The Molecular Basis of Inheritance and DNA Replication Chapter 16

DNA As the Genetic Material Introduction After T. H. Morgan proved that genes were located on chromosomes with his revolutionary fruit‐fly experiments, scientists critically debated whether proteins or DNA, the two constituents of chromosomes, were the true genetic material. Because proteins were not only extremely diverse but also carried out a variety of crucial functions on the cellular level scientists assumed that proteins were the genetic material. However various experiments conducted during the mid‐1900s proved that it was in fact DNA that was the genetic material…

1928: Frederick Griffith

1944: McCarthy, Avery, and MacLeod

1952: Hershey and Chase

• Griffith experiments with various bacteria strands and mice; he discovers that mixing a living, harmless strand with a heat‐killed pathogenic strand results in living, pathogenic cells. This process is called transformation.





• These scientists announce that DNA is the transforming material that was discovered in Griffith's experiment.

• Hershey & Chase study viruses (phages) that infect bacteria. Specifically experimenting with E. Coli and employing the radioactive isotopes of phosphorus and sulfur, elements specific to DNA and proteins, respectively, they confirm that DNA is indeed the genetic material.

Who was Rosalind Franklin?

DNA’s Double Helix Structure Once DNA is established as the genetic material the next big question regards its structure. Watson and Crick are credited with the discovery of DNA’s double helix structure; two anti‐ parallel sugar‐phosphate strands with nitrogenous bases in its interior. A brilliant scientist that was Chargoff’s Rules: researching DNA’s structure, it Erwin Chargoff discovers that was Franklin’s picture of the proportion of each base differs DNA strand, created from X‐ between species, but the ratio ray crystallography, from between bases keeps consistent… which Watson and Crick deduced DNA’s double‐helix 1. A pyrimidine must always pair with structure. Although Rosalind a purine. This results in a helix that had truly first made this maintains a consistent width revolutionary discovery, throughout its entire structure. Watson and Crick got the credit. 2. A=T and G=C describes the amounts of nitrogenous bases in every organism.

Important Vocabulary: Antiparallel: DNA strands’ 5’ and 3’ ends are oriented in opposite directions Transformation‐ A change in genotype and phenotype due to the assimilation of external DNA by a cell Double Helix‐DNA’s form; two antiparallel polynucleotide strands in a spiral shape Semiconservative Model‐ Newly replicated DNA strand consists of one old parental strand and one new stand Origins of Replication‐ place at which replication begins Replication Fork‐ Y‐shaped region on a replicating DNA strand where new strands grow Leading Strand‐New and continuous complementary DNA strand Lagging Strand‐ discontinuously synthesized DNA strand that elongates in a direction away from the replication fork Telomeres‐ The protective structure at the end of each chromosome; repetitive nucleotide sequences

DNA Replication Introduction Scientists debated between 3 models that described DNA replication: conservative, semiconservative, and dispersive. Once Meselson and Stahl held their experiment in which they employed heavy nitrogen to trace parental DNA strand it was confirmed that DNA replication followed the semiconservative model. One strand of the parental DNA acted as the template to the new strand, which would be complementary its template. Therefore, at the end of replication the copied DNA would have one parental strand as well as one new strand. Proteins and Enzymes Carry Out Replication

Enzymes are a fundamental factor of the DNA replication process. Many origins of replication are created on each chromosome; replication begins at these origins of replication; from this origin two replication forks are created and from these forks new DNA strands are synthesized in the 5’ to 3’ direction on the NEW strand. The new strand and the template parental strand are antiparallel to one another. Then enzymes begin their work… 1. Helicase unzips the two parental DNA strands 2. SSBP (single strand binding proteins) stabilize the unwinding strands 3. Topoisomerase corrects the over winding DNA 4. Primase begins replication by adding a sequence of 5‐10 RNA nucleotides to the new strand 5. DNA Polymerase elongates the new strand by replacing the RNA nucleotides and adding complementary (following Chargoff’s rules) DNA bases….this is the main copying enzyme!!! Because DNA Polymerase can only add nucleotides in the 5’  3’ direction, continuous strands cannot be created on both new strands of DNA. The leading strand is created in the direction of the replication fork while the lagging strand is created in discontinuous fragments, known as Okazaki fragments, in a direction opposite of the replication fork. DNA ligase later connects these fragments.





DNA Repair The initial nucleotide pairing in replication is not nearly as errorless as its completed state; throughout the process of replication DNA polymerase (I know, it seems like it can do everything!) proofreads the nucleotides to ensure that the correct bases are being attached. Incorrectly matched nucleotides, although quite rare, result in mutations unless corrected by other specialized enzymes. Mismatch repair involves the correction of incorrectly paired nucleotide bases. Three enzymes work in mismatch repair to fix the DNA… 1. Nuclease cuts out the “bad” or incorrectly paired DNA 2. DNA Polymerase adds the “good” or correct DNA bases 3. DNA Ligase glues the fragments together to reform a continuous strand DNA Polymerase faces a problem as it attempts to complete the 5’ ends of the replicated strand; it simply cannot. Therefore, to prevent any loss of important hereditary information, telomeres are produced. These “extra” sequences of DNA that code for nothing are actually quite necessary and useful in the process of DNA replication. Erosion of DNA is prevented through the existence of telomeres and telomerase, an enzyme that produces telomeres is often seen in gonad cells. Unfortunately, cancer cells employ telomerase, preventing cancer from ultimately self‐destructing.