Chapter 9: DNA: The Molecule of Heredity

What is DNA? Answer: Molecule that carries the blueprint of life General Features: DNA is packages in chromosomes (DNA + Proteins) Gene = Functional segment of DNA located at a particular place on a chromosome

(Figure 9.1)

Structure of DNA: Chapter 9: DNA Nucleic acid composed of individual nucleotides

Structure of DNA: Nucleic acid composed of individual nucleotides 4 types of nucleotides (nitrogen base differences): Purines (double-ring bases) Pyrimidines (single-ring bases)

How are the Nucleic Acids arranged to form DNA? 1) Chargaff (1940s): DNA contains equal amounts of adenine and thymine DNA contains equal amounts of cytosine and guanine 2) Franklin and Wilkens (early 1950s): Used X-ray diffraction DNA is a helical molecule (corkscrew) DNA has a uniform diameter (2 nm) DNA consists of repeating sub-units

How are the Nucleic Acids arranged to form DNA? 1) Chargaff (1940s): 2) Franklin and Wilkens (early 1950s): 3) Watson and Crick (1953) DNA is two separate strands of linked nucleotides Phosphate groups bonded to sugar groups (backbone) Antiparallel (strands opposite one another) DNA strands held together by hydrogen bonding between individual base pairs Complementary base pairing: Adenine Thymine Guanine Cytosine Strands twisted to form double helix

(Figure 9.3)

The order of nucleotides in DNA can encode vast amounts of information: 10 nucleotides = > 1,000,000 possible sequences Organisms have millions (bacteria) to billions (plants / animals) of nucleotides

DNA Replication The copying of DNA is a critical event in a cell s life Cell reproduce by mitosis to produce two daughter cells from a single parental cell Each daughter cell must receive an exact copy of DNA DNA replication produces two DNA double helices through process termed semiconservative replication Parental strands unwind and separate Daughter strands are formed using parental strands as templates Parental strand and new daughter strand wrap together to form new double helix

Semiconservative Replication: Chapter 9: DNA

DNA Replication - A Closer Look... Step 1: DNA Strand Separation DNA Helicase (enzyme) breaks hydrogen bonds holding nitrogen bases together (unzips) Requires energy (ATP) Multiple DNA helicases work on single chromosome form replication bubbles; time-saving mechanism DNA Helicase

DNA Replication - A Closer Look... Step 2: DNA Synthesis DNA Polymerase (enzyme) joins nucleotide sub-units to form new strands of DNA Forms complementary strands to parent strands (Recognizes nucleotide and adds appropriate base) A T G C C G T A A T C G T G A G G A T Polymerase T A A C T C C T A

DNA Replication - A Closer Look... Step 2: DNA Synthesis DNA Polymerase (enzyme) joins nucleotide sub-units to form new strands of DNA Forms complementary strands to parent strands (Recognizes nucleotide and adds appropriate base) DNA Polymerase can only move in one direction One strand synthesized as continuous molecule Second strand synthesized in small pieces DNA Polymerase DNA Ligase: Attaches pieces together

DNA Replication - Mistakes Happen... DNA replication is a rapid process 700 nucleotides / second DNA polymerase mismatches 1 bp / 10,000 bps Proof-reading enzymes correct mistakes Overall mistake rate = 1 bp / 1,000,000,000 bps Other hazards damage DNA Body temperature = 98.6 F = 10,000 bp lost / day Ultraviolet light

Sun Burns: Chapter 9: DNA 1st Degree 2nd Degree UV light from sun damages DNA If damage is great enough, cell will commit cellular suicide If damage in region of DNA controlling replication, cancer may result Asymmetry Border (irregular) Color (irregular) Diameter (large) 3rd Degree

DNA Helicase DNA Polymerase DNA Ligase: Attaches pieces together