In What Direction Does The Elongation Mrna Grow From The Template Strand Of Dna?

7.5A: Elongation and Termination in Eukaryotes

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Elongation synthesizes pre-mRNA in a 5′ to 3′ direction, and termination occurs in response to termination sequences and signals.

LEARNING OBJECTIVES

Draw what is happening during transcription elongation and termination

Central Takeaways

Key Points

RNA polymerase II (RNAPII) transcribes the major share of eukaryotic genes.
During elongation, the transcription machinery needs to move histones out of the way every time it encounters a nucleosome.
Transcription elongation occurs in a bubble of unwound Deoxyribonucleic acid, where the RNA Polymerase uses one strand of DNA every bit a template to catalyze the synthesis of a new RNA strand in the 5′ to 3′ direction.
RNA Polymerase I and RNA Polymerase III stop transcription in response to specific termination sequences in either the DNA being transcribed (RNA Polymerase I) or in the newly-synthesized RNA (RNA Polymerase 3).
RNA Polymerase Ii terminates transcription at random locations past the end of the gene beingness transcribed. The newly-synthesized RNA is cleaved at a sequence-specified location and released before transcription terminates.

Key Terms

nucleosome: any of the subunits that repeat in chromatin; a coil of Deoxyribonucleic acid surrounding a histone core
histone: any of various unproblematic water-soluble proteins that are rich in the basic amino acids lysine and arginine and are complexed with Deoxyribonucleic acid in the nucleosomes of eukaryotic chromatin
chromatin: a circuitous of Deoxyribonucleic acid, RNA, and proteins within the cell nucleus out of which chromosomes condense during prison cell division

Transcription through Nucleosomes

Following the formation of the pre-initiation complex, the polymerase is released from the other transcription factors, and elongation is allowed to proceed with the polymerase synthesizing RNA in the 5′ to 3′ direction. RNA Polymerase II (RNAPII) transcribes the major share of eukaryotic genes, so this section will mainly focus on how this specific polymerase accomplishes elongation and termination.

Although the enzymatic process of elongation is essentially the same in eukaryotes and prokaryotes, the eukaryotic Deoxyribonucleic acid template is more complex. When eukaryotic cells are not dividing, their genes be as a diffuse, but still extensively packaged and compacted mass of DNA and proteins called chromatin. The Deoxyribonucleic acid is tightly packaged around charged histone proteins at repeated intervals. These DNA–histone complexes, collectively called nucleosomes, are regularly spaced and include 146 nucleotides of Deoxyribonucleic acid wound twice effectually the viii histones in a nucleosome like thread around a spool.

For polynucleotide synthesis to occur, the transcription machinery needs to movement histones out of the style every time it encounters a nucleosome. This is accomplished by a special poly peptide dimer called FACT, which stands for "facilitates chromatin transcription." FACT partially disassembles the nucleosome immediately ahead (upstream) of a transcribing RNA Polymerase II past removing two of the eight histones (a single dimer of H2A and H2B histones is removed.) This presumably sufficiently loosens the DNA wrapped around that nucleosome then that RNA Polymerase Ii tin transcribe through information technology. FACT reassembles the nucleosome behind the RNA Polymerase Two by returning the missing histones to it. RNA Polymerase II will keep to elongate the newly-synthesized RNA until transcription terminates.

The FACT protein dimer allows RNA Polymerase II to transcribe through packaged Deoxyribonucleic acid: DNA in eukaryotes is packaged in nucleosomes, which consist of an octomer of 4 unlike histone proteins. When DNA is tightly wound twice around a nucleosome, RNA Polymerase Ii cannot access it for transcription. FACT removes ii of the histones from the nucleosome immediately ahead of RNA Polymerase, loosening the packaging then that RNA Polymerase 2 can keep transcription. FACT as well reassembles the nucleosome immediately behindd the RNA Polymerase by returning the missing histones.

Elongation

RNA Polymerase II is a circuitous of 12 protein subunits. Specific subunits within the protein let RNA Polymerase Two to act as its own helicase, sliding clamp, unmarried-stranded DNA binding protein, as well as bear out other functions. Consequently, RNA Polymerase Two does not need as many accessory proteins to catalyze the synthesis of new RNA strands during transcription elongation as DNA Polymerase does to catalyze the synthesis of new Dna strands during replication elongation.

However, RNA Polymerase Two does demand a large collection of accessory proteins to initiate transcription at cistron promoters, but one time the double-stranded DNA in the transcription start region has been unwound, the RNA Polymerase Two has been positioned at the +1 initiation nucleotide, and has started catalyzing new RNA strand synthesis, RNA Polymerase II clears or "escapes" the promoter region and leaves most of the transcription initiation proteins backside.

