Molecular Basis of Inheritance AP Biology Crosby High School

Frederick Griffith (1928) Streptococcus pneumoniae Lethal smooth strain vs. Harmless rough

Hershey – Chase Experiment T2 reprogrammed E. coli Alfred Hershey and Martha Chase (1952)

Additional Support for DNA Eu. Cells copy DNA exactly before Mitosis Diploid cells exactly twice as much DNA as Haploid gametes Chargaff’s rule %A = %T %G = %C

The Players

Watson and Crick James Watson visited Maurice Wilkins at Cambridge Noticed Rosalind Franklin’s X-ray image of DNA Took it to Francis Crick who recognized a helix structure

Double – Helix Calculations Width of helix suggested two strands One complete turn every 3.4 nm Base pairs .34 nm apart Purines must be paired specifically with Pyrimidines Supported Chargaff’s rule

The Structure

DNA Replication Proposed types 6 billion base pairs copied Conservative: Original DNA remains intact Semi-conservative: Half original and half daughter Dispersive: All four strands have old and new 6 billion base pairs copied Takes only a few hours 1 mistake out of 1 billion nucleotides

Proposed Replications

Origin of Replication Recognize specific portion to open molecule Begins copying in both directions Number of Origins Prokaryotic: 1 origin of replication Eukaryotic: May have hundreds or thousands Replication fork Y-shaped region of origin of replication

Elongation of New DNA Strands Catalyzed by DNA Polymerases Bacteria: 500 nucleotides / sec Humans: 50 nucleotides / sec Energy supplied by nucleoside triphosphates ATP, GTP, CTP, TTP Nucleotide and phosphate join strand Pyrophosphate releases energy through hydrolysis

Elongation

Antiparallel DNA Strands

Leading and Lagging Strand Only elongates in 5’  3’ Direction Lagging strand contains Okazaki fragments (100-200) DNA Ligase joins fragments

Primase Begins Replication Primase: joins RNA together to form a primer DNA Polymerase adds to the primer Another polymerase replaces the RNA primer with DNA Leading strand needs only one primer Lagging strand needs one primer for each Okazaki fragment

DNA Proofreading DNA polymerase checks for errors and corrects them as it elongates Mismatch Repair: fixes incorrectly paired nucleotides Nucleotide Excision Repair Thymine – Thymine Dimers

Excision Repair

Telomeres Exist at the ends of DNA Multiple repetitions of TTAGGG Between 100 – 1,000 repeats Telomerase replaces telomeres in germ cells If the repeats run out then the cells die

Telomerase