C A A TT
G
G
3
5
5
Newly madeRNA
Direction of transcription(“downstream”) Template
strand of DNA
Dr. Sharma Jan,09
RNA Polymerase Binding and RNA Polymerase Binding and Initiation of TranscriptionInitiation of Transcription
Promoters signal the initiation of RNA synthesisTranscription factors
Help eukaryotic RNA polymerase recognize promoter sequences
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
T A T A AA AA T A T T T T
TATA box Start point TemplateDNA strand
53
35
Transcriptionfactors
53
35
Promoter
53
355
RNA polymerase IITranscription factors
RNA transcript
Transcription initiation complex
Eukaryotic promoters1
Several transcriptionfactors
2
Additional transcriptionfactors
3
Dr. Sharma Jan,09
Elongation of the RNA Elongation of the RNA StrandStrand
It continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides
Dr. Sharma Jan,09
As RNA polymerase moves along the DNA
It continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides
Elongation of the RNA StrandElongation of the RNA Strand
Dr. Sharma Jan,09
Termination of Termination of TranscriptionTranscription
The mechanisms of termination
Are different in prokaryotes and eukaryotes
Dr. Sharma Jan,09
Eukaryotic cells modify RNA Eukaryotic cells modify RNA after transcriptionafter transcription
Enzymes in the eukaryotic Enzymes in the eukaryotic nucleusnucleus
Modify pre-mRNA in specific ways Modify pre-mRNA in specific ways before the genetic messages are before the genetic messages are dispatched to the cytoplasmdispatched to the cytoplasm
Dr. Sharma Jan,09
Alteration of mRNA EndsAlteration of mRNA Ends
Each end of a pre-mRNA molecule is modified in a particular way
The 5 end receives a modified nucleotide capThe 3 end gets a poly-A tail
A modified guanine nucleotideadded to the 5 end
50 to 250 adenine nucleotidesadded to the 3 end
Protein-coding segment Polyadenylation signal
Poly-A tail3 UTRStop codonStart codon
5 Cap 5 UTR
AAUAAA AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
G P P P
53
Dr. Sharma Jan,09
Eukaryotic mRNAs are modified. At the 5' end, a cap is added consisting of a modified GTP (guanosine triphosphate). This occurs at the beginning of transcription. The 5' cap is used as a recognition signal for ribosomes to bind to the mRNA.• At the 3' end, a poly(A) tail of 150 or more adenine nucleotides is added. The tail plays a role in the stability of the mRNA.
Dr. Sharma Jan,09
Split Genes and RNA SplicingSplit Genes and RNA Splicing
RNA splicingRemoves introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
5 Cap
ExonIntron
1
5
30 31
Exon Intron
104 105 146
Exon 3
Poly-A tail
Poly-A tail
Introns cut out andexons spliced together
Codingsegment
5 Cap
1 1463 UTR3 UTR
Pre-mRNA
mRNA
Dr. Sharma Jan,09
Is carried out by spliceosomes in some cases RNA transcript (pre-mRNA)
Exon 1 Intron Exon 2
Other proteinsProtein
snRNA
snRNPs
Spliceosome
Spliceosomecomponents
Cut-outintronmRNA
Exon 1 Exon 2
5
5
5
1
2
3
Dr. Sharma Jan,09
RibozymesRibozymes
RibozymesAre catalytic RNA molecules that function as
enzymes and can splice RNA
Dr. Sharma Jan,09
Translation is the RNA-directed synthesis of a polypeptide:
a closer look
Dr. Sharma Jan,09
Molecular Components of Molecular Components of TranslationTranslation
A cell translates an mRNA message into proteinWith the help of transfer RNA
(tRNA)
Dr. Sharma Jan,09
Translation: the basic conceptTranslation: the basic conceptTRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
Polypeptide
Polypeptide
Aminoacids
tRNA withamino acidattachedRibosome
tRNA
Anticodon
mRNA
Trp
Phe Gly
A G C
A A A
CC
G
U G G U U U G G C
Codons5 3
Dr. Sharma Jan,09
Molecules of tRNA are not all identicalMolecules of tRNA are not all identicalEach carries a specific amino acid on one Each carries a specific amino acid on one
endend
Each has an anticodon on the otherEach has an anticodon on the other endend
Dr. Sharma Jan,09
The Structure and Function of TransferThe Structure and Function of Transfer
A tRNA moleculeConsists of a single RNA strand that is only about 80 nucleotides longIs roughly L-shaped
Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3’ end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)
(a)
3
CCACGCUUAA
GACACCU*
GC
* *G U G U *CU
* G AGGU**A
*A
A GUC
AGACC*
C G A GA G G
G*
*GA
CUC*AUUUAGGCG5
Amino acidattachment site
Hydrogenbonds
Anticodon
A
Dr. Sharma Jan,09
A specific enzyme called an aminoacyl-tRNA A specific enzyme called an aminoacyl-tRNA synthetasesynthetaseJoins each amino acid to the correct tRNAJoins each amino acid to the correct tRNAAminoacyl-tRNA
synthetase (enzyme)
