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Nucleic Acids

Romolecular polymers made up of a particular type of subunit, in this case, the nucleotide. Nucleotides are more complex than the other subunits we have considered. Each one is made up of:

 

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Cytidine monophosphate (CMP), a ribonucleotide

  1. a five-carbon sugar
  2. a nitrogenous base
  3. a phosphate group 
Two general types of nucleotides are recognized, depending upon which sugar they contain. Therefore, two major types of nucleic acids can be constructed, depending upon which kind of nucleotide is used.
DNA (deoxyribonucleic acid) is made of nucleotides that contain the sugar deoxyribose.
RNA (ribonucleic acid) is made of nucleotides that contain the sugar ribose.
 
Deoxyribonucleotides are of four types, depending upon which of four possible nitrogenous bases they contain. The four bases are:
  1. Adenine
  2. Guanine
  3. Cytosine
  4. Thymine
Ribonucleotides are of four types, depending upon which of four possible nitrogenous bases they contain. The four bases are:
  1. Adenine
  2. Guanine
  3. Cytosine
  4. Uracil 
As you can see, the bases are similar in DNA and RNA, with only one exception: the thymine of DNA is replaced by uracil in RNA. Due to their chemical structures, adenine and guanine are referred to as purines, while cytosine, thymine, and uracil are called pyrimidines.
 
Nucleotides are joined to one another by phosphodiester bonds, creating long chains. As with proteins, what makes one DNA molecule different from another is the sequence of bases that makes up the primary structure. This sequence carries encoded information, and is the basis for the “genetic code” (see Chapters 5 and 6). In fact, the major functions of DNA and RNA all deal with the storage, transmission, and usage of genetic information.
It turns out that DNA normally exists in the form of a double helix in nature. Two antiparallel strands of nucleotides are held together by interactions between the bases to form a structure that resembles a twisted ladder (this structure and its consequences are examined in detail in Chapter 5). On the other hand, RNA is usually single-stranded.




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