Online College Courses for Credit

2 Tutorials that teach DNA:Structure- The Double Helix
Take your pick:
DNA:Structure- The Double Helix

DNA:Structure- The Double Helix

Author: Amanda Soderlind

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

See More

Source: Video and Images Created by Amanda Soderlind

Video Transcription

Download PDF

Welcome to this lesson today on DNA structure. Today we are going to be examining the structure and function of DNA. So we are actually going to start by discussing nucleotides. So we're going to start at the smallest level with DNA and then work our way up to see how DNA composes chromosomes.

So DNA is actually composed of nucleotides. And nucleotides are the subunits of nucleic acids. So a nucleotide is actually composed of a five-carbon sugar. In DNA, this five-carbon sugar is deoxyribose, and in RNA this five-carbon sugar is ribose.

But we're focusing on DNA today. So this five-carbon sugar that composes DNA is a deoxyribose sugar. These nucleotides also contain a phosphate group and a nitrogen base. So these nitrogen bases can be either adenine, thymine, cytosine, or guanine.

So the five-carbon sugar is a deoxyribose sugar, a nitrogen base of adenine, thymine, cytosine, and guanine, and then a phosphate group compose the chemical structure of DNA. So down here we have examples of these nucleotides. So you can see that here is the five-carbon sugar. It's a deoxyribose sugar, as I mentioned.

And the phosphate groups. So the deoxyribose sugar and the phosphate groups compose this backbone of DNA. So DNA is said to have a phosphate sugar backbone because that's what the outside part of the DNA is composed of here.

And then these nucleotides are held together in the double helix by covalent bonds. So covalent bonds play a role in holding these nucleotides together in this DNA structure. And then inside the DNA here, we have our base pairs. And our base pairs are composed of our nitrogen bases.

And the nitrogen bases pair up in specific ways. So we have two base pairs in DNA. We have the base pair of adenine and thymine and the base pair of cytosine and guanine. OK, so adenine always pairs with thymine, and cytosine always pairs with guanine.

So I'm actually going to write the name of these base pairs here just so you can see how they're spelled. So adenine and thymine will always pair together, and then cytosine and guanine will always pair together. So these are our two base pairs that we have here.

And the reason that these always pair together-- adenine with thymine and cytosine with guanine-- is because of the shape and the site for the hydrogen bond. So they're held together here by a hydrogen bond. So the shape and the site of that hydrogen bond is what allows adenine to always pair with thymine and cytosine to always pair with guanine.

So they almost fit together without hydrogen bond like a little puzzle. So adenine wouldn't properly fit with guanine, and cytosine wouldn't properly fit with thymine. So the way that that bond is allows them to fit together nicely. So these are what make up the chemical structure of DNA.

So DNA is described as being a double helix structure. So it's almost like a ladder that's been twisted. And the backbone of the ladder, as I mentioned, are the phosphate sugar. And then the rungs of the ladder are our base pairs.

And then basically what happens is that that DNA will mix with proteins and will become condensed and kind of wind around itself in order to form chromosomes. So chromosomes are kind of like all this DNA that's been twisted around and condensed to form this chromosome structure. And so chromosomes are condensed DNA that contain genes.

And genes are units of heredity that are made up of sequences of nucleotides. So the nucleotide sequence of a gene codes for a specific polypeptide chain. And polypeptide chains are the structural units of proteins, and they determine our traits.

So the base pairs that we have here pair up in certain ways to produce nucleotide sequences. And those nucleotide sequences code for specific polypeptide chains, which then produce specific proteins. So the order in which the DNA is composed basically determines different types of genes.

So this lesson has been an overview on DNA structure.

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.