Phosphate groups and sugar molecules link together to

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Phosphate groups and sugar molecules link together to form the “backbone” of a DNA strand. Bases link up to connect the two strands. The figure below shows the structure of DNA. C N N N NH HC CH C C C O NH 2 N HN NH 2 N NH C CH C C C H N HN O C C CH 3 CH C NH 2 NH N O C CH CH Purines Adenine (A) Guanine (G) Thymine (T) Cytosine (C) Pyrimidines O Phosphate group Nitrogen base Sugar (deoxyribose) P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P A A A A A A A T T T T T T T T T G G G G G G G C C C C C C C C C C P Nucleotides are the subunits of nucleic acid. Each nucleotide consists of a sugar, a phosphate, and a nitrogenous base. Sugar-phosphate bonds make up the backbone of each DNA strand. Hydrogen bonds between the base pairs hold the double helix together. Pyrimidines contain one ring of carbon and nitrogen atoms. Thymine (T) and cytosine (C) are pyrimidines. C N N N NH HC CH C C C O NH 2 N HN NH 2 N NH C CH C C C H N HN O C C CH 3 CH C NH 2 NH N O C CH CH Purines Adenine (A) Guanine (G) Thymine (T) Cytosine (C) Pyrimidines O Purines contain two rings of carbon and nitrogen atoms. Adenine (represented by the letter A) and guanine (G) are purines.
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Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 134 DNA, RNA, and Proteins Name Class Date SECTION 1 The Structure of DNA continued How Do Bases in DNA Link Together? A purine on a DNA strand always pairs with a pyrimidine on the other strand, as shown below. Weak hydrogen bonds keep those base pairs close to each other. They also hold the two strands of DNA together. You know that letters placed in a certain order can carry information in the form of words. In a similar way, the order of bases in a DNA strand carries information. Because the bases in DNA pair only in specific ways, each strand of a DNA molecule contains the same information. However, the bases on one strand are in the opposite order from those on the other strand, as shown below. P P P P P P P P P P P P P P P P P P P P P A A A A T T T T T T G G G G C C C C C Hydrogen bonds link complementary bases and hold the two strands of DNA together. A A C T T G T G A G A C A T T C T C G A C G This diagram shows how complementary base pairs join together. Note that adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). In other words, the bases are complementary . DETERMINING THE STRUCTURE OF DNA As is common in science, Watson and Crick built on information gathered by many other scientists. For example, in 1949, Erwin Chargaff found that the DNA of all organisms he studied contains the same amounts of cytosine and guanine. It also contains the same amounts of adenine and thymine. Three years later, Rosalind Franklin and Maurice Wilkins developed high-quality X-ray images that showed the shape of DNA.
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  • Fall '19
  • DNA, Holt, Rinehart

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