The Molecular Basis of Inheritance Chapter 16

The DNA Story Once the chromosomal theory of inheritance was widely accepted, scientists turned their attention to studying the chromosome.

Why did researchers think that protein was the genetic material? DNA was too simple and too uniform DNA consists of 4 different nucleotides Protein consists of 20 different amino acids Proteins have a wide variety of functions What organisms were initially studied? Bacteria and viruses, since they were so simple

Frederick Griffith Streptococcus pneumoniae, a bacteria that causes pneumonia in mice was studied

Evidence that Viral DNA can Program Cells Hershey and Chase used bacteriophage, viruses that infect bacteria. The virus consists of a protein coat on the outside and a nucleic acid in the core.

Circumstantial Evidence for DNA Prior to mitosis eukaryotic cell doubles its DNA This DNA distributes exactly evenly to the two daughter cells formed at the end of mitosis An organisms’ diploid cells have twice the DNA as its haploid gametes

Experimental Evidence for DNA Structure Chargaff (1947) analyzed DNA composition of different organisms Franklin and Wilkins X-ray crystallography pictures gave evidence for double helix DNA nucleotide backbone connection was known

DNA Replication When is the DNA replicated? During the S phase of interphase Watson and Crick realized that the DNA structure suggested a method for replication Semi-conservative replication

Messelson and Stahl devised experiments to test how DNA replicates itself

Enzymes Helicase - enzyme that untwists and unwinds the DNA Primase - enzyme that joins RNA nucleotides to make the primer Single Stranded binding Protein - molecules that keep the DNA strands apart form each other

DNA Polymerase - enzyme that adds new nucleotides to the growing strand Ligase - enzyme that joins fragments of DNA to one another

Rules for Replication DNA polymerase can only add nucleotides to an existing polynucleotide (either RNA or DNA) DNA polymerase adds new nucleotides to the 3’ end of the growing strand The helicase continues to untwist and unwind the DNA in one direction at the replication fork All of these rules combine to create a dilemma…

DNA Strands are anti-parallel

The ends of the DNA molecules of eukaryotic cell cause problems on the leading strand

Why is this problem only seen in eukaryotes? Prokaryotic DNA is circular so there is no end Eukaryotes solve this problem with the special ends called telomeres Telomeres have no genes so if they are not replicated no important material is lost

Telomerase is found in germ lines that produce gametes and in cancerous cells