DNA (Deoxyribonucleic Acid)
Introduction
Like other macromolecules, nucleic acids are composed of monomers, called nucleotides, which are polymerized to form large strands. Each nucleic acid strand contains certain nucleotides that appear in a certain order within the strand, called its base sequence. The base sequence of deoxyribonucleic acid (DNA) is responsible for carrying and retaining the hereditary information in a cell. In Mechanisms of Microbial Genetics, we will discuss in detail the ways in which DNA uses its own base sequence to direct its own synthesis, as well as the synthesis of RNA and proteins, which, in turn, gives rise to products with diverse structure and function. In this section, we will discuss the basic structure and function of DNA.
DNA Nucleotides
The building blocks of nucleic acids are nucleotides. Nucleotides that compose
DNA are called deoxyribonucleotides. The three components of a deoxyribonucleotide are a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base, a nitrogen-containing ring structure that is responsible for complementary base pairing between nucleic acid strands. The carbon atoms of the five-carbon deoxyribose are numbered 1ʹ, 2ʹ, 3ʹ, 4ʹ, and 5ʹ (1ʹ is read as “one prime”). A nucleoside comprises the five-carbon sugar and nitrogenous base.
The deoxyribonucleotide is named according to the nitrogenous bases . The nitrogenous bases adenine (A) and guanine (G) are the purines; they have a double-ring structure with a six-carbon ring fused to a five-carbon ring. The pyrimidines, cytosine (C) and thymine (T), are smaller nitrogenous bases that have only a six-carbon ring structure.
Individual nucleoside triphosphates combine with each other by covalent bonds known as 5ʹ-3ʹ phosphodiester bonds, or linkages whereby the phosphate group attached to the 5ʹ carbon of the sugar of one nucleotide bonds to the hydroxyl group of the 3ʹ carbon of the sugar of the next nucleotide. Phosphodiester bonding between nucleotides forms the sugar-phosphate backbone, the alternating sugar-phosphate structure composing the framework of a nucleic acid strand. During the polymerization process, deoxynucleotide triphosphates (dNTP) are used.
To construct the sugar-phosphate backbone, the two terminal phosphates are released from the dNTP as a pyrophosphate. The resulting strand of nucleic acid has a free phosphate group at the 5ʹ carbon end and a free hydroxyl group at the 3ʹ carbon end.
The two unused phosphate groups from the nucleotide triphosphate are released as pyrophosphate during phosphodiester bond formation. Pyrophosphate is subsequently hydrolyzed, releasing the energy used to drive nucleotide polymerization.
Discovering the Double Helix
By the early 1950s, considerable evidence had accumulated indicating that DNA was the genetic material of cells, and now the race was on to discover its three-dimensional structure. Around this time, Austrian biochemist Erwin Chargaff1(1905–2002) examined the content of DNA in different species and discovered that adenine, thymine, guanine, and cytosine were not found in equal quantities, and that it varied from species to species, but not between individuals of the same species.
He found that the amount of adenine was very close to equaling the amount of thymine, and the amount of cytosine was very close to equaling the amount of guanine, or A = T and G = C. These relationships are also known as Chargaff’s rules.
Other scientists were also actively exploring this field during the mid-20th century. In 1952, American scientist Linus Pauling (1901–1994) was the world’s leading structural chemist and odds-on favorite to solve the structure of DNA.
Pauling had earlier discovered the structure of protein α helices, using X-ray diffraction, and, based upon X-ray diffraction images of DNA made in his laboratory, he proposed a triple-stranded model of DNA.2 At the same time, British researchers Rosalind Franklin (1920–1958) and her graduate student R.G. Gosling was also using X-ray diffraction to understand the structure of DNA.
It was Franklin’s scientific expertise that resulted in the production of more well-defined X-ray diffraction images of DNA that would clearly show the overall double-helix structure of DNA.
James Watson (1928–), an American scientist, and Francis Crick (1916–2004), a British scientist, were working together in the 1950s to discover DNA’s structure.
They used Chargaff’s rules and Franklin and Wilkins’ X-ray diffractionimages of DNA fibers to piece together the purine-pyrimidine pairing of the double helical DNA molecule.
In April 1953, Watson and Crick published their model of the DNA double helix in Nature.3 The same issue additionally included papers by Wilkins and colleagues,4 as well as by Franklin and Gosling,5 each describing different aspects of the molecular structure of DNA.
In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Physiology and Medicine. Unfortunately, by then Franklin had died, and Nobel prizes at the time were not awarded posthumously.
Work continued, however, on learning about the structure of DNA. In 1973, Alexander Rich(1924–2015) and colleagues were able to analyze DNA crystals to confirm and further elucidate DNA structure.
DNA Structure
Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. The two DNA strands are antiparallel, such that the 3ʹ end of one strand faces the 5ʹ end of the other.
The 3ʹ end of each strand has a free hydroxyl group, while the 5ʹ end of each strand has a free phosphate group. The sugar and phosphate of the polymerized nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside. These nitrogenous bases on the interior of the molecule interact with each other, base pairing.
Analysis of the diffraction patterns of DNA has determined that there are approximately 10 bases per turn in DNA. The asymmetrical spacing of the sugar-phosphate backbones generates major grooves (where the backbone is far apart) and minor grooves (where the backbone is close together).
These grooves are locations where proteins can bind to DNA. The binding of these proteins can alter the structure of DNA, regulate replication, or regulate transcription of DNA into RNA.
