The structure of DNA

DNA molecules look like a twisted ladder, or spiral staircase.

The two sides of the DNA ladder are made up of molecules of sugar called deoxyribose and alternate

with molecules known as phosphates

• The sides of the ladder are often referred to as the “backbone” of the DNA molecule

Each rung of the ladder is made up of a pair of molecules called nitrogen bases.

• There are four bases.• They are adenine (A),• thymine (T), • guanine (G), and• cytosine (C). They are • represented with the • capital letters A, T, G,• and C.

These nitrogenous bases will only pair with a certain other base.

• With DNA the bases always pair like this:• A with T (adenine with thymine)• G with C (guanine with cytosine)

This pairing will help you understand how DNA replication occurs.

• What is replication? It is the process by which a cell makes a copy of the DNA in its nucleus.

• How does this replication happen?????• It begins when the two sides of the DNA

molecule unwind…………… HUH?!?!

Here is how it looks when the DNA molecule unwinds- - - -

It then unzips like a zipper….

Why is it important that the DNA molecule unzips?

• Because the base pairs separate and are left open for the floating nitrogen bases to come in, match up and build the duplicate DNA—which is replication!!

• What do you mean floating nitrogen bases?• Inside the nucleus are the other parts that are

needed to make a copy or duplicate of the existing DNA.

Once “unzipped” you have two halves of the original DNA molecule. The bases build up the missing sides = 2

molecules now----

• The duplicate DNA ---- so what?• Because of the way in which the nitrogen bases pair

with one another, the order of the bases in each new DNA molecule exactly matches the order in the original DNA molecule.

This is vital when a cell duplicates—why?

• So that there is DNA for each of the daughter cells!!!

• OH!! Now I get it. Duplicate cells mean two of the same.

Remember…..• Adenine (A) always pairs with Thymine (T)• Guanine (G) always pairs with Cytosine (C)

You will be building a DNA molecule tomorrow!!!

• Remember your base pairings-work quickly but carefully---then after you check it ---you can eat it.

• WOW it even twists up• like a real DNA molecule.