Chapter 3 – Cells: The Living Units

Definitions

This section introduces key terms related to cellular structure and function, essential for understanding the molecular basis of cell biology in Anatomy & Physiology.

• Aminoacyl-tRNA: tRNA molecule linked to its specific amino acid, used during translation.

• Anticodon: Sequence of three nucleotides in tRNA complementary to a codon in mRNA.

• Cell: The basic structural and functional unit of living organisms.

• Channel protein: Membrane protein that forms a channel for molecules to pass through.

• Cholesterol: Lipid molecule that stabilizes cell membrane fluidity.

• Coding strand: DNA strand whose sequence matches the mRNA (except T/U).

• Codon: Sequence of three nucleotides in mRNA that codes for an amino acid.

• DNA polymerase: Enzyme that synthesizes DNA during replication.

• Elongation (of transcription/translation): The stage where the RNA or polypeptide chain is extended.

• G1, G2, S phase: Phases of the cell cycle; G1 and G2 are growth phases, S is DNA synthesis.

• Gene: Segment of DNA that codes for a protein.

• Genetic code: The set of rules by which information encoded in DNA is translated into proteins.

• Glycolipid: Lipid with a carbohydrate attached, found in cell membranes.

• Hydrophilic: Water-attracting; describes molecules that interact with water.

• Hydrophobic: Water-repelling; describes molecules that do not interact with water.

• Initiation (transcription/translation): The beginning stage of RNA or protein synthesis.

• Initiator tRNA: tRNA that carries the first amino acid in translation.

• Integral protein: Protein embedded within the cell membrane.

• Interphase: The phase of the cell cycle when the cell is not dividing.

• Messenger RNA (mRNA): RNA that carries genetic information from DNA to ribosomes.

• Peripheral protein: Protein attached to the surface of the cell membrane.

• Phospholipid: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.

• Plasma membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Replication bubble/fork: Structure formed during DNA replication where the DNA is unwound.

• Ribosomal RNA (rRNA): RNA component of ribosomes.

• RNA polymerase: Enzyme that synthesizes RNA from a DNA template.

• S phase: Phase of the cell cycle where DNA is replicated.

• Semiconservative: DNA replication method where each new DNA molecule has one old and one new strand.

• Start codon: Codon (usually AUG) that signals the start of translation.

• Stop codon: Codon that signals the end of translation.

• Terminator: DNA sequence that signals the end of transcription.

• Transcription: Process of synthesizing RNA from DNA.

• Transcription factor: Protein that regulates transcription.

• Transfer RNA (tRNA): RNA that brings amino acids to the ribosome during translation.

• Translation: Process of synthesizing protein from mRNA.

• Transmembrane protein: Protein that spans the entire cell membrane.

• Triplet: Group of three nucleotides in DNA or RNA.

Plasma Membrane

Structure and Function

The plasma membrane separates the intracellular environment from the extracellular space, maintaining cellular integrity and regulating transport.

• Key Molecules: Phospholipids, cholesterol, glycolipids, and proteins are the main components.

• Phospholipid Arrangement: Phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluid Mosaic Model: The membrane is dynamic, with proteins and lipids able to move laterally.

• Functions: Controls entry/exit of substances, cell signaling, and structural support.

Example: The sodium-potassium pump is an integral membrane protein that maintains ion gradients.

Major Classes of Membrane Molecules

• Phospholipids: Form the basic structure of the membrane.

• Cholesterol: Modulates membrane fluidity.

• Glycolipids: Involved in cell recognition.

• Proteins: Serve as channels, carriers, receptors, and enzymes.

Membrane Proteins

Types and Functions

Membrane proteins are classified based on their association with the lipid bilayer and their functions in cellular processes.

• Integral Proteins: Embedded within the membrane; often span the bilayer (transmembrane).

• Peripheral Proteins: Attached to the membrane surface; often involved in signaling or structural support.

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment.

Example: Aquaporins are integral proteins that facilitate water transport.

DNA Replication

Mechanism and Key Enzymes

DNA replication is the process by which a cell duplicates its DNA before cell division, ensuring genetic continuity.

• Semiconservative Replication: Each new DNA molecule contains one original and one new strand.

• Key Enzymes: DNA polymerase, helicase, primase, ligase.

• Replication Fork: The area where DNA is unwound and replication occurs.

• Initiation: Begins at specific origins of replication.

• Elongation: New nucleotides are added to the growing DNA strand.

• Termination: Replication ends when the entire DNA molecule is copied.

Example: In human cells, replication occurs during the S phase of the cell cycle.

Equation:

Transcription

Process and Regulation

Transcription is the synthesis of RNA from a DNA template, the first step in gene expression.

• Initiation: RNA polymerase binds to the promoter region.

• Elongation: RNA polymerase adds nucleotides to the growing RNA strand.

• Termination: RNA synthesis ends at the terminator sequence.

• mRNA: Carries genetic information from DNA to ribosomes.

• Promoter: DNA sequence that signals where transcription begins.

Example: The gene for hemoglobin is transcribed in red blood cell precursors.

Equation:

Translation

Protein Synthesis

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Initiation: Ribosome assembles at the start codon (AUG) on mRNA.

• Elongation: tRNAs bring amino acids, which are joined to form a polypeptide chain.

• Termination: Occurs when a stop codon is reached.

• Codon Recognition: Each codon specifies an amino acid.

• Anticodon: tRNA sequence complementary to mRNA codon.

• Ribosomes: Site of protein synthesis; composed of rRNA and proteins.

Example: Insulin is synthesized by translation in pancreatic beta cells.

Equation:

Summary: DNA to Protein

Gene Expression Overview

Gene expression involves the flow of genetic information from DNA to RNA to protein, determining cell structure and function.

• Central Dogma: DNA → RNA → Protein

• Gene Expression: The process by which information from a gene is used to synthesize a functional gene product (protein).

• Prediction: The amino acid sequence of a protein can be predicted from the DNA template strand.

Equation:

Table: Comparison of Membrane Proteins

This table summarizes the differences between integral and peripheral membrane proteins.

Table: Key Steps in Gene Expression