Base pairing takes place between a purine and pyrimidine. In DNA, adenine (A) and thymine (T) are complementary base pairs, and cytosine (C) and guanine (G) are also complementary base pairs, explaining Chargaff’s rules. The base pairs are stabilized by hydrogen bonds; adenine and thymine form two hydrogen bonds between them, whereas cytosine and guanine form three hydrogen bonds between them.
In the laboratory, exposing the two DNA strands of the double helix to high temperatures or to certain chemicals can break the hydrogen bonds between complementary bases, thus separating the strands into two separate single strands of DNA (single-stranded DNA [ssDNA]).
This process is called DNA denaturation and is analogous to protein denaturation, as described in Proteins. The ssDNA strands can also be put back together as double-stranded DNA (dsDNA), through reannealing or renaturing by cooling or removing the chemical denaturants, allowing these hydrogen bonds to reform.
The ability to artificially manipulate DNA in this way is the basis for several important techniques in biotechnology. Because of the additional hydrogen bonding between the C = G base pair, DNA with a high GC content is more difficult to denature than DNA with a lower GC content.
Types of DNA
There are 4 major forms of DNA that are double-stranded. They are connected by the interaction of complementary base pairs.
B-form DNA
This is the most common form of DNA in which 2 strands of DNA, each in a right-hand helix, are coiled around the same axis. The H-bonding between the bases holds the 2 strands together.
A-form DNA
This is similar to B-form DNA in the sense it is also a right-handed double-helical structure but is thicker and has a shorter distance between its base pairs. When DNA is dehydrated, it takes the A form so that it can be protected from extreme conditions like desiccation. It can also be formed when protein binding removes the solvent from DNA.
Z-form DNA
This is the 3rd form of duplex DNA but is a left-handed helical structure. A sequence of alternating purines and pyrimidines form a zigzag structure in a Z-form DNA. It is present at the starting of a gene site hence it is assumed to play some role in gene regulation and transcription.
DNA Function
DNA stores the information needed to build and control the cell. The transmission of this information from mother to daughter cells is called vertical gene transfer and it occurs through the process of DNA replication. DNA is replicated when a cell makes a duplicate copy of its DNA, then the cell divides, resulting in the correct distribution of one DNA copy to each resulting cell. DNA can also be enzymatically degraded and used as a source of nucleosides and nucleotides for the cell. Unlike other macromolecules, DNA does not serve a structural role in cells.
This genetic information is carried by genes. Genes are small pieces of DNA containing approximately two million base pairs. A gene code for a polypeptide molecule counts three nitrogenous bases on a single amino acid.
Polypeptide chains are further coiled into secondary, tertiary, and quaternary structures to form different proteins. As different organisms contain numerous genes, different proteins can be formed.
Proteins are the significant structural and functional units in most organisms. Apart from conserving genetic information, The most important function of DNA is to carry genetic information from generation to generation coded in the way the nitrogen bases are arranged in the nucleotides. Some of the other functions of DNA include:
Cellular Metabolism- The metabolic reactions of the cells is regulated by DNA which uses enzymes, hormones, and specific RNAs (Ribonucleic acid which is responsible for the regulation and expression of genes) for this purpose.
Transcription- RNAs are produced from DNAs through the process of transcription
DNA Replication- Genetic information is passed from one cell to daughter cells and from one generation to the next. It produces carbon copies through replication.
Development of Organisms- By the mechanism of the internal genetic clock, the development of organisms is controlled by the DNA molecules.
DNA FingerPrinting- The DNA sequence is unique for each individual and cannot match that of another individual. This property is useful in DNA fingerprinting, which is carried out to identify an individual through his or her DNA.
Mutations- The alterations that occur in the DNA sequencing. Mutations occur due to errors during DNA replication. However, the errors can be caused due to exposure of DNA to ultraviolet radiation, deletion or insertion of DNA segments, etc.
Gene Therapy- It is a technique in which an individual's gene is modified to cure any disease. Gene therapy can work by different mechanisms like
1.Replacing a disease-causing gene with a healthy copy of a gene.
2.Inactivating a disease-causing gene that is not working properly.
3.Introducing a modified or new gene in the body to treat diseases.
Summary
Nucleic acids are composed of nucleotides, each of which contains a pentose sugar, a phosphate group, and a nitrogenous base. Deoxyribonucleotides within DNA contain deoxyribose as the pentose sugar.
DNA contains the pyrimidines cytosine and thymine, and the purines adenine and guanine.
Nucleotides are linked together by phosphodiester bonds between the 5ʹ phosphate group of one nucleotide and the 3ʹ hydroxyl group of another. A nucleic acid strand has a free phosphate group at the 5ʹ end and a free hydroxyl group at the 3ʹ end.
Chargaff discovered that the amount of adenine is approximately equal to the amount of thymine in DNA, and that the amount of the guanine is approximately equal to cytosine. These relationships were later determined to be due to complementary base pairing.
Watson and Crick, building on the work of Chargaff, Franklin and Gosling, and Wilkins, proposed the double helix model and base pairing for DNA structure.
DNA is composed of two complementary strands oriented antiparallel to each other with the phosphodiester backbones on the exterior of the molecule.
The nitrogenous bases of each strand face each other and complementary bases hydrogen bond to each other, stabilizing the double helix.
Heat or chemicals can break the hydrogen bonds between complementary bases, denaturing DNA. Cooling or removing chemicals can lead to renaturation or reannealing of DNA by allowing hydrogen bonds to reform between complementary bases.
DNA stores the instructions needed to build and control the cell. This information is transmitted from parent to offspring through vertical gene transfer.
