DNA: Structure, Types, and Functions
Last Updated : 10 Apr, 2024
DNA structure is made of nucleotide base pairs (other than RNA). DNA is the hereditary material that is possessed by all the organisms found on the Earth except certain virus species. DNA functions involve the transfer of genetic information from generation to generation.
The full form of DNA is Deoxyribonucleic acid which stands; ‘deoxy’ stands for missing oxygen, ‘ribo’ stands for ribose sugar, ‘nucleic’ represents the nucleus of a cell, and ‘acid’ represents the acidic nature of DNA due to phosphorus. In this article, we will cover DNA structure and functions, its types, and importance.
What is DNA?
DNA or Deoxyribonucleic Acid is the genetic material that codes the information for all the different processes that make an organism living like growth, replication, metabolism, etc. DNA is present in each cell (except for some viral species, RBCs, sieve cells, etc.) and is passed down from parents to their offspring. DNA is comprised of units called nucleotides. DNA is self-replicating, a long stretch of nucleotides.
DNA is a form of nucleic acid and is one of the four major macromolecules that make up the living system. In eukaryotic cells, it is found in the nucleus of the cell whereas in prokaryotes it is found free-floating in the cell cytoplasm. Other than the nucleus DNA is also found in mitochondria, chloroplast, and in smaller forms called plasmid in certain bacterial species.
Also Read: Difference Between Nucleotide and Nucleoside
Nucleotides
Nucleotides are the basic unit of the genetic material. Every nucleotide contains three parts: a phosphate group (one phosphorus bond to four oxygen particles), a sugar backbone (deoxyribose or ribose), and nitrogenous bases. The nitrogen bases are adenine (A), thymine (T) in the case of DNA, guanine (G), cytosine (C), and uracil (U) in the case of RNA. Together, these act as the “letters” that make up the hereditary code of our DNA. The nitrogenous bases are divided into two groups; Purines (G and A), and pyrimidines (C, U, and T).
Nucleotides are joined together to form two long strands that twist to form a structure called the double helix. On the off chance that you consider the twofold helix structure as a ladder, the phosphate and sugar particles would be the sides, while the base matches would be the steps. The bases on one strand pair with the bases on another strand: Adenine matches with thymine in DNA or uracil in RNA (A-T/U), and guanine matches with cytosine (G-C).
Also Read:Difference Between Purines and Pyrimidines
Who Discovered DNA?
The brief history of DNA includes
- In 1869, Friedrich Miescher identified a substance in the nuclei of human WBC which he named “nuclein”.
- In 1919, based on Miescher’s work Phoebus Levene proposed the “polynucleotide” model of nucleic acid.
- Based on Levene’s work Erwin Chargaff and Oswald Avery delved into the gene structure and composition.
- Erwin Chargaff has the “Chargaff’s rule” for A, T, G, and C arrangement and composition.
- In 1953, James Watson and Francis Crick gave the double helix model of DNA with the help of X-ray crystallography done by Rosalind Franklin and Maurice Wilkins.
Properties of DNA
DNA shows the following main characteristics;
- It is a long polymer made of repeating units of nucleotides (A, T, G, and C)
- It is made up of two strands that are held together by a hydrogen bond to form a double helical structure.
- DNA can have hundreds to millions of nucleotides in it.
- The largest human chromosome is chromosome no. 1 with around 220 million base pairs.
- The smallest human chromosome is the chromosome no. 21 with around 45 million base pairs.
- A nucleotide is formed when deoxyribose sugar is connected to nitrogenous base and a phosphate group.
- The backbone of DNA is formed when multiple units of sugar are joined by phosphodiester bonds.
- The sugars are held together by phosphate groups in such a manner that there are phosphodiester connections between the third and fifth carbon atoms of adjacent sugar rings. These are the 3′-end (three prime end) and 5′-end (five prime end) carbons.
- The two DNA strands will have opposite polarity i.e. if one strand is 3′-end to 5′-end then the other will be 5′-end to 3′-end.
- Chargaff’s rule states that “the amounts of adenine (A) and thymine (T) are usually similar, as are the amounts of guanine (G) and cytosine (C).
Structure of DNA
The DNA structure represents a helical ladder and is depicted as a double helix. It is a nucleic corrosive, and all nucleic acids are comprised of nucleotides. The structure of DNA molecule is made out of units called nucleotides, and every nucleotide is made out of three parts i.e. sugar, phosphate, and nitrogen bases. The bases A, T, G, and C are held by the sugar-phosphate complex to form the DNA strands. These 4 Nitrogenous bases pair together in an accompanying manner: A with T and C with G.
The two strands of DNA are arranged in opposite polarity. These strands are kept together by the hydrogen bond that is available between the two corresponding nitrogenous bases. There are 10.5 base pairs per turn of the helix and the pitch of every helix is 3.4 nm. Subsequently, the distance between two back-to-back base matches (i.e., hydrogen-bound bases of the opposite strands) is 0.34 nm. The helix is right-handed, meaning that the chains rotate counterclockwise as they approach an observer observing the longitudinal axis.
On one end of a strand, an exposed 5′-hydroxyl group is found, frequently with connected phosphates, whereas the other end has a free 3′-hydroxyl group present. The two strands are arranged oppositely with each other. This structure is best explained by the double-helical model given by Watson and Cricks.
Chromosomes
The double helical structure of the DNA is usually too long (almost 2 meters in length) to be placed in a nucleus linearly so the DNA is supercoiled to make it fit into the space of the nucleus. As the DNA is helical, whenever it is rotated any further it will coil many times upon itself forming a super coil. This coiling is assisted by proteins to make the long thread compactly packed to fit inside the nucleus. These packed structures are found in the form of thread-like structures called chromosomes.