1. Active site binds theamino acid and ATP.
ATP loses two P groupsand joins amino acid as AMP.2
Pyrophosphate
Phosphates
3 AppropriatetRNA covalentlyBonds to aminoAcid, displacingAMP.
AMP
Activated amino acidis released by the enzyme.4
Aminoacyl tRNA(an “activatedamino acid”)
1. Enzyme+ Amino acid+ ATP
Enzyme-Aa-AMP complex +2Pi
2. Enzyme-Aa-AMP complex +tRNA
tRNA-Aa +AMP + Enzyme
Dr. Sharma Jan,09
Dr. Sharma Jan,09
(b) Three-dimensional structure
Symbol used in the presentation
Amino acidattachment site
Hydrogen bonds
AnticodonAnticodon
A A G
5
3
3 5
(c)
Dr. Sharma Jan,09
Ribosomes
Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis
Dr. Sharma Jan,09
The ribosomal subunitsThe ribosomal subunitsAre constructed of proteins and RNA molecules Are constructed of proteins and RNA molecules named ribosomal RNA or rRNAnamed ribosomal RNA or rRNA
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide Exit tunnelGrowingpolypeptide
tRNAmolecules
EP A
Largesubunit
Smallsubunit
mRNA
Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins.
53
Dr. Sharma Jan,09
We can divide translation into three stagesWe can divide translation into three stagesInitiation, Elongation, TerminationInitiation, Elongation, Termination
P site (Peptidyl-tRNAbinding site)
E site (Exit site)
mRNAbinding site
A site (Aminoacyl-tRNA binding site)
Largesubunit
Smallsubunit
Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.
Dr. Sharma Jan,09
Amino end Growing polypeptide
Next amino acidto be added topolypeptide chain
tRNA
mRNA
Codons
3
5
Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site.
(c)
Dr. Sharma Jan,09
Ribosome Association and Initiation of Ribosome Association and Initiation of TranslationTranslation
The initiation stage of translationBrings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosomeLarge
ribosomalsubunit
The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiationfactors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid.
2
Initiator tRNA
mRNA
mRNA binding site Smallribosomalsubunit
Translation initiation complex
P site
GDPGTP
Start codon
A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met).
1
MetMet
U A CA U G
E A
35
53
35 35
Dr. Sharma Jan,09
Elongation of the Polypeptide Chain-Elongation of the Polypeptide Chain-Amino acids are Amino acids are added one by one to the preceding amino acidadded one by one to the preceding amino acid
Amino endof polypeptide
mRNA
Ribosome ready fornext aminoacyl tRNA
E
P A
E
P A
E
P A
E
P A
GDPGTP
GTP
GDP
2
2
site site5
3
TRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
Polypeptide
Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysisof GTP increases the accuracy andefficiency of this step.
1
Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site.
2
Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs,bringing the next codon to be translated into the A site.
