DNA DNA stands for Deoxyribonucleic Acid. That is a big fancy word that describes what molecules it is made up of. It is composed from a sugar – Deoxyribose A phosphate group And four different nucleic acids.

Structure Along the sides the sugar and phosphate groups alternate. Attached to each of the sugars are one of the four nucleic acids: adenine, thymine, cytosine or guanine.

Two sides The two sides of the molecule are held together by hydrogen bonds. Adenine (A) always bonds to Thymine (T) with 2 bonds. Cytosine (C) always bonds to Guanine (G) with three bonds. This helps to prevent mistakes.

5’ and 3’ The two sides of the DNA are not mirror images of each other, instead they are complementary. One side of the sugar (seen in blue) is the 5’ end ( side with the O on top) and the OH side is the 3’ end.

5’ end and 3’ end So on one side you have the 5’ and the other the 3’ end is on top. This will be important when we copy the DNA.

CHECKPOINT Here is one side of the DNA. What does the other side look like? 5’ – T A T C C G A C T G – 3’ 3’ – A T A G G C T G A C – 5’

DNA replication When it is time for a cell to divide, the cell has to make copies of all of its DNA so each new cell has them. The process of copying DNA is called DNA replication. So on the following slides you will find the process.

The steps 1. The first thing that happens is that a molecule called DNA helicase unzips and unwinds the DNA. It does this by breaking the hydrogen bonds between the nucleic acids.

RNA primer 2. A molecule called RNA primase enters and attaches a small strand of RNA primer. This then will be the location where DNA polymerase can attach.

3. A new molecule called DNA polymerase attaches to the DNA. Its job is to attach nucleotides in the 5’ to 3’ direction. So it can only work in one direction. Do you remember that the two sides are upside down????? Uh Oh.

Leading strand So in the leading strand it will be attaching bases in a continuous fashion. It attaches A to T and C to G until it is complete.

Lagging Strand On the lagging strand things are a little trickier. Instead of building straight up it has to work in short segments down, called Okazaki fragments. So here are the steps:

1. RNA primase attaches an RNA primer on the lagging strand.

2. DNA polymerase attaches nucleotides in the 5’ to 3’ direction creating an Okazaki fragment.

3. Another Okazaki fragment is synthesized.

4. When the DNA polymerase runs into another RNA primer, it stops and detaches. A new DNA polymerase attaches, cuts out the RNA primer and attaches DNA nucleotides.

5. DNA ligase glues the two fragments together.

Video Please watch Hank as he describes this procedure. M

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