Difficulty: Introductory | Prerequisites: Basic cell biology (cell structure, organelles)
Big picture: DNA is the molecule at the centre of everything in biology. It stores the instructions that build and run every living organism, and it is the physical basis of heredity, the reason offspring resemble their parents. This topic sits early in the BIO 101 sequence because nearly everything that follows (gene expression, cell division, evolution, biotechnology) depends on understanding what DNA is, how it is structured, and what it does. If you are coming in cold, know that DNA is a chemical molecule found inside cells, and its structure directly determines its function.
TL;DR: DNA is a double-stranded molecule made of nucleotide subunits, each containing a sugar, a phosphate group, and one of four nitrogenous bases (A, T, C, G). The sequence of those bases encodes genetic information. DNA is copied during cell division so each new cell inherits a complete set of instructions, and segments of DNA called genes are read out to produce proteins that do the cell's work.
DNA (deoxyribonucleic acid)
A large, double-stranded molecule that carries the genetic instructions for the development, functioning, and reproduction of all known living organisms. Think of it as the master blueprint stored inside every cell.
Nucleotide
The basic building block of DNA. Each nucleotide consists of three parts: a deoxyribose sugar, a phosphate group, and a nitrogenous base. In simple terms, nucleotides are the individual "letters" that spell out the genetic code.
Nitrogenous bases (A, T, C, G)
The four chemical bases found in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). They pair in a specific way: A always pairs with T, and C always pairs with G. These pairings are called complementary base pairs.
Gene
A segment of DNA that contains the instructions for making one specific protein (or a functional RNA molecule). Think of it as one recipe in the larger cookbook that is your genome.
Chromosome
A tightly coiled and packaged structure of DNA found in the nucleus of eukaryotic cells. Humans have 23 pairs (46 total), one set inherited from each parent.
Genetic code
The specific sequence of nitrogenous bases along a strand of DNA. This sequence determines which proteins a cell produces, and therefore which traits an organism displays.
Heredity
The passing of genetic information from parent to offspring. In simple terms, this is why children share traits with their parents.
DNA replication
The process by which a cell copies its entire DNA molecule before dividing, so each daughter cell receives a complete set of genetic instructions.
Gene expression
The process by which the information in a gene is used to produce a functional product, typically a protein. A gene that is "expressed" is one that is actively being read and translated into a protein.
Mutation
A change in the DNA sequence. Mutations can be harmful, beneficial, or neutral. They are the raw material on which natural selection acts.
DNA is a polymer, a long chain built from repeating subunits called nucleotides.
Each nucleotide has three components:
A five-carbon sugar (deoxyribose)
A phosphate group
One of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), guanine (G)
The sugar and phosphate groups form the "backbone" of the DNA strand. The bases project inward.
DNA is double-stranded: two chains run alongside each other in opposite directions (antiparallel), wound into a double helix.
The two strands are held together by hydrogen bonds between complementary base pairs:
A pairs with T (two hydrogen bonds)
C pairs with G (three hydrogen bonds)
Chargaff's rule: in any DNA molecule, the amount of A equals the amount of T, and the amount of C equals the amount of G.
In eukaryotic cells (plants, animals, fungi): DNA is housed inside the nucleus, packaged into chromosomes.
In prokaryotic cells (bacteria, archaea): DNA floats in the cytoplasm, typically as a single circular chromosome.
Humans carry 23 pairs of chromosomes (46 total). One set of 23 comes from each parent.
Each chromosome is a single, very long DNA molecule, coiled tightly around proteins called histones to fit inside the nucleus.
Before a cell divides, it must copy all of its DNA so each daughter cell gets a full set.
The double helix "unzips" as hydrogen bonds between base pairs are broken.
Each original strand serves as a template: enzymes read the exposed bases and attach the correct complementary nucleotides.
The result is two identical DNA molecules, each containing one original strand and one newly built strand. This is called semiconservative replication.
A gene is a specific stretch of DNA that codes for a protein (or functional RNA).
Gene expression occurs in two main steps:
Transcription: the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule.
Translation: the mRNA is read by ribosomes, which assemble the corresponding protein from amino acids.
Proteins carry out most of the cell's work: catalysing reactions (enzymes), providing structure, transporting molecules, and signalling between cells.
Genetics is the study of how traits are inherited from parents to offspring.
DNA is the physical substance of inheritance. Specific genes (specific DNA sequences) correspond to specific traits, such as eye colour or blood type.
