W2 L1 Transcription processing in eukaryote Flashcards
Transcript splicing : self splicing intron
Unstable so it splice its own
! RNA-catalysed reaction compared to spliceosomal-based
Two type of self splicing intron
Type 1: 5’ splice it’s own, attracted to the branch site G, 3’ is then splice, Linear intron
Type 2: 5’ end is brought close to A branch site, cleaved and attach to the branch. 3’ is then cleaved making a lariat intron
Intron loss in genome
- intron can be loss, a reverse transcriptase reverse transcribe mRNA into DNA. This new DNA can then be recombined into the dna, creating an intronless allale.
Intron gain in genome
- a gene makes mRNA, there are many intron lariat from other gene. If this intron is attached into the mRNA and is then reversed transcript, recombined or double crossover into the DNA. It could lead to intron gain
Intron sliding
- different intron position in closely related species
-a gene with intron is transcript resullt in a mRNA and lariat. The lariat can be inserted back into the mRNA at a different position. Reverse transcripted, recobine/ double crossing over
How do recombination work in double stranded organism
- our DNA repair mechanism recreated the double helix structure from the recombined dna
Intron Function
Introns may code for snoRNA (small nucleolar RNA)
! Generally transcribed by RNApol II
! Modify bases in rRNA and tRNA
The lariat is linearized and processed
How snoRNA work
- snoRNA have homology with the target RNA, able to hybirdise with its target and then modify the target
- snoRNA can be single or polycistronic ( multiple functional unit)
Ways of Transcript splicing
- a mRNA can be splice in many different ways
- Exon skip
-exon extend - intron retained
- alternative exon
SV 40
! Simian virus 40
! T antigen gene encodes two proteins (T and t)
- the DNA have a stop codon in the gene, found in the intron and another set of cryptic 5’3’ at the back
- if the intron is not spiced, it translated into t-ag protein
- if the intron is spliced, it make T-ag
Drosophila DSCAM
- gene involved in the Down syndrome
- 24 alternatively spliced exon in the gene
- 38016 possible way of combination
Role of D DSCAM many splicing method
! Two roles – neural patterning and antigen recognition. Different cell express the gene differently
! Neural patterning for self or non-self regconigtion, neurite self avoidance
Regulation type for mRNA splicing
Negative control: a repressor regconise and bind to the intron, causing it to remain in the mRNA
Positive control: activator bind to mRNA, recruit the splisosome complex, causing it to be splice. Occur in intron that is normally not splice
Regulation of mRNA processing In drosophila
- splicing control sex determination in D
Sex in Drosophila is determined by the ratio of X to autosomes
Ratio is important ! XXY=female (fertile), XO=male (sterile, Y required for fertility)
The process is control by sex lethal gene. When the gene is on. It prevent splicing on its own mRNA, creating a slt protein. The stl protein block the splice site of Tra lead to transformer to be turn functional, activating the splice site of Dsx gene, causing the dsx gene to have 30 aa of female specific, repress male differentiation gene - without slt, trans is off, dsx is unsliced and have 150 male specific aa which repress female development
RNA editing effect
- Post-transcriptional modification of mRNA sequence cause the DNA sequence differs from mRNA sequence
- Protein sequence differs from that predicted from DNA sequence
Two mechanisms - Nucleotide modification (site-specific deamination)
- Nucleotide insertion by guide RNAs
- More common than originally thought but not as common as some would have us believe.
Characteristic of alpha and A
