3Acetabularia Cut off cap….new one grows Cut off foot…no new growth Hammerling hypothesized the information for growth & development resides in the foot.AND… the nucleus resides in the foot
4Acetabularia… Transplantation Experiments In replacing caps and feet between species, Hammerling found that the nucleus-containing foot was the determining factor.
5But there were problems with believing that the nucleus is the site for genetic material It contains primarily DNA and there’s just not enough variability in DNA to explain all of life!!!So many still believed proteins must contain the genetic code…
6Hershey-Chase Experiment 1952 Worked with viruses that attacked bacteria…called BacteriophageComposed of only protein coat & DNALook like bizarre spacecraft...DNAProteinCoat
7Bacteriophage DNA Protein Coat Life cycle includes attacking bacteria cell and injecting its genetic material inside…producing thousands of new virusesText pg. 201If virus has only protein and DNA, one is the candidate genetic materialHow to determine which?Bacteria cellDNAProteinCoat
8Hershey-Chase Experiment 1952 In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to determine whether protein or DNA was the hereditary material.By labeling DNA and protein with different radioisotopes, they would be able to determine which chemical (DNA or protein) was getting into the bacteria. And such material must be the hereditary material.Since DNA contains Phosphorous (P) but no Sulfur (S), they tagged the DNA with radioactive Phosphorous-32. Conversely, protein lacks P but does have S, thus it could be tagged with radioactive Sulfur-35.
9Hershey-Chase Experiment Alfred Hershey & Martha Chase labeled the Bacteriophage protein with radioactive Sulphur (35S) and the DNA with radioactive Phosphorous (32P)Later…The newly formed viruses inside the cell had only 32P label.
11Therefore: DNA is the Genetic Material Structure of DNA: A Review…Made of Nucleotide units:Ribose sugar (5C sugar)Phosphorous group (-PO4)Nitrogen-containing base (A,G,C,T)Nucleotides are linked together by covalent (phosphodiester) bondsText pg. 205
12DNA StructureTwo anti-parallel nucleotide strands held together by H-bondsAnd twisted together to form a helical structureText pg. 204
14DNA Replication5’An enzyme termed DNA Polymerase synthesizes new DNA from original in a 5’ to 3’ direction (the end with a free –OH group)DNA replication results in two new complementary strands of DNA….One of each is from original and one new…..semi-conservative replication3’New nucleotides canOnly add to this end
155’ 3’ 5’ 3’ DNA Polymerase 5’ 3’ A A T T G G C C A A T T T T A A C C G
16DNA replication involves a great many building blocks, enzymes and a great deal of ATP energy. DNA replication in humans occurs at a rate of 50 nucleotides per second and ~500/second in prokaryotes.Nucleotides have to be assembled and available in the nucleus, along with energy to make bonds between nucleotides. DNA helicase enzymes unzip the DNA helix by breaking the H-bonds between bases. Once the polymerases have opened the DNA, an area known as the replication bubble forks (always initiated at a certain set of nucleotides, the origin of replication).New nucleotides are placed in the fork and link to the corresponding parental nucleotide already there (A with T, C with G). Prokaryotes open a single replication fork, while eukaryotes have multiple forks.
17Since the DNA strands are antiparallel, and replication proceeds in the 5' to 3' direction on EACH strand, one strand will form a continuous copy. The top strandhere….Text pg. 212
18The Lagging Strand…while the other, lagging strand will form a series of short pieces with gaps. These are called “Okazaki fragments” and require the use of other enzymes to complete the process.Text pg. 212
19Semi-conservative replication Original DNA StrandOriginal DNA StrandNew DNA Strands
20Proofreading DNA must be faithfully replicated…but mistakes occur: DNA polymerase (DNA pol) inserts the wrong nucleotide base in 1/10,000 basesDNA pol has a proofreading capability and can correct errorsMismatch repair: ‘wrong’ inserted base can be removedExcision repair: DNA may be damaged by chemicals, radiation, etc. Mechanism to cut out and replace with correct basesText pg 213
21How does DNA carry the information for life? Small segments of DNA (sequences of nucleotides) code for specific proteinsThese segments of DNA are called genesOne gene codes for one protein…(mostly!)Hundreds-Thousands of genes per chromosome