Gene Expression and Heritable Phenotype CBS520 Eric Nabity
DNA is Just the Beginning DNA was determined to be the genetic material, and the structure was identified as a (double stranded) double helix. This is a start, but doesn t tell much about how this information is translated into protein, much less a phenotype.
Relating the Problem to High Order Question: By what mechanisms does a genotype yield a phenotype in a multicellular organism with many types of cells? First Order Question: How is the information in DNA translated to protein, the biologically active building blocks of the cell?
First Order Answer Gene Transcription: ssrna Multiple Stage Process DNA + RNA polymerase --> mrna DNA is transcribed into mrna by RNA polymerase, often at or near a CpG island. mrna + trna bonded to Amino Acids --> Protein trna bonds to a particular Amino Acid (or none at all) and presents a three base anticodon region that binds to three base codon regions of the mrna.
Transcription of DNA to mrna RNA polymerase binds to DNA, mostly near TATA promoters, but not always. A pocket is formed in the DNA and an RNA strand is formed against the DNA template strand. Termination is not well understood in eukaryotes. It occurs in G-C rich regions in prokaryotes. http://porpax.bio.miami.edu/~cmallery/150/gene/c7.17.7b.transcription.jpg
mrna Transport mrna transports protein information through the nuclear membrane (if present). Facilitates delivery of information to the part of the cell where the protein is needed. http://www.answers.com/topic/mrna-interaction-png-1
trna Small size RNA ~ 75-95 base pairs Acceptor stem binds to Amino Acids Anticodon loop matches up with mrna in the Ribosome during translation. http://molbioandbiotech.files.wordpress.com/2007/09/trna1.gif
Ribosome Two Subunits 50S (large): binds trna 30S (small): binds mrna http://www.mun.ca/biology/desmid/brian/biol2060/biol2060-22/2202.jpg Three 50S Subunit binding sites A site: binds the next trna P site: binds the trna bound to the growing polypeptide chain. E site: discharges the last trna
Translation http://www.mun.ca/biology/desmid/brian/biol2060/biol2060-22/2202.jpg The trna anticodon matches up to a complementary mrna codon at the P site. An individual protein may require more than one mrna.
Specific Higher Order Questions What mechanism allows a progenitor cell to set a template of gene expression for child cells? What mechanism allows the environment of a cell to affect gene expression? Are there other factors that affect gene expression, and are they heritable?
Higher Order Regulation Must impact first order gene expression Must be selective Points of Vulnerability Transcription mrna transport
Some Methods of Regulating Transcription DNA transcription silencing DNA methylation Chromatin remodeling DNA transcription enhancement homeotic genes and homeodomain proteins
DNA Methylation Methylation is the addition of a methyl (CH3) group. In DNA methylation mostly occurs at CpGs, making them unresponsive to RNA polymerase and reducing transcription.
Chromatin Remodeling DNA wraps around histone proteins. This macro shape impacts the ability of the DNA to be transcribed. Changes in the histone structure impact if a particular gene is expressed. http://www.actrec.gov.in/pi-webpages/sanjaygupta/1chrmatin.jpg
Homeodomain Proteins A homeodomain is a region of a protein that can bind to either DNA or RNA Usually binds to promoter regions, increasing gene expression. Expressed by Homeotic genes ie. Hox genes establish anterio-posterior axis
Some Methods of Regulating mrna Transport mrna Interference micro RNA (mirna) double stranded RNA (dsrna) Repressed RNA mrna binding protein
mrna Interference mirna mirnas are single stranded, short, ~20 nucleotides long. They are bonded by argonaute proteins which catalyze the breakdown any mrna that is complementary to the mirna. mirna can be produced from transcription, in which case translation to protein doesn t occur and is instead cleaved by the endonuclease Dicer.
mrna Interference dsrna dsrna are naturally introduced by a viral invasion of the cell. The conversion of dsrna to mirna is thought to be a defense mechanism against viral infection. dsrna are also cleaved into mirna by Dicer.
Repressed RNA mrna binding protein links mrna to cellular machinery that guides it to the post synaptic density in the dendritic spine. http://www.nature.com/nrn/journal/v8/n10/images/nrn2150-f1.jpg
Non-Genetic Inheritance If gene expression in the form of protein synthesis can be passed on by means other than DNA, then those means are important to understanding cell function.
Non-Genetic Inheritance Methylation is conserved during mitosis. Allows for the specific gene expression of a parent cell to be passed on to a child cell, preserving any cell differentiation. http://www.web-books.com/mobio/free/images/ch7f2.gif
Non-Genetic Inheritance dsrna are replicated in a way similar to antibodies by proteins such as MAintenance of Killer 11 (MAK11). This prevents it from being diluted through repeated mitosis. mirna are capable of being passed between cells through gap junctions.
Higher order mechanisms impact on Gene Expression The influence of higher order mechanisms on gene expression is dependent upon the mechanics of first order transcription and translation. These higher order influences create the possibility that there are non-genetic heritable factors which influence phenotype.
Questions?