Biology Basics: Cells and DNA

What Is a Cell?

A cell is the smallest unit of life. It is the basic building block that every living organism is made of. Some organisms consist of only one cell (like bacteria). Others, like human beings, are made of trillions of cells working together.

Cells were first discovered in 1665 by Robert Hooke, who was looking at a thin slice of cork through a microscope. He called the tiny compartments he saw "cells" because they reminded him of the small rooms (cells) that monks lived in.

Two Major Types of Cells

Not all cells are the same. The most important division in biology is between two types: prokaryotic cells and eukaryotic cells.

Prokaryotic Cells

Prokaryotic cells are the simpler, older type. The word "prokaryote" comes from Greek meaning "before nucleus." These cells have no membrane-bound nucleus — their genetic material floats freely inside the cell. Bacteria and archaea (ancient single-celled organisms) are prokaryotes.

• No nucleus (DNA floats in the cytoplasm)
• No membrane-bound organelles
• Very small (typically 1–10 micrometers)
• Always single-celled organisms
• Examples: E. coli, Salmonella, Streptococcus

Eukaryotic Cells

Eukaryotic cells are more complex. "Eukaryote" means "true nucleus" — these cells have their DNA enclosed inside a membrane-bound nucleus. Plants, animals, fungi, and protists are all made of eukaryotic cells.

• True nucleus enclosed by a membrane
• Contains many specialized organelles
• Larger than prokaryotes (10–100 micrometers)
• Can be single-celled (like yeast) or multi-celled (like you)
• Examples: human cells, plant cells, fungal cells

  PROKARYOTIC CELL           EUKARYOTIC CELL
  (e.g. bacterium)           (e.g. animal cell)

  +-----------------+        +---------------------------+
  |  cell wall      |        |                           |
  |  +-----------+  |        |    +-------+              |
  |  | cytoplasm |  |        |    |nucleus|  mitochondria|
  |  |  ~DNA~    |  |        |    +-------+  [===]       |
  |  | ribosomes |  |        |  cytoplasm    ribosome .  |
  |  +-----------+  |        |                           |
  +-----------------+        +---------------------------+
  No nucleus                 Has membrane-bound nucleus
  

Parts of a Eukaryotic Cell

Eukaryotic cells contain specialized structures called organelles (meaning "little organs"). Each organelle has a specific job, just like how organs in your body each have a specific job.

Nucleus

The nucleus is the control center of the cell. It contains the cell's DNA (the instructions for building and running the cell) and is surrounded by a double membrane called the nuclear envelope. The nucleus decides which proteins the cell makes and when.

Cell Membrane

Every cell is surrounded by a cell membrane (also called the plasma membrane). It is a thin, flexible layer made of fats (lipids) and proteins. The cell membrane controls what enters and leaves the cell — letting in nutrients and kicking out waste products. Think of it as the cell's security guard.

Cytoplasm

The cytoplasm is the jelly-like fluid that fills the inside of the cell. All the organelles float in it. It is mostly water, but also contains salts, proteins, and other molecules. Chemical reactions that power the cell happen throughout the cytoplasm.

Mitochondria

Mitochondria are the power plants of the cell. They take in glucose (from the food you eat) and oxygen (from the air you breathe) and convert them into ATP — a molecule the cell uses as its main energy currency. Cells that need a lot of energy (like muscle cells) have thousands of mitochondria.

Ribosomes

Ribosomes are tiny structures (found in both prokaryotic and eukaryotic cells) that build proteins by reading instructions from the DNA. They are incredibly numerous — a single cell can have millions of ribosomes. Proteins are essential for almost every function in the body.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of folded membranes that serves as the cell's manufacturing and transport system. The rough ER is studded with ribosomes and makes proteins. The smooth ER makes lipids (fats) and processes certain toxins.

Golgi Apparatus

The Golgi apparatus (also called the Golgi body) is the cell's post office. It receives proteins from the ER, packages them, modifies them if needed, and ships them to their final destinations — either within the cell or out of it entirely.

Cell Wall (Plants Only)

Plant cells (and fungal cells) have a rigid cell wall outside the cell membrane, made mostly of cellulose. The cell wall provides structure and support, which is why plants can stand upright without a skeleton. Animal cells do not have a cell wall.

