genetic basis of development
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Genetic Basis of Development
Eukaryotic Genome Organization
Prokaryotic cell vs Eukaryotic cell
In eukaryotic cells, most DNA is nuclear & organized as follows: dsDNA (helix) is wrapped
around histone proteins which coil together to form nucleosomes
Heterochromatin – sections of DNA where nucleosomes are tightly packed together; little gene expression
Euchromatin – sections of DNA where nucleosomes are more loosely packed; high gene expression
Transcription Factors Within eukaryotes, genes are expressed in a
tissue-specific manner through transcription factors Activators - bind DNA and cause protein-mediated
bending of DNA → transcription occurs Repressors – bind DNA and prevent transcription COMBINATION EXPRESSED AT A PARTICULAR
TIME DETERMINES HOW MUCH PROTEIN IS PRODUCED
Prok vs. Euk regulation of gene expression
Prokaryotes (i.e. bacteria) use operons to regulate gene expression
Eukaryotes use transcription factors Both use regulatory sequences
In prok, the regulatory gene makes repressor In euk, the regulatory sequences are bound by various
transcription factors
Cell Differentiation Cell decides to become a particular type of cell
Cell expresses a particular set of genes that forces it to develop into a particular cell
These tissue-specific genes are located on euchromatin (expressed/loosely wound portion of DNA) within the given cell
So how does a cell decide?
Cells of zygote undergo rounds of mitosis to form stem cells (cells that are not yet differentiated and have the potential to develop into any type of cell)
Based on their location in developing zygote, stem cells produce particular proteins (i.e. transcription factors) which tell the surrounding cells what to become
Transcription Factor “Cascade”
Transcription factor binds with DNA inside nucleus Turns on the expression of particular genes Results in expression of proteins that “cascade” or
spread to neighboring cells Causes those cells in the area to develop into a specific
tissue Embryonic induction – cell will influence (induce)
surrounding cells into developing into particular tissues/organs
Example: SRY gene
Y chromosome contains SRY (sex determining region on Y chromosome) gene
In males, SRY gene produces protein causing differential development of sex organs etc
Example: Cave Fish
“eyeless cave fish” given lens of normal fish → cave fish develops eye sight
Eyeless cave fish lens transplanted into normal fish → nothing happens
Thus there is some factor inside the normal lens that causes it to “induce” surrounding cells to develop into vision-capable cells
Apoptosis During Development
Apoptosis – programmed cell death; important during embryonic development
Example: Tissues between fingers and toes dies off in utero
(called morphogenesis)
microRNA Single stranded RNA (ssRNA) found in all
organisms MicroRNA functions to disrupt mRNA and prevent
it from making proteins Example:
Drosophila mutants without microRNA: the grim, hid, and reaper genes (responsible for apoptosis) are overexpressed → significant uptick in apoptosis and embroyic death
Thus microRNA functions to repress the apoptotic genes in particular tissues
Homeotic Genes Genes found in many organisms that determine
where body parts will go in the organism Ex: HOX genes in drosophila
Mutation in ubx – two thoraxes develop Mutation in antp – leg grow from head
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