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Transcript
DNA Structure Road to Discovery 1944 Oswald Avery’s team determines that genes are composed of DNA. Road to Discovery In 1949, Erwin Chargaff noticed the amount of the nitrogen base adenine and thymine always equaled each other, and the amount of nitrogen base cytosine and guanine always equaled each other. In the 1950’s, Maurice Wilkins and Rosalind Franklin, working in London, developed a high quality X-ray photograph of strands of DNA. These photos resembled the tightly coiled helix composed of two or three chains of nucleotides. 1953, James Watson and Francis Crick used Chargaff’s findings and Franklin and Wilkin’s X-ray pictures to come up with the model of DNA made out of tin and wire. The Double Helix Watson and Crick determined that the DNA molecule is in the shape of a double helix, two strands twisted around each other, like a winding staircase. Components of DNA DNA is made up of nucleotides all linked together. Each nucleotide is composed of 3 things: a phosphate group, a five carbon sugar molecule (deoxyribose), and a nitrogen base. The Phosphate Group is always the same. The 5 carbon sugar is called deoxyribose, and that’s how DNA gets it’s name, Deoxyribonucleic acid. REMEMBER, a lot of sugar names end in “ose,” like glucose, lactose, maltose, galactose. The nitrogen bases may be any of 4. It would be adenine, guanine, thymine, or cytosine. Nitrogen Bases are grouped into 2’s. Purines Bulky, double-ringed. Either Adenine or Guanine Pyrimidines Smaller, single-ringed. Either Thymine or Cytosine. Draw this in your notes Base Pairing Watson and Crick determined that a purine on one strand of DNA is ALWAYS paired with a pyrimidine on the opposite strand. They also determined that it will always be like this: Purines Pyrimidines Adenine always with Thymine Guanine always with Cytosine Complementary Base Pairs Adenine forms two weak hydrogen bonds with Thymine, and Cytosine forms three weak hydrogen bonds with Guanine. These hydrogen bonds between the nitrogen bases keep the two strands of DNA together. Complementary Base Pairs Since the A and T can only bond with each other and C and G can only bond with each other, you can always guess what the opposite strand of DNA will be. This means they are complementary base pairs. The sugar and phosphates make up the sides of the DNA staircase (the backbone), and the nitrogen bases make up the steps of the staircase. The nitrogen bases are held together by weak hydrogen bonds. The 5 Carbon Sugar The 5 Carbon Sugar Sugar structure: A prime (') notes where a carbon is located on the sugar. Phosphate attaches to the 5' carbon. Nitrogen bases attach to the 1' carbon. The phosphate of the next nucleotide attaches to the 3’ carbon. The 5 Carbon Sugar DNA is Anti-Parallel DNA is Anti-Parallel Not only does DNA contain complementary base pairs, but it is also anti-parallel! Remember how the sugar is 5 carbon, and each carbon is numbered? Since only phosphates can attach to either the 5’ or 3’ carbons, and only bases can attach to the 1’ carbon, the two strands of DNA must run in opposite directions! DNA is Anti-Parallel If I show you ½ of a piece of DNA, can you tell me the other ½? We finally learn the reason for complementary base pairing! The Reason for Complementary Base Strands After the discovery of the double helix, scientists thought the reason for the complementary bases was in some way related to making exact copies of the DNA each time a cell divided. The Reason for Complementary Base Strands Watson & Crick thought one DNA strand serves as a template, or pattern, on which the other strand is built. They were correct! S phase of the Cell Cycle DNA Replication Before a cell divides, it duplicates its DNA in a copying process called replication. DNA Replication During DNA replication, the DNA molecule separates into its two strands, then produces two new complementary strands following the rules of base pairing. How is DNA replicated??? Step 1 of DNA Replication The DNA double helix must unwind or unzip. An enzyme called DNA helicase opens the double helix by breaking the hydrogen bonds that link the nitrogen bases between the two strands. Once the strands are broken apart, proteins move in to hold them apart. Step 2 of DNA Replication Once the strands are separated at the replication forks, enzymes known as DNA polymerases move along each DNA strand adding nucleotides to the exposed nitrogen bases. The Replication Fork http://highered.mheducation.com/olcweb/cgi/ pluginpop.cgi?it=swf::535::535::/sites/dl/free/0 072437316/120076/micro04.swf::DNA+Replic ation+Fork Step 2 of DNA Replication As DNA polymerases move along, two new double helixes are formed. Notice that there are 2 DNA polymerases, and only 1 DNA helicase! Step 3 of DNA Replication DNA polymerases continue to add nucleotides until all the DNA has been copied and the polymerases are signaled to stop. Step 3 of DNA Replication DNA replication makes two DNA molecules, each one with one old strand and one new strand of DNA. Notice that both molecules of DNA are the exact same! DNA Replication Video http://highered.mheducation.com/sites/0072943696/student_view0/chapter3/ani mation__dna_replication__quiz_1_.html Step 3 of DNA Replication This process is known as semiconservative replication, because ½ of the new molecule is from the old molecule. Semiconservative Replication What happens if there is a mistake? Errors occur during DNA replication. Thankfully, DNA polymerases are also capable of “proofreading.” The DNA polymerase can only add each next nucleotide if the one before it was the correct one. If the one before it was wrong, the DNA polymerases must go back and fix its mistake. Rate of Replication Replication does not begin at one end and end at the other. DNA replication starts in several places so that it finishes faster. Remember, a strand of DNA has about 3 billion complementary base pairs!!! Our DNA is a little over 3 feet long, in each cell!!! This would take a long time to make copies of. Replication Forks