Types of DNA
Following are some most common types of DNA found;
- Nuclear DNA (nDNA): The eukaryotic cells have a nucleus that contains the genetic material of the cell i.e. the DNA. This form of DNA that controls all the general characteristics of cell is called nuclear DNA.
- Mitochondrial DNA (mtDNA): Mitochondria are semi-autonomous as they possess their DNA, RNA, and ribosomes. This DNA is called mitochondrial DNA and it usually comes from the mother in case of sexually reproducing organisms.
- A-DNA: These are right-handed double helical DNA that have 11 bases per complete turn with a 2.3 nm diameter of the helix.
- B-DNA: It is the typical right-handed double helical DNA found in most of the cells. They have 10 bases per complete turn with a 1.9 nm diameter of the helix.
- C-DNA: It is right-handed double helical DNA that has 9.3 bases per complete turn with a 1.9 nm diameter of the helix.
- D-DNA: Lacks the Guanine (G) base unit making it an interesting variation and a rare form. It has 8 base pairs per complete turn with 2.3 nm diameter of the helix.
- E-DNA: It is an eccentric and extended form of DNA that is a right-handed helix. They have 7.5 bases per complete turn with a 2.3 nm diameter of the helix.
- Z-DNA: It has a zig-zag pattern with a left-handed helix that has 12 bases per complete turn with a 1.8 nm diameter of the helix.
Functions of DNA
Following are some of the important functions of DNA;
- It carries and transfers the genetic information.
- The genetic information code of functional proteins.
- DNA transfers the genetic information from one generation to the next.
- It regulates the metabolic reactions of the cell.
- It helps in the synthesis of RNA by transcription.
- It is self-replicative i.e. DNA can make its copies.
- It controls all the aspects of an organism’s development.
- It can be used for DNA fingerprinting during any crime or parental dispute.
- It can undergo abrupt changes or mutations due to any possible reason.
- It gives the required information to design gene-based therapy for disease.
Difference between RNA and DNA
Some of the common differences between DNA and RNA are mentioned here:
| Characteristics |
DNA |
RNA |
|---|---|---|
| Abbreviation | (DNA) Deoxyribonucleic acid | (RNA) Ribonucleic acid |
| Sugar | Deoxyribose sugar (2’OH) | Ribose sugar |
| Bases | Adenine, Thymine, Guanine, Cytosine | Adenine, Uracil, Guanine, Cytosine |
| Structure | Double-stranded | Usually single-stranded |
| Location | Mostly in the nucleus and mitochondria of the cell. | Found in the nucleus, ribosome, and cytoplasm |
| Stability | More stable and less prone to change | Less stable and more prone to change |
| Length | Longer and can be up to millions of base pairs | Shorter and typically several hundred to a few thousand nucleotides long |
| Types | There is only one type of DNA | RNA comes in a variety of forms, such as messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) |
|
Function |
Stores genetic information |
Acts as a template for protein synthesis |
|
Role in protein synthesis |
DNA provides the genetic code, which is transcribed into mRNA |
RNA serves as the template for protein synthesis (mRNA) and carries out protein synthesis (tRNA and rRNA) |
|
Examples |
Found in chromosome. |
Present in various types like mRNA, tRNA, and rRNA |
Also Read: Difference Between DNA and RNA
DNA as the Genetic Material
Both DNA and RNA contain the sugar ribose, which is a ring of carbon molecules encompassed by oxygen and hydrogen. In any case, though RNA contains a complete ribose sugar, DNA contains a ribose sugar that has lost one oxygen and one hydrogen particle. The additional oxygen and hydrogen content in RNA leaves it susceptible to hydrolysis by enzymes.
Under ordinary cell conditions, RNA goes through hydrolysis nearly multiple times quicker than DNA, which makes DNA a more stable particle. So, overall DNA is the chosen genetic material because;
- DNA is more easily repaired.
- DNA Information is more stable to the activity of enzymes.
- Double-stranded and allows double-checking for any error.
Importance of DNA
DNA contains the information that is fundamental for a living being to develop, function, and reproduce. DNA assists your body with development. The cells read this code on three bases all at once to create proteins that are fundamental for development and growth. Each combination of three nucleotide bases codes to amino acids, which are the structure blocks of proteins. Proteins are the main functional unit that directly and indirectly gets associated with all the functions of living systems when synthesized correctly.
DNA Replication
DNA replication is the process by which the DNA makes its copy. In this process, the two strands break to make two templates on which new opposite strands are formed. That means 1 double helical DNA will give rise to two new double helical DNA. As in the new DNA one strand comes from the parent DNA, the process is therefore called semi-conservative. DNA replication involves the following steps;
- Initiation: When the two strands of DNA separate to mark the beginning of the process.
- Elongation: When new bases are paired with the parental strand to form the new strand.
- Termination: When the newly attached base pair is joined by the sugar-phosphate backbone the process terminates thus forming a new double helix of DNA.
Conclusion: DNA – Structure, Types, and Functions
DNA, the hereditary material present in almost all organisms, consists of nucleotide base pairs. It is important for genetic transfer across generations. It stands for Deoxyribonucleic Acid, reflecting its composition of deoxyribose sugar, phosphate, and nitrogenous bases, with adenine pairing with thymine and guanine with cytosine. The structure of DNA, resembling a twisted ladder or double helix, comprises nucleotides forming long strands, held together by hydrogen bonds between complementary base pairs. It plays vital roles in encoding genetic information, regulating metabolic processes, and facilitating cellular activities, essential for life’s continuity and diversity.
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