DNA Structure and Replication

• Frederick Griffith (1927) showed that avirulent strains of Diplococcus pneumoniae could be transformed to virulence.

Section 10.3

• Avery, MacLeod, and McCarty (1944) demonstrated that the transforming principle was DNA and not protein.

Figure 10.4

• Avery, Macleod, and McCarty Experiment (1944)

• DNA not protein is the heritable material

Section 10.3

• Hershey and Chase (1952) demonstrated that DNA, and not protein, enters the bacterial cell during bacteriophage infection and directs viral reproduction.

Figure 10.5

Figure 10.6

Genes Are Made of DNA

1. Heritable information is carried in units called genes

2. Genes are parts of structures called chromosomes

3. Chromosomes are made of deoxyribonucleic acid (DNA) and protein

DNA

• DNA is made of chains of small subunits called nucleotides

DNA

• Each nucleotide has three components:1. Phosphate group

2. Deoxyribose sugar

3. One of four nitrogen-containing bases– Thymine– Cytosine– Adenine– Guanine

Chargaff’s Rule

• In 1940, biochemist E. Chargaff determined that:

– In a DNA molecule, amounts of

A = T, G = C– “Chargaff’s Rule”

Early DNA Studies• Rosalind Franklin and Maurice Wilkins studied DNA

structure using X-ray scattering

• From X-ray diffraction patterns they deduced that DNA – Is long and thin– Has a uniform diameter of 2 nanometers – Is helical, and is twisted like a corkscrew– Consists of repeating subunits

DNA Is a Double Helix

• James Watson and Francis Crick :

– DNA is made of two strands of nucleotides

– Deoxyribose and phosphate portions make up the sugar-phosphate backbone

Bases Form Ladder Rungs in DNA

• Nitrogen-containing bases protrude inward from sugar-phosphate backbone

ghr.nlm.nih.gov

Hydrogen Bonds

• Hydrogen bonds hold certain nitrogenous base pairs together

– A bonds with T, G bonds with C

– Bonding bases called complementary base pairs

DNA is a Double Helix

• Ladder-like structure of the two DNA strands are twisted into a double helix

• How can a molecule with only 4 simple parts be the carrier of

genetic information?

DNA Replication

• All cells come from pre-existing cells

• Cells reproduce by dividing in half

• Each of two daughter cells gets an exact copy of parent cell’s genetic information

• Duplication of the parent cell DNA is called replication

DNA Replication

• DNA helicases

• DNA polymerases

ncbi.nlm.nih.gov

DNA Replication

• Base pairing is the foundation of DNA replication

Semiconservative Replication

• The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication)

Replication And Proofreading

Types of Mutations

• Point mutation - individual nucleotide in the DNA sequence is changed

• Insertion mutation - one or more nucleotide pairs are inserted into the DNA double helix

• Deletion mutation - one or more nucleotide pairs are removed from the double helix

• Inversion - piece of DNA is cut out of a chromosome, turned around, and re-inserted into the gap

• Translocation - chunk of DNA (often very large) is removed from one chromosome and attached to another

The End