DNA Structure & Replication Deoxyribonucleic acid

“A Winding Staircase” DNA is a long molecule that is shaped like a spiral staircase It is composed of two parallel strands of subunits Known as a double helix

1.) Sugar 2.) Phosphate 3.) Nitrogen Base Nucleotides Nucleotides are the subunits that make up DNA Each nucleotide is made up of three parts: Sugar & Phosphate make up “back bone” (hand rails of the staircase) The Nitrogen Bases pair together to connect the strands (steps of the staircase)

Nitrogen Bases In DNA, each nucleotide has the same sugar molecule and phosphate group, but it can have one of 4 bases –Adenine –Thymine –Cytosine –Guanine The base-pairing rules state that –Adenine only pairs with Thymine (complementary) –Cytosine only pairs with Guanine (complementary) Base pairs are held together by hydrogen bonds

How do cells make more DNA? Remember that before mitosis or meiosis can occur, DNA must be copied!! This process is called REPLICATIONREPLICATION Steps of DNA replication 1.An enzyme (helicase) unwinds & separates the 2 strands of DNA 2.New complementary nucleotides are added to each side 3.Another enzyme (DNA polymerase) joins the newly added nucleotides together and “proof-reads” the new strands

Watch this: