Describe the process of transcription (initiation, elongation, termination, posttranscriptional modifications) Flashcards
Initiation
-enzyme RNA polymerase binds to the DNA and unwinds it near the beginning of a gene
-binding occurs at a promoter (nucleotide sequence that lies before a gene and allows for binding of RNA
polymerase) located upstream of the start of a gene.
- a key element of the promoter in eukaryotes is the TATA BOX (section of DNA with lots of thymine and adenine which is recognized by RNA polymerase. RNA polymerase can break these 2 hydrogen bonds easier where in C-G there are 3 hydrogen bonds which would expend more energy.
-part of the gene transcribed into RNA is the transcription unit
Elongation
- RNA is made in the 5’ to 3’ direction using the 3’ to 5’ DNA template strand. The strand not being copied is known as the coding strand since it has the same sequence as the new RNA strand.
-As RNA polymerase moves along the DNA it unwinds DNA at the forward end of the enzyme.
-The new RNA molecule elongates as nucleotides are added, The new RNA winds temporarily with the template DNA strand into a hybrid RNA-DNA double helix.
-beyond this pairing the RNA strand unwinds and extends past as a single nucleotide chain as the double
DNA helix reforms behind it.
-Once RNA polymerase has started transcription and progressed past the beginning of a gene, another
RNA polymerase may start producing another RNA strand if there is room at the promoter.
Termination
-transcription is terminated when RNA polymerase recognizes a termination sequence.
-in prokaryotes a termination mechanism is a protein binding to the mRNA and stopping transcription, another mechanism is mRNA binding with itself in a hairpin loop and stopping transcription
-in Eukaryotes one termination sequence is a string of adenosine which is transcribed as a string or uracils on the RNA. Nuclear proteins bind to the poly uracil site and stop transcription. The newly synthesized RNA then dissociates from the DNA template strand.
-Transcription ceases and RNA polymerase is free to bind to another promoter region
Post-transcriptional modifications
-At this point we have newly transcribed eukaryotic RNA or pre-mRNA
-this pre-mRNA is vulnerable to enzymes and conditions outside the cell nucleus, it must undergo additional modifications before it can exit the nucleus and reach the ribosome
-One modification is the addition of a chain of 50-250 adenine nucleotides, added one at a time to the 3’ end by an enzyme poly-A polymerase. This adenine chain is called the poly(A) tail, and enables mRNA to be translated efficiently and protects it from attacks by RNA digesting enzymes in the cytosol. (poly A tail acts as a shield, gets eaten away to protect amino acid regions)
-modifications are also made at the start of the pre-mRNA transcript, where a 5’ cap consisting of seven G’s is added by a different enzyme. This functions as the initial attachment site for mRNA;s to ribosomes to allow for translation, this entire process is known as capping and tailing.
-it is still not ready to leave the nucleus, DNA of a eukaryotic cell is composed of coding regions known as exons and non coding region introns. The introns are interspersed and transcribed into mRNA’s. Introns do not code for part of the protein if they were left in the mRNA they would alter the amino acid sequence which builds the protein. This would result in additional amino acids which would not allow the protein to fold and preventing its proper function.
-a process called mRNA splicing removes the introns from the pre-mRNA and joins the exons together, this process occurs in the spliceosome, a complex formed between the pre-mRNA and a bunch of small ribonucleoproteins called snRNPs. (these snRNP’s singal the introns for removal)
- the exons can be joined in different combinations, this is called alternative splicing, this xan increase the variety of protein created from a single gene. (this is what humans with only 20k genes can create 100k proteins)
