lecture09 - Biology 172 Lecture 9 Wednesday Todays Outline...

Lecture 9: Wednesday, May 26, 2010
Biology 172
Today’s Outline Announcements
RNA Processing
- cap, tail, splicing
Translation
- Overview
- The Genetic Code
- tRNA Structure/Function
- Ribosomes
Tomorrow’s
Discussion: Central
Dogma
Quiz on Lectures 8
and 9

RNA Processing
- prokaryotes use RNA immediately in translation
- eukaryotes “process” RNA before it is used

RNA Processing Events in Eukaryotes
- addition of “cap” to 5’ end of RNA
- addition of poly A “tail” to 3’ end of RNA
A modified guanine nucleotide
added to the 5 ′ end
50 to 250 adenine nucleotides
added to the 3 ′ end
Protein-coding segment
Polyadenylation signal
Poly-A tail
3 ′ UTR
Stop codon Start codon
5 ′ Cap
5 ′ UTR
AAUAAA AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
G P P P
5 ′
3 ′

RNA Processing Events in Eukaryotes
- addition of “cap” to 5’ end of RNA
- addition of poly A “tail” to 3’ end of RNA
- RNA splicing - removal of introns…. portions of the
RNA not used in translation
- conducted by spliceosome (RNA/protein complex)
Figure 16-7a
Primary RNA transcript
DNA
Promoter
Intron 1 Intron 2
Exon 1
Exon 2
Exon 3
Spliced transcript
3 ′
5 ′
5 ′
3 ′
5 ′
3 ′

Figure 16-7b
snRNPs ARE THE EDITORS
.
Primary
RNA
snRNPs
Exon 1 Exon 2 Intron
Spliceosome
3 ′
3 ′
5 ′
5 ′
1. Several snRNPs and proteins
assemble to form a spliceosome . An A
(adenine) reacts with the 5’ end of the
intron.
A
A
snRNPs =
“ snurps ” = small
nuclear ribo-
nucleoproteins =
complexes of
proteins and
small RNAs

Figure 16-7b
Excised
intron
snRNPs ARE THE EDITORS
.
Exon 1 Exon 2
Mature mRNA
5 ′
5 ′
5 ′
5 ′
3 ′
3 ′
3 ′
2
. The primary transcript (pre-mRNA) is
processed in the spliceosome, which is
made up of snRNPs.
3
. The ends of adjacent exons bond
covalently, releasing the intron
(which is then degraded).
A
A

RNA Processing Events in Eukaryotes
- addition of “cap” to 5’ end of RNA
- addition of poly A “tail” to 3’ end of RNA
- RNA splicing - removal of introns…. portions of the
RNA not used in translation
- conducted by spliceosome (RNA/protein complex)
Human Dystrophin Gene
X chromosome
Largest known mammalian gene
• 2.7 million bp
• 78 introns
• mRNA is 14,000 bp

RNA Processing Events in Eukaryotes
- addition of “cap” to 5’ end of RNA
- RNA splicing - removal of introns…. portions of the
RNA not used in translation
- conducted by spliceosome (RNA/protein complex)
- addition of poly A “tail” to 3’ end of RNA
- RNA export to cytosol

DNA
Replication
RNA Protein
Transcription Translation
THE CENTRAL DOGMA

Translation – Basic Features
- synthesis of a polypeptide using information in RNAs
- carried out by a ribosome
- occurs in the cytosol
How is a nucleic acid
“language” converted into
a protein “language”?

Translation – Requires 3 Types of RNA
mRNA (messenger) – specifies the sequence of amino
acids in a protein
rRNA (ribosomal RNA) – component of ribosomes
tRNA (transfer RNA) – links nucleic acid information to
amino acids