Lecture-6-RNA-modifification - NUCLEIC ACID BIOCHEMISTRY...

NUCLEIC’ACID’BIOCHEMISTRY’
BCHS’4306’
’
Lecture’6:’
RNA-splicing/RNA-modiGication-Editing’

The’Central’Dogma’
Germline’
(Gametes)’
’
’
Somatic’Cells’
Meiosis’
Mitosis’
Express’traits’
Transmit traits

A’View’of’the’Gene’Structure’in’Eukaryotes’
• Transcription’always’occurs’in’the’5 ` ’to’3 ` ’direction’just’like’replication’
• Genes’are’split’

Questions to ponder today
• Why is it advantageous to have split genes that then
have to be spliced
• We can increase genome complexity without
increasing the number of genes
– Allows us to increase genome complexity without increasing gene number
– 30,000 genes/human genome – many isoforms…100,000 coding RNA
• We can exert more control over gene expression

Genes Transcription
• Genes on the same chromosome can be transcribed in
different directions.
• Each gene is always transcribed only in one direction.

Chromosome 1
Allele’C’
Allele’-’A’ Allele’-’B’
5±’
’
3±’
3±’
’
5±’
Allele’C’
Allele’-’A’
Gene’b’
5±’
’
3±’
3±’
’
5±’
Paternal’Homolog’
Maternal’Homolog’
Gene’A’
Allele’-’b’
Gene’B’ Gene’C’
Coding strand of allele B =’5±-ATGGCTATGAGTTCAGTCC-3±’
’
’
Coding strand of allele b ’=’5±-ATGGC a ATGAGTTCAGTCC-3±’
’

Remember
• All of the DNA is replicated.
• Only 50-90% of the DNA in our genome is transcribed
into RNA
• Only 1-2% of the genome is expressed into protein
coding RNA
• Therefore the majority of RNA expressed from our
genome is non-coding RNAs

Eukaryotic’RNA’Processing’
(Introns’are’spliced’out)

The’Gene’for’Dystrophin’is’the’Largest’in’the’
Human’Genome’
• Gene = 2,500,000 bases (2.5 Mb) + promoter
• Primary transcript = pre-mRNA = 2.5 Mb
• Mature transcript = mRNA = 14,000 bases (14 Kb)
• Total DNA in introns = 2,486,000 bases (2.486 Mb)
• Mutations result in muscular dystrophy

Requirements for intron splicing
• 5 ` ’donor’site’–’typically’GU’
• 3 ` ’acceptor’site’–’typically’AG’
• Branch’point’sequence.’

Splicing: How it works

Intron Splicing:
Lariat is an important intermediate structure

http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html

http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html

How does the splicesome know where to go?
http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html

http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html

http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html

http://bcs.whfreeman.com/thelifewire/content/chp14/1402001.html ’

Eukaryotic Gene Regulation
1. Controls
§ How much
§ When – time during development
§ Where – tissue/cell type
2. Levels
§ RNA
v At transcription initiation – most common
v During splicing or processing
v Post-transcriptional stability – common-
miRNA-mediated
§ Protein
• Translation initiation/elongation
• protein stability –ubiquitin system
• protein modifcation
19

snRNA: Small Nuclear RNAs
• Introns
– Removed during pre-mRNA maturation through two transesterifcation reactions.