Types of Nucleic Acids

• Nucleic acids function primarily as informational molecules, for the storage and retrieval of information regarding the primary sequence of polypeptides.
• There are two types of nucleic acids:
  1. Deoxyribonucleic acid (DNA), which serves as a cellular database by storing an immense amount of information regarding all possible polypeptides a cell can make.
  2. Ribonucleic acid (RNA), which occurs in several different forms (messenger RNA, ribosomal RNA, transfer RNA) and is needed to convert DNA information into polypeptide sequences. In some viruses, RNA serves as the primary database with no DNA involvement. Certain RNAs have catalytic ability similar to that of protein enzymes; these are called ribozymes.

Nucleotide anatomy

• Nucleic acids are built from subunits called nucleotides.
• Each nucleotide includes three components:
  1. a ring-shaped molecule belonging to the class of purine or pyrimidine bases
  2. a 5-carbon, or pentose, sugar
  3. one or more phosphate groups

Purines & Pyrimidines

• Every nucleotide contains a nitrogenous base. These bases are classified as purines (two ring-shaped molecules joined together, one with 6 and one with 5 atoms) and pyrimidines (a single ring made from 6 atoms).
• In DNA, there are four different bases: Adenine (A) and Guanine (G) are the larger purines. Cytosine (C) and Thymine (T) are the smaller pyrimidines. These are frequently symbolized by their single letter abbreviations.
• RNA also contains four different bases. Three of these are the same as in DNA: Adenine, Guanine, and Cytosine. RNA contains Uracil (U) instead of Thymine (T).

View purines and pyrimidines

How did Purines and Pyrimidines evolve? A possible origin for Adenine.

• At first glance, molecules such as adenine look very complex. How did they evolve to become part of nucleic acids? We can only speculate about such questions, but there are reasons for thinking that adenine is not all that complex a molecule.
• We know that the primitive Earth evolved around 4.5 billion years ago, and that initially there was no free oxygen. Compounds such as water (H₂O), ammonia (NH₃) and methane (CH₄) were abundant, as was energy in the forms of heat, UV radiation, lightning, radioactivity, etc.
• Scientists have tried simulating such environments, boiling mixtures of gases and water with electric spark discharges. In some such experiments, purine and pyrimidine bases have been formed! A possible mechanism is shown in the accompanying diagram. This is not the way adenine is synthesized in cells today, but it suggests why such molecules may have been available in the primitive earth when life first evolved.

View Possible Mechanism for Adenine evolution

Ribose & Deoxyribose

• The sugars found in nucleic acids are pentose sugars, with five Carbon atoms.
• Ribose, found in Ribonucleic acid (RNA), is a "normal" sugar, with one oxygen atom attached to each carbon atom.
• Deoxyribose, found in Deoxyribonucleic acid (DNA), is a modified sugar, lacking one oxygen atom (hence the name "de-oxy"). This difference of one oxygen atom is an important one for the enzymes which recognize DNA and RNA, allowing these two molecules to be easily distinguished inside organisms.

Phosphate

• Phosphate groups can be joined together to form phosphodiester bonds.
• Nucleotides typically have one, two, or three phosphate groups, and are named monophosphate, diphosphate, or triphosphate accordingly.
• When phosphate groups are joined together, they have a strong tendency to repel each other, because of the high concentration of negative charge in the very polar and usually ionized oxygen atoms. As a result, molecules with two or three phosphate groups are good energy donors, readily releasing energy along with the transfer of phosphate groups. Nucleotides such as ATP and GTP are used not just for RNA or DNA synthesis, but also as energy donors for many cellular reactions.

ATP, ADP, AMP

• Let's examine a set of nucleotides built with the purine base Adenine, the sugar deoxyribose, and one, two, and three phosphate groups.
• A combination of a base and a sugar is called a nucleoside.
• When the base Adenine is added to the sugar deoxyribose, the resulting nucleoside is deoxyadenosine. When one or more phosphates are added to this nucleoside, we have a nucleotide.
• We can name nucleotides by combining the nucleoside name (Deoxyadenosine) with the number of phosphates (mono-, di-, or tri-phosphate), as shown in the figure.
  1. Adenosine monophosphate, or AMP
  2. Adenosine diphosphate, or ADP
  3. Adenosine triphosphate, or ATP

The following tables gives the names of bases and their corresponding nucleosides and nucleotides

Nucleotides involved in DNA

Nucleotides involved in RNA

DNA and RNA

• Nucleotides can serve as monomers for the assembly of polymeric nucleic acids. When the nucleotides contain deoxyribonucleotides, the polymer is deoxyribonucleic acid, or DNA. DNA is normally double-stranded (although some viruses contain single-stranded DNA).
• Each strand of DNA consists of a "backbone" of alternating units of phosphate and deoxyribose. Purine or pyrimidine bases are attached to the 5-C deoxyribose sugar, and form base pairs with purine or pyrimidine bases from the opposite strand. The only effective pairs are Adenine with Thymine (A-T pairs) and Guanine with Cytosine (G-C pairs).

  View static DNA model
  View interactive DNA tutorial (if Chime plug-in is installed) - highly recommeded! Click on each box with an "x" in it to see various features. Use "reset" button to restore intial view. (See plugins page).

• When ribonucleotides serve as monomers, the resulting polymer is ribonucleic acid, or RNA. RNA is normally single-stranded (although some viruses contain double-stranded RNA). Many bases in RNA molecules such as ribosomal RNA and transfer RNA are chemically modified after polymerization, a process which makes these molecules more stable.
• An RNA consists of a "backbone" of alternating units of phosphate and deoxyribose. Purine or pyrimidine bases are attached to the 5-C ribose sugar.

  View RNA model