Answered Review Questions The Recipe of Life 1. Describe the Griffith experiment (1928). What transformed the rough strain into a smooth strain? As shown above, Frederick Griffith experimented with two strains of Streptococcus pneumoniae bacteria. The cells of the smooth strain (S) are encapsulated and cause a lethal case of pneumonia. The rough strain (R) lack a capsule and are harmless. Mice injected with S cells die. Mice injected with R cells live. Mice injected with S cells that have been heat-killed also live. However, if you inject the mice with a mix harmless R cells with dead S cells, the mice die. These mice when examined post-mortem have both living R and S cells inside of them. What made the harmless R cells grow capsuies? Griffith suspected the transformation factor was DNA. DNA from the dead S cells was picked up by R cells and the new genetic information allowed them to grow capsules. 2. What was the contribution of Oswald Avery (1944)? Avery tested Griffith s prediction by chemically isolating all the organic compounds in the heat-killed S cells and introducing each one in turn into R cells to see which compound transformed the cells. Avery discovered that it was DNA that transformed the cells.
3. Why didn t the scientific community accept Griffith s and Avery s hypothesis? Back in 1869, a German bacteriologist explored the chemical make-up of the nucleus. Friedrich Mieschner discovered that a nucleus contains two compounds: DNA and Proteins. Thomas Hunt Morgan determined that something in the nucleus was responsible for inheritance of traits. Most scientists thought that DNA was too simple a molecule to be the recipe for life. Most sided with proteins because proteins are complicated and the there are thousands of different kinds. 4. What changed their mind? Martha Chase and Alfred Hershey comfirmed that DNA was the genetic material. By labeling the protein coat of a bacteriophage with radioactive sulphur (just found in proteins) and the viral DNA inside the capsid with radioactive phosphorus (in DNA but not in proteins), they discovered that phages inject their DNA into bacteria. The phages take over the cellular machinery and make new viruses. Chase and Hershey found the radioactive DNA in not only the bacteria plus in the new phages made after the infection. Their results convinced the scientific community that DNA was the genetic material.
5. Describe Chargaff s Rules. Back in the 1940 s, Edwin Chargaff, an Austrian American biochemist, examined the relative amounts of adenine, guanine, cytosine, and thymine in the DNA of a variety of organisms. Chargaff compared the amounts of these bases in everything from bacteria to humans. What stood out was that for each species, the amount of adenine always equaled the amount of thymine. Likewise, the amount of guanine was the same as cytosine. The pattern is constant regardless of the species, and so it is known as Chargaff s rules. Later, in the early 1950 s, James Watson and Francis Crick concluded from Chargaff s rules that in the DNA molecule every adenine binds to thymine and every guanine always binds to cytosine forming rungs of the double helix.
6. Explain the contributions of Franklin, Wilkins, Crick, and Watson. Rosalind Franklin and Maurice Wilkins were trying to determine the structure of DNA. They did an x-ray diffraction on the DNA molecule. In 1953, James Watson and Francis Crick, inferred from the x-ray diffraction picture and Chargaff s rules the structure of DNA. In the 1960 s, Crick, Watson, and Wilkins received Nobel prizes for their work. Unfortunately, Rosalind Franklin had died from cancer a few years earlier. 7. Describe the structure of DNA DNA is built of subunits called nucleotides. A nucleotide is composed of three parts: deoxyribose sugar, a nitrogenous base (A, C, G, T), and a phosphate group.
DNA is a long chain of nucleotides covalently bonded together forming a polynucleotide. A polynucleotide is a chain of nucleotides covalently bonded to each other. DNA is a double helix composed of two polynucleotides that spiral around one another. DNA looks like a twisted rope ladder. Deoxyribose is a five-carbon monosaccharide. Biochemists have mapped the molecule by numbering each carbon in the sugar. Next to the number, there is also what looks like an apostrophe. For example, the first carbon in ribose is called the 1 (one prime) carbon. The term prime refers to the apostrophe. The apostrophe represents that the carbon is part of a carbohydrate. For every kind of nucleotide, the nitrogenous base is always attached to the 1 carbon, the phosphate
group is always attached to the 5 carbon, and the 3 carbon always holds a hydroxyl group ( OH). The DNA molecule looks like a twisted rope ladder. The sides of the ladder are called the sugar-phosphate backbone. Each nucleotide is attached to the one above by means of a covalent bond between the phosphate group and the 3 carbon. The repeating pattern of sugar hooked to phosphate hooked to sugar gives the backbone its name. Just like other organic compounds, the sugar-phosphate backbone is built by dehydration synthesis. A DNA molecule is described as anti-parallel because one polynucleotide is oriented in the opposite direction to the other. A DNA
molecule kind of resembles a divided highway with traffic moving in opposite directions. Since DNA is anti-parallel, one polynucleotide is oriented with the 5 end of the ribose sugars pointing up and the 3 end pointing down. This is the 5 strand. The other polynucleotide is oriented in the opposite direction with the 3 end pointing up and the 5 end pointed down. This is called the 3 strand.
5 strand 3 strand A synonym of the 5 strand is the leading strand. The 3 strand is also known as the lagging strand. 8. Where is DNA located in a cell? Within the cells of animals, plants, fungi, and protists, DNA is stored in the nucleus. Bacteria lack a nucleus and so store their DNA in a portion of the cytoplasm called the nucleoid region.
9. What is a chromosome? A chromosome is a very long piece of DNA. A typical chromosome contains hundreds of genes. 10. What is a gene? Technically, a gene is a length of DNA that codes for a polypeptide. Later in the semester, we will slightly modify this definition.
11. What is the function of DNA? DNA carries the information for building proteins. The sequence of nucleotides in a DNA molecule determines the sequence of amino acids in a polypeptide of a protein. A DNA molecule is a collection of recipes for making proteins. Since proteins are the machinery of life, DNA is the recipe for making an organism. 12. Explain semi-conservative replication. In 1954, Crick and Watson proposed a mechanism for DNA replication. This hypothesis was later confirmed by Meselson and Stahl in 1958. See a diagram of their experiment below.
Prior to cell division, a cell must make a copy of its DNA to pass along to the next generation. Copying DNA is called replication. Rather than build a DNA molecule from scratch, the new DNA is composed of one old DNA strand (used as the template) and one brand new strand. Semi-conservative means that half of the new DNA molecule is old DNA. 13. How can the speed of DNA replication increase while the rate of replication remains constant? The conundrum of DNA replication is that in humans the replication enzymes can copy at a rate of 50 base pairs per second. That may seem like a fast rate but there are 3.1 billion base pairs in the human genome. At that rate, if the machinery started at one end of the DNA and replicated all the way down to the other end, it would take ~ 2 years to copy one DNA molecule. Replication occurs much faster than that. How? Well, the answer is that DNA replication starts at many places along the molecule. These separate origins of replication form
replication bubbles. Once a bubble forms, replication moves in both directions. These expanding bubbles of replication will eventually meet and the whole genome will be copied in a matter of hours rather than years. 14. Describe DNA replication. Replication begins with an enzyme called helicase. Helicase moves along the double helix unwinding and unzipping the double helix breaking the hydrogen bonds between the nitrogenous bases. Next, a group of enzymes, called DNA Polymerase, attach complementary nucleotides to the DNA strands and build the sugar phosphate backbone.