From Gene to RNA

Kanokporn Boonsirichai

Molecular Biology of the Cell. 2002. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

Figure 7-10 Essential Cell Biology (© Garland Science 2010)

Orientation matters in transcription

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?

RNA Processing

Kanokporn Boonsirichai

Molecular Biology of the Cell. 2002. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

Figure 7-17 Essential Cell Biology (© Garland Science 2010)

Gene organization Bacteria

Uninterrupted coding sequence Genes are grouped together in an “operon”.

Eukaryotes Coding sequence is interrupted by “introns”. Coding sequence is broken into pieces of

“exons”.

gene A gene B gene C

promoter

mRNA Processing

Pre-mRNA

Mature mRNA

Eukaryotic mRNA Processing Capping

Addition of 7-methylguanosine at the 5’ end

Splicing Removal of introns

3’cleavage 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?