transcription transcription gene sequence (dna) recopied or transcribed to rna sequence gene...
TRANSCRIPT
Transcription Transcription
transcriptiontranscription
Gene sequence (DNA) recopied or Gene sequence (DNA) recopied or transcribed to RNA sequencetranscribed to RNA sequence
Products of TranscriptionProducts of Transcription
1. Ribosomal RNA (rRNA)1. Ribosomal RNA (rRNA) - - Several Several rRNAs arerRNAs are
vital constituents of vital constituents of ribosomesribosomes
22 Transfer RNA (tRNA)Transfer RNA (tRNA) - - The molecule thatThe molecule that
physically couples nucleic acid codons with physically couples nucleic acid codons with specific amino acids specific amino acids
33 Messenger RNA (mRNA)Messenger RNA (mRNA) - - The nucleic The nucleic acidacid
messenger that carries encoded information messenger that carries encoded information from genes on DNA to the protein from genes on DNA to the protein manufacturing ribosomesmanufacturing ribosomes
overviewoverview
Transcription requires:Transcription requires: ribonucleoside 5´ triphosphates: ribonucleoside 5´ triphosphates:
ATP, GTP, CTP and UTP ATP, GTP, CTP and UTP bases are adenine, guanine, cytosine and bases are adenine, guanine, cytosine and
uraciluracil sugar is ribose (not deoxyribose)sugar is ribose (not deoxyribose)
DNA-dependent RNA polymeraseDNA-dependent RNA polymerase Template (sense) DNA strandTemplate (sense) DNA strand Animation of transcriptionAnimation of transcription
overviewoverview
Features of transcription:Features of transcription: RNA polymerase catalyzes sugar- catalyzes sugar-
phosphate bond between 3´-OH of ribose phosphate bond between 3´-OH of ribose and the 5´-POand the 5´-PO44. .
Order of bases in DNA template strand Order of bases in DNA template strand determines order of bases in transcript.determines order of bases in transcript.
Nucleotides are added to the 3´-OH of Nucleotides are added to the 3´-OH of the growing chain.the growing chain.
RNA synthesis does not require a primer.RNA synthesis does not require a primer.
overviewoverview
In prokaryotes transcription and In prokaryotes transcription and translation are coupled. Proteins are translation are coupled. Proteins are synthesized directly from the synthesized directly from the primary transcript as it is made.primary transcript as it is made.
In eukaryotes transcription and In eukaryotes transcription and translation are separated. translation are separated. Transcription occurs in the nucleus, Transcription occurs in the nucleus, and translation occurs in the and translation occurs in the cytoplasm on ribosomes.cytoplasm on ribosomes.
RNA PolymeraseRNA Polymerase DNA-dependent DNA-dependent
DNA template, ribonucleoside 5´ triphosphates, DNA template, ribonucleoside 5´ triphosphates, and Mgand Mg2+2+
Synthesizes RNA in 5´ to 3´ directionSynthesizes RNA in 5´ to 3´ direction E. coliE. coli RNA polymerase consists of 5 subunits RNA polymerase consists of 5 subunits Sigma factors are a subunit of RNA Sigma factors are a subunit of RNA
polymerase.polymerase. Sigma factors are needed for promoter Sigma factors are needed for promoter
binding, but after transcription starts they binding, but after transcription starts they
dissociate.dissociate.
Eukaryotes have three RNA Eukaryotes have three RNA polymerasespolymerases RNA polymerase II is responsible for RNA polymerase II is responsible for
transcription of protein-coding genes transcription of protein-coding genes and some snRNA moleculesand some snRNA molecules
RNA polymerase II has 12 subunits RNA polymerase II has 12 subunits Requires accessory proteins Requires accessory proteins
(transcription factors)(transcription factors) Does not require a primerDoes not require a primer
Stages of TranscriptionStages of Transcription
Promoter Recognition Chain Initiation Chain Elongation Chain Termination
promoter recognitionpromoter recognition Transcription factors bind to promoter s
equences and recruit RNA polymerase..
DNA is bound first in a DNA is bound first in a closed complexclosed complex. . Then, RNA polymerase denatures a 12–Then, RNA polymerase denatures a 12–15 bp segment of the DNA (open 15 bp segment of the DNA (open complex).complex).
The site where the first base is The site where the first base is incorporated into the transcription is incorporated into the transcription is numbered “+1” and is called the numbered “+1” and is called the transcription start sitetranscription start site..
Defined regions are transcribed
upstream region
transcribedregion
downstream region
promoter(RNA polymerase
binding site)
transcriptionstart site
terminationsite
gene dsDNA
TB
Transcription factors that are Transcription factors that are required at every promoter site for required at every promoter site for RNA polymerase interaction are RNA polymerase interaction are called basal transcription factors.called basal transcription factors.
promoter sequencespromoter sequences Promoter sequences vary considerably.Promoter sequences vary considerably. RNA polymerase binds to different RNA polymerase binds to different
promoters with different strengths; binding promoters with different strengths; binding strength relates to the level of gene strength relates to the level of gene expressionexpression
There are some common There are some common consensus sequences for promoters: for promoters: Example: Example: E. coliE. coli –35 sequence (found 35 bases 5 –35 sequence (found 35 bases 5
´ to the start of transcription)´ to the start of transcription) Example: Example: E. coliE. coli TATA box (found 10 bases 5´ to TATA box (found 10 bases 5´ to
the start of transcription)the start of transcription)
“-35 squence” “Pribnow box (-10 Sequence)
Ptrp TTGACA----17bp----TTAACTA---transcriptionPlac uv5 TTTACA----18bp----TATAATG---transcriptionPtac TTGACA----16bp---TATAATG---transcriptionProkaryotic TTGACA TATAATconsensusHuman ß-globin CCAAT-----39bp----CATAAA----transcriptionEukaryotic CCAAT ATAconsensus
Sekuen DNA dan beberapa promotor bakteri
enhancersenhancers
Eukaryotic genes may also have Eukaryotic genes may also have enhancers.enhancers.
