Physiology

Genetic Code & : Chromosomes I. Fundamental Concepts A. The Structure and Components of Nucleic Acids: DNA & RNA At the heart of all life is information, and in biological systems, this information is stored and transmitted by nucleic acids. There are two primary types: Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). Both are polymers made up of repeating monomer units called nucleotides. 1. The Nucleotide: The Building Block Each nucleotide is composed of three main components: A. A Pentose Sugar: In DNA: The sugar is 2′-deoxyribose (lacks a hydroxyl group at the 2′ carbon). In RNA: The sugar is ribose (has a hydroxyl group at the 2′ carbon). Significance: The presence or absence of this 2′-OH group is critical. The 2′-OH group in RNA makes it more reactive and less stable than DNA. B. A Nitrogenous Base: These are nitrogen-containing heterocyclic compounds. They fall into two categories: Purines (double-ring structure): Adenine (A) Guanine (G) Pyrimidines (single-ring structure): Cytosine (C) Thymine (T) (found only in DNA) Uracil (U) (found only in RNA, replaces Thymine) Memory Aid CUT the PY: Cytosine, Uracil, Thymine are Pyrimidines. AG is PUre: Adenine, Guanine are Purines. C. A Phosphate Group: Consists of a phosphorus atom bonded to four oxygen atoms. Attached to the 5′ carbon of the pentose sugar. Significance: Phosphate groups give nucleic acids their negative charge and allow them to form the backbone of the polymer. Combining these components: A base + sugar = Nucleoside (e.g., Adenosine, Guanosine, Cytidine, Uridine for RNA; Deoxyadenosine, Deoxyguanosine, Deoxycytidine, Deoxythymidine for DNA). A base + sugar + phosphate = Nucleotide (e.g., Adenosine Monophosphate (AMP), Deoxyadenosine Monophosphate (dAMP)). These are often referred to by their triphosphate forms (ATP, GTP, etc.) when they are free in the cell, as these are the forms used for synthesis. 2. Polynucleotide Chains: The Backbone Nucleotides are linked together to form long polynucleotide chains. This linkage occurs via a phosphodiester bond. A phosphodiester bond is formed between the 5′-phosphate group of one nucleotide and the 3′-hydroxyl group of the sugar of the adjacent nucleotide. This creates a sugar-phosphate backbone, with the nitrogenous bases extending off this backbone. Polarity: Because of this linkage, each polynucleotide strand has a distinct directionality or polarity: One end has a free phosphate group attached to the 5′ carbon of the sugar (the 5′ end). The other end has a free hydroxyl group attached to the 3′ carbon of the sugar (the 3′ end). Significance: All nucleic acid synthesis (DNA replication, RNA transcription) occurs in the 5′ to 3′ direction. 3. DNA vs. RNA: Key Differences Feature DNA (Deoxyribonucleic Acid) RNA (Ribonucleic Acid) Primary Function Long-term storage and transmission of genetic information Gene expression (carrying genetic message, making proteins) Sugar 2′-deoxyribose Ribose Bases Adenine (A), Guanine (G), Cytosine (C), Thymine (T) Adenine (A), Guanine (G), Cytosine (C), Uracil (U) Structure Typically double-stranded helix Typically single-stranded, but can fold into complex 3D shapes Stability Very stable (due to deoxyribose and double helix) Less stable (due to ribose and often single-stranded) Location Primarily in the nucleus (eukaryotes), mitochondria, chloroplasts Nucleus, cytoplasm, ribosomes (multiple forms) 4. The DNA Double Helix: Watson and Crick Model The most iconic structure in molecular biology is the DNA double helix, elucidated by Watson and Crick (with crucial contributions from Rosalind Franklin and Maurice Wilkins). Two Polynucleotide Strands: DNA consists of two long polynucleotide strands wound around each other to form a right-handed double helix. Antiparallel Orientation: The two strands run in opposite directions; one strand runs 5′ to 3′, and its complementary strand runs 3′ to 5′. This is crucial for replication and transcription. Sugar-Phosphate Backbone: The sugar-phosphate backbones are on the outside of the helix, forming the structural framework. Nitrogenous Bases Inside: The nitrogenous bases are stacked in the interior of the helix, like steps on a spiral staircase. Complementary Base Pairing: This is the most critical feature. Bases on one strand form specific hydrogen bonds with bases on the opposite strand: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds (A=T). Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds (G≡C). Significance: This pairing ensures that the two strands are complementary, meaning the sequence of one strand dictates the sequence of the other. It’s vital for accurate DNA replication and repair. Hydrogen Bonds: These weak bonds hold the two strands together. While individually weak, their collective strength along the entire DNA molecule provides significant stability. Major and Minor Grooves: The helical twisting of the DNA strands creates two grooves on the surface: a wider major groove and a narrower minor groove. These grooves are important for protein binding, allowing regulatory proteins to access and interact with specific base sequences without having to unwrap the helix. B. The Central Dogma of Molecular Biology The concept of the Central Dogma, first proposed by Francis Crick, describes the fundamental flow of genetic information within a biological system. It states: DNA → RNA → Protein Let’s break down each arrow: DNA → DNA (Replication): The process by which a cell makes an exact copy of its entire DNA content. Essential for cell division, ensuring that each daughter cell receives a complete set of genetic instructions. Occurs in the nucleus (eukaryotes) during the S phase of the cell cycle. DNA → RNA (Transcription): The process by which the genetic information encoded in a gene (segment of DNA) is copied into an RNA molecule. This RNA molecule acts as an intermediary, carrying the genetic message from the DNA (which stays in the nucleus) to the protein-synthesizing machinery in the cytoplasm. Occurs in the nucleus (eukaryotes). RNA → Protein (Translation): The process by which the genetic code carried by messenger RNA (mRNA) is decoded to synthesize a specific protein. This is where the “language” of nucleic acids (sequence of nucleotides) is translated into the “language” of proteins (sequence of amino acids). Occurs in the cytoplasm on ribosomes. Overall Significance of the Central Dogma: It defines the sequential flow of genetic information that ultimately leads to the