DNA Replication

DNA is the unit of heredity in living things. It is the storehouse of information that is needed for the proper development and functioning of organisms. So, it is important that the number and sequence of genes is always maintained. Just before cell division, the DNA in a cell replicates, so that each daughter cell has the same number of chromosomes as those in the parent cell. During DNA replication, the sequence of deoxyribonucleotides is based on the rule that Adenine (A) pairs with Thymine (T) and Cytosine (C) pairs with Guanine (G).

DNA Replication Steps

• DNA replication begins at specific points known as the Origins. These sites are recognized by certain proteins in the cell. The Origins are rich in Adenine and Thymine bases, as breaking two hydrogen bonds between the Adenine and Thymine is easier than breaking the triple hydrogen bonds between Cytosine and Guanine.
• Helicase (an enzyme) separates the two strands of a DNA double helix by breaking the hydrogen bonds between them. This results in the formation of a replication fork. Both the strands of the parent DNA molecule serve as templates for a new DNA molecule. Hence, during replication, two new DNA molecules are formed from a single DNA molecule.
• DNA polymerase (an enzyme) starts synthesizing a new strand called the Leading Strand by adding new deoxyribonucleotides at a continuous stretch against one of the parent strands. The new strand that is formed using the other parent strand as a template is known as the Lagging Strand, as it is formed in segments referred to as the Okazaki Fragments. The synthesis of the Lagging Strand begins with the addition of RNA primers. DNA Polymerase continues with the synthesis of the new DNA strand between these primers. Once all the gaps between the RNA primers are filled with new deoxyribonucleotides, the RNA primers are removed by DNA Polymerase. New deoxyribonucleotides are added in place of the RNA primers. Finally, DNA ligase (an enzyme) joins the deoxyribonucleotides together, thus completing the lagging strand.

Nuclear DNA

Most of the DNA in a cell is present in the nucleus. For most of the cell cycle, nuclear DNA is present in the form of the chromatin network. However, during cell division the chromatin network condenses and is visible as separate entities known as chromosomes. Nuclear DNA carries both paternal and maternal DNA that are rearranged as a result of the recombination of genetic material. Hence, determining ancestry using nuclear DNA is difficult. The blueprint of most of the proteins in an individual is contained in the nuclear DNA. Majority of the 20,000 to 25,000 genes in the human genome are present in the nuclear DNA.

Mitochondrial DNA (mtDNA)

As already stated, a majority of the cell's genetic material is present in the nucleus. However, some amount of DNA is also present in another cell organelle known as the Mitochondrion. This DNA is called mitochondrial DNA, (mtDNA) or the maternal DNA as it is inherited matrilineally. This happens either because the sperm mtDNA cannot enter the egg or it gets dissolved once a sperm enters an egg during fertilization. Each mitochondrion has 2-10 mtDNA copies which contain 16,500 base pairs. Mitochondrial DNA is supposed to have evolved from bacterial genome. However, it is believed that in this process, though some DNA is lost, some amount gets transferred to the nucleus. The rest forms the mtDNA. Though majority of the proteins of mitochondria are coded by nuclear DNA, the mtDNA in human beings has 37 genes. Of these, 13 code for proteins, 22 for transfer RNA (tRNA) and the remaining two code for the subunits of ribosomal RNA (rRNA).

Importance of mtDNA

• Study Lineage and Evolution: The importance of mtDNA lies in the fact that it is inherited only from the mother. Thus, unlike the nuclear DNA in which there is a shuffle between the paternal and maternal genes, there is no recombination of the genetic material in case of mtDNA. This makes mtDNA an important tool in studying genetic lineage along the maternal line. This property of mtDNA has led to the evolution of the concept of Mitochondrial Eve - a woman who lived in Africa about 170,000 years ago and who is believed to be the most recent common matrilineal ancestor of all human beings. Also, the mutation rate of mtDNA is much higher than that of nuclear DNA. Hence, mtDNA can be used to study the diversity in human population.
• Use of mtDNA in Forensic Science: Studies of Human mtDNA coupled with other anthropological and forensic methods, are used to identify unclaimed skeletal remains and exclude the possible matches. Mitochondrial DNA is also useful in identifying the old remains as the cell has a higher number of copies of mtDNA as compared to nuclear DNA.
• Diseases: Mutations in the mtDNA cause several diseases. Point mutations of mtDNA cause MELAS syndrome and Leber's hereditary optic neuropathy, whereas Kearns-Sayre syndrome and Pearson's syndrome are a result of large-scale arrangement of mtDNA segments.

Discovery of DNA and the developments in the field of genetics have opened an interesting branch of study. Not only can it help us find remedies to a number of diseases that were till now considered incurable, but it has also opened new vistas to answer questions related to human evolution and ancestry.