DNA vs RNA: Structure, Function, and Cellular Location

DNA vs. RNA: Key Differences

DNA (Deoxyribonucleic Acid)

• Composition: The pentose sugar is D-deoxyribose. Contains the nitrogenous bases Adenine (A), Guanine (G), Thymine (T), and Cytosine (C) in similar proportions.
• Structure: DNA is a double-stranded molecule consisting of two polynucleotide chains.
• Configuration: Forms a double helix structure with the two chains connected by base pairing (A-T and C-G).
• Location: In eukaryotic cells, DNA is located in the nucleus, forming chromosomes. In prokaryotic cells, it forms a single, circular chromosome located in the cytoplasm.
• Function: DNA encodes the information that determines the order of amino acids required to synthesize all cellular proteins. A gene, a defined segment of DNA, contains the information for the synthesis of a single polypeptide chain. During transcription, the information from DNA is transferred to another molecule: messenger RNA (mRNA).

RNA (Ribonucleic Acid)

• Composition: The pentose sugar is D-ribose. Contains the nitrogenous bases Adenine (A), Guanine (G), Uracil (U), and Cytosine (C) in varying proportions.
• Structure: RNA molecules are typically single-stranded, consisting of a single polynucleotide chain.
• Configuration: RNA does not have a specific spatial structure, except for tRNA, which adopts a cloverleaf shape.
• Location: RNA is synthesized in the nucleus through a process called transcription and then moves to the cytoplasm where it functions.
• Function: RNA acts as an intermediary, carrying information from the nucleus to the cytoplasm. During translation, the sequence of bases in mRNA directs the synthesis of proteins in the ribosomes. Specific tRNA molecules transport amino acids and place them in the correct order to form the protein.

Cellular Level

The Cell Theory

The first description of cells was made by Robert Hooke in 1665 when he observed a thin slice of cork under a microscope. He named the structures he saw "cells" because they resembled the cells of a honeycomb. Later, the existence of free-living cells was discovered. In 1838 and 1839, Matthias Schleiden and Theodor Schwann formulated the cell theory, which states that the cell is the fundamental anatomical and functional unit of all living organisms. It is the smallest unit of life with its own properties. The cell theory is based on the following principles:

• All living organisms are composed of one or more cells.
• The cell is the basic unit of structure and function in living organisms.
• All cells arise from pre-existing cells through reproduction (germ cells).

In multicellular organisms, each cell has a life of its own.

Types of Cells: Eukaryotic and Prokaryotic

• Eukaryotic cells: Have a complex cytoplasm with membrane-bound organelles. They possess a distinct nucleus where DNA is associated with proteins.
• Prokaryotic cells: Lack a true nucleus. Their cytoplasm is less complex.
• Viruses: Their simplicity and lack of metabolic structure place them in a category between living and non-living organisms.

The Eukaryotic Cell: Plasma Membrane

The plasma membrane defines the cell's external environment, regulating the passage of substances through it. It is composed of four layers: two layers of globular proteins and two layers of bipolar lipids. The lipids that make up the membrane are phospholipids and steroids. The membrane proteins are either:

• Peripheral: Located on the surface of the lipid bilayer.
• Integral: Span the entire membrane, protruding on both sides.

Properties of the Plasma Membrane

• Self-assembly: The membrane components have a natural tendency to self-assemble and form bilayers.
• Self-sealing: If the membrane is broken, the phospholipids will spontaneously rearrange and reseal.
• Fluidity: The absence of covalent bonds and the presence of weak interactions allow for lateral movement of membrane components, giving the membrane fluidity and modularity.
• Impermeability: Due to the hydrophobic nature of the lipid bilayer, the membrane acts as a barrier, preventing the escape of most water-soluble contents from the cell.

Function: Selective Permeability

The plasma membrane controls the movement of substances between the cell's interior and exterior. The transport mechanisms differ depending on whether the substances are small molecules or macromolecules. Transport can be either passive or active.

Passive Transport

Passive transport is based on the phenomenon of diffusion and does not require energy input from the cell. It involves the movement of molecules down their concentration gradient, from an area of higher concentration to an area of lower concentration. Some molecules diffuse more rapidly because they are assisted by transport proteins. This form of passive transport is called facilitated diffusion.