Gene Activity: How Genes Work Mader Chapter 14

Gene Expression Two Steps Transcription 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 steps Initiation Elongation 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: Degenerate/redundant Unambiguous Start and Stop Signals Universal