All RNA Polymerases travel forth the template Deoxyribonucleic acid strand in the iii′ to v′ direction and catalyze the synthesis of new RNA strands in the 5′ to iii′ management, calculation new nucleotides to the iii′ end of the growing RNA strand.

RNA Polymerases unwind the double stranded Deoxyribonucleic acid ahead of them and let the unwound DNA behind them to rewind. Every bit a result, RNA strand synthesis occurs in a transcription bubble of well-nigh 25 unwound DNA basebairs. Merely about 8 nucleotides of newly-synthesized RNA remain basepaired to the template Dna. The rest of the RNA molecules falls off the template to allow the Deoxyribonucleic acid behind it to rewind.

RNA Polymerases apply the Deoxyribonucleic acid strand below them as a template to directly which nucleotide to add to the 3′ terminate of the growing RNA strand at each signal in the sequence. The RNA Polymerase travels along the template Deoxyribonucleic acid one nucleotide at at fourth dimension. Whichever RNA nucleotide is capable of basepairing to the template nucleotide below the RNA Polymerase is the next nucleotide to exist added. Once the addition of a new nucleotide to the 3′ cease of the growing strand has been catalyzed, the RNA Polymerase moves to the next Deoxyribonucleic acid nucleotide on the template below it. This process continues until transcription termination occurs.

Termination

Transcription termination by RNA Polymerase II on a protein-encoding gene.: RNA Polymerase Two has no specific signals that stop its transcription. In the case of protein-encoding genes, a protein complex volition bind to two locations on the growing pre-mRNA one time the RNA Polymerase has transcribed past the finish of the factor. CPSF in the complex will bind a AAUAAA sequence, and CstF in the circuitous will bind a GU-rich sequence (acme effigy). CPSF in the complex will cleave the pre-mRNA at a site between the 2 leap sequences, releasing the pre-mRNA (heart figure). Poly(A) Polymerase is a part of the same circuitous and will begin to add together a poly-A tail to the pre-mRNA. At the aforementioned fourth dimension, Xrn2 protein, which is an exonuclease, attacks the v′ stop of the RNA strand still associated with the RNA Polymerase. Xrn2 volition showtime digesting the non-released portion of the newly synthesized RNA until Xrn2 reaches the RNA Polymerase, where it aids in displacing the RNA Polymerase from the template Dna strand. This terminates transcription at some random location downstream from the true stop of the gene (bottom effigy).

The termination of transcription is different for the three different eukaryotic RNA polymerases.

The ribosomal rRNA genes transcribed by RNA Polymerase I contain a specific sequence of basepairs (11 bp long in humans; xviii bp in mice) that is recognized by a termination protein chosen TTF-ane (Transcription Termination Factor for RNA Polymerase I.) This protein binds the DNA at its recognition sequence and blocks farther transcription, causing the RNA Polymerase I to disengage from the template Dna strand and to release its newly-synthesized RNA.

The protein-encoding, structural RNA, and regulatory RNA genes transcribed by RNA Polymerse 2 lack any specific signals or sequences that straight RNA Polymerase 2 to terminate at specific locations. RNA Polymerase Ii tin keep to transcribe RNA anywhere from a few bp to thousands of bp past the actual cease of the cistron. However, the transcript is cleaved at an internal site before RNA Polymerase II finishes transcribing. This releases the upstream portion of the transcript, which will serve every bit the initial RNA prior to further processing (the pre-mRNA in the example of protein-encoding genes.) This cleavage site is considered the "cease" of the gene. The residuum of the transcript is digested by a 5′-exonuclease (called Xrn2 in humans) while information technology is still being transcribed by the RNA Polymerase Ii. When the 5′-exonulease "catches up" to RNA Polymerase 2 by digesting away all the overhanging RNA, it helps disengage the polymerase from its DNA template strand, finally terminating that circular of transcription.

In the case of protein-encoding genes, the cleavage site which determines the "end" of the emerging pre-mRNA occurs between an upstream AAUAAA sequence and a downstream GU-rich sequence separated past about xl-60 nucleotides in the emerging RNA. Once both of these sequences have been transcribed, a poly peptide called CPSF in humans binds the AAUAAA sequence and a protein called CstF in humans binds the GU-rich sequence. These two proteins form the base of a complicated protein circuitous that forms in this region before CPSF cleaves the nascent pre-mRNA at a site 10-thirty nucleotides downstream from the AAUAAA site. The Poly(A) Polymerase enzyme which catalyzes the addition of a three′ poly-A tail on the pre-mRNA is part of the complex that forms with CPSF and CstF.

The tRNA, 5S rRNA, and structural RNAs genes transcribed by RNA Polymerase III have a not-entirely-understood termination signal. The RNAs transcribed by RNA Polymerase III have a brusque stretch of four to seven U'due south at their 3′ cease. This somehow triggers RNA Polymerase Iii to both release the nascent RNA and disengage from the template Dna strand.