EUKARYOTIC GENE CONTROL
THE BIG QUESTIONS How are genes turned on and off? How do cells with the same DNA/ genes differentiate to perform completely different and specialized functions?
GENE EXPRESSION The control of gene expression takes place along a specific pathway. 1. Packing/unpacking DNA 2. Transcription 3. mrna processing 4. mrna transport 5. Translation 6. Protein processing 7. Protein degradation/mrna degradation
1. DNA PACKING The double stranded DNA needs to coil and fold to fit inside of the nucleus. Double stranded DNA DNA wraps around a histone protein forming a nucleosome Looped domains Chromosome
HISTONE/NUCLEOSOME Positively charged amino acids on the histone proteins bind to the negatively charged DNA.
LOOPED DOMAIN
CHROMOSOME
DNA PACKING CONTROLS TRANSCRIPTION Heterochromatin (H)-The DNA is tightly packed and will appear dark in an image. No transcription can take place so the genes in that region are turned off An example would be the insulin gene in brain or bone cells.
HETEROCHROMATIN
DNA METHYLATION Methylation of DNA in the promoter region blocks transcription factors so the genes are turned off Occurs when a methyl group (CH 3 ) is attached to a cytosine (C) This process in nearly permanent. ex. Inactivation of mammalian X chromosome=barr body
DNA PACKING CONTROLS TRANSCRIPTION Euchromatin (E)-The DNA is loosely packed and will appear light in an image. Transcription can take place so the genes in that region are turned on
EUCHROMATIN
LOOSEN THE DNA BY HISTONE ACETYLATION Attachment of an acetyl group (COOCH 3 ) to a lysine of the histone which will help unwind the DNA. The DNA will be loosely wrapped which will allow for transcription and the genes to be turned on.
2. TRANSCRIPTION INITIATION Control regions of the DNA Enhancer is a short region of DNA bound by regulatory proteins to increase the likelihood of transcription. Promoter is a nearby control sequence of DNA where transcription factors and RNA Polymerase can bind. This is called the transcription complex. The DNA will fold forming the transcription complex and the mrna can begin to be produced.
3. POST TRANSCRIPTIONAL CONTROL Removal of introns using spliceosomes. Add a 5 G cap and 3 poly A tail.
4. REGULATION OF MRNA DEGREGATION Life span of mrna determines the amount of protein synthesis. The mrna can last from hours to weeks.
DEATH OF MRNA Small interfering RNA (sirna) Short segments of RNA that bind to mrna and create double stranded mrna. Triggers the cell to degrade the mrna. The death tag for mrna. Causes gene silencing. The gene is turned off so no protein is produced.
ACTION OF SIRNA Hot Hot new topic in biology dicer enzyme mrna for translation sirna breakdown enzyme (RISC) double-stranded mirna + sirna mrna degraded functionally turns gene off
5. CONTROL OF TRANSLATION Block initiation of translation tag Regulatory proteins attach to the 5 end of mrna Prevents the attachment of ribosomal subunits and initiator trna Blocks translation of mrna to protein
6. PROTEIN PROCESSING AND DEGRADATION Protein processing Folding, cleaving, adding sugar groups and targeting for transport. Protein degradation Ubiquitin-Tags the protein for degradation by proteases Proteasomes-Responsible for the degradation of the protein.
UBIQUITIN Death Tag A 76 amino acid polypeptide that marks unwanted proteins with a label to be broken down by proteasomes.
PROTEASOMES Protein-degrading machine. Breaks down any protein into 7-9 amino acid fragments that can be recycled.
THE WHOLE STORY
6 7 protein processing & degradation Gene Regulation 1 & 2. transcription - DNA packing - transcription factors 1 2 initiation of transcription 5 initiation of translation 3 & 4. post-transcription - mrna processing 4 - splicing - 5 cap & poly-a tail mrna processing - breakdown by sirna 5. translation - block start of translation 6 & 7. post-translation - protein processing - protein degradation 3 mrna splicing 4 mrna protection