Mitochondrial Genetics and Forensic DNA Analysis Flashcards
1
Q
Mitochondrial DNA: The Forgotten Genomes
A
- since the nuclear chromosomal DNA is so prominent these days, it is often easy to forget there is a second set of genetic information located in cells
- Mitochondria are organelles located in the cytoplasm of the cell and contain their own DNA and are thus another source of DNA
2
Q
Mitochondrial Diseases
A
- mutations of ox/phosp
- most serious in CNS and muscle
- commonly include: neuropathies, encephalopathies, myopathies
- the effects of mitochondrial mutations can occur in a broad range of tissues and organs, but they usually are related to tissues in which there is a high degree of ox/pho activity.
3
Q
Mitochondria
A
- full function linked to nuclear genes
- defects of mitochondrial function may show autosomal or X-linked pattern of inheritance
- mitochondrial function is dependent on nuclear genes
- some processes are governed by mitochondrial genes and others by nuclear genes
- therefore defects in mitochondrial function could be due to autosomal or X-linked mutations of the nuclear genome, or mutations in the mitochondrial DNA itself
4
Q
Matrilineal Inheritance
A
- mitochondria transmitted in egg cytoplasm
- few if any mitochondria present in the pronucleus of the sperm
5
Q
Homoplasy
A
- homozygosity for one or more cytoplasmic gene
- usually refers to a population of mitochondrial that all have the same genetic composition
6
Q
Heteroplasmy
A
- when there are two or more different populations of mitochondria are present in a cell.
- heteroplasmy in the cytoplasm in analogous heterozygosity in the nucleus
- if the mother is heteroplasmic, the rules of inheritance change slightly
- the mother may be unaffected and yet have affected children. This can occur if the mother has a low frequency of mutant cells but passes higher frequencies on to her children who then express the disease
- if the mother is heteroplasmic and affected, she may pass a low frequency of mutant mitochondria to her children who would be unaffected
- thus, within families, there may be a difference in the expression of the same disease
- in general, the more mutant mitochondria, the more severe the disease
- in order for any given cell to express dysfunction, there must be a high proportion of mutant mitochondria present in that cell (85%)
- for the disease to be manifested in an individual, there must be a large number of such dysfunctional mitochondria throughout the body or within a target organ or tissue
7
Q
Mitochondria disorders progressive
A
- mitochondrial disorders often are progressive with late onset
- this is due to an increase, over time, in the numbers of mutations per cell and the numbers of mutant cells
- one mechanism that explains this is replicative segregation. As cells divide, the relative proportions of mutant mitochondria may change over time
- the frequency of mutation within a clone can increase or decrease. If an individual had a low, non-pathogenic frequency initially, it is possible that as he/she gets older, the disease may manifest due to the growing population of defective mitochondria that did not previously exist
- because there is high mutation rate in mitochondrial DNA, it is possible for a mutation to occur as a new event (acquired mutation) and then proliferate in the population, again generating an subpopulation of mutant mitochondria. This type of mutation is analogous to acquired nuclear mutations associated with cancer. Because these mutations occur in the somatic cells, they would not be transmitted to progeny
- these mechanisms also predict that under certain conditions, mutations may be lost from the cell population
8
Q
Conclusion of Mitochondrial Stuff
A
- complex group of diseases
- diagnosis complicated by: heteroplasmy and variable expression, maternal inheritance, progressive nature of disease
- include possibility of mitochondrial disorder in the differential diagnosis of a patient with unexplained neurological defects
9
Q
Forensic DNA Analysis
A
- Use of DNA technologies (PCR, RFLP, sequencing) to obtain information on the genetic identity of an individual and how that relates to a criminal, medical, or scientific investigation
- popularized by TV
- originally brought to the attention of the public with the OJ Simpson trial- 1995
- currently accepted in many disciplines
10
Q
DNA Analysis
A
- sequence variability with a high degree of polymorphism
- nuclear DNA: hypervariable minisatellite regions, DNA fingerprinting
- both nuclear and mitochondrial DNA are used in forensics
- key features include the ability to uniquely identify the DNA
- to do this, you want to examine regions of the DNA that have the highest degree of polymorphism
- in the nuclear DNA, it has been shown that the hypervariable ministatellite regions have the most variability and these are the regions that have been utilized in DNA fingerprinting
11
Q
Sources of Error
A
- quality of specimen
- poor sample collection
- mislabeling or handling error
- poor sample preparation
- degradation
- contamination
- statistical analysis and interpretation
- analyze a sufficient number of polymorphisms with a high degree of variability
- consider that allele frequencies vary between populations
- consider differences in allele frequencies in different racial and ethnic groups
12
Q
Guidelines
A
- standards for collecting and preserving specimens- limit possible contamination
- chain of custody for specimens
- standardization of techniques for handling and processing samples
- accreditation of laboratories
- relevance of technology must be established
13
Q
Mitochondrial DNA Analysis
A
- human identity:
- maternal inheritance
- siblings carry the same mitochondrial DNA
- link individuals by comparing maternal mitochondrial lineages
- family identity- not individual identity
14
Q
Tomb of Unknown Soldier
A
- bones placed in Tomb of Unknown Soldier were known to be from one of 3 different individuals
- testing of mitochondrial DNA from the bones and maternally related individuals from all 3 families provided a positive match to one family. The bones were removed and buried by the family because now the identity was known
- these types of tests are excellent at matching family members but are not specific enough to identify of a particular individual. Any testing that is done must be confirmed using at least 2 different known individuals from the family- all maternally related
15
Q
Identification
A
- forensic testing of nuclear DNA will allow unique identification of individuals
- military
- mass disaster
- criminal cases
- medical cases
