Here's a lesson on transcription. Enjoy.... I'll put some new pictures up tomorrow.
Gene expression is accomplished through proteins.
The sequence of nucleotides in
You all know that
A set of three nucleotides in mRNA codes is called a codon. There are 4 nucleotides possible meaning there are a total of 64 possible codons (4*4*4). However only 61 code for amino acids. The remaining codons serve as stop codons (UAA,
Transcription in Prokaryotes is catalyzed by RNA polymerase. Bacterial cells only have a single type of RNA polymerase. It consists of 5 subunits: 2 alpha, 2 beta, and one sigma. The sigma factor is critical for accurate transcription as it promotes RNA polymerase binding to the promoter sequence. The alpha and beta subunits are virtually the same in all RNA polymerase molecules, but the sigma varies depending on the gene category.
The promoter sequence is a specific sequence of
The place where transcription begins is called the startpoint/site. Some sequences help identify this spot. Upstream about 10bp (-10bp) there is a sequence known as the Pribnow box (TATAAT). Even farther upstream at -35 there is the -35 sequence (TTGACA). This is an example for one sigma factor. Basically you need to know that sigma factors recognize various sequences specific to that sigma factor.
So… the RNA polymerase binds to the
The chunk of
RNA polymerase moves along the
This elongation continues for awhile…until RNA polymerase reaches a termination signal (stop codon). RNA polymerase dissociates and an RNA transcript is born.
When RNA poly runs into a termination sequence, the end of transcription is triggered. There are two types of termination signals in prokaryotes that are classified by their independence or dependence on a protein called a
Transcription in Eukaryotes (aka…humans, dogs, cats, Betta fish, zebras, pigmy goats, poison arrow frogs, etc).
Of course, Eukaryotes have to outdo prokaryotes so they make things a little more complicated. Eukaryotes have three (3) types of RNA polymerase. Each one uses a different promoter. These promoters can be downstream OR upstream. For RNA polymerase to even bind to
Here’s some stuff about the different types. RNA pol I involves ribosomalRNA. II is mRNA. II is tRNA. Mitochondria nd Chloroplasts have their own RNA pol molecules. All these buggers have different promoters – good thing I don’t have to know them!
RNA polymerase I has 2 parts. Remember this is the RNA pol involved with rRNA. The first part is called the core promoter, which is the minimum
And you thought RNA pol I was complicated. Well, RNA polymerase II has 4 types of
The four of those are categorized into 2 types of core promoters. There are the TATA driven promoters – have an Inr and a TATA (may or may not have the
Transcription factors….These guys are always involved with the transcription of all nuclear genes. A transcription factor is a protein that is required for transcription. YUCK! There are so many of these little buggers. 1st TATA binding protein (
After RNA pol II has bound…E, and H (helicase) bind too. Now you have A, B, D, E, F, and H bound to
Termination!! Of course it’s different depending on the RNA pol…
RNA pol I termination occurs when the protein binds to 18 nucleotide termination signal in the RNA chain.
**RNA pol II – cleaved by an endonuclease at a specific site before transcription is even finished/terminated. This occurs 10-35 nucleotides downstream of AAUAAA. OR the cleavage site is where the poly A tail is added to the mRNA.
After all this, this is only the primary transcript…which has to go through more modification post transcription. rRNAs are cleaved from a common rRNA precursor. Processing includes removing chunks from the primary transcript and chemical modifications. rRNA is the most abundant/stable form of RNA in a cell. 70-80% of total cellular RNA is rRNA, 10-20% is tRNA, <10% style=""> Ribosomes have multiple typs of rRNA (28S, 18S, 5.8S, and 5S).
28S, 18S, and 5.8S are all encoded by a single transcription unit that is transcribed by RNA pol I and produces a single primary transcript (pre-rRNA). These rRNA are separated by spacer regions. Human haploid genes contain 150-200 copies that are separated by nontranscribed spacer regions on the
tRNA processing has a lot of stuff that happens too: removal, addition, and chemical modification of nucleotides. There are several dozen types of tRNA…each one brings a particular AA or more to a codon of mRNA during translation. tRNAs are funky little dudes. They all share general structure and are about 70-90nt in length. The have 2 hairpin loops that have complementary base pairing.
tRNAs are processed too. First a 16nt sequence is removed from the 5’ end (head/leader). Two terminal nucleotides are removed from the 3’ (tail) end…and are replaced with CCA. Some ways nucleotides are processed: methlyation, unusual bases (dihdrouracil, ribothymine, pseudouridine, inosine)
mRNA processing generally involves capping (polyA tail) and intron removal. Most of this occurs in the nucleus so the mRNA is ready for translation when it reaches the cytoplasm. Translation (cytoplasm) and transcription (nucleus) in eukaryotes are separated by time and space. The pre-mRNA is often much longer than the final mRNA. Ends are modified. 5’ end has a 5’cap and then they have a poly A 3’ tail. Poly A = lots and lots of Adenine.
The 5’ cap is a methylated at position 7 of the purine ring Guanosine nucleotide. This added shortly after the initiation of transcription. This protects from 5’ nucleases…and helps postion the mRNA on the ribosome. It’s essential for translation initiation.
More..The Poly A tail is normally from 50-250 nucleotides long, but normally is around 200. Present at most 3’ tail ends of eukaryotic mRNA. Histone mRNAs lack a poly A tail. Completed after transcription. Genes do not have long stretches of T. PolyA polymerase catalyzes this addition of polyA sequences to mRNA..independently.
The signal that says add a poly A tail is AAUAAA. The mRNA is cleaved about 10-35 nucleotides downstream of AAUAAA. Poly A protects mRNA from attack at 3’ end, recognized by specific proteins that help mRNA get out of the nucleous, and help the ribosomes recognized mRNA for translation.
Introns are sequences that are within the primary transcript that don’t appear in the RNA. Exons appear in the final functional RNA…Introns are out. Exons stay in.
What removes introns? Splicesomes remove introns from pre-mRNA in a process called RNA splicing. A splicesome is an assembly of an RNA-protein complex known as a snRNP (small nuclear robonucleoproteins…great vocab word, eh?). Remove introns and shove the exons together.
Some introns are self slicing. Suicidal introns are called ribozymes.