Nucleic Acids Overview, Composition & Functions PDF

Nucleic Acids: Overview, Composition, and Functions Nucleic acids are essential biomolecules that serve as the genetic blueprint for all living organisms. They are long polymers made up of repeating units called nucleotides, each consisting of three key components: nitrogenous bases, a pentose suga...

Nucleic Acids: Overview, Composition, and Functions Nucleic acids are essential biomolecules that serve as the genetic blueprint for all living organisms. They are long polymers made up of repeating units called nucleotides, each consisting of three key components: nitrogenous bases, a pentose sugar, and a phosphate group. Classification of Nucleic Acids Nucleic acids are classified based on their sugar type and function: Deoxyribonucleic Acid (DNA): The primary genetic material in most organisms, found in the nucleus, mitochondria, and chloroplasts. DNA is a double-stranded helix with complementary base pairing: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Its primary functions include storing genetic information, providing a template for RNA synthesis, and facilitating replication during cell division. Ribonucleic Acid (RNA): Involved in protein synthesis and gene regulation. RNA is single- stranded and contains uracil (U) instead of thymine (T). RNA can be further classified into: I. Messenger RNA (mRNA) (1-5% of total RNA): mRNA carries genetic instructions from DNA to ribosomes in the cytoplasm, where it directs protein synthesis, making up about 1-5% of the total RNA in the cell. II. Transfer RNA (tRNA) (10-15% of total RNA): tRNA transports specific amino acids to the ribosome during translation, contributing about 10-15% of the total RNA in the cell. III. Ribosomal RNA (rRNA) (80-90% of total RNA): rRNA forms the structural and catalytic components of ribosomes, which are responsible for protein synthesis, making up the majority (about 80-90%) of RNA in the cell. IV. Regulatory RNAs (miRNA and siRNA) (1-2% of total RNA): Regulatory RNAs, including miRNA and siRNA, regulate gene expression by binding to mRNA to inhibit its translation or promote its degradation, accounting for roughly 1-2% of the total RNA in the cell. The Central Dogma of Molecular Biology The central dogma of molecular biology outlines the flow of genetic information in a biological system, describing the process by which information in DNA is used to produce proteins. The flow follows a one-way path: DNA → RNA → Protein  DNA Replication: The process by which a cell copies its DNA before cell division, ensuring that daughter cells inherit identical genetic material. Transcription: The process by which a segment of DNA is copied into RNA, specifically mRNA. Translation: The process by which mRNA is translated into proteins at the ribosome, where amino acids are assembled according to the mRNA sequence. Differences Between DNA and RNA Feature DNA RNA Sugar Deoxyribose (lacks -OH at 2' carbon) Ribose (has -OH at 2' carbon) Contains uracil (U) instead of thymine Nitrogenous Bases Contains thymine (T) (T) Strand Structure Double-stranded (double helix) Single-stranded Molecular More stable in alkaline conditions Less stable, prone to hydrolysis Stability Involved in protein synthesis and Function Stores genetic information regulation Length Longer, millions of nucleotides Shorter, varies with type and function Feature DNA RNA Found in both the nucleus and Location Found in the nucleus (and mitochondria) cytoplasm Composition of Nucleic Acids Nucleic acids are complex macromolecules composed of repeating units called nucleotides, which are the basic building blocks of both DNA and RNA. The combination and arrangement of these components enable nucleic acids to store and transmit genetic information. 1. Nitrogenous Bases: Purines: These are larger, double-ring structures including; Adenine (A) (6-amino purine) and guanine (G) (2-oxo, 6-amino purine) are present in both DNA and RNA. Pyrimidines: These are smaller, single-ring structures. They include; Cytosine (C), thymine (T, in DNA), and uracil (U, in RNA). 2. Pentose Sugar: Deoxyribose: Found in DNA; lacks an -OH group at the 2' carbon. Ribose: Found in RNA; has an -OH group at the 2' carbon.  Nucleosides are formed when nitrogenous bases are attached to the pentose sugar (D- ribose or 2-deoxy-D-ribose) via a beta-N-glycosidic bond between the sugar's 1st carbon and the nitrogen atom of the base. Purine nucleosides end in "-sine" and pyrimidine nucleosides in "-dine," with deoxy nucleosides prefixed by "d-" and uracil combining with ribose while thymine pairs with deoxyribose. 