Lac operon, homeobox genes and apoptosis Flashcards
What are transcription factors?
Species that control the transcription of a gene by either inhibiting action of RNA polymerase or promoting it.
What is an operon?
A length of DNA made up of structural genes and control sites.
What are structural genes?
Gene coding for a protein (by coding for its primary structure).
What are control sites?
Operator and control regions that control the expression of the structural genes.
What is the purpose of the Lac operon?
To control the metabolism of the sugar lactose in prokaryotic cells (especially in the presence of glucose).
What is the structure of the Lac operon?
Control sites:
- P (promotor) region: A length of DNA onto which RNA polymerase binds to and begins the transcription of the structural genes.
- O (operator) region: A length of DNA next to the structural genes responsible for controlling their expression (binding site of the lac repressor protein).
Structural genes:
- Z region: Contains the gene coding for the enzyme β-galactosidase, an enzyme that catalyses the hydrolysis of lactose to glucose and galactose.
- Y region: Contains the gene coding for the lactose permease, a transporter protein which fuses with plasma membrane and increases its permeability to lactose, increasing rate of lactose transport into cell cytoplasm.
I region (regulatory gene): Not part of the lac operon, but contains gene coding for the lac repressor protein, responsible for controlling the expression of the lac structural genes.
What are the sequence of events that occur when lactose is absent from the medium that surrounds the prokaryote?
- The regulatory gene is transcribed and translated, resulting in the production of the lac repressor protein.
- Lac repressor protein has 2 binding sites, 1 for lactose and the other for the operator region.
- Lac repressor protein binds to the operator region on the lac operon, covering part of the promotor region and preventing the binding of RNA polymerase onto operon.
- RNA polymerase cannot bind, so structural (Z and Y) genes cannot be transcribed.
- No mRNA for β-galactosidase and lactose permease produced, so no translation occurs at ribosome, and the 2 enzymes are not produced.
What are the sequence of events that occur when lactose is present from the medium that surrounds the prokaryote?
- A few molecules of lactose diffuse into cytoplasm of prokaryote from medium.
- Lactose bind to the allosteric site on the lac repressor region, causing conformational change in 3D structure of suppressor.
- This results in unbound repressor proteins no longer being able to bind to operator region, and already bound repressor proteins unbind (dissociate) from operator region.
- Promotor region fully exposed, allowing RNA polymerase to bind to promotor region and begin the transcription of the structural genes.
- mRNA produced by transcription then translated into the enzymes lactose permease and β-galacosidase at ribosome.
- Lactose permease fuse with plasma membrane and increases rate of transport of lactose into cytoplasm of prokaryote.
- β-galactosidase breaks down lactose into glucose and galactose, which can then be respired to by cell to produce ATP.
- Lactose acts as inducer of lac operon as its presence induces the expression of the structural genes coding for enzymes required to metabolise lactose.
What are homeobox genes?
Genes that control the development of the body plan of an organism, including the polarity (head and tail) as well as the positions of organs.
What features do homeobox genes control?
- Some genes determine the polarity of a developing embryo; that is, which end is the head (anterior), tail (posterior), back (dorsal) and front (ventral).
- Homeotic selector genes determine the identity of each segment of the developing embryo and determine which body segment they develop into. These are the master genes that control the expression of families of genes regulating the development of the thorax and abdomen, head and thorax.
- Segmentation genes specify the polarity of each developing segment.
What is the structure of homeobox genes?
- Each homeobox gene contains a sequence of 180 base pairs that code for a sequence of 60 amino acids on the polypeptide coded for by the gene.
- This region is called the homeobox sequence and codes for the homeodomain, the part of the polypeptide that binds to DNA and regulates the expression of genes further upstream associated with morphogenesis.
What is the mechanism of action of homeobox genes?
- Homeobox genes code for transcription factors that regulate the expression of other genes associated with morphogenesis by binding onto DNA.
- These genes in turn produce proteins that regulate other genes associated with a number of aspects of morphogenesis.
- Binding of the homeobox gene thus triggers a cascade of events resulting in the production of thousands of proteins associated with the development of the body plan.
Why are homeobox genes regarded as ‘highly conserved’?
Because these genes are very similar between different organisms (including plants, animals and fungi) so have been conserved throughout evolution. The body plans of these organisms develop in very similar ways.
What are Hox clusters?
Homeobox genes are always located on the chromosome in clusters called Hox clusters. Simple organisms like nematode worms have only 1 whereas complex vertebrates have 4.
What is the order of expression of homeobox genes during development?
Homeobox genes are always activated and expressed in the same order in both vertebrates and invertebrates; from anterior to posterior. This means that embryos from all organisms in these groups develop from head to tail.
