**2. Mitochondrial hereditary material**

### **2.1 Small circular genome**

Majority of animal DNA as genetic information (about 3.3x109 base pairs) is stored in chromosomes within the cell nucleus. However, a minor part of DNA is located in chromosomes of mitochondria, outside the nucleus, in the cytosol. The circular mitochondrial genome is also built up of double-stranded DNA like nuclear genome, and consists of about 16,500 base-pairs. It is a semi-autonomous asexually reproducing genome in eukaryotic organisms [7]. Mitochondria are late descendants of free-living bacteria capable of metabolizing oxygen maintained by endosymbiosis in eukaryotic cells.

The mitochondrial DNA (mtDNA) which is not enveloped like nuclear DNA in chromosomes, is located in the mitochondrial matrix which can be found inside the inner mitochondrial membrane. The outer compartment of a mitochondrion is surrounded by the outer and inner membrane. The outer membrane contains porins through which smaller or larger proteins can enter the mitochondria. While, the inner mitochondrial membrane has all the elements of the electron transport system and the ATP synthase complex [8].

Considering that a cell has multiple mitochondria, and that a mitochondrion carries multiple copies of its own genome as opposed to the nuclear genome, the difference between the two remains significant.

#### **2.2 High mutation rate**

The mitochondrial genome or mitogenome mutates more frequently (approx. 100 times more often) than nuclear ones causing divergence in mtDNA at withinmitochondrion and between-mitochondrion level. Therefore, the mitogenome can be considered heterogeneous and heteroplasmic homoplasmic instead.

Little is known about the movement and segregation of mitochondrial DNA during mitotic growth or meiotic divisions. When a cell divides, mitochondria enter the progeny cells at random. If the DNA of the mitochondria of the dividing cell differs for several mitochondria, it is possible that the two daughter cells will receive the same genetic information, but it is also conceivable that they will not. Thus, it is hard to estimate the outcome of the transfer of genetic information, including defects. The random mutations that occurred further complicate that situation.

Because of the constantly frequent mutation rate of mitogenome, it has been widely used as a phylogenetic marker for both cladogram building and molecular dating. Brown et al. [9] first implemented the mitochondrial molecular clock in primates using fossil data. According to that work, scientists considered the 2% substitution rate per one million year as a reasonable reference in case of missing of relevant fossil data in vertebrates [10]. Since then, studies (e.g. [11]) have reported significant differences between species, it was found that because of the not fully clock-like evolution of the species the mtDNA mutation rate is of limited use in a comparison. The median (from 3 to 14.3 million years) of divergence dates

**181**

*Reality of Mitogenome Investigation in Preservation of Native Domestic Sheep Breeds*

between species are not related to body mass or generation interval [12]. However, Galtier et al. [13] found a proven correlation between mutation and longevity, which is closely related to the generation interval, and suggested a low (somatic) mutation rate could be responsible to achieve long life, in concordance with the mitochondrial theory of ageing. According to Song et al. [14] mitochondria may have a nucleotide imbalance that leads to higher mitochondrial DNA mutation rates. Their research suggests increased dGTP (deoxyguanosine triphosphate) level in free deoxynucleotide triphosphate (dNTP) pool which increases the rate of T to

The offspring receive mostly mtDNA from the maternal ooplasm (sometimes this material from the sperm can also be included), but in the adult embryo only the maternal mtDNA remains functional. So, mitochondrial inheritance is considered as clonal or maternal, as one of the cases of uniparental inheritance. Paternal mtDNA, even if it enters the ovum, loses its function before (in crayfish [15]), during (in Ascidia [16]) or after (in mouse [17]) fertilization. That condition prevents an effective recombination. However, the paternal inheritance of mtDNA was displayed by Zhao et al. [18], what is to show the mtDNA patterns of progeny were identical to that of its male parent. Systematic surveys of within-species mtDNA data revealed departure from the clonality assumption in several species [19]. These prove that mitochondrial recombination is possible, and caution when in constructing and