Enhancers can be Enhancers can be located at great at great distances from the gene they regulate, distances from the gene they regulate, either 5´ or 3´ of the transcription start, either 5´ or 3´ of the transcription start, in introns or even on the noncoding in introns or even on the noncoding strand.strand.
One of the most common ways to One of the most common ways to identify promoters and enhancers is to identify promoters and enhancers is to use a reporter gene.use a reporter gene.
other playersother players Many proteins can regulate gene Many proteins can regulate gene
expression by modulating the strength of expression by modulating the strength of interaction between the promoter and interaction between the promoter and RNA polymerase.RNA polymerase.
Some proteins can activate transcription Some proteins can activate transcription (upregulate gene expression).(upregulate gene expression).
Some proteins can inhibit transcription by Some proteins can inhibit transcription by blocking polymerase activity.blocking polymerase activity.
Some proteins can act both as repressors Some proteins can act both as repressors and activators of transcription.and activators of transcription.
chain initiationchain initiation RNA polymerase locally denatures the RNA polymerase locally denatures the
DNA.DNA. The first base of the new RNA strand is The first base of the new RNA strand is
placed complementary to the +1 site.placed complementary to the +1 site. RNA polymerase does not require a RNA polymerase does not require a
primer.primer. The first 8 or 9 bases of the transcript are The first 8 or 9 bases of the transcript are
linked. Transcription factors are released, linked. Transcription factors are released, and the polymerase leaves the promoter and the polymerase leaves the promoter region.region.
RNA Pol.
InitiationInitiation
T. F.
RNA Pol.
5’RNA
Promoter
T. F.
T. F.
chain elongationchain elongation
RNA polymerase moves along the RNA polymerase moves along the transcribed or template DNA strand.transcribed or template DNA strand.
The new RNA molecule (primary The new RNA molecule (primary transcript) forms a short RNA-DNA transcript) forms a short RNA-DNA hybrid molecule with the DNA hybrid molecule with the DNA template.template.
chain terminationchain termination
Most known about Most known about bacterial chain termination
Termination is signaled by a Termination is signaled by a sequence that can form a hairpin sequence that can form a hairpin loop.loop.
The polymerase and the new RNA The polymerase and the new RNA molecule are released upon molecule are released upon formation of the loop.formation of the loop.
UUUURNA
3' end of RNA
Termination siteTermination site
Rho and TerminationRho and Termination
Rho independentRho independent termination depends termination depends on both slowing down the elongation on both slowing down the elongation complex, and an AT rich region that complex, and an AT rich region that destabilizes the elongation complexdestabilizes the elongation complex
Rho dependentRho dependent requires a protein requires a protein called called RhoRho, that binds to and slides along , that binds to and slides along the RNA transcript. The terminator the RNA transcript. The terminator sequence slows down the elongation sequence slows down the elongation complex, complex, Rho Rho catches up and knocks it off catches up and knocks it off the DNAthe DNA
RNAPol.
5’RNA
RNA Pol.
5’RNA
RNA Pol.
5’RNA
TerminationTerminationRho IndependentRho Independent
Terminator
RNAPol.
5’RNA
TerminationTerminationRho DependentRho Dependent
Terminator
RNA Pol.
5’RNA
RNA Pol.
5’RNA
Help, rhohit me!
mRNA mRNA synthesis/processingsynthesis/processing
Prokaryotes: mRNA transcribed directly Prokaryotes: mRNA transcribed directly from DNA template and used from DNA template and used immediately in protein synthesisimmediately in protein synthesis
Eukaryotes: primary transcript must be Eukaryotes: primary transcript must be processed to produce the mRNA to produce the mRNA Noncoding sequences (introns) are removedNoncoding sequences (introns) are removed Coding sequences (exons) spliced togetherCoding sequences (exons) spliced together 5´-methylguanosine cap added5´-methylguanosine cap added 3´-polyadenosine tail added3´-polyadenosine tail added
mRNA mRNA synthesis/processingsynthesis/processing
Removal of introns and splicing of exons can Removal of introns and splicing of exons can occur several waysoccur several ways For introns within a nuclear transcript, a For introns within a nuclear transcript, a
spliceosome is required. spliceosome is required. Splicesomes protein and small nuclear RNA (snRNA)Splicesomes protein and small nuclear RNA (snRNA) Specificity of splicing comes from the snRNA, some of Specificity of splicing comes from the snRNA, some of
which contain sequences complementary to the splice which contain sequences complementary to the splice junctions between introns and exonsjunctions between introns and exons
Alternative splicing can produce different forms of a Alternative splicing can produce different forms of a protein from the same geneprotein from the same gene
Mutations at the splice sites can cause diseaseMutations at the splice sites can cause disease ThalassemiaThalassemia • Breast cancer (BRCA 1)• Breast cancer (BRCA 1)