Homeobox genes and body plans
Homeobox genes = a fragment of DNA (about 180 base pairs, not counting the introns) found within genes. Homeoboxes code for proteins (Transcription factors) that typically switch on cascades of other genes.
- Maternal-effect genes = determine embryo polarity = position of the head (anterior) and the tail (posterior)
- Segmentation genes = specify the polarity of each segment
- Homeotic selector genes = identity of each segment and the direction of development for each individual body segment. Master genes = control regulatory genes. (Control of development of thorax and abdomen segments and development of head and thorax segments.)
Transcription factors = bind to genes further along the DNA, initiate transcription so they regulate the expression of other genes.
Regulatory proteins are transcription factors.
- RNA polymerase uses the regulatory protein to start coding
- Hormones can switch on transcription factors = Cyclic AMP activates the transcription factor
- Some hormones diffuse directly into a cell, others need receptor sites.
Prokaryote = Repressor protein = turns off gene
Eukaryote = Transcription factors = turns on gene
Introns = non-coding part of the gene.
Hox genes = particular group of Homeobox genes. They are responsible for (limb) development.