DNA TRANSCRIPTION Flashcards
what is transcription
the process of making RNA from a DNA template
In eukaryotes DNA doesn’t leave the nucleus -so RNA is made in the nucleus .
transcription is divided into 3 phases :
initiation , elongation and termination.
pre-mRNA is produced that needs to be further processed .
main steps in transcription
RNA polymerase binds the promoter first forming a closed promoter complex and then forming an open promoter complex with the localised melting of the DNA. initiation then occurs by the synthesis of the first phosphodiester bond. Elongation proceeds by translocation of RNA polymerase down the gene. Each nucleotide of the template strand is transcribed into a complementary nucleotide in the growing polynucleotide chain. Termination occurs at the end of the gene. at this point the RNA product is complete and the complex dissociates.
a gene contains a region that codes for RNA
this region begins with a promoter and ends with a terminator
The Gene
the promoter region of a gene contains recognition sites for RNA polymerase to bind.
this is where the majority of gene expression is controlled; be either permitting or blocking access to this site by RNA polymerase .
several protein complexes , known as transcription factors are required for transcription. they are usually named TF(something )
the promoter is therefore a region that can be recognised by transcription factors.
several of them bind the promoter and form what is called an initiation complex.
that attracts and includes the RNA polymerase that will then lead to the elongation phase ; the making of RNA.
the RNA polymerase will help unzip the double strands and form a transcription bubble.
the transcription bubble includes 2 different strands necessary for the elongation process.
there is a + strand also known as sense strand or coding strand because the transcription will resemble its code
there is a - strand , the anti sense strand , also known as template strand . this is one that RNA polymerase slides over to produce the RNA that is complementary to it : the transcript
the template strand is used to make the RNA transcript
Binding of the RNA polymerase causes
the DNA double helix to unwind and open .
then during elongation , the RNA polymerase slides along the template DNA strand.
as the complementary bases pair up , the RNA molecules links nucleotides to the 3’ end of the growing RNA molecule.
MRNA
once the RNA polymerase reaches the terminator portion of the gene , the messenger RNA transcript is complete .
the mRNA here produced includes regions that code for proteins called exons and non coding sections called introns.
for the MRNA to be used in translation
the non coding introns need to be removed.
modifications such as 5’ cap and 3’ poly A tail are added
the process is called intron splicing and is performed by a complex made by proteins and RNA called a spliceosome
structure of eukaryotic pre-mRNA
all the mRNA’s have a 5’ cap and all mRNAs (with the exception of the histone mRNA) contain a poly(a) tail
the 5’ cap and 3’poly (a) tail prevent mRNA degradation
loss of the cap and poly(a) tail results in mRNA degradation
all eukaryotic mRNAs have a 5’ cap which is synthesised co-transcriptionally , that is , immediately following initiation of transcription. it contains a 7-methyl guanosine residue that is coupled to the 5’ nucleotide through a 5’-5’ triphosphate bond. In addition to the methyl group on the guanine base , there are two other methylations that are on the 2’ ribose sugars of the first two nucleotides.
to produce mature mRNA
the complex removes the introns and joins the adjacent exons. that can leave the nucleus through a nuclear pore and enter the citoplasm to begin translation.
how is the information in the mature mRNA strand translated into a protein ?
the nitrogenous bases are grouped into 3 letter codes called codons .
sequence elements within a typical eukaryotic gene
TATA box (TATAAA)
located approximately 25-30 bp upstream of the +1 start site
determines the exact start site (not in all promoters)
binds the TATA binding protein which is a sub united of TFIID
GC box (CCGCCC)
binds sp1
CAAT box (GGCCAATCT)
binds CTF
Octamer (ATTTGCAT)
binds otf
proteins regulating eukaryotic mRNA synthesis
general transcription factors
TFIID binds to the TATA box to begin the assembly of the transcription apparatus
the TATA binding protein directly binds the Tata box
TBP associated factors (TAFS) bind to TBP
TFIIA , TFIIB , TFIIE , TFIIF , TFIIH , TFIIJ assemble with TFIID
rna polymerase II binds the promoter region via the TFII’s
Proteins that regulate transcription are called transcription factors. The general transcription factors function at all mRNA promoters and are abbreviated TFII for transcription factors regulating RNA polymerase II promoters. Each (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and TFIIJ) is a multisubunit protein. They function to recognize and bind to the promoter so as to help stabilize the binding of RNA polymerase. They also bind to other transcription factors through protein-protein interactions, which are bound to their own specific DNA elements.
continuation of proteins regulating eukaryotic mRNA synthesis
a component of TFIID ,TATA binding protein (TBP) binds to TATA box.
it uses the TATA box to position the TFIID near the transcription start site.
other transcription factors then attach .
this complex provides the anchoring allowing the successful binding of RNA polymerase
other transcription factors are added completing the complex to start transcription
now energy must be added for the process to start
this energy is provided by the reduction of ATP to ADP and Pi.
the process of making mRNA starts.
Eukaryotic protein coding genes
Upstream region (with regulatory signals)
Promoter region, with transcription initiation site (e.g. TATA box)
5’ untranslated region (5’ UTR)
Translation initiation site (includes start codon)
Alternating sequence of exons (protein-coding) and introns
Translation start site (start codon)
3’ UTR
Polyadenylation (poly-A) signal
Translation stop site (stop codon)
Mechanisms for inhibit of protein synthesis by antibiotics targeting transcription
Rifamycin: Blocks transcription by blocking binding of RNA polymerase in prokaryotes
Rifampicin (derived from rifamycin), inhibits the beta subunit of RNA polymerase.
Single step high level resistance to the rifamycins occurs as the result of a single amino acid change in the bacterial DNA dependent RNA polymerase