3. Phosphate Group:  Contributes to the acidic nature and negative charge of nucleic acids. It forms the sugar- phosphate backbone of nucleic acids through phosphodiester bonds between the 5' and 3' carbons of adjacent sugars. 4. Nucleotides: Nucleotides are composed of: o A nitrogenous base (purine or pyrimidine), o A pentose sugar (deoxyribose or ribose), o One or more phosphate groups. When a nucleoside is esterified to a phosphate group, it forms a nucleotide or nucleoside monophosphate. If a second phosphate group is added, it becomes a nucleoside diphosphate, and the addition of a third phosphate group results in a nucleoside triphosphate. Nucleic acids (DNA and RNA) are polymers made of nucleoside monophosphates. Functions of Nucleic Acids 1. Genetic Information Storage: DNA serves as the repository of genetic information, encoding instructions necessary for organismal development, functioning, and reproduction. For example, DNA in humans’ controls protein synthesis for various bodily functions. 2. Replication: DNA replicates itself to ensure that genetic information is passed on during cell division, allowing for accurate inheritance. 3. Transcription and Translation: DNA directs the synthesis of RNA which in turn directs protein synthesis, processes necessary for cell growth and function. 4. Energy Transfer: Nucleotides such as ATP (adenosine triphosphate) and GTP (guanosine triphosphate) are critical for cellular energy transfer, fueling various biochemical processes. 5. Role in Coenzymes: Nucleic acids form part of coenzymes including NAD+, NADP, FAD and coenzyme A which are essential for redox reactions, metabolism, and energy production. 6. Cell Signaling: Nucleic acids like cyclic AMP (cAMP) act as second messengers in cell signaling, influencing cellular responses to external stimuli. 7. Catalysis (Ribozymes): Certain RNA molecules, like the ribosome, act as enzymes. Ribozymes catalyze biochemical reactions, such as the formation of peptide bonds during protein synthesis. Structure of DNA Deoxyribonucleic acid (DNA) is composed of four deoxyribonucleotides, i.e. deoxyadenylate (A), deoxyguany late (G), deoxycytidylate (C), and thymidylate (T). These units are linked by 3′ to 5′ phosphodiester bonds to form a long polypeptide chain Watson-Crick Model of DNA Structure In the year 1953, Two scientists, James Watson (an American molecular biologist, geneticist and zoologist) along with Francis Crick (a British molecular biologist, biophysicist and neurologist) have demonstrated the double-helical structure of DNA. James Watson and Francis Crick collaborated with the Maurice Wilkins and Rosalind Franklin to introduce the DNA model. The model explained by Watson and Crick is considered to be the best model. Watson and Crick gain a lot of attention and appreciation for their work in the discovery of the DNA double-helical structure. They won Nobel Prize for their contribution in the year 1962 which they shared with the Wilkins. According to the Crick and Watson model, DNA consists of two polydeoxyribonucleotide chains twisted into a right-handed helix. The sugar-phosphate backbone forms the outer rails of the helix, while the nitrogenous bases—adenine, thymine, guanine, and cytosine—form the inner steps. The sugar-phosphate backbone holds both the polynucleotide strands of DNA by means of “Phosphodiester bond”. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. The base pairing (A with T; G with C) is called Chargaff’s rule, which states that the number of purines is equal to the number of pyrimidines. These complementary base pairings ensure uniform helix width and stability. The two strands run antiparallel to each other, with one strand oriented from 5′ to 3′ and the other from 3′ to 5′. The hydrogen bonds and complementary base pairing maintain the structure and allow for accurate replication and transcription. Turning of DNA causes a formation of wide indentations, i.e. “Major groove”. The distance between the two strands forms a narrow indentation, i.e. “Minor groove”. The formation of major and minor grooves result after the DNA coiling and the grooves also act as a site of DNA binding proteins.The DNA helix has a pitch of 3.4 nanometers per turn, containing ten base pairs per turn. Adjacent bases are spaced 0.34 nanometers apart, and the helix diameter measures approximately 1.9 to 2.0 nanometers.