BIO282 Lecture Notes - Lecture 12: Rna Editing, Intron, Spliceosome
Learning objectives:
In this topic/lecture you will learn about:
• RNA processing and how different RNAs get processed,
• Capping of mRNA and functions of the ‘Cap’,
• How and why the ‘poly A’ tail is made,
• Introns and how are they spliced out,
• The removal of introns by spliceosomes and rhibozymes,
• Role of introns,
• RNA editing and why and how it is accomplished.
• Processing of tRNA and rRNA
RNA PROCESSING
• The RNA molecule (primary transcript), produced by transcription, usually undergoes through
several chemical changes before it can function in the cell.
• The term RNA processing refers to all chemical modifications required to generate the final
RNA product.
The coding sequences (exons) of many eukaryotic genes are disrupted by noncoding regions
(introns).
• In many eukaryotic genes the coding sequences are not continuous (they get interrupted)
• The coding sequences (exons) are interrupted by non-coding sequences (introns); both exons
and introns are transcribed into RNA
• RNA processing steps are required to remove intron to generate mRNA for translation; the
mRNA has only the sequences of the exons.
• Number of introns and exons varies in different genes.
The human dystrophin gene is 2 million bases long and contains 78 introns (99% DNA of the
gene)
Processing of mRNA-
1. RNA is modified in the nucleus by
a. Capping at the 5` end
b. Addition of a poly A tail (lots of A bases) at the 3` end
c. Removal of introns
d. RNA editing
2. Mature mRNA is transported through nuclear pores to the cytoplasm, where it is translated.
These introns are only present in eukaryotes (i.e. bacteria do not have introns)… WHY?
Bacteria do not have a separate nucleus, meaning as soon as the DNA sequence is produce
ribosomes will translate it giving it the wrong protein.
Eukaryotes have these two steps separated so are able to remove introns before translation
(A)
• This is the exception to the rule that we always add to the 3’ end. 5' to 5' pairing
• It unusual bonding protects the DNA from the restriction enzymes
o Allows its escape from the attack of nucleases
• The figure is just to show you that some of the bases have been added and some of
the sugars have been methylated. You need to remember that guanine is added in the
reverse way.
Methylation of bases and sugar
What is the role of cap?
• enhances splicing of the mRNA.
• required for transport of the mRNA to the cytoplasm.
• increases the translatability of the mRNA.
• Protects the mRNA against nuclease degradation. RNA is very sensitive to nucleases and
by having this unusual attachment it provided protection.
(B)
Document Summary
Rna processing: the rna molecule (primary transcript), produced by transcription, usually undergoes through several chemical changes before it can function in the cell, the term rna processing refers to all chemical modifications required to generate the final. The coding sequences (exons) of many eukaryotic genes are disrupted by noncoding regions (introns). The human dystrophin gene is 2 million bases long and contains 78 introns (99% dna of the gene) Bacteria do not have a separate nucleus, meaning as soon as the dna sequence is produce ribosomes will translate it giving it the wrong protein. Eukaryotes have these two steps separated so are able to remove introns before translation (a: this is the exception to the rule that we always add to the 3" end. You need to remember that guanine is added in the reverse way. What is the role of cap: enhances splicing of the mrna, protects the mrna against nuclease degradation.
