DNA and DNA Replication As you take notes, just write down the text that is in yellow.
Griffith’s Experiment Initial Part of Experiment (first two steps) Independent Variable: ______________________ Dependent Variable: ________________________ Controlled Variable (constant): ________________ Summary of Entire Experiment: ________________________________________
Griffith concluded that some part of the heat-killed cells( a gene ) was transferred to the live cells. Harmless cells became harmful. • This process was called Transformation(gene transfer).
Canadian biologist Oswald Avery took Griffith’s experiment one step further. • Avery made an extract from the heat-killed bacteria: • Treated extract with enzymes that destroy proteins, lipids, carbohydrates, and RNA transformation still occurred • Treated extract with enzymes that break down DNA no transformation
Conclusion: • Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.
The Hershey-Chase Experiment • Bacteriophage: One kind of virus that infects and kills bacteria. • American scientists, Alfred Hershey and Martha Chase performed this experiment in 1952. • Bacteriophages consist of a DNA or RNA core and a protein coat.
Problem: Are genes made of DNA or protein? • A virus injects genes into the bacteria They used radioactive markers to see if it was DNA or protein • Avery already concluded that genes are found on DNA. Why are Hershey and Chase doing this experiment?
Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.
Structure of DNA • DNA is a long molecule made up of nucleotides. • 3 parts of a nucleotide: • A 5-carbon sugar • called deoxyribose • A phosphate group • A nitrogenous (nitrogen-containing) base (There are four kinds of nitrogenous bases in DNA)
Erwin Chargaff, an American biochemist, made observations about DNA’s nucleotides that developed into Chargaff’s Rule: • [A] = [T] • [G] = [C]
Watson and Crick • In 1953, Francis Crick, a British physicist, and James Watson, an American biologist, made 3D models to try and understand the structure of DNA. • They discovered that DNA was a double helix (two strands wound around each other)
Problem: What forces hold the two strands together? • Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases and provide just enough force to hold the two strands together. • Base Pairing: hydrogen bonds form between adenine and thymine and also between guanine and cytosine. (Chargaff’s rule)
DNA Length • Most prokaryotes, such as an E. Coli bacterium, have a single circular chromosome. This chromosome must hold about 4,639,221 base pairs of DNA.
Chromosome Structure in Eukaryotes • DNA in Eukaryotic cells must be packed even more tightly. The nucleus of a human cell contains more than 1 meter of DNA. • Chromatin: tightly packed DNA and protein • Histones: the proteins in which the DNA is tightly coiled around • Nucleosome: a beadlike structure formed by the DNA and histones.
DNA Replication • Problem: How can DNA be copied, or replicated? • After Watson and Crick discovered the DNA structure, they also realized that each strand of the DNA double helix has all the information needed to reconstruct the other half by base pairing. • The strands are complementary, each strand can be used to make the other.
Replication: the copying process that duplicates DNA before a cell divides. • During DNA replication, the DNA molecule separates into two strands • Then produces two new complementary strands following the rules of base pairing. • Each strand of the double helix of DNA serves as a template, or model, for the new strand. (semi-conservative)
Replication fork: the sites on DNA where separation and replication occur. • DNA Helicase: the enzyme that “unzips” a molecule of DNA. Hydrogen bonds between the base pairs are broken and the two strands unwind.
DNA Polymerase: enzyme that brings more nucleotides and also proof-reads each new DNA strand, helping to ensure each is a perfect copy. • DNA ligase: enzyme that “glues” the two DNA strands back together • Replication proceeds in both directions until each chromosome is completely copied.
Replication Practice • DNA strand: TACGTT Complementary strand: ? • DNA strand: CAGGCC Complementary strand: ? • DNA strand: GTAGGC Complementary strand: ?