Nucleic Acids Function

Nucleic Acids - Functions of RNA and DNA

Nucleic acid is a natural chemical compound that can be broken down to produce phosphoric acid, sugars and a combination of organic bases (nucleotide, purines, and pyrimidines). They are the cell's main information-carrying molecules and they ultimately determine the inherited traits of every living organism by guiding the entire process of protein synthesis. There are two types of nucleic acid: RNA and DNA. In RNA the nucleotide bases are ribose, and the common pyrimidine bases are uracil and cytosine. In DNA, the nucleotides contain 2-deoxyribose, and the common pyrimidine bases are thymine and cytosine. The primary purines are adenine and guanine in both RNA and DNA.

Function of DNA

DNA functions are vital to inheritance, protein coding, and life's genetic blueprint. It is not surprising, given the enormity of the functions of DNA in the human body and its responsibility for the growth and maintenance of life that the discovery of DNA has led to so many developments in the treatment of various types of diseases. DNA is guided by the development and reproduction of an organism - ultimately, it’s survival.

DNA contains the genetic information

In a series of experiments by Avery, MacLeod, and McCarty, the demonstration that DNA contained the genetic information was first made in 1944. They showed that by introducing purified DNA from the former coccus into the latter, the genetic determination of the character (type) of the capsule of specific pneumococcus could be transmitted to another capsular type. These scholars actually referred to the agent (later shown to be DNA) that accomplished the change as a "transforming factor." Thereafter, this form of genetic manipulation became commonplace. Recently, similar studies have been conducted using yeast, cultivated mammalian cells, and insect and mammalian embryos as recipients and cloned DNA as a donor of genetic material.

Replication & transcription

There are two purposes in the genetic information stored in the DNA nucleotide sequence. It is the source of information for the synthesis of all cell and organism protein molecules, and it provides the information that daughter cells or offspring have inherited. Both of these functions require the DNA molecule to represent as a template — in the first case for the transcription of the information into RNA and in the second case for the daughter DNA molecules. The complementarity of the double-stranded DNA model Watson and Crick strongly suggests that semi-conservative replication of the DNA molecule occurs. Thus, when each strand of the double-stranded parental DNA molecule separates during replication from its complement, each serves as a template for synthesizing a new complementary strand. The two newly created double-stranded daughter DNA molecules are then sorted between the two daughter cells (Figure 35–5), each containing one strand (but complementary rather than identical) from the parent dual - stranded DNA molecules (Figure35–5). Each daughter cell contains DNA molecules with specific information remarkably similar to that possessed by the parent; however, the parent cell's DNA molecule was only semi-conserved in each daughter cell.

Function of RNA

Protein synthesis

Nearly all of the various RNA species are involved in some aspects of the synthesis of proteins. Messenger RNAs or mRNAs are designated as those cytoplasmic RNA molecules that serve as templates for protein synthesis (i.e., transferring DNA genetic information to protein synthesizing machinery). Many other cytoplasmic RNA molecules (ribosomal RNAs; rRNAs) have major structural roles in which they contribute to the formation and function of ribosomes (organellar protein synthesis machinery) or serve as adapter molecules (transfer RNAs; tRNAs) for translating RNA information into specific polymerized amino acid sequences.

Some RNA molecules have catalytic activity intrinsic to them. These ribozymes ' activity often involves a nucleic acid's cleavage. An illustration is the role of RNA in facilitating the retrieval into mature messenger RNA of the primary transcript of a gene. Within the nucleus, much of the RNA synthesized from DNA templates in eukaryotic cells, including mammalian cells, is degraded and never serves as either a structural or informational entity within the cell cytoplasm.

Small nuclear RNA (snRNA) species are found in all eukaryotic cells that are not directly involved in protein synthesis but play pivotal roles in the processing of RNA. These relatively small molecules vary in size between 90 and 300 nucleotides. For some animal and plant viruses, the genetic material is RNA rather than DNA. Though some RNA viruses never transcribe their information into a DNA molecule, many animal RNA viruses— specifically, the retroviruses (for example, the HIV virus)—are transcribed by an RNA-dependent DNA polymerase, the so-called reverse transcriptase, to produce a dual-stranded DNA copy of their RNA genome. The resulting double-stranded DNA transcript is integrated into the host genome in many cases and subsequently serves as a template for gene expression from which to transcribe new viral RNA genomes.

