DNA

Deoxyribonucleic Acid

Components, Structure and

Replication

Components and Structure

DNA looks like a twisted

ladder.

Two strands of DNA spiral

around one another to form

a double helix.

The strands are held

together like two sides of a

zipper by hydrogen bonds.

The double helix model was

developed in 1953 by

Watson and Crick

Components and Structure

Each strand is made up of units called nucleotides

5-carbon sugar called deoxyribose

Phosphate group

Nitrogenous base

Label the picture on your notes

Components and Structure

4 kinds of bases: A, G, C and T

Purines

Adenine and Guanine

Two rings

Pyrimidines

Cytosine and Thymine

One ring

Components and Structure

The bases hydrogen bond with each

other to form base pairs in the double

helix, creating the “rungs” of the

ladder.

Adenine pairs with Thymine

Cytosine pairs with Guanine

Chargaff’s Rule - The percentage of A and T

will always be equal in a sample of DNA. The

percentage of C and G will also always equal.

DNA Replication

During S Phase of the Cell Cycle, a cell must replicate its genetic material in preparation for cell division.

Why does the cell need two copies?

Replication – the process of copying or duplicating DNA

The strands of a double helix are complementary; because of the base pairing rules, the sequence of one strand can be used to determine the sequence of the other strand.

DNA Replication

The two strands are “unzipped” at the replication fork. Prokaryotes start at one point and replicate in

both directions.

Eukaryotes start at many points and have several replication forks at once since they have so much DNA to copy.

An enzyme called DNA polymerase uses an old strand of DNA as a template to create a new strand of DNA. Each old strand is a template for a new strand.

DNA polymerase proofreads its work to make sure that it has created an accurate complementary strand.

DNA Replication

The end result of replication – two double

helices, each with one old strand and one new

strand.

Replication

http://www.youtube.com/watch?v=rpwjZX_

z5rg

Question

Why is it important for DNA polymerase to

proofread its copies?

What would happen if DNA polymerase

had a greater frequency for mistakes?

Let’s Practice!

Template DNA: AGCTAA

Complementary DNA: TCGATT

Template DNA: TTGCGA

Complementary DNA: AACGCT

Template DNA: AGCATG

Complementary DNA: TCGTAC