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DNA: Definition, Structure and Functions

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Victor Anunobi
Victor Anunobi is an environmentalist, writer and movie enthusiast living in the city of Lagos, Nigeria. He enjoys long walks, food and alternative music.

Deoxyribonucleic acid (DNA) is the chemical name of a molecule present in all living organisms which carry the hereditary attribute that determines reproduction, life, functioning, growth, and development of all living things.

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It is the blueprint of inherited characteristics that are passed from one generation to another. All forms of life and some viruses contain DNA, and its principal role is the storage of genetic information.

There are four nitrogenous bases that code for differences in all living organisms on earth. They include cytosine, guanine, adenine, and thymine.

The four bases line up in a specific order to form the double helix structure of the DNA. The order of these amino acids determines the sort of protein that is created.

Interestingly enough, only four nitrogenous bases account for all the diversity of life on the planet. There exists no other system or code found in living or previously existing organisms on earth.

This means that from the smallest bacteria to humans and even to dinosaurs, all share the same DNA as a genetic code. This implies that all life evolved from a single common ancestor.

Many great scientists have contributed to the discovery of DNA. It was first isolated by Swiss scientists Friedrich Miescher in 1869 during his experiment to isolate phosphate-rich chemicals he called nuclein (the modern-day nucleic acid) from the nuclei of white blood cells.

This gave way for the identification of DNA as a mechanism of inheritance. However, its role in genetic inheritance was not demonstrated until 1953 by James Watson and Francis Crick at the University of Cambridge, England.

Aided by the work of biophysicists Rosalind Franklin and Maurice Wilkins, they identified the molecular structure of the DNA as a double-helix polymer, a spiral that consists of two DNA strands wound around each other.

This discovery led to a significant breakthrough in the field of science and the understanding of DNA as an agent of inheritance.

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Structure of DNA

DNA consists of long-chain polymers of repeating cells known as nucleotides. Each nucleotide contains a nitrogen group, a phosphate group, a sugar group, and a nitrogen base. The four types of nitrogen bases are thymine (T), adenine (A), cytosine (C), and guanine (G).

The order of the four bases determines the genetic code. According to the U.S. National Library of Medicine, the human DNA has about three billion bases, and 99% of these bases are the same in everyone.

The DNA sequence forms genes similar to the way the order of letters in the alphabet can be used for words. Ribonucleic acid, another type of nucleic acid, translates the genetic information from the DNA into proteins.

The nucleotides are attached and wound up together to form a ladder-like structure called a double helix. The bases from one strand pair with the bases on another strand — for example, guanine pairs with cytosine and adenine pairs with thymine.

The molecules of DNA are so long; they cannot fit into cells without the appropriate packaging. For this to occur, the DNA strands are coiled tightly to form structures known as chromosome. Each chromosome contains a single DNA molecule. There are 23 pairs of chromosomes in humans, and these are found in the cell’s nucleus.

For cell division to occur, these chromosomes would be duplicated in a process known as DNA replication. This provides a complete set of chromosomes for each daughter cell.

Eukaryotic organisms – that is protists, fungi, plants, and animals – store most of their DNA inside the cell nucleus, some store theirs in the mitochondria as mitochondrial DNA and others in the chloroplasts as chloroplast DNA.

However, prokaryotes like archaea and bacteria store theirs only in the cytoplasm. The structure of DNA can be compromised in a process known as mutation.

This can be caused by mutagens, which change the sequence of DNA. Mutagens include alkylating agents, oxidizing agents, and also electromagnetic radiation such as ultraviolet light and X-rays.

For instance, high energy irradiation (exposure to radiation) to UV light can break the covalent bonds holding the DNA strands together, and this can cause mutations.

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These mutagens can cause diseases such as cancer, brain tumors, and disorders that affect the central nervous system. However, due to the ability to hinder the replication of DNA, these same mutagens can be used in chemotherapy for the treatment of cancer.

Functions of DNA

Chromosomes in a cell make up its genome, and this has about three billion based pair sequences arranged in it. The genetic information contained in the DNA is held in the genes.

The transmission of genetic information occurs through base pairing. RNA in a cell transcribes and copies the genetic information into proteins in a process known as DNA replication. Given below are the main functions of DNA

Biological Inheritance

DNA molecule is distinguished by the order in which nitrogenous bases appear within the DNA polymer with each DNA strand contained in the chromosomes. For replication to occur, two strands of DNA separate along a short stretch, creating a bubble-like structure.

This new single strand joins another via a hydrogen bond. They continue alongside each other, forming a new double-helix structure until the entire DNA structure is replicated.

When the fertilization of an egg by a sperm occurs, a zygote is formed. This zygote undergoes cell division multiple times, creating an entirely different array of cells and tissues. When cell division occurs, genetic material is duplicated.

This means that about 3 billion nucleotides are accurately read and copied by the RNA. Thus, a new life form is created with the same number of DNA molecules but with a different order of nucleotides.

Transcription

Proteins, peptides, and RNA perform functional and catalytic roles in the body. Nucleotides in the DNA determines the function and structure of these molecules.

Transcription occurs when molecules of RNA or proteins are to be reproduced. Similar to DNA replication, this happens by the formation of a single strand of RNA. This single strand then acts as a guide for the polymerization of a complementary polymer of RNA.

Genetic Engineering

The role of the DNA as genetic material and the understanding of the chemistry behind it allows us to modify and manipulate it and enhance the quality of life with the use of biotechnology.

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New DNA forms are obtained either by isolating and copying the genetic material of interest using recombinant DNA or by synthesizing the DNA artificially. For instance, genetically modified crop species that are pest and disease resistant.

Evolution and Mutation

One of the principal functions of genes is replication and inheritance. For a new generation to emerge, genetic information needs to be copied accurately and then transmitted.

The DNA structure ensures that the information coded in each DNA strand is replicated with astonishing accuracy. Although it is important that DNA is duplicated accurately, the process of evolution requires the presence of variability in the genes within every species.

One of the ways by which this occurs is via mutation. Changes in the DNA sequence in genetic material allow the formation of a new allele. Alleles are the functional variant forms of genes.

For example, people with AB blood group have proteins distinct from the antigens of those present in blood group A or B. Also, people with sickle cell anemia have a different hemoglobin allele from those with genotype AA or AS.

This variability indicates that some populations survive when there are drastic changes in the environment.

For instance, people living with sickle cell have a higher chance of survival in areas where malaria is endemic. These mutations allow populations to evolve and adapt to a changing environment and circumstances.

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