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DNA:Structure- The Double Helix

DNA:Structure- The Double Helix

Author: Aaron Mullally

This lesson will examine the structure of DNA as being in the form of a double helix composed of nucleotides.

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The Double Helix

Video Images: Double Helix image, Creative Commons:; Nucleotides, Public Domain:; GC DNA Base Pair image, Public Domain:; AT DNA Base Pair, Public Domain:; DNA Chemical Structure, Creative Commons:

Source: Aaron M

Terms to Know
Adenine (A)

A nucleotide building block of DNA and RNA, adenine is classified as a purine and complements thymine (T) in DNA and uracil (U) in RNA.

Base Pair

The way that nucleotides interact with one another, A bonds with T and C bonds with G in DNA, while C bonds with G and A bonds with U (uracil) in RNA. The sequence of base pairs creates the genetic code that is transcribed and translated into proteins.

Cytosine (C)

A nucleotide building block of DNA and RNA, it is classified as a pyrimidine and complements guanine (G) in DNA and RNA.

Double Helix

The shape of the DNA molecule, often times is referred to as the “twisted ladder” and is the title to the book about Watson & Crick's discovery of DNA's structure.


A segment of DNA that codes for a specific protein, genes are a sequence of nucleotides.

Guanine (G)

A nucleotide building block of DNA and RNA, guanine is classified as a purine and complements cytosine (C) in DNA and RNA.


Organic molecules that consist of a 5 carbon sugar (ribose in the case of RNA, and deoxyribose in the case of DNA), a phosphate group and a nitrogenous base; nucleotides are the building blocks of nucleic acids (DNA & RNA).

Nucleotide Sequence

The arrangement of nucleotides that form genes in strands of DNA.

Thymine (T)

A nucleotide building block of DNA, thymine is classified as a pyrimidine and complements adenine (A) in DNA; thymine is not found in RNA.