Transcript Biol 178 Lecture 30

Bio 178 Lecture 30
Gene Expression
http://ccgb.umn.edu/~mwd/cell_www/chapter2/ribosome.html
Reading
•
Chapters 15 & 20 (P 410-411)
Quiz Material
•
Questions on P 318 & 432
•
Chapters 15 & 20 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Gene Expression
 Transcription (Cntd.)
 Translation
• Mutation
# Somatic and Germ-Line
# Types of Mutation
Transcription Initiation
RNA pol has no proofreading ability - why is this not a problem?
Transcription (Cntd.)
4. Termination
At the end of the gene “stop” sequences cause
transcription to stop, the RNA-DNA hybrid to dissociate,
& the DNA in the transcription bubble to rewind.
Example of a Stop Signal:
Series of GC base pairs, followed by series of AT base pairs
 GC hairpin followed by a polyU sequence.
Hairpin causes RNA pol to pause, & the pairing of U with
A is weak  Dissociation.
GC Hairpin
Transcription in Eukaryotes
Some of the important differences from prokaryotes include:
• Transcription Factors
Additional proteins that are required for transcription.
• RNA Polymerases
3 different RNA polymerases with different functions.
• Promoters
The different RNA polymerases use different promoters.
The RNA pol II promoter (for mRNAs) has a core
sequence called a TATA box.
Initiation of Transcription in Eukaryotes
Transcription in Eukaryotes (Cntd.)
• Posttranscriptional Modifications
1. 5 Cap
GTP is added to the 5 end of the transcript. A methyl
group is then added to the G of the GTP.
Function - Stability & initiation of translation.
2. 3  Poly-A Tail
The transcript is cleaved downstream of a specific site.
Many A’s are added to the 3 end.
Function - Stability.
Posttranscriptional Modifications
Eukaryotic Posttranscriptional Modifications
(Cntd.)
3. Intron Removal
• Introns
Region of the DNA that is transcribed but does not code
for protein.
Removed from the transcript prior to translation.
• Exons
Coding regions of DNA.
Posttranscriptional Modification
Prokaryotic Translation - Initiation
Translation (Prokaryotes)
1. Leader sequence of mRNA binds with rRNA of the
small ribosomal subunit.
2. The first codon is AUG. The complementary anticodon
on tRNA binds to the codon. This tRNA carries the amino
acid fMet.
3. The large ribosomal subunit binds. tRNAfMet is in the P
site.
Prokaryotic Translation - Elongation & Translocation
Translation (Cntd.)
4. tRNA with the appropriate anticodon binds to the
codon in the A site.
5. fmet is released from its tRNA and a peptide bond is
formed between it and the second amino acid.
6. Ribosome moves 3 nts 53. Initial tRNA now in E
site, from where it leaves.
7. Appropriate tRNA binds to A site and the process
continues until a stop codon is reached.
Translation - Termination
Translation (Cntd.)
8. When a stop codon is reached it binds a release factor.
9. The bond between the protein and the tRNA in the P
site is broken and the protein released.
Translation in Eukaryotes
Principle differences from prokaryotes:
First amino acid is met.
5 cap binds to rRNA.
Secreted Proteins
http://cellbio.utmb.edu/cellbio/ribosome.htm
Summary of Gene Expression
Question Example
For the DNA sequence (on the template strand):
3-ACTACTGAGTAGCCCAGCTAACTCCTGTT-5 
Give the mRNA sequence, and using the genetic code, the
amino acid sequence of the corresponding protein.
Translation
McGraw-Hill Video
Mutation
Changes in the hereditary message of an organism.
• Somatic Mutations
Not inherited, but can have a profound effect on the
individual, eg. Cancer.
• Germ-Line Mutations
Inherited.
Importance
Starting point for evolution - mutation produces new
alleles and recombination shuffles them.
Do all Germ Line Mutations Increase Genetic Fitness?
No - May reduce, maintain, or increase progeny number.