This is DNA, or deoxyribonucleic acid.

By the end of this video, you will

be able to identify the key structural features of DNA,

as well as describe the importance

of those features for function.

During this video, we will look at different representations

of the DNA molecule to better view certain details,

but all views represent this same structure.

Inside the cell, you will most commonly

find double- stranded DNA, in which two strands intertwine

to form a double helix.

The most common form of the DNA double helix,

which is what we will discuss here,

is also called B-form DNA.

Now, let's move to a more simplified representation

of DNA to discuss the details.

We can unwind the double helix like this

so that we can see the chemical structure inside.

Each strand is a polynucleotide, meaning

the strand is made up of many individual units called

nucleotides.

A nucleotide has three components: the five-carbon sugar,

a phosphate group, and one

of four possible nitrogenous bases--

adenine, guanine, thymine, and cytosine.

The nitrogenous base is always attached at the

1' carbon of the sugar.

If we count from there, we can see

that there is a phosphate between the 5' carbon

of one sugar and the 3' carbon of the neighboring sugar.

The sugar is called deoxyribose because it

is missing a hydroxyl group at the 2' carbon which

is present in ribose.

Because of this, nucleotides in DNA, deoxyribonucleic acid,

are called deoxynucleotides.

Nucleotides attach to each other in the DNA strand

by phosphodiester bonds.

The phosphate group of one nucleotide

binds to the 3' oxygen of the neighboring nucleotide.

Thus, we can see that the sugars and phosphate groups make up

the DNA backbone.

The carbon numbering is key to describing

the directionality of the DNA strand, 5' to 3'.

Looking within the sugars, there is an intrinsic orientation

difference between the two strands.

On the top strand, you can see that the 5' carbon

of each sugar is on the left and the 3' carbon is

on the right.

The opposite is true for the bottom strand.

Reading left to right, that makes

the top strand orientation 5' to 3'

and the bottom strand orientation 3' to 5'.

These strands are also sometimes called Watson and Crick.

Keep in mind that this double-stranded DNA is still

a double helix and we have simplified the representation

by flattening and unwinding the helix here to better see

the atomic structure.

Although the nucleotides come together

through covalent bonds in the backbone,

the two DNA strands interact through non-covalent hydrogen

bonds between the bases.

Each base forms multiple hydrogen bonds

with its complementary base on the opposite strand.

Bound together by hydrogen bonds,

each unit is called a base pair.

The hydrogen bonding contributes to the specificity

of base pairing.

Thymine preferentially pairs with adenine

through two hydrogen bonds and cytosine preferentially pairs

with guanine through three hydrogen bonds.

Thymine and cytosine are called pyrimidines, characterized

by their single ring structure, and adenine and guanine

are called purines, which have double rings.

The geometry of the AT or TA and GC or CG base pairs

is the same, allowing for symmetry and base stacking

in the helix.

This mostly has to do with the distance between the backbones

and the angles to which the bases attach to the backbone.

Other base pairs, like GT, for example,

do not have the same geometry, cannot form strong hydrogen

bonds, and disturb the helix.

The double helix structure of DNA is highly regular.

Each turn of the helix measures approximately ten base pairs.

In addition to the hydrogen bonding between the bases,

the stacking of the bases also stabilizes the double helix

structure.

These pi-pi interactions form when

the aromatic rings of the bases stack

next to each other and share electron probabilities.

The regularity of the helical structure

forms two repeating and alternating

spaces, called the major and minor grooves.

These grooves act as base pair recognition

and binding sites for proteins.

The major groove contains base pair specific information

while the minor groove is largely base pair nonspecific.

This is because of the patterns of hydrogen bond acceptors

and donors that proteins can interact with in the grooves.

In this way, the DNA can be acted upon

in either a sequence specific or non-sequence specific manner,

allowing proteins to position themselves correctly

in the genome to carry out their designated tasks.

This is the DNA double helix, and you've now

learned the structural features that influence its function.

We hope you've enjoyed exploring this amazing molecule with us.