Part 2: RNA Processing Flashcards
Do bacteria have introns and exons?
- No.
Exons:
- nucleic acid sequences that are transcribed and retained in the corresponding mature mRNA
Introns:
- nucleic acid sequences that are transcribed but spliced from the primary transcript to yield the mature mRNA
- removed BEFORE translation
Transcription:
DNA → RNA
Translation:
mRNA → protein
introns REMOVED before translation
Start and stop codons are in which portion of a gene: introns or exons?
- exons
- start and stop codons are utilized by the ribosome during translation (protein synthesis)
- since splicing of introns occurs before translation, start and stop codons must be in the exons in order to reach the ribosome
Splicing:
- the removal of introns from mRNA
- occurs before translation
- splices adjacent exons to each other
When does splicing occur?
- co-transcriptionally or post-transcriptionally
- BEFORE translation
Schematic summary of splicing:
How are introns recognized by spliceosomes?
- GU always at 5’ splice site of intron
- AG always at 3’ splice site of intron
- consensus sequences
- branch-site (conserved sequence)
Length of introns is highly variable, ranging from:
<100 to several thousand nucleotides
Consensus sequences:
- conserved, but not invariant sequences found at 5’- and 3’-ends of introns
- used for intron recognition and splicing
Branch-point:
- conserved sequence 20 – 50 nucleotides from the 3-end of the intron
- used for intron recognition and splicing
- an adenine residue that is catalytic/nucleophilic
How does splicing occur?
- two transesterification reactions:
- cleaving of phosphodiester bond between exon 1 and intron 1
- cleaving of phosphodiester bond between intron 1 and exon 2
Splicing Mechanism (Text):
- 2’-OH of an adenylate residue at the branch site attacks phosphate at 5’ intron splice site
- new 2’– 5’ phosphodiester bond formed
- 3’-OH of exon 1 attacks phosphate at 3’ intron splice site
- cleaves phosphodiester bond between the intron and exon 2
- Exon 1 and Exon 2 now joined
- Intron forms lariat structure - degraded
Splicing Mechanism (Draw):
The splicing reaction is catalyzed by:
- the spliceosome
- assembly of ribonucleoprotein particles (SNURPs) that recognize the 5’ splice site, the 3’ splice site and the branch site.
The spliceosome is:
- an assembly of ribonucleoprotein particles (SNURPs) that recognize the 5’ splice site, the 3’ splice site, and the branch site.
Spliceosome assembly:
- de novo
- SNURPs come together on pre-mRNA
- requires ATP
SNURPs that make up a spliceosome:
- U1 and U2
- U4-U5-U6 complex
The U1 SNURP of the spliceosome recognizes:
- the GU sequence at the start of an intron
- U1 then binds to GU
The U2 SNURP of the spliceosome recognizes:
- the 2’-OH adenylate residue of the branchpoint
- U2 then binds to branchpoint
Steps of spliceosome assembly (Text):
- U1 binds to GU sequence at intron start
- U2 binds to 2’OH adenylate residue branch point
- U1 and U2 come together with the U4-U5-U6 complex to form a complete spliceosome
REQUIRES ATP
(ATP HYDROLYSIS)
Steps of spliceosome assembly (Draw):
A conserved GU sequence is always at:
- the 5’ splice site of an intron (the start)
A conserved AG sequence is always at:
- the 3’ splice site of an intron (the end)
Alternative splicing:
- can produce multiple, related proteins from a single gene
- although structurally related, can have completely different functions
There are 100,000 proteins in the human body, but only 20,000 genes. This difference is accounted for partly by:
alternative splicing
