INTRODUCTION
Deoxyribonucleic acid abbreviated scientific name is DNA. It
is a polymer made up of two polynucleotide chains. These
polynucleotides are coil around each other to form a double helix structure
having genetic information for the development, functioning, growth
and reproduction of the organism.
Polynucleotides are strands two DNA consist of simpler monomeric units
called nucleotides. The nucleotides are linked to one another in a chain
by covalent bonds, called the phospho-diester linkage. This double-stranded
DNA is the composition of bases the two separate nitrogenous bases polynucleotide
strands which bound together with hydrogen bonds.
Both strands of double-stranded DNA are having the same biological
instructions. These instructions are replicated when the two strands
separate from each other. For human more than 98% part of DNA is non-coding.
Timeline of
Discovery of DNA
The exact answer to the question that who discovered DNA? is complex, because in truth, its discovery is the series of contribution of many researchers and scientists to what we know about it. So the researchers and scientists continue to expound on his work to this day, as we are still learning more about its mysteries. A simple timeline of this great discovery is as under: -
- In 1866, Gregor Johann Mendel, who is known as the “Father of Genetics,” was actually the first to suggest that characteristics are passed down from generation to generation.
- It is also considered that the Swiss physician Friedrich Miescher in 1869 was the first who isolated the DNA. He discovered a microscopic substance in the pus and he called it "nuclein" being found in nuclei of the cell.
- Nobel Prize winner and German biochemist Albrecht Kossel in 1878, isolated the non-protein component of "nuclein", nucleic acid, and later isolated its five primary nucleobases: adenine (A), cytosine (C), guanine (G), and thymine (T) (which is replaced by uracil (U) in RNA).
- In 1882, Walther Flemming a German Biologist, discovered mitosis and he was the first biologist to execute a wholly systematic study of the division of chromosomes and suggested that chromosomes double is very significant.
- In 1909, the Russian-American biochemist Phoebus Levene identified the base, sugar, and phosphate nucleotide unit of the RNA and named it "yeast nucleic acid".
- In 1927, a Russian biologist and a pioneer of modern genetics, Nikolai Koltsov proposed that inherited traits would be inherited via a "giant hereditary molecule" made up of "two mirror strands that would replicate in a semi-conservative fashion using each strand as a template".
- 1928, a British bacteriologist, Frederick Griffith in his experiment discovered that traits of the "smooth" form of Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form. This system provided the first clear suggestion that DNA carries genetic information.
- In 1929, Phoebus Levene identified deoxyribose sugar in "thymus nucleic acid" (DNA) and suggested that DNA consisted of a string of four nucleotide units linked together through the phosphate groups ("tetranucleotide hypothesis"). He suggested that the chain was short and the bases repeated in a fixed order.
- In 1933, a Belgian biochemist Jean Brachet while studying virgin sea urchin eggs suggested that DNA is found in the cell nucleus and that RNA is present exclusively in the cytoplasm.
- In 1937, William Astbury an English physicist and molecular biologist who made pioneering X-ray diffraction studies of biological molecules. He produced the first X-ray diffraction patterns that showed that DNA had a regular structure.
- In 1943, Oswald Avery a Canadian-American physician and medical researcher, along with his co-workers Colin MacLeod and Maclyn McCarty, identified DNA as the transforming principle, which essentially means that it’s DNA, not proteins, that transform cell properties.
- 1944 – 1950, Erwin Chargaff an Austro-Hungarian-born American biochemist developed and published observations now known as Chargaff's rules, stating that DNA is responsible for heredity and that it varies between species. He proved that guanine and cytosine units, as well as adenine and thymine units, were the same in double-stranded DNA, and he also discovered that DNA varies among species.
- 1951-1952, Roslind Franklin’s and James Watson showed the helical form during X-ray crystallography of DNA and gave DNA’s double helix structure that twists to form the ladder-like structure we think of when we picture DNA.
- In February 1953, Roslind Franklin’s and James Watson announce that they had "discovered the secret of life".
- 1953 - What we know about DNA today can be largely credited to James Watson and Francis Crick, they discovered the structure of DNA. Despite there being many important and contributing discoveries both before and after their work, this is the year they discovered DNA’s double helix, or spiraling, intertwined structure, which is fundamental to our current understanding of DNA as a whole.
Characteristics and Structure of DNA
DNA is a long polymer made from repeating units
called nucleotides. The structure of DNA is dynamic along its length,
being capable of coiling into tight loops and other shapes. It is made up
of two helical chains and bound to each other by hydrogen bonds in all
species. Although each individual nucleotide is very small, but a DNA
polymer can be very long and may possess hundreds of millions of nucleotides
just like in chromosome 1. In human chromosome 1 is the largest
chromosome having approximately 220 million base pairs. If
straightened it would be 85 mm long.
DNA does not usually exist as a single strand. It exists as a pair
of strands that are tightly coil around each other, in the shape of
a double helix. The backbone of the DNA strand is composed from
alternating phosphate and sugar groups called 2-deoxyribose,
which is a pentose (means five-carbon) sugar. Therefore, any DNA strand
normally has one end at which there is a phosphate group attached to the 5′
carbon of a ribose are called the 5′ phosphoryl and another end at which there
is a free hydroxyl group attached to the 3′ carbon of a ribose are called the
3′ hydroxyl. One of the major difference between DNA and RNA is the
sugar, with the 2-deoxyribose in DNA being substituted by the related pentose
sugar ribose in RNA.
