What is DNA? DNA stands for Deoxyribonucleic Acid. In humans, it is found in almost every cell, mostly tucked away safely inside the nucleus (Nuclear DNA), with a tiny bit also found in the mitochondria (mtDNA).
The Structure: The "Twisted Ladder"
The shape of DNA is a double helix. Think of it like a ladder that has been twisted into a spiral.
A. The Backbone (The Sides of the Ladder) The "rails" of the DNA ladder are made of two alternating parts: Sugar molecule (Deoxyribose) Phosphate group
B. The Rungs (The Steps of the Ladder) The "steps" are made of four chemical bases. These bases follow very strict pairing rules:
Adenine (A) always pairs with Thymine (T).
Cytosine (C) always pairs with Guanine (G).
Tip: A simple way to remember the pairs is: "Apples in the Tree, Cars in the Garage."
C. The Nucleotide One single building block of DNA is called a nucleotide. It consists of one sugar, one phosphate, and one base.
How DNA is Organized Since human DNA is about 2 meters long but must fit into a microscopic cell, it is packed very tightly:
DNA wraps around proteins called histones. These wrap further to form thread-like structures called Chromosomes.
Humans have 46 chromosomes (23 pairs) in total. A specific section of DNA that codes for a trait (like eye color) is called a Gene.
DNA Replication (Making Copies)
Before a cell divides (Mitosis), it must copy its DNA so both new cells have the instructions. This process is semi-conservative, meaning each new DNA molecule has one old strand and one new strand.
Step 1: Unzipping – An enzyme called Helicase "unzips" the double helix by breaking the hydrogen bonds between the bases.
Step 2: Priming – An enzyme called Primase makes a small piece of RNA to show the starting point.
Step 3: Building – DNA Polymerase travels along the strand, adding matching bases (A to T, C to G) to build the new side.
Step 4: Zipping – The new strands are sealed together.
Summary of Functions
Storage: It holds all your genetic information.
Protein Synthesis: DNA provides the code for making proteins, which do all the work in your body.
Inheritance: It is how traits are passed from parents to children.