3
Dr. Sharma Jan,09
Termination of Translation-When the Termination of Translation-When the ribosome reaches a stop codon in the mRNAribosome reaches a stop codon in the mRNA
Release factor
Freepolypeptide
Stop codon(UAG, UAA, or UGA)
5
3 35
35
When a ribosome reaches a stop When a ribosome reaches a stop codon on mRNA, the A site of the codon on mRNA, the A site of the ribosome accepts a protein called ribosome accepts a protein called a release factor instead of tRNAa release factor instead of tRNA.
1 The release factor hydrolyzes The release factor hydrolyzes the bond between the tRNA in the bond between the tRNA in the P site and the last amino the P site and the last amino acid of the polypeptide chain. acid of the polypeptide chain. The polypeptide is thus freed The polypeptide is thus freed from the ribosome.from the ribosome.
2 3 The two ribosomal subunits The two ribosomal subunits and the other components of and the other components of the assembly dissociate.the assembly dissociate.
Dr. Sharma Jan,09
A number of ribosomes can translate a single A number of ribosomes can translate a single mRNA molecule simultaneously- mRNA molecule simultaneously- PolyribosomePolyribosome
Growingpolypeptides
Completedpolypeptide
Incomingribosomalsubunits
Start of mRNA(5 end)
End of mRNA(3 end)
Polyribosome
An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes.
(a)
Ribosomes
mRNA
This micrograph shows a large polyribosome in a prokaryotic cell (TEM).
0.1 µm
(b)
Dr. Sharma Jan,09
Completing and Targeting the Functional Protein
Polypeptide chainsUndergo modifications after the translation process
Dr. Sharma Jan,09
Protein Folding and Post-Protein Folding and Post-Translational ModificationsTranslational Modifications
After translationProteins may be modified in ways that affect their three-dimensional shape
Dr. Sharma Jan,09
Targeting Polypeptides to Specific Targeting Polypeptides to Specific LocationsLocations
Two populations of ribosomes are evident in cells
Free and bound
Free ribosomes in the cytosolInitiate the synthesis of all proteins
Dr. Sharma Jan,09
• Proteins destined for the endomembrane Proteins destined for the endomembrane system or for secretionsystem or for secretion– Must be transported into the ERMust be transported into the ER– Have signal peptides to which a signal-recognition Have signal peptides to which a signal-recognition
particle (SRP) binds, enabling the translation particle (SRP) binds, enabling the translation ribosome to bind to the ERribosome to bind to the ER
Dr. Sharma Jan,09
The signal mechanism for The signal mechanism for targeting proteins to the ERtargeting proteins to the ER
Figure 17.21
Ribosome
mRNASignalpeptide
Signal-recognitionparticle(SRP) SRP
receptorprotein
Translocationcomplex
CYTOSOL
Signalpeptideremoved
ERmembrane
Protein
ERLUMEN
PolypeptidePolypeptidesynthesis beginssynthesis beginson a freeon a freeribosome inribosome inthe cytosol.the cytosol.
1
An SRP binds An SRP binds to the signal to the signal peptide, halting peptide, halting synthesissynthesismomentarily.momentarily.
2 The SRP binds to aThe SRP binds to areceptor protein in the ERreceptor protein in the ERmembrane. This receptormembrane. This receptoris part of a protein complexis part of a protein complex(a translocation complex)(a translocation complex)that has a membrane porethat has a membrane poreand a signal-cleaving enzymeand a signal-cleaving enzyme.
3 The SRP leaves, andThe SRP leaves, andthe polypeptide resumesthe polypeptide resumesgrowing, meanwhilegrowing, meanwhiletranslocating across thetranslocating across themembrane. (The signalmembrane. (The signalpeptide stays attachedpeptide stays attachedto the membrane.)to the membrane.)
4
The signal-The signal-cleaving cleaving enzymeenzymecuts off thecuts off thesignal peptidesignal peptide.
5 The rest ofThe rest ofthe completedthe completedpolypeptide leaves polypeptide leaves the ribosome andthe ribosome andfolds into its finalfolds into its finalconformation.conformation.
6