1. Gene Activity: How Genes Work
Mader Chapter 14

2. Gene Expression Two Steps Transcription Translation
Can you describe the basics of each of these steps?

3. Gene Expression -Transcription and Translation

4. Transcription -The First Step
DNA “unzips”
DNA has a template or “sense” strand
RNA Polymerase adds complimentary RNA bases to the growing 3’ end
Begins at a promoter and ends at the “Stop Sequence”

5. RNA Processing -Primary RNA modifications
Modified guanine 5’ cap helps ribosome attach
Poly A tail is added adenines that delay degradation and facilitate transport
Introns are spliced out by spliceosomes which act as a ribozyme and contain snRNA that recognize the site to be cut
Capped and tailed exons exit the nucleus through a nuclear pore

6. Function of Introns -The RNA “Left Behind”
For many years biologists thought that introns were simply “wasted space” within genes
Now… we understand that the presence of introns allow for alternative mRNA splicing
Now we know that one gene can code for multiple polypeptides
Some introns also give rise to microRNAs (miRNAs) that can bind to mRNA and prevent translation

7. Translation -The Second Step
Occurs in three steps
Initiation
Elongation
Termination

8. Initiation
The small subunit of the ribosome binds near the “start codon” (AUG)
Initiator tRNA binds
Large subunit binds
This requires energy and enzymes

9. Elongation Incoming tRNA binds to the “A site”
rRNA, part of the large subunit, transfers energy to create the peptide bond between amino acids of the growing polypeptide
Translocation moves the ribosome down the mRNA and the spent tRNA is ejected

10. Termination Occurs when the “Stop Codon” is reached
Subunits disassociate and polypeptide begins to fold into its 3D shape

11. Transferring the Amino Acid
What is this and what does it do?
What do we call these three bases?

12. What are these and how do they work?
The Code:
Degenerate/redundant
Unambiguous
Start and Stop Signals
Universal