Essential Cell Biology

1. Cells: Life's Universal Building Blocks

All living things (or organisms) are built from cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two.

Fundamental units. Cells are the basic structural and functional units of all known living organisms. They are diverse in size, shape, and function, from tiny bacteria to large nerve cells, reflecting their adaptation to various roles and environments. Despite this diversity, all cells share fundamental characteristics.

Common chemistry. At their core, all living cells share a remarkably similar basic chemistry. They are composed of the same types of molecules, participate in similar chemical reactions, and use DNA as their genetic material, written in the same chemical code. This underlying unity points to a common ancestry.

Self-replication. The defining feature of life is the ability to reproduce. Cells achieve this through self-replication, duplicating their genetic material and dividing. This process, driven by the interplay of DNA, RNA, and proteins, allows life to persist and diversify.

2. DNA: The Blueprint of Life

In all organisms, genetic information—in the form of genes—is carried in DNA molecules.

Genetic instructions. DNA, or deoxyribonucleic acid, serves as the cell's instruction manual. It's a long polymer made of four nucleotide subunits (A, T, G, C), whose specific sequence encodes the genetic information required to build and maintain an organism. This sequence is the basis of heredity.

Double helix structure. DNA typically exists as a double helix, two complementary strands held together by specific base pairing (A with T, G with C). This structure is not only stable but also provides a simple mechanism for accurate copying, as each strand can serve as a template for a new one.

Genes are segments. Genes are specific segments of this DNA molecule. Each gene contains the instructions for making a particular protein or functional RNA molecule, which ultimately dictates the cell's characteristics and functions. The entire collection of genes in an organism is its genome.

3. From Gene to Protein: The Central Dogma

This flow of information—from DNA to RNA to protein—is so fundamental to life that it is referred to as the central dogma.

Information flow. The genetic information stored in DNA is accessed and used through a two-step process. First, the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule in a process called transcription. This is like transcribing a message from one format to another within the same language (nucleotides).

Translation to protein. Second, the nucleotide sequence of the mRNA molecule is used to direct the synthesis of a protein in a process called translation. This is like translating a message into a different language (amino acids). This occurs on ribosomes, with the help of transfer RNA (tRNA) adaptors.

Proteins execute function. Proteins are the workhorses of the cell, determining its structure and carrying out most of its functions. The specific sequence of amino acids in a protein, dictated by the genetic code in the mRNA, determines its unique three-dimensional shape and activity.

4. Energy: Fueling the Cell's Work

To accomplish this remarkable feat, the cells in a living organism must continuously carry out a never-ending stream of chemical reactions to maintain their structure, meet their metabolic needs, and stave off unrelenting chemical decay.

Maintaining order. Living cells are highly organized systems that resist the natural tendency towards disorder (entropy). Maintaining this order requires a constant input of energy to power the thousands of chemical reactions that build and maintain cellular components.

Sources of energy. Cells obtain energy from their environment.

• Photosynthetic organisms (plants, algae, some bacteria) capture light energy from the sun.
• Animals and other organisms obtain chemical energy by breaking down food molecules (sugars, fats).

Oxidation and ATP. Cells extract energy from food molecules through controlled oxidation reactions. This energy is captured in the chemical bonds of activated carrier molecules, primarily ATP (adenosine triphosphate). ATP serves as the cell's main energy currency, readily releasing energy to fuel other cellular processes.

5. Proteins: The Cell's Molecular Machines

The appearance and behavior of a cell are dictated largely by its protein molecules, which serve as structural supports, chemical catalysts, molecular motors, and much more.

Diverse functions. Proteins are the most versatile macromolecules in the cell, performing a vast array of tasks.

• Enzymes catalyze nearly all cellular chemical reactions.
• Structural proteins provide shape and support (e.g., cytoskeleton).
• Motor proteins generate movement.
• Receptors detect signals.
• Transporters move molecules across membranes.

Shape determines function. A protein's function is intimately linked to its precise three-dimensional shape, or conformation. This shape is determined by the linear sequence of amino acids in its polypeptide chain, stabilized by various noncovalent bonds and hydrophobic forces.

Regulation and interaction. Protein activity is tightly controlled within the cell. This often involves changes in their conformation triggered by binding to other molecules (ligands), including other proteins. Many proteins work together in large complexes or "machines" to perform complex tasks.

6. Organization: Compartments and Frameworks

Internal Membranes Create Intracellular Compartments with Different Functions

Membrane-enclosed organelles. Eukaryotic cells are subdivided by internal membranes into specialized compartments called organelles.

