DNA Replication Hydrogen bonds Nucleotide Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Opposite sides of the DNA double helix are called complementary strands.

Why replicate DNA? Copying of the DNA occurs before a cell divides Parent cell pinches off into two daughter cells Each daughter has the exact same DNA (the full amount, not half!)

DNA Replication – step 1 Hydrogen bonds are weak between bases Easily broken by an enzyme – like unzipping a zipper

DNA Replication – Step 2 Free floating nucleotides in nucleus form hydrogen bonds with the complementary bases on the parent strands

DNA Replication – Step 3 DNA winds back up End result is 2 identical DNA molecules – each with one old strand and one new strand (SEMICONSERVATIVE)

DNA Replication Original strand New strand DNA polymerase Growth Replication fork Replication fork New strand Original strand

DNA Replication Step 1: Helicase binds to the DNA and unwinds the 2 strands (exposing nitrogenous bases) What kinds of bonds does helicase break? Helicase

DNA Replication Step 2: DNA Polymerase adds complementary nucleotides to the exposed bases on the original strand This forms a NEW strand attached to the OLD one What kinds of bonds are being formed? Helicase DNA Polymerase DNA Polymerase

DNA Replication Step 3: New DNA molecules wind back up to form a double helix Why do we say DNA replication is “Semi-conservative?” There are now two DNA molecules, each containing one OLD strand and one NEW strand Helicase DNA Polymerase DNA Polymerase

DNA Replication - Photo

Self-Quiz! The sequence of bases on one strand of DNA is: ACTGTACCGA TGACATGGCT What is the sequence of bases on the complementary strand?

DNA Replication - Summary When? Where? How? Before cell division In the nucleus of the cell DNA unzips, complementary free nucleotides latch on, (A=T, G=C), two complete strands are produced.