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Genetic Information Flow: Post-Transcriptional Gene C ontrol. YILDIRIM BEYAZIT UNIVERSITY FACULTY OF MEDICINE THE DEPARTMENT OF MEDICAL BIOLOGY ASST. PROF. DR. ENDER ŞİMŞEK. Post-Transcriptional Gene Control.
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Genetic Information Flow:Post-Transcriptional Gene Control YILDIRIM BEYAZIT UNIVERSITY FACULTY OF MEDICINE THE DEPARTMENT OF MEDICAL BIOLOGY ASST. PROF. DR. ENDER ŞİMŞEK
Post-Transcriptional Gene Control The control of protein synthesis, in prokaryotes, generally occurs in the level of transcription, not the translational level and it is a qualitative control.
Post-Transcriptional Gene Control The control of protein synthesis, in prokaryotes, generally occurs in the level of transcription, not the translational level and it is a qualitative control. After synthesizing a protein,this protein synthesis is restricted or stopped until needed again, while other proteins are synthesized.
Post-Transcriptional Gene Control The control of protein synthesis, in prokaryotes, generally occurs in the level of transcription, not the translational level and is a qualitative control. After synthesizing a protein,this protein synthesis is restricted or stopped until needed again, while other proteins are synthesized. The half-life of mRNA is short in prokaryotes. Generally, when the protein synthesis needs to be decreased, the gene transcription is turned off and mRNA is not produced.
The regulation of gene expression, at the level of transcription, is defined as ‘’Operon’’. Genes on chromosomes, according to their functions, can be divided into four groups: 1) Structural genes: synthesizemRNA. STRUCTURAL GENES
The regulation of gene expression, at the level of transcription, is defined as ‘’Operon’’. Genes on chromosomes, according to their functions, can be divided into four groups: 1) Structural genes: synthesize mRNA. 2) Operator genes: control the function of the structural genes. OPERATOR GENES
The regulation of gene expression, at the level of transcription, is defined as ‘’Operon’’. Genes on chromosomes, according to their functions, can be divided into four groups: 1) Structural genes: synthesize mRNA. 2) Operator genes: control the function of the structural genes. 3) Promoter genes: contain the binding sites of RNA polymerase and cAMP-receptor protein. PROMOTER GENES
The regulation of gene expression, at the level of transcription, is defined as ‘’Operon’’. Genes on chromosomes, according to their functions, can be divided into four groups: 1) Structural genes: synthesize mRNA. 2) Operator genes: control the function of the structural genes. 3) Promoter genes: contain the binding sites of RNA polymerase and cAMP-receptor protein. 4) Regulatory genes: remotely control the operon. REGULATORY GENES
The arrangement of the prokaryotic gene expression, at the level of transcription, can be divided into two groups: the induction model or 2. the repression model
The induction model as the positive control For example: on the lac operon of E. coli, there are three genes (Z, Y and A ) close to each other and these genes are controlled by the operator gene. (PI: promotorfor I gene;I: repressor; P: promotor; O: operator) (Z:-galactosidase, Y:galactosidepermease, and A:transacetylase).
Control of transcription of the lac operon. • Prokaryotes can control transcriptional initiation in complex manners. • Example: E. coli lac operon. • Has 3 consecutive genes (Z, Y, and A) that are necessary to metabolize lactose.
Control of transcription of the lac operon. • In the absence of lactose, the lac repressor protein binds a control site in the lac operon called an operator. • This prevents the RNA polymerase from initiating transcription.
Control of transcription of the lac operon. • If lactose is present, some of the lactose is converted to allo-lactose which binds to the lac repressor causing it to fall of the operator sequence. • This allows RNA polymerase to initiate transcription of the genes.
POZİTİF KONTROL Positive control of gene expression in prokaryotes
Negative control of Lac operon (w/o lactose) NEGATİF KONTROL
Negative control of Lac operon (with lactose) NEGATİF KONTROL
The repression model as the negative control For example, on E. coli-tryptophan operon system; during the extreme tryptophan synthesis, the regulator gene transcribes mRNA synthesizing an apo-repressor to stop the synthesis of tryptophan.
The repression model as the negative control. For example, on E. coli-tryptophan operon system; during the extreme tryptophan synthesis, the regulator gene transcribes mRNA synthesizing an apo-repressor to stop the synthesis of tryptophan. The newly synthesized apo-repressor is inactive but after binding to tryptophan, it can bind to the operator in order to stop the synthesis of tryptophan.
The control of protein synthesis, in eukaryotes, generally occurs in the translational level. This is a quantitative control and is provided with feedback inhibition and density of ribosomes on the mRNA.
The control of protein synthesis, in eukaryotes, generally occurs in the translational level. This is a quantitative control and is provided with feedback inhibition and density of ribosomes on the mRNA. The accumulation of the inhibitor inhibits the formation of the initiation complex and reduces the protein synthesis. The amount of protein synthesis is increased by more ribosomes on the mRNA.
rRNAprocessing in eukaryotes • rRNA in eukaryotes is also generated from a single, long precursor molecule by specific modification and cleavage steps
18S 5.8S 28S 47S ETS1 ITS1 ITS2 ETS2 45S 41S 20S and 32S Mature rRNAs 18S rRNA 5.8S rRNA 28S rRNA Mammalian pre-rRNA processing Indicates RNase cleavage
tRNA processing in eukaryotes The pre-tRNA is synthesized with a • 16 nt 5’-leader, • a 14 nt intron and • two extra 3’-nucleotides.
Processing of mRNA in eukaryotes • In eukaryotes, mRNA is synthesized by RNA Pol II as longer precursors (pre-mRNA), the population of different RNA Pol II transcripts are called heterogeneous nuclear RNA (hnRNA). • Among hnRNA, those processed to give mature mRNAs are called pre-mRNAs
Eukaryotic mRNA undergoes post-transcriptional modification • In order for mRNAs in eukaryotes to become functional, they must undergo modifications. • 7-methylguanosine-containing “cap” is added to the 5’ end. • 250 nucleotide polyadenylic acid [poly(A)] tail is added to the 3’ end. • Undergo gene splicing in which RNA segments called introns are excised from the RNA and the remaining exons are rejoined to form the mature mRNA.
Pre-mRNA molecules are processed to mature mRNAs by 5’-capping, 3’-cleavage and polyadenylation, splicing and methylation.
5’ Capping • Very soon after RNA Pol II starts making a transcript, and before the RNA chain is more then 20 -30 nt long, the 5’-end is chemically modified. • 7-methylguanosine is covalently to the 5´ end of pre-mRNA. • Linked 5´ 5´ • Occurs shortly after initiation
Function of 5´cap • Protection from degradation • Increased translational efficiency • Transport to cytoplasm • Splicing of first exon
3’ Cleavage and polyadenylation • In most pre-mRNAs, the mature 3’-end of the molecule is generated by cleavage followed by the addition of a run, or tail, of A residues which is called the poly(A) tail.
Function of poly(A) tail • Increased mRNA stability • Increased translational efficiency • Splicing of last intron
Splicing • the process of cutting the pre-mRNA to remove the introns and joining together of the exons is called splicing. • it takes place in the nucleus before the mature mRNA can be exported to the cytoplasm.
Introns: non-coding sequences • Exons: coding sequences • RNA splicing: removal of introns and joining of exons • Splicing mechanism must be precise to maintain open reading frame • Catalyzed by spliceosome(RNA + protein)
