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The Structure of DNA
Transcript of The Structure of DNA
The Structure of DNA
DNA is Composed of Four Types of Nucleotides
Since the 1920's, scientists have known that the DNA molecule is a very long polymer, or string of repeating units. The small units, or monomers that make up DNA are called nucleotides.
Each nucleotide consists of three parts.
1) A phosphate group (one phosphorus with four oxygen atoms).
2) A ring-shaped sugar called deoxyribose.
3) A nitrogen-containing base (a single or double ring built around nitrogen and carbon atoms).
The Structure of DNA
One molecule of DNA contains billions of nucleotides, but there are only four types of nucleotides in DNA. These nucleotides differ only in their nitrogen-containing bases. The four bases in DNA are shown in the picture. Notice that the bases cytosine (C) and thymine (T) have a single-ring structure. Adenine (A) and guanine (G) have a larger, double-ring structure. The letter abbreviations refer to both the bases and the nucleotides that contain the bases.
Previous Thoughts About DNA
For a long time, scientists hypothesized that DNA was made up of equal amounts of the four nucleotides, and so the DNA in all organisms was exactly the same. The hypothesis was one of the reasons why it was so hard to convince scientists that DNA was genetic material. They reasoned that identical molecules could not carry different instructions across all organisms. By 1950, Erwin Chargaff changed the thinking about DNA by analyzing the DNA of several different organisms. Chargaff found that same four bases are found in the DNA of all organisms, but the proportion of the bases differs from organism to organism. In the DNA of each organism, the amount of adenine approximately equals the amount of thymine. Similarly, the amount of cytosine roughly equals the amount of guanine. This A=T and C=G relationships became known as Chargaff's rules.
DNA's 3-dimensional Structure
The breakthrough in understanding the structure of DNA came in the early 1950's through the teamwork of American geneticist James Watson and British physicist Francis Crick. Both men were fascinated by the challenge of figuring out the structure of DNA. Their interest was sparked by the work of biochemist Linus Pauling. Pauling had found the structure of some proteins was a helix, or spiral. Watson and Crick hypothesized that the structure of DNA might also be a helix.
Rosalind Franklin and Maurice Wilkins were studying DNA using a technique called x-ray crystallography. When DNA is bombarded with x-rays, the atoms in DNA diffract the x-rays in a pattern that can be caputred on film. Franklin's x-ray photographs of DNA showed an X surrounded by a circle. Franklin's data gave Watson and Crick the clues they needed. The patterns and angle of the X suggested that DNA is a helix consisting of two strands that are a regular, consistent distance apart.
The Double Helix
In their own laboratory, Watson and Crick made models of metal and wood to figure out the structure of DNA. Their model placed the sugar phosphate backbones on the outside and the bases on the inside. At first, Watson reasoned that A might pair with A, T with T and so on. But the bases A and G are about twice as big as C and T, so this produced a helix that varied in width. Finally, Watson and Crick found that if they paired double-ringed nucleotides with single-ringed nucleotides, the bases fit like a puzzle. The structure became known as a double helix, in which two strands of DNA wind around each other like a spiral staircase. The strands are complementary in that they fit together and are the opposite of each other. That is, if one strand is ACACAC, the other is TGTGTG. The pairing of bases in their model finally explained Chargaff's rules.
Nucleotides Always Pair in the Same Way
The DNA nucleotides of a single strand are joined together by covalent bonds that connect the sugar of one nucleotide to the phosphate of the next nucleotide. The alternating sugars and phosphates form the sides of a double helix, like a spiral staircase. The DNA double helix is held together by hydrogen bonds between the bases in the middle. Individually, each hydrogen bond is weak, but together, they maintain the DNA structure.
DNA Base Pairing Rules