Exam 2: Topic 7 Flashcards
Explain the mechanisms by which UV radiation and X-ray radiation generates mutations.
- UV Radiation:
Induces thymine dimers between adjacent thymine bases.
Can stall replication and transcription and introduce frameshift mutations or point mutations. - X-ray Radiation:
Low exposure: causes DNA base pairs to ionize
High exposure: shatters DNA
Describe the use of the enzyme photolyase in repairing DNA damage from UV light. Why is photolyase activated by light.
- Photolyase recognizes the distortion in the DNA helix caused by the thymine dimer and binds to the dimer.
- Then, in the presence of visible light, the photolyase enzyme changes conformation and breaks apart the thymine dimer, allowing the thymines to again correctly base pair with the adenines on the complementary strand.
Understand the purpose of the Ames test.
Ames test: a test method to determine if a substance is a mutagen; DOES NOT determine carcinogenicity.
Describe the Ames test in detail. Be sure to explain the role of histidine (note: this is NOT histamine), Salmonella, and “back mutations”.
- Start with HIS- Salmonella.
2. HIS- Salmonella:
§ Cannot synthesize histidine.
§ Requires histidine in the media
3. Goal induce a back mutation in HIS gene so that the original (wild-type) sequence is regained: HIS- —-> HIS+
○ The Ames test uses the test organism a strain of Salmonella typhimurium that is a histidine auxotroph, unable to synthesize its own histidine because of a mutation in an essential gene required for its synthesis. After exposure to a potential mutagen, these bacteria are plated onto a medium lacking histidine, and the number of mutants regaining the ability to synthesize histidine is recorded and compared with the number of such mutants that arise in the absence of the potential mutagen.
○ Chemicals that are more mutagenic will bring about more mutants with restored histidine synthesis in the Ames test.
Are all mutagens also carcinogens? Explain your answer
○ Nearly all carcinogens are mutagenic, not all mutagens are necessarily carcinogens.
○ Mutagens can be caused by various types of chemical agents or radiation, but may not necessarily cause cancer. These mutagens can have some other impact on genes and the expression of protein that may not result in cancer.
Define gene expression.
○ Gene expression refers to the process by which the information encoded within a gene is used to produce a functional product, typically a protein, by going through the steps of transcription (DNA to RNA) and translation (RNA to protein), ultimately affecting an organism’s observable traits (phenotype).
○ Gene expression is controlled at the transcriptional level (i.e. by whether or not RNA polymerase synthesizes mRNA).
Describe eukaryotic gene expression with respect to (1) arrangement of genes involved in a metabolic pathway and (2) how their expression is controlled (e.g. one operator, many transcription factors).
Eukaryotes: Transcription factors are the cell signaling molecules that dictate the specific gene being expressed.
Genes involved in the same pathway may be located on different chromosomes but can still be regulated together through shared transcription factor binding sites in their promoter regions
Describe prokaryotic gene expression with respect to (1) arrangement of genes involved in a metabolic pathway and (2) how their expression is controlled (e.g. one operator, many transcription factors)
Arrange their genes into operons.
○ Access to genes in an operon is determined by the operator: specifically, if there is a blockade preventing RNA polymerase from transcribing.
What does “turning on” a gene mean? What does “turning off” a gene mean? Is control over gene expression exerted on the transcription or translation level?
- Gene expression is controlled at the transcriptional level (i.e. by whether or not RNA polymerase synthesizes mRNA).
- When genes are turned on, it means that the information encoded in a gene is being expressed, or turned into a function.
- When a gene is turned off, it stops providing instructions for making proteins. This means that the proteins that are needed to perform a specific function are not produced.
Explain the concept of an operon.
○ Prokaryotes arrange their genes into operons.
○ Access to genes in an operon is determined by the operator: specifically, if there is a blockade preventing RNA polymerase from transcribing.
Distinguish between an inducible operon and a repressible operon. Which type is more common.
- Inducible operon:
§ Default position is off (not making any mRNA–no gene expression).
§ Boulder in place by default.
§ An inducible operon is a gene system that increases its expression in response to an inducer, enhancer, or positive regulator. Inducible operons are often involved in bacterial metabolic pathways and are turned on by a metabolite that regulates the operon - Repressible operon (more common):
§ Default position is on; more common for products the cells needs often
§ A repressible operon is a cluster of genes that can be turned off by a repressor protein in response to environmental conditions. Repressible operons are usually on, but can be repressed by the presence of a co-repressor, which is often the end product of a metabolic pathway.
Briefly diagram the lac operon. Is the lac operon inducible or repressible?
○ The lac operon is an inducible operon.
○ For the lac operon to be expressed, lactose must be present. This makes sense for the cell because it would be energetically wasteful to create the enzymes to process lactose if lactose was not available.
○ In the absence of lactose, the lac repressor is bound to the operator region of the lac operon, physically preventing RNA polymerase from transcribing the structural genes. However, when lactose is present, the lactose inside the cell is converted to allolactose. Allolactose serves as an inducer molecule, binding to the repressor and changing its shape so that it is no longer able to bind to the operator DNA. Removal of the repressor in the presence of lactose allows RNA polymerase to move through the operator region and begin transcription of the lac structural genes.
Contrast the general prokaryotic and eukaryotic mechanisms of gene transfer.
- Vertical gene transfer :
§ Genetic information transferred from parent to offspring.
§ the transmission of this information from mother to daughter cells is called vertical gene transfer and it occurs through the process of DNA replication. DNA is replicated when a cell makes a duplicate copy of its DNA, then the cell divides, resulting in the correct distribution o fone DNA copy to each resulting cell. - Lateral gene transfer:
□ Transformation:Occurs with plasmid or parts of a chromosomal DNA.
Uptake of naked DNA from environment.
Occurs with competent cells (capable of transformation), cells must be under stress, and a high density of cells so it is more likely to encounter DNA.
□ Conjugation: Direct and purpose, always one-way from donor to recipient.
Donor makes conjugation pilus, as a recipient cannot.
Often the gene to synthesize a conjugation pilus is on a plasmid (F-factor).
□ Transduction: a bacteriophage (virus that infects bacteria) accidentally brings in DNA from its prior host while trying to infect a new bacterium.
What does recombinant mean?
A molecule of DNA that contains fragments of DNA from different organisms is called recombinant DNA.
Describe Fredrick Griffith’s experiments that first demonstrated “transformation”.
○ In his famous series of experiments, Griffith used two strains of S. pneumoniae. The S strain is pathogenic and causes death.Mice injected with the nonpathogenic R strain or the heat-killed S strain survive. However, a combination of the heat-killed S strain and thelive R strain causes the mice to die. The S strain recovered from the dead mouse showed that something had passed from the heat-killed Sstrain to the R strain, transforming the R strain into an S strain in the process.
