BIO 105 Gene Expression
The Central Dogma • DNA RNA PROTEIN • One gene makes one RNA that directs the synthesis of one protein. • Human genome contains ~ 30,000 genes. • Not all cells make all 30,000 proteins. • Some cells have specific genes turned off and other cells have different genes turned off. • That’s how we get cells to be different from each other. • Stem cells have all genes still turned on.
A. How are Genes Controlled? • There are 6 levels of control that determine if a gene will get expressed into a protein. • Transcriptional control • RNA polymerase binds to a promoter sequence on the template strand of DNA.
Primary RNA Transcript • Single strand of RNA synthesized by RNA polymerase. This is called transcription.
2. Posttranscriptional Control • The primary RNA transcript contains coding regions called exons and non-coding regions called introns. • The introns must be excised (cut out) and the exons spliced together. • This process happens in the nucleus. • Less than 10% of a gene is comprised of exons. • The final product is the mature mRNA transcript.
Gene Splicing • Although there are only 30,000 genes in the human genome, exons from a single gene can be reordered in the nucleus to form over 100,000 mRNA
mRNA Transcript • Transport of mRNA from nucleus to ribosomes • Transport into cytoplasm is done through nuclear pores of the nuclear envelope. • Only about half of the transcripts leave the nucleus.
mRNA Selection • Translational control • Not all mRNA transcripts become associated with ribosomes • The process of the ribosomes “reading” mRNA and synthesizing proteins is called translation. • Control of mRNA half life • mRNA gets degraded by cytoplasmic enzymes. • Half lifes can range from 3 minutes up to 10 hours.
6. Posttranslational Modification of Protein • After synthesis, some proteins need to be modified in order to become functional. • Two examples of modification: • Phosphorylation – adding phosphate groups to AA such as tyrosine. This is how protein kinases become active. • Glycosylation – adding sugar molecules to proteins within the Golgi apparatus.
B. The Genetic Code • How do four nucleotides (ATCG) translate into proteins that may contain several hundred AA? • Crick concluded that the genetic code is read in increments of 3 nucleotides or a codon….the triplet code. • Every 3 nucleotides (codon) on a mRNA would code for 1 AA.
So • If a mRNA had a base sequence as follows: • AUGUUUUUAGGGUAAUAG • The sequence of the protein would be: • PHE-LEU-GLY-stop-damnit stop • But how do AA how to be placed in order on the ribosome? • And, how do they connected to each other (dehydration synthesis).
Protein Synthesis • As the ribosome moves along the mRNA, one codon (a triplet) is exposed. • This is where tRNA (transfer RNA) comes into play. • Specific tRNAs bind to specific AA. • tRNAs also have a region of 3 bases called an anticodon. • The anticodon will bind to the appropriate codon when it becomes exposed by the ribosome.
Codon/Anticodon • Go back to the Genetic Code • The codon (on the mRNA) UGU codes for cysteine. • The tRNA that binds cysteine contains what anticodon? • And the answer is….. • ACA
tRNA Cysteine ACA
tRNA Cysteine ACA UGU mRNA