mitochondria being a semi-autonomous organelle having endosymbiotic origin has certain note worthy characteristic features.

1. DR. VIBHA KHANNA
ASSO. PROF. (BOTANY)
S.P.C. GOVERNMENT COLLEGE
AJMER (RAJASTHAN)

2. CYTOGENETICS
• BLOCK 1
• PRESENTATION 2:
Genetics Of Mitochondria

3. INTRODUCTION
• The presence of extrachromosomal genes was
hypothesised in 1950s, while explaining the inheritance of
certain phenotypes of yeast (Saccharomyces cerevisiae),
red mould ( Neurospora crassa) and the algae
(Chlamydomonas reinhardtii).
• This was later confirmed by experimental studies in early
1960s.
• The presence of organelle genome strongly support the
endosymbiotic theory.
(https://www.slideshare.net/vibhakhanna1/endosymbiotic-
theory)
• Mitochondria and chloroplast are the two semi-
autonomous organelles present in the eukaryotic cell.
• Mitochondrial DNA (mtDNA) varies enormously in size,
whereas chloroplast DNA (ctDNA) ranges in size between
120 and 200 kb.

4. MITOCHONDRIAL GENOME:
• The multiple copies of mitochondrial DNA are present
within the matrix. They are usually distributed in several
clusters, called nucleoids.
• Nucleoids are thought to be attached to the inner
mitochondrial membrane.
• Most mitochondrial DNAs (mtDNA) consist of a closed
circular double stranded supercoiled DNA molecules , but
in some eukaryotic microbes, like Chlamydomonas, many of
the yeasts, Paramecium etc. it is linear, with terminal and
sub-terminal repeats.
• The mitochondrial DNA is not associated with histones.
• Its size is variable and is not related to the complexity of
the organism. {In mammalian cells mitochondrial DNA
makes up 1% of the total cellular DNA, but its proportion
higher in some plants or amphibian eggs (99%)}.

5. MITOCHONDRIAL GENOME: (Contd.)
• The lengths of a few of the mitochondrial DNAs, which
have been sequenced, are shown to scale as circles for
circular genomes and lines for linear genomes. The
largest circle represents that of Rickettsia prowazekii, a
small pathogenic bacterium whose genome most
closely resembles that of mitochondria.

6. MITOCHONDRIAL GENOME: (Contd.)
• Although most mtDNAs are in the size range of 15 to 60 kb,
mtDNA in malarial parasite (Plasmodium spp) is only 6 kb
long, while that of rice (Oryza sativa) is 490 kb, and
cucurbits 2 Mb.
• There are about 40 to 50 coding genes in mitochondrial
DNA, Plasmodium being an exception with 5 coding genes.
• The size of mitochondrial genomes does not correlate well
with the number of proteins encoded in them:
– while human mitochondrial DNA encodes 13 proteins, the 22-
fold larger mitochondrial DNA of Arabidopsis encodes only 32
proteins—that is, about 2.5-fold as many as human
mitochondrial DNA.
– The extra DNA that is found in Arabidopsis, Marchantia, and
other plant mitochondria may be “junk DNA”.
– The mitochondrial DNA of the protozoan Reclinomonas
americana has 97 genes, more than the mitochondrion of any
other organism analyzed so far.

7. MITOCHONDRIAL GENOME: (Contd.)
• Relaxed usage of codon: only 22 tRNAs are required for
mitochondrial protein synthesis. The normal codon-
anticodon pairing rules are relaxed in mitochondria, so that
many t-RNA molecules recognize any one of the four
nucleotides in the third (wobble) position. Such “2 out of 3”
pairing allows one tRNA to pair with any one of four codons
and permits protein synthesis with fewer tRNA molecules.
• Variant genetic code: Perhaps most surprising, comparisons
of mitochondrial gene sequences and the amino
acid sequences of the corresponding proteins indicate that
the genetic code is different: 4 of the 64 codons have
different “meanings” from those of the same codons in
other genomes

8. Variant genetic codes in mt-DNA:
• One of the characteristic feature of the mitochondrial genome is
the deviation of its genetic code from the universal genetic code.
• It exhibit extreme form of ‘wobble’ wherein U in the anticodon of
the tRNA is able to pair with any of the four bases in the third
codon of mRNA.
• Some of the mitochondrial codons specify for different amino acids,
for example UGA in human mitochondria codes for tryptophan and
is not the stop codon, instead AGA and AGG are the STOP codons
and do not code for arginine.

