3. definition
is the DNA located in mitochondria, cellular
organelles within eukaryotic cells that convert
chemical energy from food into a form that cells
can use, adenosine triphosphate (ATP).
Mitochondrial DNA is only a small portion of the
DNA in a eukaryotic cell; most of the DNA can be
found in the cell nucleus and, in plants, in
thechloroplast.
4. In humans, mitochondrial DNA can be assessed as the
smallest chromosome coding for 37 genes and
containing approximately 16,600 base pairs. Human
mitochondrial DNA was the first significant part of
the human genome to be sequenced. In most species,
including humans, mtDNA is inherited solely from the
mother.
5. The DNA sequence of mtDNA has been determined from a large
number of organisms and individuals (including some organisms
that are extinct), and the comparison of those DNA sequences
represents a mainstay of phylogenetics, in that it allows
biologists to elucidate theevolutionary relationships among
species. It also permits an examination of the relatedness of
populations, and so has become important in anthropology and
field biology
6. Origin
Nuclear and mitochondrial DNA are thought to be of
separate evolutionary origin, with the mtDNA being derived from
the circular genomes of the bacteria that were engulfed by the early
ancestors of today's eukaryotic cells. This theory is called
the endosymbiotic theory. Each mitochondrion is estimated to
contain 2–10 mtDNA copies.[4] In the cells of extant organisms, the
vast majority of the proteins present in the mitochondria (numbering
approximately 1500 different types in mammals) are coded for
by nuclear DNA, but the genes for some of them, if not most, are
thought to have originally been of bacterial origin, having since been
transferred to the eukaryoticnucleus during evolution.
7. Mitochondrial inheritance
In most multicellular organisms, mtDNA is inherited from the mother (maternally
inherited). Mechanisms for this include simple dilution (an egg contains on
average 200,000 mtDNA molecules, whereas a healthy human sperm was
reported to contain on average 5 molecules[5]), degradation of sperm mtDNA in
the male genital tract, in the fertilized egg, and, at least in a few organisms, failure
of sperm mtDNA to enter the egg. Whatever the mechanism, this single parent
(uniparental) pattern of mtDNA inheritance is found in most animals, most plants
and in fungi as well.
8. Female inheritance
In sexual reproduction, mitochondria are normally inherited exclusively from the
mother; the mitochondria in mammalian sperm are usually destroyed by the egg
cell after fertilization. Also, most mitochondria are present at the base of the
sperm's tail, which is used for propelling the sperm cells; sometimes the tail is lost
during fertilization. In 1999 it was reported that paternal sperm mitochondria
(containing mtDNA) are marked with ubiquitin to select them for later destruction
inside the embryo.[6] Some in vitro fertilization techniques, particularly injecting a
sperm into an oocyte, may interfere with this.
9. The fact that mitochondrial DNA is maternally inherited
enables genealogical researchers to trace maternal lineage far back in time.
(Y-chromosomal DNA, paternally inherited, is used in an analogous way to
determine the patrilineal history.) This is accomplished on human
mitochondrial DNA by sequencing one or more of the hypervariable control
regions (HVR1 or HVR2) of the mitochondrial DNA, as with agenealogical DNA
test. HVR1 consists of about 440 base pairs. These 440 base pairs are then
compared to the control regions of other individuals (either specific people or
subjects in a database) to determine maternal lineage. Most often, the
comparison is made to the revised Cambridge Reference Sequence. Vilà et
al. have published studies tracing the matrilineal descent of domestic dogs to
wolves.[7]The concept of the Mitochondrial Eve is based on the same type of
analysis, attempting to discover the origin of humanity by tracking the lineage
back in time.
As mtDNA is not highly conserved and has a rapid mutation rate, it is useful
for studying the evolutionary relationships—phylogeny—of organisms.
Biologists can determine and then compare mtDNA sequences among
different species and use the comparisons to build anevolutionary tree for the
species examined.
10. Male inheritance
Doubly uniparental inheritance of mtDNA is
observed in bivalve mollusks. In those species,
females have only one type of mtDNA (F),
whereas males have F type mtDNA in their
somatic cells, but M type of mtDNA (which can be
as much as 30% divergent) in germline
cells.[8] Paternally inherited mitochondria have
additionally been reported in some insects such
as fruit flies,[9] honeybees,[10] andperiodical
cicadas
