Gene Activity: How Genes Work Mader Chapter 14
Dec 28, 2015
Gene Activity: How Genes Work
Mader Chapter 14
Gene Expression
Two Steps1. Transcription2. Translation
Can you describe the basics of each of these steps?
Gene Expression-Transcription and Translation
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”
RNA Processing-Primary RNA modifications
• Modified guanine 5’ cap helps ribosome attach
• Poly A tail is added- 150-200 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
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
Translation-The Second Step
Occurs in three steps1. Initiation2. Elongation 3. Termination
Initiation• The small subunit
of the ribosome binds near the “start codon” (AUG)
• Initiator tRNA binds
• Large subunit binds
• This requires energy and enzymes
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
Termination• Occurs when the “Stop
Codon” is reached• Subunits disassociate
and polypeptide begins to fold into its 3D shape
Transferring the Amino Acid What is this and what does it do?
What do we call these three bases?
What are these and how do they work?
The Code:1. Degenerate/redundant2. Unambiguous3. Start and Stop Signals4. Universal