 Homework:  Lab 6B Analysis Questions – due tomorrow  Problem Set will be due next Wednesday  Do Now: With your lab group…  Take out lab packet  Get gloves, get your gel, wait for instructions.  Today’s Goals:  Interpret gel electrophoresis results (who killed the virus??!)  Explain how a gene is transcribed and translated into a protein!  Agenda:  Gel results  Lecture: Gene Expression

DNA Chromosomes: DNA tightly coiled around little protein balls (histones) to make it very compact Gene: Instructions for making a protein that determines a trait Another Gene: instructions for making a different protein that determines a different trait Nucleotide Base Pairs Chromosomes unraveling into DNA

 The process of transcribing and translating a gene to make a protein  All cells carry all the genes of an organism, but only SOME genes are expressed in cells based on when and where particular proteins are needed

Transcription

RNA Processing:

5’ Cap Poly-A tail Introns cut out and exons spliced together Signals mRNA’s destination (cytoplasm) Protects from degrading enzymes Helps ribosomes attach Non-coding sequences interspersed with coding sequences that get translated into proteins

Translation

Translation: RNA runs the show messenger RNA – carries transcript of codons w/ amino acid sequence – processed inside nucleus – mature mRNA leaves nucleus, diffuses to ribosomes ribosomal RNA – forms the ribosome and acts as a catalyst to form peptide bonds connecting amino acids transfer RNA – carries amino acids to ribosome – each tRNA must get “charged” with a specific amino acid

tRNA:

Ribosome Anatomy:

 Homework:  PS 13 due tomorrow  Do Now:  Find a partner sitting near you  For each stage of translation shown in your notes packet:  Study the diagram  Write a bullet-point summary of what’s happening  Today’s Goals:  Explain how RNA is translated into a protein  Explain how cells regulate gene expression  Agenda:  Lecture: Translation, and Control of Gene Expression

3 Steps of Translation: Initiation (start) Elongation (making the polypeptide chain) Termination (end)

3 Steps of Translation: Initiation:

3 Steps of Translation: Initiation: – tRNA binds to start codon (AUG) in P site of ribosome – Ribosome assembles around mRNA at beginning of coding sequence

3 Steps of Translation: Elongation

3 Steps of Translation: Elongation – 2nd tRNA enters A site – Ribosome catalyzes peptide bond formation between amino acids in P and A sites (growing amino acid chain shifts to A site tRNA) – Ribosome shifts over – 1 st tRNA exits – new tRNA enters – Process repeats

3 Steps of Translation: Termination

3 Steps of Translation: Termination – Release factor binds to STOP codon – Completed polypeptide chain is cut from final tRNA – Ribosome dis-assembles

Control of Gene Expression In Prokaryotes: OPERONS with ON/OFF switches Operon – a chain of genes, all in a line, that all relate to one metabolic pathway

Operons can be inducible or repressible

Control of Gene Expression In Eukaryotes: Multiple Levels of Control

Control of Gene Expression In Eukaryotes: Multiple Levels of Control 1.Prevent or enhance transcription Histone acetylation – loosens the coiling of DNA to activate genes Transcription factors - help RNA polymerase bind to the promoter, activating genes DNA methylation - permanently inactivates genes

Control of Gene Expression In Eukaryotes: Multiple Levels of Control Histones – protein “beads” that DNA is wrapped around in eukaryotic (and archaean) chromosomes Chromatin – the DNA-protein combo that forms chromosomes

Control of Gene Expression In Eukaryotes: Multiple Levels of Control 1.Prevent or enhance transcription 2.Activate or inactivate RNA after transcription Some small RNA molecules don’t get translated but help regulate other RNAs miRNA (microRNA) siRNA (small interfering RNA) shRNA (short hairpin RNA) These can bind to mRNA strands to enhance or prevent translation. 2.Post-translational control: Modify or degrade proteins