Thus, mtDNA testing can reveal the maternal background of individuals; which maternal lineage of mitochondria they belong to. But, of course, it can also be used to prove maternal kinship. Mitochondrial DNA sequencing has gained significance

Mitochondrial gene content is strongly conserved across animals, with very few duplications, no intron, and very short intergenic regions [22]. At the same time, mitogenome also contains a very limited presence of non-coding regions, approximately 3%, as opposed to nuclear DNA, where its proportion is 93%. These highly variable non-coding regions (e.g. the control region) are typically flanked by highly conserved ones (e.g. ribosomal DNA). The elevated mutation rate of highly variable regions creates the condition for monitoring the population history over relatively

One of the perspectives of mitogenome research is therefore the discovery of mitochondrial genetic disorders (next to accelerated ageing, neurodegenerative disease, cancer, diabetes), and the study of their mechanism of action (e.g. [23]). Another promising area is the mapping of evolutionary branches and the determination of the more precise taxonomic location and movement of different species (including humans [24]). Mutations will be passed over into all maternal progenies homoplasmic making individuals of a maternal lineages the same in mitogenome,

For the third time, it is worth mentioning the unfolding of the microevolutionary web of our domesticated animals and the knowledge of the origin (at the same time

especially, when they share entirely homoplasmic mitochondrial pool.

geographical) of the breeds that have developed today (e.g. [25]).

It was observed that some characteristics (e.g. behavioral) do not follow Mendelian segregation. If a trait is such, it is either polygenic or extranuclear.

*DOI: http://dx.doi.org/10.5772/intechopen.95768*

interpreting within-species mtDNA genealogies [20].

C substitutions.

**2.3 Maternal inheritance**

also in human rights cases [21].

short time frames.

**2.4 Perspectives of mtDNA research**

*Reality of Mitogenome Investigation in Preservation of Native Domestic Sheep Breeds DOI: http://dx.doi.org/10.5772/intechopen.95768*

between species are not related to body mass or generation interval [12]. However, Galtier et al. [13] found a proven correlation between mutation and longevity, which is closely related to the generation interval, and suggested a low (somatic) mutation rate could be responsible to achieve long life, in concordance with the mitochondrial theory of ageing. According to Song et al. [14] mitochondria may have a nucleotide imbalance that leads to higher mitochondrial DNA mutation rates. Their research suggests increased dGTP (deoxyguanosine triphosphate) level in free deoxynucleotide triphosphate (dNTP) pool which increases the rate of T to C substitutions.

#### **2.3 Maternal inheritance**

*Landraces - Traditional Variety and Natural Breed*

**2. Mitochondrial hereditary material**

**2.1 Small circular genome**

endosymbiosis in eukaryotic cells.

**2.2 High mutation rate**

system and the ATP synthase complex [8].

difference between the two remains significant.

the world.

characterization and differentiation of our animal species living with us. Looking to the future, we can believe that this will be essential for the conservation of genetic resources and preservation of endangered autochthonous animal breeds all over

base pairs) is

Majority of animal DNA as genetic information (about 3.3x109

stored in chromosomes within the cell nucleus. However, a minor part of DNA is located in chromosomes of mitochondria, outside the nucleus, in the cytosol. The circular mitochondrial genome is also built up of double-stranded DNA like nuclear genome, and consists of about 16,500 base-pairs. It is a semi-autonomous asexually reproducing genome in eukaryotic organisms [7]. Mitochondria are late descendants of free-living bacteria capable of metabolizing oxygen maintained by

The mitochondrial DNA (mtDNA) which is not enveloped like nuclear DNA in chromosomes, is located in the mitochondrial matrix which can be found inside the inner mitochondrial membrane. The outer compartment of a mitochondrion is surrounded by the outer and inner membrane. The outer membrane contains porins through which smaller or larger proteins can enter the mitochondria. While, the inner mitochondrial membrane has all the elements of the electron transport

