Extra-nuclear Genome- Mitchondrial DNA

1. Extra-nuclear Genome
Mitochondrial DNA
Vaishali S.Patil
Assosiate Professor, Department of Botany
Shri Shivaji College of Arts, Commerce & Science
Akola

2. Introduction
• Mitochondrial DNA(mtDNAor mDNA) is the DNAlocated in mitochondria,
cellularorganelleswithineukaryoticcells.
• It convert chemical energy from food into energy in a form
usable to the host cells. Adenosine triphosphate (ATP) is an
energy that every cell in our body can use.
• Mitochondrial DNA is only a small portion of the DNA in a
eukaryotic cell.
• Human mitochondrial DNA was the first significant part of the human
genome to be sequenced. This sequencing revealed that the human
mtDNA includes 16,569 base pairs and encodes 13 proteins.
• Animal mtDNA evolves faster than nuclear genetic markers.
• This DNA is small and circular. Mitochondrial DNA, unlike nuclear
DNA, is inherited from the mother, while nuclear DNA is
inherited from both parents.

3. • mtDNA being derived from the circular genomes of
the bacteria that were engulfed by the early ancestors of today's
eukaryotic cells.
•There are six main genome types found in mitochondrial genomes,
classified by their structure (e.g. circular versus linear), size, presence
of introns or plasmid like structures, and whether the genetic material is a
singular molecule or collection.
•In many unicellular organisms (e.g., the ciliate Tetrahymena and
the green alga Chlamydomonas reinhardtii), and in rare cases also in
multicellular organisms (e.g. in some species of Cnidaria), the mtDNA is
found as linearly organized DNA.
•Most of these linear mtDNAs possess telomerase-independent telomeres
(i.e., the ends of the linear DNA) with different modes of replication.
•The smallest mitochondrial genome sequenced to date is the 5,967 bp

4. mtDNA of the parasite Plasmodium falciparum.
•Most animals, specifically bilaterian animals, have a circular
mitochondrial genome. Medusozoa and calcarea clades however have
species with linear mitochondrial chromosomes.
• In Silene conica, mtDNA containing as many as 11,300,000 base pairs.
In cucumber (Cucumis sativus) consists of three circular chromosomes
(lengths 1556, 84 and 45 kilobases).
•In terms of base pairs, the anemone Isarachnanthus nocturnus has the
largest mitochondrial genome of any animal at 80,923 bp.
•A jellyfish-related parasite – Henneguya salminicola was discovered
that lacks mitochondrial genome but retains structures deemed
mitochondrion-related organelles. In mitochondrial DNA, replication and
transcription were either absent or present only as pseudogenes. This is
the first multicellular organism known to have this absence of aerobic
respiration and lives completely free of oxygen dependency.

5. Structure-
Mitochondrial DNA contains 37 genes, all of which are essential for
normal mitochondrial function. Thirteen of these genes provide
instructions for making enzymes involved in oxidative
phosphorylation. Oxidative phosphorylation is a process that uses
oxygen and simple sugars to create adenosine triphosphate (ATP),
the cell's main energy source. The remaining genes 24, provide
instructions for making molecules called transfer RNA (tRNA) and
ribosomal RNA (rRNA), which are chemical cousins of DNA. These
types of RNA help assemble protein building blocks (amino acids)
into functioning proteins.
Each mitochondrion is estimated to contain 2-10 mtDNA
copies. 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 eukaryotic nucleus
during evolution.

6. Among multicellular animals (metazoans), nearly all of the mtDNA in
a fertilized egg (zygote) is inherited from only one parent - the female.
In humans (and probably in metazoans in general), 100-10,000
separate copies of mtDNA are usually present per cell (egg and sperm
cells are exceptions). The human mitochondrial genome is built of a
mere 16,569 base pairs.
In mammals, each circular mtDNA molecule consists of 15,000-
17,000 base pairs, which encode the same 37 genes: 13 for proteins
(polypeptides), 22 for transfer RNA (tRNA) and one each for the small
and large subunits of ribosomal RNA (rRNA).
In some cases one or more of the 37 genes is absent and the mtDNA
size range is greater.
Some plant species have enormous mtDNAs (as many as 2,500,000
base pairs per mtDNA molecule!)

