Section 2: Carbon Atoms and Large Biological Molecules

Nucleic Acids: Structure and Function

Nucleic acids are essential biological macromolecules that store, transmit, and help express hereditary information in all living organisms. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• Gene: A discrete unit of inheritance, located on chromosomes, composed of DNA.

• Nucleic acids: Polymers called polynucleotides, made up of monomers called nucleotides.

• DNA: Stores genetic information and directs its own replication. It is passed from parent to offspring and from cell to cell.

• RNA: Functions in gene expression by directing the synthesis of proteins.

Roles of Nucleic Acids

• Information Storage: DNA carries genetic information for all cellular activities.

• Gene Expression: DNA directs the synthesis of messenger RNA (mRNA) in a process called transcription.

• Translation: mRNA is translated into a polypeptide with a specific amino acid sequence, forming proteins that perform cellular functions.

• Central Dogma: The flow of genetic information is typically described as DNA → RNA → Protein.

Components of Nucleic Acids

Nucleic acids are composed of long chains of nucleotides. Each nucleotide consists of three components:

• Nitrogenous base: Can be a purine (adenine [A], guanine [G]) or a pyrimidine (cytosine [C], thymine [T] in DNA, uracil [U] in RNA).

• Pentose sugar: Deoxyribose in DNA, ribose in RNA. Ribose has an OH group at the 2' carbon; deoxyribose lacks this group.

• Phosphate group: Links the nucleotides together.

Nucleoside: A nitrogenous base plus a sugar (no phosphate). Nucleotide: A nucleoside plus one or more phosphate groups.

Nucleotide Polymers and Directionality

• Nucleotides are joined by phosphodiester linkages (covalent bonds between the phosphate of one nucleotide and the sugar of the next).

• This forms a sugar-phosphate backbone with nitrogenous bases as side groups.

• Polynucleotides have directionality: a free 5' phosphate group at one end (5' end) and a free 3' hydroxyl group at the other (3' end).

• The sequence of bases encodes genetic information.

Structures of DNA and RNA Molecules

• DNA: Double-stranded helix with two antiparallel polynucleotide strands. The sugar-phosphate backbone is on the outside, and the bases pair in the interior via hydrogen bonds.

• Base pairing rules: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Complementarity: The sequence of one strand determines the sequence of the other.

• RNA: Usually single-stranded, but can form complex 3D shapes through internal base pairing. Uracil (U) replaces thymine (T) in RNA.

Example: Complementary DNA Strands

If one DNA strand has the sequence 5'-TAGGCCT-3', the complementary strand will be 3'-ATCCGGA-5'.

Section 3: Membrane Structure and Function

Overview of Membrane Structure

Cellular membranes are dynamic structures composed primarily of lipids and proteins. They function as selective barriers, regulating the movement of substances into and out of cells.

• Fluid mosaic model: Describes the membrane as a mosaic of proteins drifting laterally in a fluid bilayer of phospholipids.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer that creates a stable boundary between two aqueous environments.

• Other components: Cholesterol (in animal cells), carbohydrates (attached to lipids and proteins).

Membrane Fluidity

• Membranes are held together by hydrophobic interactions, allowing lipids and some proteins to move laterally.

• Fluidity is influenced by the type of fatty acids (saturated vs. unsaturated) and cholesterol content.

• Membranes must remain fluid to function properly; organisms adapt membrane composition to environmental temperatures.

Membrane Proteins and Their Functions

Proteins embedded in the membrane perform a variety of functions essential for cell survival and communication.

• Integral proteins: Penetrate the hydrophobic core of the lipid bilayer; many span the membrane (transmembrane proteins).

• Peripheral proteins: Loosely bound to the membrane surface, often attached to integral proteins or the cytoskeleton.

Role of Membrane Carbohydrates in Cell Recognition

• Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) serve as identification markers.

• Important for sorting cells into tissues and for immune system recognition.

• Usually short, branched chains of fewer than 15-16 sugar units.

Synthesis and Sidedness of Membranes

• Membranes have distinct inside (cytoplasmic) and outside (extracellular) faces.

• Lipid and protein composition can differ between the two sides.

• Glycoproteins and glycolipids are typically found on the extracellular surface.

Membrane Transport Mechanisms

• Passive transport: Movement of substances across the membrane without energy input (e.g., diffusion, osmosis, facilitated diffusion).

• Active transport: Uses energy (usually ATP) to move substances against their concentration gradients.

• Bulk transport: Large molecules are transported via exocytosis and endocytosis.

Example: Phospholipid Interaction with Water

• Phospholipid heads are hydrophilic and interact with water; tails are hydrophobic and avoid water, leading to bilayer formation.

Additional info:

• HIV resistance can be linked to the absence of certain membrane proteins (e.g., CCR5 co-receptor).

• Membrane composition is an evolutionary adaptation to environmental conditions.