The DNA contained in the nucleus of each cell is organized into structures called chromosomes. In human cells, there are 46 chromosomes and they are equal two to two, for this they are called homologous chromosomes, so the chromosome set is called diploid. All somatic cells contain 22 pairs of chromosomes, called autosomes, as well as two sex chromosomes, or heterochromosomes. The sex chromosomes are two identical XX copies in females and an X chromosome and a Y chromosome in males, which contains the genetic information for sexual differentiation. The set of metaphase chromosomes in a cell is called the karyotype. In the normal growing cell, the chromosomes are not distinguishable but dispersed in the nucleus in the form of chromatin granules. The DNA that makes up each chromosome is a linear, double-helix, unbroken molecule and is complexed with histone and non-histone proteins: the set of DNA and proteins is called chromatin. The DNA wraps around the histone proteins to form nucleosomes, which in turn wrap around to form the chromatin fibre. The chromatin fibre further folds to form the chromosome structure.
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The human genome, therefore, consists of two types of genetic material: nuclear DNA and mitochondrial DNA. Human mitochondrial DNA is a closed circular supercoiled double helix molecule. The molecule is 93-95% coding and contains 37 genes: 22 for tRNAs required for mitochondrial protein synthesis, 2 for rRNAs (12S and 16S), and 13 for proteins. DNA is the distinguishing and characterizing element of all individuals: All individuals share 99.5% of the entire human genome and ONLY 0.5% of the entire genome differs between individuals.
The nuclear DNA is present in the nucleus of all cells and allows exclusive identification of a subject. It allows phenotypic investigations, which can be used to create the genetic profile and for this reason has identification power and validity in Forensic Court.
The mitochondrial DNA, instead, is present only in the cytoplasm of all cells and has validity for maternal line analysis. Doesn’t allow phenotypic investigations and for this reason, has no identification power in Forensic Court.
The role of mitochondrial DNA analysis is to provide evidence when nuclear DNA gives negative results or is compromised or when only maternally related samples are available. Mitochondrial DNA was used and it is still used in disasters for the identification of all the victims.
In forensic law, DNA can be applied in both civil and criminal law. In fact, DNA can identify a subject in a unique and exclusive way, can give information about the family line of descent of a subject that can be used to construct the genetic profile of the subject itself.
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In a forensic context, the Scientific Proof – defined as “queen” – is proof that must answer to four fundamental requirements:
A single DNA trace can give only information about Who it arrived at the location where it is found. For this reason, it is necessary to study all the traces and especially their context in order to gain scientific information that can provide the necessary answers to figure out a scientific proof that is complete and unquestionable.
About the Author
Ms. Chiara Lucanto is a Forensic Biologist and Academic Content writer (firstname.lastname@example.org).