In this article we will discuss about:- 1. Meaning of DNA 2. Features of DNA 3. Molecular Structure 4. Components 5. Forms.
Contents:
- Essay on the Meaning of DNA
- Essay on the Features of DNA
- Essay on the Molecular Structure of DNA
- Essay on the Components of DNA
- Essay on the Forms of DNA
Essay # 1. Meaning of DNA:
A nucleic acid that carries the genetic information in the cell and is capable of self-replication and RNA synthesis is referred to as DNA. In other words, DNA refers to the molecules inside cells that carry genetic information and pass it from one generation to the next. The scientific name for DNA is deoxyribonucleic acid.
Essay # 2. Features of DNA:
The main features of DNA are given below:
i. Location:
In eukaryotes, DNA is found both in nucleus and cytoplasm. In the nucleus it is a major component of chromosome, whereas in cytoplasm it is found in mitochondria and chloroplasts. In prokaryotes, it is found in the cytoplasm.
ii. Structure:
Mostly the DNA structure is double stranded in both eukaryotes and prokaryotes. However, in some viruses DNA is single stranded. DNA is a double stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
iii. Shape:
In eukaryotes, the DNA is of linear shape. In prokaryotes and mitochondria; the DNA is circular.
iv. Replication:
The DNA is capable of self-replication. This is the only chemical which has self-replicating capacity. The DNA replicates in semi-conservative manner. In eukaryotes, DNA replicates during the S-Phase of the cell cycle.
v. Types:
There are different forms of DNA such as A, B, C, D and Z DNA. The DNA may be linear or circular. It may be double stranded or single stranded. It may be right handed or left handed. The DNA may be repetitive or unique. It may be nuclear or cytoplasmic.
vi. Functions:
DNA plays important role in various ways. It is used in transcription i.e. synthesis of mRNA which in turn is used in protein synthesis. It carries genetic information from one generation to the next generation. DNA stores and transmits the genetic information in cells.
It forms the basis for genetic code. The genes are made of DNA and are responsible for passing on traits from generation to generation. DNA contains the genetic instructions for the development and functioning of living organisms. Thus it is the substance of heredity.
Essay # 3. Molecular Structure of DNA:
The double helical structure of DNA was identified by Watson and Crick in 1953. This brilliant research work resulted in significant breakthrough in understanding the gene function. This structure has been verified in many different ways and is universally accepted. James Watson and Francis Crick were awarded Nobel prize in 1958 for this significant contribution in the field of molecular biology.
The important features of their model of DNA are as follows:
1. Two helical polynucleotide chains are coiled around a common axis, the chains run in opposite directions.
2. The purine and pyrimidine bases are on the inside of the helix whereas the phosphate and deoxyribose units are situated on the outside. Also the planes of the base residues are perpendicular to the helix axis. While the planes of the sugar residues are almost at right angles to those of the bases.
3. The diameter of the helix is 2 nm. Adjacent bases are separated by 0.34 nm along the helix axis. Hence the helix repeats itself every 10 residues on each chain at intervals of 3.4 nm.
4. The two chains are held together by hydrogen bonds formed between pairs of bases. Pairing is highly specific. Adenine pairs with thymine, guanine always pairs with cytosine. A = T, G = C.
5. The sequence of bases along the polynucleotide chain is not restricted. The precise sequence of bases carries the genetic information.
6. The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a spiral staircase.
7. The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.
Essay # 4. Components of DNA:
DNA molecule is a polymer which is composed of several thousand pairs of nucleotide monomers. Union of several nucleotides together leads to the formation of polynucleotide chain. The monomer units of DNA are nucleotides, and the polymer is known as a “polynucleotide.” Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group.
There are three components of DNA, viz:
(1) Nitrogenous bases,
(2) Deoxyribose sugar, and
(3) Phosphate group.
These are briefly discussed below.
i. Nitrogenous Bases:
Nucleotides are also known as nitrogenous bases or DNA bases. Nitrogenous base are of two types, viz. pyrimidines and purines.