Considering that a cell has multiple mitochondria, and that a mitochondrion carries multiple copies of its own genome as opposed to the nuclear genome, the

The mitochondrial genome or mitogenome mutates more frequently (approx. 100 times more often) than nuclear ones causing divergence in mtDNA at withinmitochondrion and between-mitochondrion level. Therefore, the mitogenome can

Little is known about the movement and segregation of mitochondrial DNA during mitotic growth or meiotic divisions. When a cell divides, mitochondria enter the progeny cells at random. If the DNA of the mitochondria of the dividing cell differs for several mitochondria, it is possible that the two daughter cells will receive the same genetic information, but it is also conceivable that they will not. Thus, it is hard to estimate the outcome of the transfer of genetic information, including defects. The random mutations that occurred further complicate that

Because of the constantly frequent mutation rate of mitogenome, it has been widely used as a phylogenetic marker for both cladogram building and molecular dating. Brown et al. [9] first implemented the mitochondrial molecular clock in primates using fossil data. According to that work, scientists considered the 2% substitution rate per one million year as a reasonable reference in case of missing of relevant fossil data in vertebrates [10]. Since then, studies (e.g. [11]) have reported significant differences between species, it was found that because of the not fully clock-like evolution of the species the mtDNA mutation rate is of limited use in a comparison. The median (from 3 to 14.3 million years) of divergence dates

be considered heterogeneous and heteroplasmic homoplasmic instead.

**180**

situation.

The offspring receive mostly mtDNA from the maternal ooplasm (sometimes this material from the sperm can also be included), but in the adult embryo only the maternal mtDNA remains functional. So, mitochondrial inheritance is considered as clonal or maternal, as one of the cases of uniparental inheritance. Paternal mtDNA, even if it enters the ovum, loses its function before (in crayfish [15]), during (in Ascidia [16]) or after (in mouse [17]) fertilization. That condition prevents an effective recombination. However, the paternal inheritance of mtDNA was displayed by Zhao et al. [18], what is to show the mtDNA patterns of progeny were identical to that of its male parent. Systematic surveys of within-species mtDNA data revealed departure from the clonality assumption in several species [19]. These prove that mitochondrial recombination is possible, and caution when in constructing and interpreting within-species mtDNA genealogies [20].

Thus, mtDNA testing can reveal the maternal background of individuals; which maternal lineage of mitochondria they belong to. But, of course, it can also be used to prove maternal kinship. Mitochondrial DNA sequencing has gained significance also in human rights cases [21].

It was observed that some characteristics (e.g. behavioral) do not follow Mendelian segregation. If a trait is such, it is either polygenic or extranuclear.

#### **2.4 Perspectives of mtDNA research**

Mitochondrial gene content is strongly conserved across animals, with very few duplications, no intron, and very short intergenic regions [22]. At the same time, mitogenome also contains a very limited presence of non-coding regions, approximately 3%, as opposed to nuclear DNA, where its proportion is 93%. These highly variable non-coding regions (e.g. the control region) are typically flanked by highly conserved ones (e.g. ribosomal DNA). The elevated mutation rate of highly variable regions creates the condition for monitoring the population history over relatively short time frames.

One of the perspectives of mitogenome research is therefore the discovery of mitochondrial genetic disorders (next to accelerated ageing, neurodegenerative disease, cancer, diabetes), and the study of their mechanism of action (e.g. [23]).

Another promising area is the mapping of evolutionary branches and the determination of the more precise taxonomic location and movement of different species (including humans [24]). Mutations will be passed over into all maternal progenies homoplasmic making individuals of a maternal lineages the same in mitogenome, especially, when they share entirely homoplasmic mitochondrial pool.

For the third time, it is worth mentioning the unfolding of the microevolutionary web of our domesticated animals and the knowledge of the origin (at the same time geographical) of the breeds that have developed today (e.g. [25]).
