Mitochondria
Mitochondria are present in nearly all eucaryotic cells-in plant , animals, and
most eucaryotic microorgnisms and most of a cell's ATP is produced in these
organelles.
In mitochondria, the metabolism of sugars is completed, and the energy
released is harnessed so efficiently that about 30 molecules of ATP are
produced for each molecule of glucose oxidized.
• Mitochondria are generally similar in size and
shape to bacteria, although these attributes
can vary depending on the cell type.
• DNA and RNA, and a complete transcription and translation system including ribosomes.
• Mitochondria as remarkably mobile organelles, constantly changing shape and position.
• Organelles can form long, moving chains in association with the microtubules of the
cytoskeleton.
• In other cells, they remain fixed in one location to target ATP directly to a site of unusually
high ATP consumption.
• The number of mitochondria present in different cell types varies dramatically, and can charge
with the energy needs of the cell.
A mitochondrion is organized into four separate compartments
Citric Acid Cycle generates high-energy electrons.
Nearly all the energy available from burning
carbohydrates, fats, and other foodstuffs in the
earlier stages of their oxidation is initially saved in
the form of the activated carrier molecules generated
during glycolysis and the citric acid cycle-NADH and FADH2 .
Carrier molecules donate their high-energy
electrons to the electron-transport chain in the mitochondria
membrane, and thus become oxidized to NAD+ and FAD.
The electrons are quickly passed along the chain to molecular oxygen (O2) to form water(H2O).
Passage of the high-energy electrons along the electron-transport chain releases energy that
is harnessed to pump protons across the inner mitochondrial membrane.
Resulting proton gradient in turn drives the synthesis of ATP.
Inner mitochondrial membrane thus serves as a device that converts the energy
contained in the high-energy electrons of NADH into the high-energy phosphate
bond of ATP.
This chemiosmotic mechanism of ATP synthesis is called Oxidative Phosphorylation,
because it involves both the consumption of O2 and the addition of a phosphate
group to ADP to form APT
Mitochondrion DNA
About 1,000 mitochondria in each cell, and about 10 mitochondria
DNA in each mitochondria.
Human mitochondrial DNA (mtDNA) is a circular
double-stranded molecule, 16,569 base pairs (bp) in length that
codes for 13 subunits of the oxidative phosphorylation
system, 2 ribosomal RNAs (rRNAs), and 22 transfer RNAs (tRNAs).
It is present in hundreds to thousands of copies in each cell,
not within the nucleus, but within the cell’s energy-generating
organelles, the mitochondria.
mtDNA consists predominantly of coding DNA, with the Exception of
a ∼1100-bp long fragment that has mainly regulatory functions and is therefore termed the
control region.
Since the first in-depth study of human mtDNA variation 25 years ago, it has become widely
used for studies of human evolution, migration, and population.
This widespread use is due to unique features of mtDNA that make it particularly amenable to
evolutionary studies. These features include a high copy number, maternal inheritance, lack of
recombination, and a generally higher mutation rate than found in nuclear DNA.
Reactive Oxygen species (ROS)
In the mitochondrial respiratory chain,
Complex IV (cytochrome oxidase) retains all
partially reduced intermediates until full
reduction is achieved.
Other redox centres in the electron transport
chain, however, may leak electrons to oxygen,
partially reducing this molecule to superoxide anion (O2−•).
Even though O2−• is not a strong oxidant, it is a precursor of most other reactive oxygen
species, and it also becomes involved in the propagation of oxidative chain reactions.
Despite the presence of various antioxidant defences, the mitochondrion appears to be the
main intracellular source of these oxidants.
In aerobic organisms the energy needed to fuel biological functions is produced in the
mitochondria via the electron transport chain.
In addition to energy, reactive oxygen species (ROS) with the potential to cause cellular
damage are produced.
ROS can damage DNA, RNA, and proteins, which, in theory, contributes to the physiology of
ageing.
Mitochondrial Haplogroups
Mitochondrial DNA (mtDNA) variation has recently been suggested to have an association
with various cancers.
Human mitochondria DNA is passed down generations only through females.
Haplogroups represent related groups of sequences that are defined by shared mutations
and which tend to show regional specificity.
mtDNA is haploid and lacks recombination, specific mutations in the mtDNA genome
associated with human diseases arise and remain in particular genetic backgrounds referred
to as haplogroups.
mtDNA haplogroup
A, B, C, CZ, D, E, F, G, H, HV, I, J, pre-JT, JT, K, L, L0, L1, L2, L3, L4, L5, L6, M, N, P, Q, R, R0, S, T, U, V, W, X, Y, Z
n = 37