DNA, Genes, and Chromosomes

Learning Objectives

Describe the basic double helix structure of a DNA molecule.

Explain the relationship between DNA, genes, and chromosomes.

Understand that genes carry the instructions for making proteins.

The Blueprint of Life

Every living organism has a set of instructions that tells its cells what to do. This master blueprint is a molecule called DNA (Deoxyribonucleic Acid). DNA contains all the information needed for an organism to grow, survive, and reproduce.

The Structure of DNA: The Double Helix

The structure of a DNA molecule is a double helix. Imagine a ladder that has been twisted into a spiral shape.

The sides of the ladder are made of a repeating pattern of sugar and phosphate molecules. This is called the sugar-phosphate backbone.

The rungs of the ladder are made of pairs of molecules called nitrogenous bases.

There are four different bases in DNA:

Adenine (A)

Guanine (G)

Cytosine (C)

Thymine (T)

These bases always pair up in a specific way, known as the base-pairing rule:

Adenine (A) always pairs with Thymine (T).

Guanine (G) always pairs with Cytosine (C).

(A simple way to remember is that the 'curvy' letters G and C go together, and the 'straight' letters A and T go together).

The order of these bases along the DNA ladder is what makes up the genetic code.

From DNA to Chromosomes

A single cell contains an enormous amount of DNA. In a human cell, it's about 2 meters long! To fit all of this DNA inside the tiny nucleus, it has to be very tightly packaged.

A gene is a specific segment of DNA that carries the instructions for building one specific protein. You can think of a gene as a single 'recipe' in the giant DNA 'cookbook'. Proteins are the workhorse molecules of the cell; they do almost everything, from building structures to sending signals. The sequence of bases in a gene determines the sequence of amino acids in a protein.

A chromosome is a single, long molecule of DNA that has been tightly coiled and condensed. The DNA wraps around proteins, which helps to organize and compact it. Humans have 46 chromosomes in most of their cells (23 pairs).

So, the relationship is:

DNA is the molecule. A gene is a section of that DNA. A chromosome is the tightly packaged structure containing the long DNA molecule.

You have chromosomes in the nucleus of your cells. The chromosomes are made of DNA. The DNA is divided into sections called genes. And the genes contain the instructions to make you, you!

Key Terms