DNA Chapter 12. DNA and RNA B.1.4, B.1.9, B.1.21, B.1.26, B DNA and RNA B.1.4, B.1.9, B.1.21, B.1.26, B Griffith s Experiment

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1 DNA Chapter 12 DNA and RNA B.1.4, B.1.9, B.1.21, B.1.26, B.1.27 To truly understand genetics, biologists after Mendel had to discover the chemical nature of the gene. In 1928, Frederick Griffith was trying to figure out how bacteria caused disease. He set up an experiment involving mice and the bacteria that causes pneumonia. DNA and RNA B.1.4, B.1.9, B.1.21, B.1.26, B.1.27 Griffith Isolated two different strains (or types) of bacteria. One strain of bacteria was caused disease (smooth colonies), while the other one didn t (rough colonies). Griffith s Experiment Griffith s Experiment Transformation: One strain of bacteria changing permanently into another. Griffith showed that the Smooth strain that was heat-killed must have transferred some type of factor to the Rough strain in order to make them harmful. Since the ability to cause disease was inherited by the transformed bacteria s offspring, the transforming factor might be a gene. Avery and DNA In 1944, a group of scientists led by Oswald Avery decided to repeat the work done by Griffith. They wanted to determine what the most important molecule was during bacteria transformation Avery made an extract from heatkilled bacteria and treated it with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules. Oswald Avery 1

2 Avery and DNA After adding the enzyme, transformation still occurred. This means the molecules that were destroyed were not responsible for transformation. When they repeated the experiment with an enzyme that destroyed DNA, transformation did not occur. Avery discovered that the nucleic acid DNA stores and transmits the genetic information from one generation to the next. The Hershey-Chase Experiment 1952 Alfred Hershey and Martha Chase collaborated in studying bacteriophages. Bacteriophages- a kind of virus that attacks bacteria. Bacteriophages are made up of protein and DNA or RNA. The protein part protects the nucleic acid. One way that bacteriophages attack bacteria is by attaching to the outside of the bacterial cell and injecting the cell with its own DNA. The Hershey-Chase Experiment Radioactive markers- radioactive isotopes that are used by scientists to trace molecules during life processes. Hershey and Chase used radioactive markers of phosphorous and sulfur. Phosphorous is found in DNA and sulfur is found in protein. By observing the phosphorous markers inside of the cell and sulfur markers outside of the cell, they deduced that DNA is responsible for the passing of traits. Hershey and Chase Experiment 12.2 The Structure of DNA 2

3 Knowing that genes were made of DNA was not enough for scientists. They wanted to know how DNA could do the three critical things that genes were known to do: Store Information Copy Information Transmit information DNA is a long molecule made up of units called nucleotides. Each nucleotide contains three parts: 5-carbon sugar called deoxyribose A phosphate group A nitrogenous (nitrogencontaining) base. There are four kinds of nitrogenous bases in DNA, and they are divided into two groups: Purines and Pyrimidines Purines Two ring structures Adenine and Guanine Pyrimidines Single ring Structures Cytosine and Thymine The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide. The nitrogenous bases stick out sideways from the chain and can be joined together in any order. Chargaff s Rule One puzzling fact about DNA was its curious relationship between nucleotides. American biochemist Erwin Chargaff discovered that the percentages of Guanine (G) and Cytosine (C) bases are almost equal in amount in any sample of DNA. The same thing is true for the other two nucleotides: Adenine (A) = Thymine (T) The observation that A=T and G=C became known as Chargaff s Rule, despite Chargaff knowing why DNA obeyed this rule 3

4 X-Ray Evidence In the early 1950 s, British scientist Rosalind Franklin began to study DNA. She used a technique called X-Ray diffraction to gather information about DNA s structure. She concluded that DNA strands are twisted (helix) and there are two strands in the structure with the nitrogenous bases near the center. The Double Helix Watson and Crick (1953) Francis Crick and James Watson were trying to understand the structure of DNA at the same time Franklin was continuing her research. Watson used clues from Franklin s X-ray pattern to build their model. Watson and Crick built their model of the DNA double helix and published their results in a historic one-page paper April 2,

5 Watson and Crick 1953 James Watson and Francis Crick used Franklin s X-Ray evidence and built a model of DNA. They discovered the shape to be a double helix. A double helix has two strands wound around each other. They received the Nobel Prize for discovering the structure and shape of DNA. Watson and Crick The double helix looks like a twisted ladder. Each side of the ladder is held together by hydrogen bonds. Base-pairing means that A s only go with T s and C s only go with G s. This explains why Chargaff noticed them to be in equal numbers. DNA Composition Watson and Crick discovered how DNA can replicate itself based on what they learned about its structure. Each strand has all of the information needed to double itself based on base pairing. Each strand can be used to make the other strand by base pairing. In prokaryotes, DNA replication starts at one point and moves in opposite directions around the circular DNA. 5

6 In Eukaryotes, replication occurs at hundreds of places. Replication goes in both directions until the chromosome is completely copied. Replication forks- the sites where separation and replication occur. Duplicating DNA Replication- the duplication of the entire DNA of a cell. During replication, the DNA separates into two strands, and then copies each side to make two identical strands. Each strand serves as a template for the new strand, and base paring allows that to happen. How Replication Occurs Enzymes and regulatory proteins control DNA replication. Enzymes break the hydrogen bonds that hold the two strands together and they read the code so that base pairing can occur. DNA polymerase- joins individual nucleotides to produce a DNA molecule, and proofreads its work to make sure they are all correct. 6

7 DNA and Chromosomes Standards: B.1.3, B.1.4, B.1.9, B.1.29, B.1.26 Prokaryotic cells lack nuclei and many of the organelles found in eukaryotes. Most prokaryotes have a single circular DNA molecule that contains nearly all of the cell s genetic information. This large DNA molecule is referred to as the cell s chromosome. DNA and Chromosomes Eukaryotic DNA is more complicated. Many eukaryotes have as much as 1000x more DNA than prokaryotes. DNA is found in the nucleus of eukaryotic cells in the form of chromosomes. The number of chromosomes varies greatly between species. Ex: Humans have 46 chromosomes, while fruit flies only have 8 chromosomes. 7

8 DNA and Chromosomes Chromosome Structure DNA is a very long molecule. E. coli bacteria contain 4,639,221 base pairs. The length of this molecule is 1.6mm (1000X longer than the length of the bacterium itself) To get an Idea of the size of the DNA compared to the bacteria cell, this would be like trying to pack 1000 ft. of rope into your backpack. Human DNA contains around 1000x as many base pairs of DNA than bacteria. A strand of Human DNA can be around 6ft in length when stretched out. Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin. Chromatin consists of DNA that is tightly coiled around proteins called histones. The DNA and histone molecules form a beadlike structure called a nucleosome. Nucleosomes pack with one another to form a thick fiber which is shortened by loops and coils. 8