The DNA double helix is stabilized mainly by two forces, base-stacking links
among aromatic nucleobases and hydrogen bonds between
nucleotides. Till now four bases are found in DNA are adenine (A), cytosine (C), guanine (G)
and thymine (T) which are attached to the sugar-phosphate to form the
complete nucleotide. When adenine pairs with thymine it forms A-T base pair and
guanine pairs with cytosine it forms G-C base pairs.
Classification
of Nucleobases
The nucleobases are classified into two types: the purines
(pair of A and G) and the pyrimidines (the six-membered rings of C and T).
Grooves
or Spaces
There are spaces, or grooves, between the strands of double helix.
These spaces are adjacent to the base pairs and may provide a binding area.
As the strands of double helix are not symmetrically located with respect to
each other, therefore spaces are not equal in size. The size of spaces may vary
from 12 to 22 ångströms (or 1.2nm to 2.2 nm) wide. The edges of the bases
of larger spaces are more accessible than in the minor groove.
DNA Sequence
or Sense Sequence
A DNA sequence is also called
a "sense" sequence. Similarly, the sequence on the opposite
strand is called the "antisense" sequence. Both strands of DNA can
contain both sense and antisense sequences.
Supercoiling Characteristics
The process in which DNA can be twisted like a rope called DNA supercoiling. If the DNA is twisted in the direction of the double helix, this is called positive supercoiling which makes the bases held together more tightly. If they are twisted in the opposite direction, this is called negative supercoiling, and the bases are loosely wound. Naturally, most of the DNA has shown slight negative supercoiling.
Alternative
structures of DNA
The DNA may exist in many
possible conformations which include A-DNA, B-DNA, and Z-DNA forms.
B-DNA and Z-DNA have directly been observed in functional organisms. The
conformation of DNA depends on the DNA sequence, hydration level, chemical
modifications of the bases, the amount and direction of supercoiling, the type
and concentration of metal ions and the presence of polyamines in
solution. Other structure also includes Quadruplex structure and branched
structures and artificial based structure.
Acidity
The
phosphate groups of DNA have phosphoric acid properties which is
considered as a strong acid and fully ionized at a normal cellular pH.
Base
modifications and DNA packaging
Packing of DNA in a structure called
chromatin of chromosomes influence the expression of genes.
Damages
and Harms
The DNA can be damaged by many sorts
of mutagens, that may
change the DNA sequence. These mutagens include alkylating agents, oxidizing
agents and also high-energy electromagnetic radiation such as ultraviolet light and X-rays.
Genes and
genomes
Genomic DNA is tightly and orderly
packed in the process called DNA condensation, to fit the small available volumes of
the cell.
Translation
and Transcription
A gene is a sequence of DNA having genetic
instructions and may influence the phenotype of an organism. Relationship
between the amino-acid sequences of proteins and the nucleotide sequences of genes is
determined by the rules of translation are collectively known as the genetic code. The
genetic code comprises of three-letter 'words' called codons formed from a
sequence of three nucleotides (e.g. ACT, CAG, TTT).
The codons of a gene are copied into
messenger RNA by RNA polymerase during transcription. This copy of RNA is further
decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA
to transfer RNA.
Replication
of DNA
The
process of cell division is necessary
for every organism to grow. During cell division, cell must replicate the DNA
in its genome so that the two daughter cells have the same genetic information
as their parent. DNA replicate in simple mechanism due to its double-stranded
structure. Firstly, the two strands are separated from each other. Then each
strand's complementary DNA sequence is re-formed by an enzyme known
as DNA polymerase which help
the complementary strand by finding the correct base through complementary base
pairing and bonding it onto the original strand. Many different mechanisms are
used to copy the antiparallel strands of the double helix, as DNA polymerases enzyme
can only extend a DNA strand in a 5′ to 3′ direction. During this process,
the base on the old strand directs which base appears on the new strand and
resultantly a perfect copy of its DNA in formed in the cell.
Chromosomal Genetic recombination
The helix of DNA usually does not
interact with other segments of DNA. However, in human cells the different
chromosomes even occupy separate areas in the nucleus known as "chromosome territories". In DNA this physical separation of different
chromosomes provides an important ability to function as a stable source for instructions,
because during sexual reproduction when genetic
recombination occurs sometimes
chromosomes interact is in chromosomal
crossover. This Chromosomal
crossover is occur only when two DNA helices break, swap a section and then
rejoin.
This recombination permits chromosomes
to exchange genetic instructions and produces new combinations of genes. This
genetic recombination increases the efficiency of natural selection, important
in the rapid evolution of new proteins and involved in DNA repair, mainly in
the cell's response to double-strand breaks.
Evolution of DNA
The DNA carry the genetic instructions
that permits all forms of life to function, grow and reproduce. However, it is not
clear that how long in the 4-billion-year history of life, the DNA has performed this function, because
as it has been proposed after various researches that the earliest forms of
life may have used RNA as their genetic material.