Pyrimidines:
Main features of pyrimidines are given below:
(i) These are single ring structures,
(ii) These are of two types namely cytosine and thymine,
(iii) They occupy less space in DNA structure,
(iv) Pyrimidine is linked with deoxyribose sugar at position 3.
Purine:
Main features or purines are given below:
(i) They are double ring compounds.
(ii) They are of two types, viz. adenine and guanine.
(iii) They occupy more space in DNA structure.
(iv) Deoxyribose sugar is linked at position 9 of purine.
Thus, in DNA there are four different types of nitrogenous bases, viz. adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, the pyrimidine base thymine is replace by uracil.
Base Pairing:
The purine and pyrimidine bases always pair in a definite fashion. Adenine will always pair with thymine and guanine with cytosine. Adenine and thymine are joined by double hydrogen bonds while guanine and cytosine are joined by triple hydrogen bonds. However, these bonds are weak which help in separation of DNA strands during replication.
ii. Deoxyribose Sugar:
This is a pentose sugar having five carbon atoms. The four carbon atoms are inside the ring and the fifth one is with CH2 group. This has three OH groups on 1, 3 and 5 carbon positions. Hydrogen atoms are attached to carbon atoms one to four. In RNA, the sugar ribose is similar to deoxyribose except that it has OH group on carbon atom 2 instead of H group.
iii. Phosphate:
The phosphate molecule is arranged in an alternate manner to deoxyribose molecule. Thus there is deoxyribose on both sides of phosphate. The phosphate is joined with carbon atom 3 of deoxyribose at one side and with carbon atom 5 of deoxyribose on the other side.
Nucleosides and Nucleotides:
A combination of deoxyribose sugar and nitrogenous base is known as nucleoside and a combination of nucleoside and phosphate is called nucleotide.
Nucleoside = Deoxyribose Sugar + Nitrogenous base
Nucleotide = Deoxyribose + Nitrogenous base + Phosphate
Thus, a nucleotide is a nucleoside with one or more phosphate groups covalently attached to it. Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and deoxythymidine (dT).
DNA Backbone:
The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3′-hydroxyl and 5′-hydroxyl groups to phosphate groups in ester links, also known as “phosphodiester” bonds.
Essay # 5. Forms of DNA:
Depending upon the nucleotide base per turn of the helix, pitch of the helix, tilt of the base pair and humidity of the sample, the DNA can be observed in four different forms namely, A, B, C and D. The comparison of A, B and Z forms of DNA is presented in Table 15.1.
i. B-form:
This is the same form of DNA proposed by Watson and Crick.
Main features of B form of DNA are given below:
1. This is the most common form of DNA.
2. It is observed when humidity is 92% and salt concentration is high.
3. The coiling is in the right direction.
4. The number of base is 10 per turn of helix.
5. The pitch is 3.4 nm.
6. The sugar phosphate linkage is normal.
7. The helix is narrower and more elongated than A form.
8. The major groove is wide which is easily accessible to proteins.
9. The minor groove is narrow.
10. The conformation is favored at high water concentrations.
11. The base pairing is nearly perpendicular to helix axis.
12. The sugar puckering is C2′-endo.
ii. A-form:
1. This form is observed when the humidity of the sample is 75%.
2. The coiling is in the right direction.
3. The number of bases is 10.7 per turn of helix.
4. The pitch is 2.8 nm.
5. The sugar phosphate linkage is normal.
6. The major groove is deep and narrow which is not easily accessible to proteins.
7. The minor groove is wide and shallow which is accessible to proteins, but information content is lower than major groove.
8. The helix is shorter and wider than B form.
9. The conformation is favored at low water concentrations.
10. The base pairs are tilted to helix axis.
11. The sugar puckering is C3′-endo.
iii. Z-form:
1. The helix has left-handed coiling pattern.
2. The number of bases is 12 per turn of helix.
3. The pitch is 4.5 nm.
4. The sugar phosphate linkage is zigzag.
5. The major “groove” is not really a groove.
6. The minor groove is narrow.
7. The helix is narrower and more elongated than A or B form.
8. The base pairing is nearly perpendicular to helix axis.
9. The conformation is favored by high salt concentrations.
10. The sugar puckering is C: C2 endo, G : C2 exo.