8. •There are three different mitochondrial genome types found in
plants and fungi.
1.Circular genome that has introns (type 2) range from 19 to 1000 kbp
in length.
2.Acircular genome (about 20–1000 kbp) that also has a plasmid-like
structure (1 kb) (type 3).
3. A linear genome made up of homogeneous DNA molecules (type 5).
•Type 2, 3 and type 4, types 5 and 6 each range from 1–200 kbp in size
genomes exist in some protists.

9. Replication-
•Mitochondrial DNA is replicated by the DNA polymerase gamma
complex which is composed of a 140 kDa catalytic DNA polymerase
encoded by the POLG gene and two 55 kDa accessory subunits encoded
by the POLG2 gene.
•The replisome machinery is formed by DNA
polymerase, TWINKLE and mitochondrial SSB proteins. TWINKLE is
a helicase, which unwinds short stretches of dsDNA in the 5' to 3'
direction.
• All these polypeptides are encoded in the nuclear genome.
During embryogenesis, replication of mtDNA is strictly down-regulated
from the fertilized oocyte through the preimplantation embryo.
•The resulting reduction in per-cell copy number of mtDNA plays a role
in the mitochondrial bottleneck, exploiting cell-to-cell variability to
ameliorate the inheritance of damaging mutations.

10. • The cells of the inner cell mass restrict mtDNA replication until they
receive the signals to differentiate to specific cell types.
Human mtDNA- The two strands of the human mitochondrial DNA are
distinguished as the heavy strand and the light strand.
•The heavy strand is rich in guanine and encodes 12 subunits of the
oxidative phosphorylation system, two ribosomal RNAs (12S and 16S),
and 14 tRNAs.
•The light strand encodes one subunit, and 8 tRNAs. So, altogether
mtDNA encodes for two rRNAs, 22 tRNAs, and 13 proteins subunits, all
of which are involved in the oxidative phosphorylation process.

12. Advantage
•It is a powerful tool for tracking ancestry through females (matrilineage)
and has been used in this role to track the ancestry of many species back
hundreds of generations.
•Human mtDNA can be used to identify individuals.
•Mitochondria are responsible for the production of cellular energy.
•They play an important role in the regulation of cellular metabolism,
apoptosis and oxydative stress control.
•In anthropological genetics, mtDNA is useful to trace geographic
distribution of genetic variation, for the investigation of expansions,
migrations and other pattern of gene flow.
•It is widely applicated in forensic science. It is a powerful implement to
identify human remains.

13. •It is characterized by the high rate of polymorphisms and mutations.
• Some of which are increasingly recognized as an important cause of
human pathology such as oxidative phosphorylation (OXPHOS)
disorders, maternally inherited diabetes and deafness (MIDD), Type 2
diabetes mellitus, neurodegenerative disorders, heart failure and cancer.
The total genomic DNA is extracted from biological material, such as a
tooth, blood sample, or hair by using the polymerase chain reaction
(PCR).
•The mtDNA control region is an area of the mitochondrial
genome which is non-coding DNA. This region controls RNA
and DNA synthesis. ... The mtDNA control region contains the origin of
replication of one strand, and the origin of transcription for both strands.
•If there's a defect in some of those mitochondrial DNA bases, that is to
say a mutation,it will have a mitochondrial disease, which will involve
the inability to produce sufficient energy in things like the muscle and
the brain, and the kidney.It is very helpful sometimes in determining how
a person has a certain disorder in the family.

14. Disadvantage-
• It can only follow maternal line back i.e. only trace back a single
maternal line. Mitochondrial DNA is passed from mother to children.
•It possess lower discrimination power compared to multiple
nuclear DnA markers.
•mtDNA mutations lead to some serious diseases.