3.10 Glossary

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Chromosomal Rearrangement: Any process altering the structure of the native chromosome. Such processes include: deletions, duplications, inversions, and translocations.

Continuum Model: A model by which a tumour suppressor gene might act. This model suggests that progression towards a cancer phenotype is dependent upon concentrations of the functional tumour suppressor element. At normal concentrations, the function is optimal, whereby cancer progression is halted. As concentrations drop, the cell's ability to halt cancer progression deteriorates; however, past a certain point, where concentrations of the element has dropped too much, the cell will not be able to survive due to the existence of fail-safe mechanisms, or the triggering of senescence or cell death pathways. Therefore, malignancy is optimized at some point between no expression of the element and normal expression of the element.

Competing endogenous RNA (ceRNA): ceRNAs competitively inhibit miRNAs by binding to their specific mRNA targets, thereby preventing miRNAs from carrying out their action. This results in gene expression at a post-transcriptional level.

DNA Base Excision Repair: Mechanism of removing helix-distorting regions from the genome and replacing with the proper bases. Occurs throughout the cell-cycle.

Fibroblast: Type of cell responsible for production of the extracellular matrix. Most common cell found in the connective tissue.

Gene conversion: An event which can occur during S phase, where DNA polymerase replicating a DNA strand may skip between homologous chromosomes, thus resulting in the possibility of an unequal segregation of alleles between daughter cells.

Haploinsufficiency Hypothesis: A model by which a tumour suppressor gene might act. This model suggests that the tumour suppressor gene activity is not effective in preventing malignancy even if one functional allele for the gene is present. Furthermore, having no functional allele for the gene may cause the cell to be even more incapable of preventing malignancy.

Knockdown Model: Use of molecular reagents to reduce the expression of a gene of interest in an organism or cell line.

Knockout Model: Genetically engineered organism or cell line with an inactivated gene of interest.

Loss of heterozygosity: A scenario where a cell existing in a person heterozygous for a functional and non-functional version of an allele loses function of the only working allele, thereby resulting in a cell which is homozygous for non-functional versions of a gene. This scenario applies to cancer genetics, as a person who is heterozygous for a tumour suppressor gene is at a relatively higher risk for developing cancers, as one or more somatic cells may lose function of the one working allele, thus predisposing it to cancer progression.

Mismatch Repair: Process of recognizing and removing improper base pairings and subsequent replacement of proper base pair.

MicroRNA (miRNA): Short RNA elements which repress gene expression post-transcriptionally by inducing mRNA degradation events catalyzed by an RNA-induced silencing complex (RISC) when binding mRNA targets. 

Mitotic recombination: An event where chromosomal crossover occurs during mitosis in a cell. This event can result in the unequal segregation of alleles between daughter cells.

Non-Homologous End Joining: Repair process for double stranded breaks in the DNA.

Nucleotide Excision Repair: Repair process for damaged DNA, such as UV induced thymine dimers. 

Obligate haploinsufficiency: A hypothetical point where relative malignancy is at its maximum. If tumour suppressor element concentration or expression is lowered further past this point, relative malignancy of the cell decreases because of fail-safe mechanisms leading to cell death or senescence.

One-hit Hypothesis: A model by which a tumour suppressor gene might act. This model suggests that gene activity is ineffective in preventing malignancy at the cellular level even when one functional copy of the allele is present. If both copies of the allele are non-functional, greater risk for malignancy is possible.

Quasi-sufficiency model: A model by which a tumour suppressor gene may act. This model states that function of the tumour suppressor gene is continuous and dependent upon the level of expression or concentration of the functional version of the tumour suppressing element. As expression or concentration of the element lowers, the function of the relative malignancy of the cell will increase.

Sister Chromatid Exchanges: The exchange of material from one chromatid to it's identical sister chromatid.

Two-Hit Hypothesis: A model by which a tumour suppressor gene might act. This model suggests that the tumour suppressor gene activity is effective in preventing malignancy when one or more functional alleles for the gene are present. Relative malignancy is high only when there is no functional copy of the gene present.