Chapter 12 DNA STRUCTURE AND REPLICATION

A. DNA Structure DNA is a nucleic acid composed of nucleotide monomers. DNA nucleotide consists of: F one phosphate group F one deoxyribose sugar (5 carbon sugar) F one nitrogenous base (G, A, C or T)

DNA is a double-stranded helix (Watson & Crick 1953). ] Sides of ladder make up sugar- phosphate “backbone”. ] Rungs of ladder composed of base pairs joined by hydrogen bonds.

Pyrimidines (T & C) form hydrogen bonds with purines (A & G). ] Thymine pairs with Adenine, forming 2 hydrogen bonds. ] Cytosine pairs with Guanine, forming 3 hydrogen bonds.

DNA strands are antiparallel. 5’ to 3’ strand 3’ to 5’ strand Numbering of strands is based on position of deoxyribose sugars.

DNA is highly condensed. ] DNA is wrapped tightly around proteins & folded. ] DNA must unwind for replication to occur.

B. DNA Replication Process by which DNA is duplicated. F occurs during the S phase of Interphase F is semiconservative (Meselson & Stahl)

Overview of DNA Replication:  Unreplicated DNA.  Strands “unzip” at several points creating replication forks.  Each strand serves as template for complementary nucleotides to H-bond.  New nucleotides of each daughter strand are linked.

Steps in DNA Replication:  Helicase breaks hydrogen bonds.  Binding proteins stabilize strands; prevent them from rejoining.  Primase makes an RNA primer.

 Free nucleotides move in & H-bond; DNA polymerase links nucleotides to each other starting at primer & working in the 5’ to 3’ direction.  DNA polymerase “proofreads” new strand (replaces incorrect bases).

 DNA replication is continuous on one strand.  DNA replication is discontinuous on other strand, producing Okazaki fragments.

 Repair enzymes remove RNA primers; Ligase connects Okazaki fragments. Ligase

Determine the base sequence of daughter DNA replicated from the following parental DNA strand. parental DNA C T A G G T A C T daughter DNA G A T C C A T G A

C. DNA Repair UV radiation damages DNA by causing thymine dimers to form. DNA damage can be repaired by photoreactivation or excision repair.

1. Photoreactivation – photolyase (enzyme) uses light energy to split dimer. 2. Excision repair - repair enzyme cuts out damaged area; DNA polymerase inserts replacement sequence & ligase seals backbone.

3. Mismatch repair - enzymes proofread newly replicated DNA for base mispairing & correct the error. Faulty DNA repair results in chromosome breaks & an increased susceptibility to cancer. Ex. Xeroderma pigmentosum