Biology: Structure and Location of DNA

Where is DNA Found?

• DNA is mainly found in the nucleus of a cell, forming part of the chromatin network.
• Small amounts of DNA are also found outside the nucleus in mitochondria and chloroplasts, known as extranuclear DNA.

Structure of DNA

• DNA is a long, twisted ladder-like molecule that forms a stable, 3-dimensional double helix.
• The DNA molecule is made up of nucleotides, which are building blocks consisting of a sugar molecule, a phosphate molecule, and a nitrogenous base.

Nucleotides

• There are four types of nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
• These four bases are the foundation of the genetic code, instructing cells on how to synthesize enzymes and other proteins.

Double Helix Structure

• The outer two strands of the ladder are formed by a chain of alternating sugar and phosphate links.
• The rungs of the ladder are formed from pairs of bases linked by weak hydrogen bonds.
• The base pairs are attached to the sugar molecules.

Base Pairing

• Adenine (A) pairs with thymine (T) or uracil (U) using two hydrogen bonds.
• Cytosine (C) pairs with guanine (G) using three hydrogen bonds.

Classification of Nitrogenous Bases

• There are two groups of nitrogenous bases: purines and pyrimidines.
• Purines consist of two fused rings of nitrogen, carbon, and hydrogen atoms (e.g. guanine and adenine).
• Pyrimidines consist of one ring of similar atoms and are smaller than purines (e.g. cytosine and uracil).

Universality of DNA

• The four nucleotides are the same in all animals and plants.

Genetic Code

• The sequence of nucleotides determines the genetic code of an organism.
• The sequence of nucleotides in certain sections of DNA in a human is different from the same sections in every other human being (except in identical twins).

Role of DNA

• DNA molecules carry hereditary information in the form of genes.
• DNA provides a blueprint for an organism's growth and development by coding for protein synthesis.
• DNA can replicate, making a copy of itself to pass on genetic information to each daughter cell during cell division.

Non-Coding DNA

• Less than 2% of human DNA codes for proteins; the rest consists of non-coding DNA.
• Non-coding regions are interrupted by protein-coding regions called exons.
• Complex organisms contain more non-coding DNA than less complex organisms.