topic 20 Flashcards
what’s the purpose of RNA processing? describe RNA processing
RNA processing is required for the formation of mature RNA from pre-mRNA. in eukaryotic cells, these steps include capping, polyadenylation, and splicing.
describe the steps of RNA processing (4)
- from DNA, a pre-mRNA is transcribed. this primary transcript is capped at the 5’ end and shortened and polyadenylated at the 3’ end. it’s then spliced to take out unnecessary sections.
- the 5’ capping protects the 5’ end of the mRNA from degradation and is necessary for the export of mRNA to the nucleus. during 5’ capping, a reverse methylated G nucleotide is added to the 5’ end. this nucleotide can’t easily be cleaved by endonucleases, protecting the 5’ end from degradation.
- on the 3’ end of the mRNA, along the tail, A nucleotides are added: after the stop codon, which specifies the end of the coding message, an AU rich sequence is found. when the RNA polymerase passes this sequence, another enzyme, the poly(A) polymerase attached to the mRNA and cleaves the growing RNA chain approximately 30 bases after the AU rich sequence. the poly(A) polymerase then starts adding untemplated A nucleotides to produce the poly(A) tail. the function of the poly(A) polymerase includes cleaving RNA and adding A nucleotides. this protects RNA from degradation and extends the RNA lifespan.
- unnecessary non-coding RNA sequences are excised from the pre-mRNA through a process called splicing. introns are spliced out of the primary transcript to give the mature RNA. a pre-mRNA gene can be spliced into different variants, making a number of related but non-identical proteins (alternative splicing). all variants must include the 1st exon (from the 5’ end). within a single cell, usually only 1 variant exists.
what is the purpose of splicing?
splicing increases the coding capacity of the genome
what is the importance of 5’ capping and 3’ polyadenylation? (3)
- marks the 5’ and 3’ ends of the mRNA as being intact
- required for mRNA export and translation
- protects the mRNA from degradation by stabilization
differentiate between exons and introns
exons: coding sequences (expressed)
introns: non-coding sequences (interruptions)
how are introns removed in splicing?
introns are removed in 2 consecutive transesterification reactions.
the 1st reaction is between the adenosine branch point (within the intron) and the 5’ splice site. the 2’ hydroxyl group of the A branch point, attaches the phosphate at the 5’ end of the intron. this frees the 3’ end of the 1st exon and generates a lariat shaped intermediate.
in the 2nd reaction, the free 3’ hydroxyl group of the 1st exon attaches the 5’ phosphate of the 2nd exon. the excised intron diffuses away and is degraded. results in 2 fused exons, a mature RNA.
what is a spliceosome?
splicing rarely happens without help of a spliceosome (snRNPs), large protein RNA complex that consists of both RNA and protein subunits.
an example is yeast spliceosome. it binds to the splice site and facilitates the splicing process.
when is alternative splicing used?
alternative splicing works when the spliceosome can’t recognize the 3’ site due to a modified A branch point. the spliceosome will skip to the next splice site, omitting an exon.
when does capping, splicing, and polyadenylation occur? what about translation?
capping, splicing, and polyadenylation are sequential events that all occur while RNA is being transcribed.
capping happens as soon as the mRNA 5’ end leaves the polymerase. (splicing) the first exon is already excised before polyadenylation occurs. once the RNA is fully processed, it’s transported out of the nucleus to the cytosol, where it’s translated into protein. that only occurs in eukaryotes as bacteria does not have a nucleus. in bacteria, translation could happen as soon as mRNA is made.
what is sequence specific RNA degradation? describe it
sequence specific RNA degradation is termed RNA interference. RNA interference requires the binding of a second complementary RNA strand (microRNA) to the mRNA. with the help of several proteins the mRNA is then cleaved and degraded. the cell is able to use these RNA degradation mechanisms to turn off production of gene products.
how is mRNA exported into the cytoplasm?
to export mRNA into the cytoplasm, several proteins interact with the mRNA. a protein binds to the 5’ cap and multiple poly(A) binding proteins bind to the 3’ poly(A) tail. a transport receptor binds to the coding message.
the transport receptor is recognized by the nuclear pore complex, a huge multi-subunit complex that acts as a funnel through the nuclear envelope. the nuclear pore complex doesn’t recognize the mRNA itself, but the proteins associated with it. the mRNA with a cap binding protein, transport receptor, and poly(A) binding protein passes through the nuclear pore and the proteins dissociate from the mRNA. 2 transport proteins are then returned to the nucleus. new proteins such as initiation factors, bind to the mRNA and initiate translation
how is mRNA degraded?
mRNA degradation could start at either 5’ or 3’ end. deadenylase shortens the poly(A) tail and a decapping enzyme cleaves the 5’ end cap. without the cap and poly(A) tail, the RNA quickly gets degraded by the 5’ and 3’ exonucleases.
how are rRNAs transcribed in eukaryotes? describe the structure of rRNAs. how are they protected from degradation?
in eukaryotes, most rRNA are transcribed from a single pre-RNA. this pre-RNA is spliced into 3 separate RNAs. ** this splicing doesn’t yield a fused message but separate RNA products.
while there usually aren’t operons in eukaryotes, rRNAs are an exception, where 3 RNAS are encoded in a message. rRNAs aren’t capped or adenylated. they are protected from degradation due to the tight binding to ribosome proteins.
describe modifications in the different RNAs (mRNA, tRNA, rRNA)
rRNAs are highly modified. meaning that the nucleotide building blocks are altered after they are chained together.
similarly, the tRNAs that help in translation are highly modified. uridine is a very common modification in tRNA. without modifications, tRNAs and rRNA are often not functional.
mRNAs are rarely modified, and if they are, they usually fulfil specific functions such as the alteration of a splice site.