9. MITOCHONDRIAL GENOME: (Contd.)
• Dense gene packing: Unlike other genomes,
nearly every nucleotide seems to be part of a
coding sequence, either for a protein or for one
of the rRNAs or tRNAs. Since these coding
sequences run directly into each other, regulatory
DNA sequences are very few.
• The large size of mitochondrial genomes in plants
are due to noncoding inter-genic regions and
their content of tandem repeats. Introns are
present in many mtDNAs

10. Introns in mt-Genome
• The presence of introns in organelle genes is not expected, because
introns are not common in the genes of the bacteria whose
ancestors are thought to have given rise to mitochondria.
• In yeasts, the same mitochondrial gene may have an intron in one
strain but not in another. Such “optional introns” seem to be able to
move in and out of genomes like transposable elements.
• In contrast, introns in other yeast mitochondrial genes have also
been found in a corresponding position in the mitochondria
of Aspergillus and Neurospora, implying that they were inherited
from a common ancestor of these three fungi.
• It is possible that these intron sequences are of ancient origin—
tracing back to a bacterial ancestor—and that, although they have
been lost from many bacteria, they have been preferentially
retained in some organelle genomes where splicing of RNA is
regulated to help control gene expression.

11. ORGANISATION OF MITOCHONDRIAL
GENOME:
• Mitochondria, the double membrane-bound organelles found in
most eukaryotic cells and commonly described as cellular “power
plants” for their role in the generation of ATP by oxidative
phosphorylation.
• In addition the mitochondria perform a variety of additional cellular
functions, such as biosynthesis of amino acids and steroids, β-
oxidation of fatty acids, FeS metabolism, and initiation of apoptosis.
• However, oxidative phosphorylation appears to be the only
function that requires the presence of a mitochondrial genome (mt-
genome).
• The mt-genomes codes for
– some protein subunits of the electron transport chain
– ribosomal RNA (rRNA) components of mitochondrial ribosomes
– some mitochondrial transfer RNA (mt-tRNA),
– some encode the ribosomal subunit of RNaseP and proteins involved
in mitochondrial transcription, translation, protein import and
maturation.

12. ORGANISATION OF MITOCHONDRIAL
GENOME:

13. Heteroplasmy in mt-DNA
• In the mitochondrial DNA the rate of mutation is higher
because of the presence of oxygen free radicals which
are released during oxidative phosphorylation. These
free radicals damage the mitochondrial DNA which is
unable to repair itself in the absence of DNA repair
enzymes.
• Since the number of mitochondrial DNA per
mitochondria varies and is usually more than one,
heteroplasmy may be observed.
• Heteroplasmy is the state in which a specific mutation
may be present in some copies of the mitochondrial
DNA but not in others, resulting in the presence of
more than one type of genome in the organelle.

14. mt-DNA & Apoptosis
• The protein traffic between the cytosol and these
organelles seems to be unidirectional, as no known
proteins are exported from mitochondria or
chloroplasts to the cytosol.
• An exception occurs under special conditions when a
cell is about to undergo apoptosis.
• The release of intermembrane space proteins
(including cytochrome c) from mitochondria through
the outer mitochondrial membrane is part of a
signaling pathway that is triggered in cells
undergoing programmed cell death (i.e. apoptosis).

15. Promiscuous DNA
• The mitochondrial DNA of a few plant species contain
a few partial sequences of the chloroplast genome. The
sequences are called promiscuous DNA. (They may be
several kilobases in length and may even contain
complete copies of certain chloroplast genes.)
• *Reasons for the higher rate of nucleotide
substitutions in mitochondrial genes as compared to
that of nuclear genes :
– Fidelity of replication / repair low
– Low selective pressure
– Multiple copies

16. Significance of mitochondrial DNA:
• Mitochondrial DNA is of immense use in forensic
science. It is helpful in cases of scarcity of DNA or when
the samples are quite deteriorated. (For example, in
cases of dead war soldiers, mass natural disasters etc.
when the biological material is limited.)
• The inheritance of mitochondrial DNA is of maternal
type since cytoplasm is furnished only by the egg
during fertilization. Therefore, the mitochondrial DNA
may also be used to determine the maternal line of
inheritance and also the maternal ancient ancestry.

17. REFERENCES
• Alberts B, Johnson A, Lewis J, et al. Molecular
Biology of the Cell. 4th edition. New York:
Garland Science; 2002. The Genetic Systems
of Mitochondria and Plastids. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK269
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• T.A. Brown. Genetics: A Molecular Approach.