Transcript From gene to protein 2

Molecular Biology of the Cell. 2002.
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts,
and Peter Walter.
From Gene to RNA
Kanokporn Boonsirichai
Orientation matters in transcription
Figure 7-10 Essential Cell Biology (© Garland Science 2010)
Question to consider

Could RNA polymerase used for transcription be used as
the polymerase (primase) that makes RNA primer
required for replication? Why or why not?
Molecular Biology of the Cell. 2002.
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts,
and Peter Walter.
RNA Processing
Kanokporn Boonsirichai
Gene organization

Bacteria


Uninterrupted coding sequence
Genes are grouped together in an “operon”.
promoter
gene A

gene B
gene C
Eukaryotes


Coding sequence is interrupted by “introns”.
Coding sequence is broken into pieces of “exons”.
Figure 7-17 Essential Cell Biology (© Garland Science 2010)
mRNA Processing
Pre-mRNA
Mature mRNA
Eukaryotic mRNA Processing

Capping


Splicing


Removal of introns
3’cleavage


Addition of 7-methylguanosine at the 5’ end
Cleavage of the transcript at the 3’end of the gene
Polyadenylation

Addition of 150-250 adenine ribonucleotides at the 3’ end
1. Addition of 5’ cap
2. Pre-mRNA Splicing: removal of introns

Splicing signals are located in the transcript itself.
R = purine (A or G)
Y = pyrimidine (C or T)
N = any one of the 4 bases

2’-OH is utilized to form
a lariat structure
Branch point
Spliceosome

A complex of snRNPs
(small ribonucleoproteins)
Interactions among snRNPs are mediated by
their snRNAs
How is Fidelity Achieved?

snRNAs are associated with snRNPs.

Several components of the spliceosome are carried on
the phosphorylated tail of RNA polymerase(to keep
track of introns and exons).

The “exon definition hypothesis”


Exon size is somewhat uniform (averaging about 150
nucleotide pairs).
Exon-intron boundaries are marked with spliceosome
components
Other Splicing Systems
Transplicing => attachment of
two different RNAs
What might be the importance
of RNA splicing?
Alternative splicing allows for the production of
multiple related proteins from a single transcript.
Processing
of the 3’ end

Cleavage factors

Poly-A polymerase

Cleavage signals

Poly-A addition
Mature mRNAs are exported from the
nucleus
Questions to consider

What do you think might be the reasons for the
eukaryotes to have their mRNA processed?