Unit 6 Notes:

DNA Replication

(1) Why Replicate DNA?• Necessary for cell division.

• During the S Phase of the Cell Cycle, the cell needs to make a copy of its DNA for the new cell.

• Errors:– Can have ZERO impact on the cells.– Could provide a positive advantage to cell.– Could kill the cell.

(2) Semi-Conservative Replication

• Messelson & Stahl discovered that DNA Replication is a Semi-Conservative process:– Semi = ½ or Partial– Conserve = To Keep or To Save

• You start with 1 whole, old molecule of DNA…– You end with 2 new molecules of DNA.– Each new molecule has:

• 1- old strand• 1- new strand• (1/2 of the old molecule is conserved in each new one)

(3) Replication Basics:• Overall Goal: Make an Identical copy of

cell’s DNA molecule.

• Basic Steps:1- Un-twist strands.2- Separate strands.3- Make a complimentary copy of each strand.

• End Result:2 DNA molecules, each with 1 old strand and 1 new strand.

(4) Untwisting & Separating

• DNA is untwisted and separated by the enzyme Helicase.

• DNA is opened in small chunks with replication forks in between…– Several helicase enzymes open up DNA at once.– Results in several open “bubbles” along helix.– This increases replicating efficiency!

(5) Complimentary Copies• The enzyme DNA POLYMERASE will lay down

the new DNA nucleotides.– DNA Polymerase can only lay down new DNA

nucleotides starting with a 5’ End!– This means the enzyme must move along the original

strand 3’5’ to be able to lay down new nucleotides.

• The Leading Strand:– Original strand that is positioned 3’ 5’.– DNA is first laid down along this strand.

• The Lagging Strand:– Original strand that is positioned 5’ 3’.– RNA nucleotides are first laid down along this strand.– Then DNA Polymerase replaces them with DNA .

(6) Complimentary Bases

• REMEMBER:– A bonds with T– G bonds with C

• EXAMPLE:A

C

G

T

T