RNA as Hereditary Information

Though RNA in most cells do not really serve as genetic information, for many viruses that do not contain DNA, RNA holds this function. Therefore, RNA clearly has the extra ability to serve as genetic information. Although RNA is typically single cell stranded, there is considerable diversity in viruses. Rhinoviruses causing a common cold; influenza viruses; and one - stranded RNA viruses are Ebola viruses. Examples of double-stranded RNA viruses are rotaviruses that cause severe gastroenteritis in children and other immunocompromised individuals. Because in eukaryotic cells double-stranded RNA is uncommon, its presence serves as an indicator of viral infection. Viruses analyze in more detail the implications for a virus having an RNA genome instead of a DNA genome.

Other Functions of Nucleic Acid

Nucleotides Carry Chemical Energy in Cells

One or two additional phosphates may be attached to the phosphate group covalently linked to a ribonucleotide's 5' hydroxyl. The resulting 
molecules are called mono-, di-, and triphosphate nucleoside. Nucleoside triphosphate hydrolysis provides the chemical energy needed to drive a wide range of cellular reactions. Adenosine 5'-ATP, triphosphate, is by far the most widely used for this purpose, but in some reactions, UTP, GTP, and CTP are also used. The triphosphate group structure accounts for the energy released by ATP hydrolysis and the other nucleoside triphosphates. The bond between the α-phosphate and the ribose is an ester bond. Under standard conditions, hydrolysis of the ester bond yields about 14 kJ / mol, whereas hydrolysis of each bond yields about 30 kJ / mol. In biosynthesis, ATP hydrolysis often plays a significant thermodynamic role. When combined with a reaction with a positive change in free energy, ATP hydrolysis shifts the overall process balance to favor product formation.

Adenine Nucleotides are Components of many Enzyme Cofactors

Various types of enzyme cofactors that serve a wide range of chemical functions comprise of adenosine as part of their structure. They are structurally unrelated and the presence of adenosine is the only common factor. In none of these cofactors does the adenosine portion participate directly in the primary function, but the removal of adenosine generally results in a drastic reduction of cofactor activities. Although this requirement for adenosine has not been investigated in detail, it must involve the binding energy between enzyme and substrate (or cofactor) that is used both in catalysis and in stabilizing the initial enzyme-substrate complex. In the case of ketoacyl-CoA transferase, the nucleotide moiety of coenzyme A appears to be a binding “handle” that helps to pull the substrate (acetoacetyl-CoA) into the active site.

Adenosine is certainly not unique in the amount of potential energy it can contribute. The importance of adenosine probably lies not so much in some special chemical characteristic as in the evolutionary advantage of using one compound for multiple roles. Once ATP became the universal source of chemical energy, systems developed to synthesize ATP in greater abundance than the other nucleotides; because it is abundant, it becomes the logical choice for incorporation into a wide variety of structures. A single protein domain that binds adenosine can be used in a wide variety of enzymes. Such a domain called a nucleotide-binding fold, is found in many enzymes that bind ATP and nucleotide cofactors.

Some Nucleotides Are Regulatory Molecules

By taking signals from hormones or other external chemical signals, cells respond to their environment. The interaction between these extracellular chemical signals ("first messengers") and cell surface receptors often leads to the production of second messengers inside the cell, which in turn leads to adaptive changes inside the cell. The second messenger is often a nucleotide. Adenosine 3', 5'-cyclic monophosphate (cyclic AMP, or cAMP), formed from ATP in a reaction catalyzed by adenylyl cyclase, an enzyme associated with the plasma membrane's inner face, is one of the most common. In virtually every cell outside the plant kingdom, Cyclic AMP serves regulatory functions. In many cells, Guanosine 3 ', 5 ' - cyclic monophosphate (cGMP) occurs and also has regulatory functions.