Bio 178 Lecture 30Gene 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 53. 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 ExampleFor 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

MutationChanges in the hereditary message of an organism.

• Somatic MutationsNot inherited, but can have a profound effect on the individual, eg. Cancer.

• Germ-Line MutationsInherited.

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.