Final Flashcards
Explain the role of caspases in cell death
They remove small parts of other proteins by cleaving peptide bonds.
Through this enzymatic trimming, the target proteins are inactivated.
The caspases attack different kinds of proteins, including the lamins, which make
up the inner lining of the nuclear envelope, and several components of the cytoskeleton.
The cells then lose their integrity; their chromatin becomes fragmented, blebs of cytoplasm form at their surfaces, and they begin to shrink.
They are then engulfed by phagocytes, which are scavenger cells of the immune system, and are then destroyed
Explain the observations that led to people believing that cancer was due to genetic malfunctions
- The cancerous state is clonally inherited.
ex. When cancer cells are grown in culture, their descendants are all cancerous. The cancerous condition is therefore transmitted from each cell to its daughters at the time of division—a phenomenon indicating that cancer has a genetic (or epigenetic) basis.
- Certain types of viruses can induce the formation of tumors in experimental animals.
ex. The induction of cancer by viruses implies that the proteins encoded by viral genes are involved in the production of the cancerous state.
- Cancer can be induced by agents capable of causing mutations.
ex. Mutagenic chemicals and ionizing radiation had been shown to induce tumors in experimental animals.
- Certain types of cancer tend to run in
families.
ex. Susceptibility to retinoblastoma, a rare cancer of the eye, and colon cancer appeared to be inherited as simple dominant conditions, albeit with incomplete penetrance and variable expressivity. (All cancers might have their basis in genetic defects—either inherited mutations or somatic mutations acquired during a person’s lifetime.)
- Certain types of white blood cell cancers (leukemias and lymphomas) are associated with particular chromosomal aberrations.
Cancer researchers have identified two
broad classes of genes that, when mutated, can contribute to the development of a
cancerous state. What are they?
- Oncogenes
-mutant genes actively promote cell division - Tumor Suppressor Genes
- mutant genes actively fail to repress cell division
Each type of viral oncogene appears to encode a protein that could theoretically play a role in regulating the expression of cellular genes, including those involved in the processes of growth and division. What are the functions?
- Some of these proteins may act as signals to stimulate certain types of cellular activity
- Some may act as receptors to pick up these signals or as intracellular agents to convey them from the plasma membrane to the nucleus
- Viral oncogene proteins may act as transcription factors to stimulate gene expression.
What are the cellular homologues of v-oncs called?
Proto-oncogenes
or
Cellular oncogenes, c-onc
Why do c-oncs have introns whereas v-oncs do not?
v-oncs were derived from c-oncs by the insertion of a fully processed c-onc mRNA
into the genome of a retrovirus.
A virion that packaged such a recombinant molecule would then be able to transduce the c-onc gene whenever it infected another cell.
During infection, the recombinant RNA would be reverse-transcribed into DNA and then
integrated into the cell’s chromosomes
Why do v-oncs induce tumors, whereas normal c-oncs do not?
Give and example with chickens
Viral oncogene produces much more protein than its cellular counterpart, perhaps because it has been transcriptionally activated by enhancers embedded in the viral genome.
ex. The v-src gene produces 100 times
as much tyrosine kinase as the c-src gene. This oversupply of the kinase upsets the delicate signaling mechanisms that control cell division, causing unregulated growth.
Other v-onc genes may induce tumors by expressing their proteins at inappropriate times, or by expressing mutant forms of proteins.
Explain the Transfection Test
- DNA was extracted from the cancerous tissue and fragmented into small pieces
- Each of these pieces was joined to a segment of bacterial DNA, which served as a molecular marker.
- The marked DNA fragments were then transfected, into cells growing in culture to determine if any of them could transform the cells into a cancerous state.
4.This state could be recognized by the cancer cells forming in small clumps, or foci, when grown on soft agar plates.
- The DNA from such cells was extracted and screened to see if it carried the molecular marker. If it did, this DNA was retested for its ability to induce the cancerous state.
Explain what was found with the transfection test in bladder cancer cells.
