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AP Bio: Thursday, 2.9.12Gene Expression
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
Gene Expression
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
Transcription
RNA Processing:
RNA Processing:
• 5’ Cap• Poly-A tail• Introns cut out and exons spliced together
Signals mRNA’s destination (cytoplasm)Protects from degrading enzymesHelps ribosomes attach
Non-coding sequences interspersed with coding sequences that get translated into proteins
Translation
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:
AP Bio: Tuesday, 2.14.12Gene Expression
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– 1st 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 ExpressionIn Prokaryotes: OPERONS with ON/OFF switchesOperon – a chain of genes, all in a line, that all relate to one metabolic pathway
Operons can be inducible or repressible
Control of Gene ExpressionIn Eukaryotes: Multiple Levels of Control
Control of Gene ExpressionIn Eukaryotes: Multiple Levels of Control1. 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 ExpressionIn 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 ExpressionIn Eukaryotes: Multiple Levels of Control1. Prevent or enhance transcription
2. Activate or inactivate RNA after transcriptionSome 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.
3. Post-translational control: Modify or degrade proteins