Mutations, changes in the DNA sequence, can alter a gene's product and therefore change a trait. Mutations can be:
Beneficial (confer a survival advantage)
Neutral (no observable effect)
Harmful (cause disease or reduced fitness)
The field of molecular biology examines how genes are regulated, how they are switched on and off, and how proteins are produced. This knowledge underpins medicine, agriculture, and biotechnology.
DNA knowledge is the foundation of forensic science: crime-scene investigators match DNA samples to identify suspects, because each person's DNA sequence is (almost) unique. In medicine, understanding mutations in specific genes allows doctors to diagnose genetic disorders such as cystic fibrosis and sickle-cell anaemia, and to develop targeted therapies. In agriculture, scientists modify plant DNA to produce crops that resist pests or tolerate drought, a practice known as genetic engineering. Biotechnology companies use DNA manipulation to manufacture insulin, produce vaccines, and develop gene therapies.
Students often think DNA and genes are the same thing. They are not. DNA is the entire molecule; a gene is one specific segment of that molecule that codes for a particular protein.
Students sometimes believe that every cell in your body uses all of its DNA all the time. In reality, most genes are switched off in any given cell. A liver cell and a nerve cell contain the same DNA but express different genes.
"Mutation" does not mean "harmful." Many mutations have no effect at all, and some are beneficial. The word simply means a change in the DNA sequence.
Students often confuse DNA replication (copying all the DNA before cell division) with gene expression (reading one gene to make a protein). These are different processes with different purposes.
⚠️ Know the three components of a nucleotide (sugar, phosphate, base) and be able to name all four bases. This is a near-certain exam question.
⚠️ Understand complementary base pairing (A–T, C–G) and be ready to determine the complementary strand when given a DNA sequence.
⚠️ Be clear on the difference between DNA replication and gene expression. Exams frequently ask students to distinguish the two.
⚠️ Know where DNA is found in eukaryotic vs. prokaryotic cells.
⚠️ Understand that chromosomes are DNA plus packaging proteins, not separate from DNA. "Chromosome" and "DNA molecule" refer to the same physical thing at different levels of organisation.
True or false: A nucleotide consists of a sugar, a phosphate group, and an amino acid.
Answer: False. A nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
Fill in the blank: If one strand of DNA reads A–T–C–G, the complementary strand reads ____.
Answer: T–A–G–C.
True or false: DNA is found in the nucleus of prokaryotic cells.
Answer: False. Prokaryotic cells do not have a nucleus. Their DNA is in the cytoplasm.
Fill in the blank: Humans have ____ pairs of chromosomes.
Answer: 23.
True or false: Every mutation in DNA is harmful.
Answer: False. Mutations can be harmful, beneficial, or neutral.
Q: What are the four nitrogenous bases found in DNA, and how do they pair?
A: Adenine (A), thymine (T), cytosine (C), and guanine (G). A pairs with T; C pairs with G.
Q: Describe the three components of a single nucleotide.
A: A five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base.
Q: Explain why DNA replication is described as "semiconservative."
A: Each new DNA molecule contains one original (parent) strand and one newly synthesised strand. Half the original molecule is conserved in each copy.
Q: Where is DNA located in a eukaryotic cell, and how is it organised?
A: In the nucleus, packaged into chromosomes. Each chromosome is a long DNA molecule coiled around histone proteins.
Q: What is the difference between a gene and a chromosome?
A: A gene is a specific segment of DNA that codes for one protein (or functional RNA). A chromosome is an entire DNA molecule, containing many genes, coiled and packaged with proteins.
Q: A student claims that because liver cells and brain cells have different functions, they must contain different DNA. Is this correct? Explain.
A: This is incorrect. All cells in an organism contain the same DNA. The difference in function arises because different genes are expressed (switched on) in different cell types.
This material connects directly to evolution (covered in the companion study notes): mutations in DNA are the source of genetic variation, and natural selection acts on that variation. It also connects to cell division (mitosis and meiosis), where DNA replication ensures each daughter cell gets a full copy. Later in the course, gene expression links to protein structure and enzyme function.
Related Terms / Search Tags
DNA, deoxyribonucleic acid, nucleotide, nitrogenous base, adenine, thymine, cytosine, guanine, complementary base pairing, double helix, chromosome, gene, genetic code, heredity, inheritance, DNA replication, semiconservative replication, gene expression, transcription, translation, mutation, molecular biology, genetics, BIO 101, intro biology, University of Washington