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Imagine a combination lock with just the numbers 1-4 on it. In the space below, write as many combinations of those 4 numbers as you can think of. What 3 items form a nucleotide? 1. 2. 3. What is DNA? Living things contain proteins. Your skin, muscles and bones all contain protein. All the actions you perform such as eating, funning and thinking depend on proteins called enzymes. But how is this related to DNA? Within DNA is all the information for life. DNA contains all the instructions to make all the different proteins an organism needs. DNA Structure What are the 4 nitrogen bases found in DNA? 1. 2. 3. 4. Which bases are Purines? Pyramidines? DNA is made of nucleotides. In the 1920s, biochemist P.A. Levene showed that each DNA nucleotide contains the sugar deoxyribose (dee ahk sih RI bos), phosphate group, and one of four nitrogenous bases —adenine (A dun een), guanine (GWAH neen), cytosine (SI tuh seen), or thymine (THI meen). RNA also is made of nucleotides. Each RNA nucleotide contains the sugar ribose, a phosphate, and one of four nitrogenous bases—adenine, guanine, cytosine, and uracil (YOO ruh sihl). The figure on the left shows the structure of a nucleotide. Adenine (A) and guanine (G) are double-ringed bases, which are called purine bases. Thymine (T), cytosine (C), and uracil (U) are single-ringed bases, which are called pyrimidine bases. Chargaff’s Rule Erwin Chargaff discovered that the amount of guanine is almost equal to the amount of cytosine, and the amount of adenine is almost equal to the amount of thymine for the DNA of any given species. This finding is known as Chargaff ’s rule: According to Chargaff’s Rule if 42% of an organisms genome (DNA) is Adenine, than what percentage would be Thymine? Guanine? C = G and T = A. Cytosine? What does the DNA molecule look like? In 1953, James Watson and Francis Crick published a paper correctly describing the shape of a DNA molecule for the first time. They said that DNA is made of two strands of nucleotides held together by the nitrogen bases, the parts that stick out like zipper teeth. Adenine on one strand always joins with thymine on the other strand. Likewise, guanine always joins with cytosine. They also noted that DNA is shaped like a long zipper that is twisted into a coil like a spring. The two strand of twisted DNA are referred to as a double helix. How can DNA do so much with so little? If every organisms is made up of the same four nucleotides, how can organisms be so different from one another? The key to variety in organisms is the sequence , or order of the four nucleotides. For Example, a nucleotide sequence of A-T-T-G-A-C carries different information that the sequence T-C-C-A-A-A. It is the sequence of nucleotides that forms the unique genetic information for every organism. You can think of it like this some words have the same letters, but different meanings. TEA for example is not the same as ATE or EAT. The order of nucleotides in a DNA molecule therefore determines the enzymes produced (remember! enzymes are proteins) which in turn control many of the biochemical reactions in your body. So in a way the sequence of nitrogen bases controls everything. What do you call the twisted ladder structure of a DNA molecule? In DNA, what determines the traits (characteristics) of an organism?