Chloroplasts (Plants Only)

Chloroplasts are found in plant cells (and algae). They contain a green pigment called chlorophyll and are where photosynthesis happens — the process of converting sunlight and carbon dioxide into glucose and oxygen. In a sense, chloroplasts do for plants what mitochondria do for animals.

  ANIMAL CELL (simplified cross-section)

                 cell membrane
                _____________
               /             \
              |  [nucleus]    |
              |    nucleus    |   ribosome (.)
              |               |
              |  [mito]       |   endoplasmic
              |  mitochondria |   reticulum (~)
              |          [G]  |
              |          Golgi|
               \_____________/

  PLANT CELL — adds:
    - cell wall (outer rigid layer)
    - chloroplasts (green, for photosynthesis)
    - large central vacuole (water storage)
  

What Is DNA?

DNA stands for Deoxyribonucleic Acid. It is the molecule that stores the instructions for building and operating every living thing. If you think of a cell as a factory, DNA is the master blueprint that tells the factory what to make and how to make it.

DNA has a distinctive shape called a double helix — imagine a twisted ladder. The sides of the ladder are made of alternating sugar and phosphate molecules. The rungs of the ladder are made of pairs of chemical bases: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). The sequence of these bases encodes information.

  DNA DOUBLE HELIX (simplified)

  5'--A--T--G--C--A--T--3'
      |  |  |  |  |  |
  3'--T--A--C--G--T--A--5'

  A always pairs with T
  G always pairs with C
  

If you unraveled all the DNA from a single human cell and stretched it out, it would be about 2 meters long — yet it is packed into a nucleus that is invisible to the naked eye. DNA achieves this by wrapping tightly around proteins called histones, forming compact structures called chromosomes.

Genes and What They Do

A gene is a specific section of DNA that contains the instructions for making one protein (or sometimes a few related proteins). Humans have approximately 20,000 to 25,000 genes. Together, all the genes in an organism make up its genome.

Genes determine many of your traits: your blood type, the color of your eyes, how tall you are likely to grow. However, genes do not work alone — your environment, diet, experiences, and random chance also play significant roles in who you become.

Cell Division: Mitosis

Cells reproduce by dividing. The most common type of cell division is called mitosis. It is the process your body uses to grow, repair damaged tissue, and replace old cells. In mitosis, one parent cell divides to produce two daughter cells that are genetically identical to each other and to the parent.

The Phases of Mitosis

• Interphase — the cell grows and makes a complete copy of its DNA (this happens before mitosis officially begins)
• Prophase — the chromosomes condense and become visible; the nuclear envelope breaks down
• Metaphase — chromosomes line up along the middle of the cell
• Anaphase — the chromosomes are pulled apart toward opposite ends of the cell
• Telophase — two new nuclei form around each set of chromosomes
• Cytokinesis — the cytoplasm divides and two separate daughter cells are produced

  MITOSIS OVERVIEW

  [Parent Cell]  →  DNA copied  →  [Cell with 2 sets of DNA]
        |
        v
   Chromosomes condense (Prophase)
        |
        v
   Line up in middle (Metaphase)
        |
        v
   Pull apart (Anaphase)
        |
        v
   Two nuclei form (Telophase)
        |
        v
   Cell splits (Cytokinesis)
        |
        v
  [Daughter 1]            [Daughter 2]
  (identical to parent)   (identical to parent)
  

Why This Matters

Understanding cells and DNA is not just a classroom exercise. These concepts underlie virtually all of modern medicine and biology:

• Cancer is a disease of uncontrolled cell division — cells that ignore the normal signals to stop dividing
• Vaccines work by giving your immune cells the information they need to recognize a pathogen
• Genetic diseases like cystic fibrosis are caused by mutations (errors) in DNA sequences
• CRISPR gene editing technology works by precisely cutting and modifying DNA sequences inside cells
• COVID-19 mRNA vaccines work by delivering mRNA instructions that teach your cells to make a viral protein

For related chemistry concepts — like how molecules are built from atoms — visit our Atoms and Molecules guide. The same bonding principles that govern chemistry also govern how DNA holds its double helix shape and how proteins fold into their functional forms.

Frequently Asked Questions