1Biology 1 Exam 3Chapter 161. What is the genetic material of organisms?- After many experiments, it was proven that DNA was the genetic material and that our DNA is our genetic endowment; hereditary information is encoded in the chemical language that is DNA. DNA effects the development of our biochemical, physiological, and behavioral traits. How does it pass down from generation to generation? Though a process called DNA replication that we will learn about It was thought for a long time that proteins were the genetic material; it was thought that nucleic acids were way to uniform and simple to be in charge of such a huge role. It might be worth knowing the experiments that made this discovery; the first experiment was done by studying bacteria and viruses.- The first study was done by looking at vaccines for pneumonia. When the pathogenic (disease causing) strain was killed with heat, was combined with a non-pathogenic strain, the mouse still died of pneumonia. Why? The pathogenic strain was killed. Clearly, some chemical component caused the new generation of bacteria to be pathogenic; so it transformed the new generation. This was called transformation.- Now the task was to identify that chemical that caused the transformation. Oswald Avery discovered that DNA was the cause by looking at DNA, RNA, or proteins. He did this by treating the pathogenic cells with chemicals that made the DNA, RNA or the proteins inactive, and only when the DNA was left active did the transformation occur. 2. How was this confirmed?- Now a study was done by Hersey and Chase looking at bacteriophages, viruses that attacked bacteria. The phages were made out of a protein capsule with DNA inside the capsule, and they work by attacking a cell and taking over that cell’s metabolic machinery, so that cell is programmedto make more viruses. The phage, called T2, was made of DNA and protein. The T2 would attack a bacteria and turn it into a T2 producing factory, but what was responsible for that? The DNA or the protein of the T2?2 This was proven via radioactive staining. What this means is that the protein was first stained with radioactive sulfur (35S), and this makes it possible for us to follow that radioactive molecule. Then the let the virus infect the bacteria and then they took the bacteria and centrifuged it in a test tube. This process puts the densest material (the cells) in the bottom of the test tube. So all the cells (which contain the material that the virus injected in the cell) are on the bottom, and the radioactive sulfur was seen in the fluid. Meaning that the protein is not what causes the cell to transform into a virus producing factory. The test is repeated with radioactive phosphate (this is part of the DNA) and the radioactive phosphate was seen in the bottom after being centrifuged. Proving that DNA was responsible.3. How was the idea of DNA base pairing discovered?- After DNA was proven to contain the genetic information, a fellow named Chargaff did an experiment. He already knew that DNA consisted of a nitrogenous base, a sugar called deoxyribose, and a negative charged phosphate. The base can either be adenine, thymine, guanine, or cytosine. After analzing many organism’s, he found that the ratio of A (adenine) was exactly equal to T (thymine) or vice versa for C and G. This meant that the number of A bases ALWAYS equaled thenumber of T bases; he called this Chargaff’s Rule. - So if you are given a question saying the percent of C bases is 19.9%, find the other percent’s. Since we know that the percent of C and G are equal, G must be 19.9%. We know that there are four bases and they must equal 100, so doing math: 100% - (19.9 + 19.9) =30.3 %. So there are 60.2% of T and A’s; and again they are qual. So 30.1% of T, 30.1 of A.4. How and what is the structure of DNA?- So they knew that DNA was the genetic material, but what was the structure? After an experiment with x-ray crystallography, Watson and Crick discovered that a DNA strand formed a double helix with another DNA strand. Even better was that their model explain Chargaff’s Rules perfectly: A strand of DNA contains the bases, and it hydrogen bonds with a complementary strand containing bases following Chargaff’s Rule. And the strands were also found to be anti-parallel.3 What this means in if there was a strand of bases ATGC, then the complementary strand that it hydrogen binds to would be TACG. This forms a double helix. And they are anti-parallel, with the strands being defined by a 3’ and 5’ end. So the complimentary strand will be 5’ to 3’. They proved Chargaff’s Rule by seeing that A can’t complimentary bind with G or C, since they will cause the helix to either be too short or too wide. A and G are called the purines and T and Care called the pyridimines. After trial and error, they saw that a purine can only bind with a pyridimine, and Chargaff already proved that A binds with T and vice versa. 5. Now to duplication- That was the model of DNA, but how was it passed down to generations? Watson and Crick proposed that the helix was broken apart and each strand was used as a template to make a new daughter strand that is complimentary to the template. So where you started with one helix, you have two, and each new helix has one parent strand from the original and a new daughter strand. This is called the semi-conservative method; each new strand has one daughter strand. Another theory was the conservative method, stating that afterduplication, the two parent strands come back together—crazy talk.6. How does this process work in prokaryotes?- In prokaryotes (bacteria) the DNA is a double stranded circular model and the area that the replication starts is called the origin of replication. The origin is a short stretch of nucleotides (the A,C,G,C). Proteins then attach to the origin of replication, separating the two stands, forming a “bubble”. The replication starts at the bubble, proceeding in both directions. The proteins do this by adding the complementary bases to the parent strands of DNA and eventually it is all copied. Bacteria are very simple; now to eukaryote’s.7. Now to eukaryote’s.- In animal cells, the DNA has many origins of replication, whereas bacteria only have one. This speeds up the process of replication. At each replication bubble, a replication fork is formed; this is the area that the helix is being unwound. The
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