Cell Biology 101: Cells, DNA and Genetics Explained
By Dr. Zubair Khalid, DVM, MS, PhD ·
This page is a map of the basics. Each section below covers one part of introductory cell biology in the order most courses teach it, with a link to a full guide for every topic. If you are new to biology, read the sections top to bottom. If you are revising for a test, jump to the topic you need.
Key Takeaways
- Every living thing is made of one or more cells, and every cell comes from an existing cell. That is cell theory, and the rest of biology builds on it.
- Organelles split the work of a eukaryotic cell: the nucleus stores DNA, ribosomes make proteins, and mitochondria and chloroplasts handle energy.
- DNA carries genetic information. Cells copy it before they divide, transcribe it into RNA, and translate that RNA into protein.
- Genetics and evolution explain how traits pass from parents to offspring and how populations change over generations.
1. What a Cell Is
- The 3 parts of cell theory: the three statements every biology course starts with, and who proposed them.
- Levels of organization: how cells build tissues, organs, organ systems and organisms.
- Animal vs plant cells: the cell wall, chloroplasts and large central vacuole that set plant cells apart.
- Parts of a plant cell: each structure and what it does.
2. Organelles and Their Jobs
- Cell organelles: an overview of the structures inside a eukaryotic cell.
- Nucleus vs nucleolus: where DNA is stored and where ribosomes are assembled.
- Ribosomes: the machines that build proteins.
- Endoplasmic reticulum: rough ER for protein folding, smooth ER for lipids.
- Lysosomes: the cell's digestive compartment.
- Vacuoles: storage and turgor pressure in plant and animal cells.
- Chloroplasts: where photosynthesis happens.
- Cytoplasm: the fluid and structures between the membrane and the nucleus.
3. The Cell Membrane and Transport
- Cell membrane function: how the membrane controls what enters and leaves.
- The fluid mosaic model: the standard picture of membrane structure.
- Passive transport: diffusion, facilitated diffusion and osmosis.
- Active transport and the sodium-potassium pump: moving molecules against a gradient with ATP.
4. Energy: Respiration and Photosynthesis
- Cellular respiration: the stages and how much ATP each one makes.
- Glycolysis: the first stage, in the cytoplasm.
- Light-dependent reactions: how chloroplasts capture light energy.
- Photosynthesis diagram: the light and dark reactions side by side.
5. Cell Division
- Prophase and the stages of mitosis: what happens to chromosomes at each step.
- Chromatid vs chromosome: how to count each before and after replication.
- Cell cycle checkpoints: how the cell checks itself at G1, G2 and M.
6. DNA and How It Is Copied
- What is DNA?: its meaning, structure and function.
- Nucleotide diagram: the phosphate, sugar and base that make up each unit.
- The double helix: how two strands pair and twist.
- DNA vs RNA: structural and functional differences.
- Semiconservative replication: why each new DNA molecule keeps one old strand.
- The replication fork: the leading and lagging strands.
7. From Gene to Protein
- Transcription: copying a gene from DNA into RNA.
- Where transcription takes place: nucleus in eukaryotes, cytoplasm in prokaryotes.
- Translation: reading mRNA to build a protein.
- Protein synthesis: transcription and translation as one process.
8. Genetics and Evolution
- Alleles: the different versions of a gene.
- How to use a Punnett square, or try the Punnett square calculator.
- Mutations: changes in DNA sequence and their effects.
- Natural selection: how heritable traits that help survival become more common.
- Gene flow vs genetic drift: two other forces that change allele frequencies.
Frequently Asked Questions
What should I learn first in cell biology?
Start with cell theory and the parts of a cell, then the membrane, then energy, division, DNA and genetics. Each topic depends on the one before it, which is why the sections above follow that order.
What is the difference between prokaryotic and eukaryotic cells?
Eukaryotic cells keep their DNA inside a nucleus and have membrane-bound organelles. Prokaryotic cells, such as bacteria, have no nucleus and no membrane-bound organelles, and their DNA sits in a region of the cytoplasm called the nucleoid.