The fragment carried an allele of the c-H-ras oncogene, a homologue of an oncogene in the rat sarcoma virus.
DNA sequence analysis showed that a nt in codon 12 of this allele had been mutated, with a substitution of a valine for the glycine.
Unlike viral oncogenes, the mutant c-H-ras gene does not synthesize abnormally large amounts of protein.
Instead, the valine-for-glycine substitution
at position 12 impairs the ability of the mutant c-H-ras protein to hydrolyze one of its substrates, GTP.
Because of this impairment, the mutant
protein is kept in an active signaling mode, transmitting information that ultimately
stimulates the cells to divide in an uncontrolled way.
What is different about mutated c-ras oncgenes?
In all cases, the mutations involve amino acid changes in 12, 59, or 61.
Each of these amino acid changes impairs the ability of the mutant Ras protein to switch out of its active signaling mode. Therefore stimulating cells to grow and divide.
In these types of cancer, only one of the two copies of the c-ras gene has been mutated. The single mutant allele is dominant in its ability to bring about the cancerous state.
Mutations in c-ras and other cellular oncogenes that lead to cancer in this way are therefore dominant activators of uncontrolled cell growth.
What causes the predisposition of tumor suppressor gene cancers?
A predisposition to develop the cancer follows a dominant pattern of inheritance.
This predisposition is due to heterozygosity for an inherited loss of-function mutation in the tumor suppressor gene.
What is necessary for cancer to develop?
A cancer develops only if a second mutation occurs in the somatic cells and if this mutation knocks out the function of the wild-type allele of the tumor suppressor gene.
Thus, development of the cancer requires two loss-of-function mutations—that is, two inactivating “hits,” one in each of the two copies of the tumor suppressor gene.
What were Knudsons findings and what was his hypothesis?
He proposed an explanation of retinoblastoma.
His pedigree analysis indicated that:
-40 percent of the cases = inherited mutation -60 percent of the cases = sporadic
(cannot be traced to a specific inherited mutation.)
Knudson proposed that both the inherited and sporadic cases of retinoblastoma occur because the two copies of a particular gene have been inactivated.
Inherited cases: one of the inactivating mutations has been transmitted through the germ line, and the other occurs during the development of the somatic tissues of the eye.
Sporadic cases: both of the inactivating mutations occur during eye development.
Thus, in either type of retinoblastoma, two mutational “hits” are required to knock out a gene that normally functions to suppress tumor formation in the eye.
Explain the 3 findings that proved Knudsons hypothesis
- Some were due to small deletions in the long arm of chromosome 13. The gene that normally prevents retinoblastoma—symbolized RB—must therefore be located in the region defined by this deletion
- Positional cloning techniques were used to isolate a candidate RB gene. Once isolated, the gene’s structure, sequence, and expression patterns were determined.
- The structure of the candidate gene was examined in cells taken from tumorous eye tissue.
- Cell culture experiments demonstrated that a cDNA from the wild-type allele of the candidate gene could revert the cancerous properties of cultured tumor cells.
What is the percentage of hereditary cancers?
How many inherited cancer syndromes are there? What defect causes them?
Only about 1 percent of all cancers are hereditary.
More than 20 different inherited cancer syndromes have been identified, and in nearly all of them the underlying defect is in a tumor suppressor gene rather than in an oncogene
What is the main function of the pRB tumor suppressor protein?
What has happened when cancer forms?
pRB joins with E2F (a TF) which stops it from binding to specific enhancer sequences in their target genes.
(The cell-cycle factors encoded by these genes are not produced, and the machinery for DNA synthesis and cell division remains quiet)
In cancer, both copies of the RB gene
have been inactivated and the inability of pRB to bind to these transcription factors. It allows E2F free to activate their target genes.
In the absence of this brake (one fail), cells have a tendency to move through their cycle quickly.
If other cell-cycle brakes fail, the cells divide ceaselessly to form tumors.
What are the simplest bacterial transposons
insertion sequences
What are the three main types of bacterial transposons
the insertion sequences, or IS elements
the composite transposons
Tn3-like elements
