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