Nucleic Acids
Introduction
Nucleic acids are complex, high-molecular-weight biochemical macromolecules that serve crucial functions in all living organisms. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main types of nucleic acids are DNA and RNA. DNA carries the genetic information in the cell, while RNA is involved in protein synthesis and sometimes in the transmission of genetic information.
Structure of Nucleic Acids
Nucleic acids are polymers of nucleotides. The nucleotides in DNA are composed of a deoxyribose sugar, a phosphate, and a nitrogenous base. The four types of nitrogenous bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, the sugar is ribose instead of deoxyribose, and the base uracil (U) replaces thymine.
DNA Structure
DNA is a double-stranded molecule held together by hydrogen bonds between base pairs. The structure of DNA is a double helix, like a twisted ladder. The sides of the ladder are made of alternating sugar and phosphate molecules, and the rungs are formed by base pairs connected by hydrogen bonding. The sequence of these bases constitutes the genetic code, which carries the instructions for synthesizing proteins and for the replication of the DNA molecule itself.
RNA Structure
RNA is typically a single-stranded molecule, although it can form localized double-stranded regions. Its structure is more variable than that of DNA, enabling it to perform a wider range of functions. There are several types of RNA, each with a specific function in the cell. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), among others.
Functions of Nucleic Acids
Nucleic acids play a critical role in the storage and expression of genetic information. They are also key players in the synthesis of proteins.
DNA Functions
The primary function of DNA is to store genetic information. The sequence of bases in a DNA molecule codes for the sequence of amino acids in a protein. This information is used during protein synthesis, a process in which the genetic code in DNA is translated into a sequence of amino acids to form a protein.
RNA Functions
RNA plays several roles in the cell, primarily involving the synthesis of proteins. Messenger RNA (mRNA) carries the genetic information from DNA to the ribosome, where it is used to assemble a protein. Transfer RNA (tRNA) carries amino acids to the ribosome to be added to the growing protein chain. Ribosomal RNA (rRNA) is a component of the ribosome and plays a role in the catalysis of peptide bond formation.
Nucleic Acid Synthesis
The synthesis of nucleic acids involves a series of complex, enzyme-catalyzed reactions. DNA synthesis, or DNA replication, is a semi-conservative process in which each strand of the original DNA molecule serves as a template for the synthesis of a new strand. RNA synthesis, or transcription, involves the synthesis of an RNA molecule from a DNA template.
DNA Synthesis
DNA synthesis occurs during the S phase of the cell cycle. The process begins with the unwinding of the DNA double helix by the enzyme helicase. Each strand of the unwound DNA serves as a template for the synthesis of a new strand by the enzyme DNA polymerase. The result is two identical DNA molecules, each composed of one original strand and one newly synthesized strand.
RNA Synthesis
RNA synthesis, or transcription, is the process by which an RNA molecule is synthesized from a DNA template. This process is catalyzed by the enzyme RNA polymerase, which synthesizes the RNA molecule in a 5' to 3' direction. The resulting RNA molecule is complementary to the DNA template strand and identical to the DNA coding strand, except that uracil replaces thymine.
Conclusion
Nucleic acids are essential to all forms of life. They carry the genetic information that determines the characteristics of organisms and direct the synthesis of proteins, which perform most of the functions in a cell. Understanding the structure and function of nucleic acids is fundamental to understanding life at the molecular level.