• Nucleus: Contains the DNA.
• Endoplasmic Reticulum (ER): Synthesis of lipids and proteins.
• Golgi apparatus: Protein/lipid modification and sorting.
• Lysosomes: Degradation of waste.
• Mitochondria: ATP synthesis (oxidative phosphorylation).
• Chloroplasts (plants): Photosynthesis.

Cytosol. The space outside the organelles, enclosed by the plasma membrane, is the cytosol. It's a concentrated gel where many metabolic pathways and protein synthesis occur.

Cytoskeleton. A dynamic network of protein filaments (intermediate filaments, microtubules, actin filaments) extends throughout the cytoplasm. It provides mechanical support, determines cell shape, organizes organelles, and drives cell movement and transport.

7. Communication: Cells Talking to Each Other

Individual cells, like multicellular organisms, need to sense and respond to their environment.

Sensing signals. Cells receive information from their surroundings through extracellular signal molecules. These signals can come from other cells (hormones, local mediators, neurotransmitters, contact-dependent signals) or from the environment (light, chemicals).

Receptors. Target cells have specific receptor proteins that bind to these signal molecules. This binding initiates a process of signal transduction, converting the external signal into internal signals.

Intracellular pathways. Signal transduction involves intracellular signaling molecules that relay, amplify, integrate, and distribute the signal within the cell. These pathways often involve molecular switches, like proteins activated by phosphorylation or GTP binding, ultimately altering the activity of effector proteins to change cell behavior.

8. Reproduction: Copying and Dividing

A cell reproduces by carrying out an orderly sequence of events in which it duplicates its contents and then divides in two.

The cell cycle. Eukaryotic cells reproduce through a highly regulated sequence of events called the cell cycle. It consists of interphase (G1, S, G2) and M phase (mitosis and cytokinesis).

Duplication and segregation. During S phase, the cell replicates its DNA. In M phase, the duplicated chromosomes are accurately segregated into two daughter nuclei (mitosis), and the cytoplasm divides (cytokinesis) to form two daughter cells.

Control system. The cell-cycle control system, based on cyclin-dependent protein kinases (Cdks) and cyclins, orchestrates these events. It includes checkpoints to ensure DNA is replicated and repaired, and chromosomes are properly attached to the mitotic spindle before division proceeds.

9. Evolution: The Engine of Diversity

Charles Darwin provided the key insight that makes this history comprehensible. His theory of evolution, published in 1859, explains how random variation and natural selection gave rise to diversity among organisms that share a common ancestry.

Genetic variation. Differences in DNA sequence are the raw material for evolution. Variation arises through mechanisms like point mutations, gene duplication, exon shuffling, transposition of mobile elements, and horizontal gene transfer.

Natural selection. These variations are tested by natural selection. Changes that provide a selective advantage (increased survival or reproduction) are more likely to be passed on and become common in the population. Deleterious changes are usually eliminated.

Common ancestry. The fundamental similarities in cell chemistry and molecular processes across all life-forms provide strong evidence for a common ancestor. Comparing genome sequences allows us to trace evolutionary relationships and identify genes and sequences that have been conserved over billions of years.

10. Tissues: Cells Working Together

Most of the cells in multicellular organisms are organized into cooperative assemblies called tissues...

Cell communities. In multicellular organisms, cells are organized into tissues and organs. This requires cells to adhere to each other and to an extracellular matrix, a secreted material that provides structural support.

Adhesion and junctions. Animal cells use cell junctions (tight junctions, adherens junctions, desmosomes, hemidesmosomes, gap junctions) to connect to each other and the matrix, transmitting forces and allowing communication. Plant cells use cell walls and plasmodesmata.

Renewal and stem cells. Many tissues require continuous renewal. This is achieved by stem cells, undifferentiated cells that can divide to produce more stem cells (self-renewal) and specialized precursor cells that differentiate into the various cell types needed by the tissue.

11. Cancer: When Cells Go Rogue

Disorders of tissue renewal are a major medical concern, and those due to the misbehavior of mutant cells underlie the development of cancer.

Loss of control. Cancer is a disease characterized by uncontrolled cell proliferation and the ability of cells to invade surrounding tissues and spread to distant sites (metastasis). It arises from genetic changes in somatic cells.

Accumulation of mutations. Cancer is typically caused by the accumulation of multiple mutations over time, affecting genes that control cell growth, division, survival, and DNA repair. These mutations give cancer cells a competitive advantage.

Oncogenes and tumor suppressors. Cancer-critical genes fall into two classes: proto-oncogenes (gain-of-function mutations create oncogenes that promote cancer) and tumor suppressor genes (loss-of-function mutations remove brakes on cell growth). Understanding these genes and the pathways they control is key to developing targeted cancer therapies.

Last updated: May 7, 2025