Nucleic Acids

Nucleic Acids:Molecular basis of Inheritance

The life of a cell depends on its ability to produce a large number 
of proteins, each with a specific sequence. The information necessary to produce the correct proteins at the correct time is encoded
by the cell within nucleic acids.

Cells contain two kinds of nucleic acids: deoxyribonucleic acid

(DNA), which is the genetic material, and ribonucleic acid (RNA), 

which functions in protein synthesis. Both DNA and RNA are 

polymers built of repeated units called nucleotides. Each nucleotide contains three parts: a sugar, a nitrogenous base, and a 

phosphate group. The sugar is a pentose (5-carbon) sugar; in 

DNA it is deoxyribose and in RNA it is ribose ( Figure.1 )

         

Ribose and deoxyribose, the pentose sugars of nucleic acids. A carbon 

atom lies in each of the four corners of the pentagon (labeled 1′ to 4′). 

Ribose has a hydroxyl group (—OH) and a hydrogen on the number 2′

carbon; deoxyribose has two hydrogens at this position.

Nitrogenous bases of nucleotides are also of two types: pyrimidines, are smaller, 

single-ring molecules, whose characteristic structure is a single, 6-membered 

ring, and purines,are large, double-ring molecules 

found in both DNA and RNA; which contain two fused rings. Purines and 

pyrimidines contain nitrogen as well as carbon in their rings, 

which is why they are called “nitrogenous” bases. The purines 

in both RNA and DNA are adenine and guanine ( See Table ).  

                                    The 

pyrimidines in DNA are thymine and cytosine, and in RNA they 

are uracil and cytosine. Carbon atoms in the bases are numbered 

(for identification) according to standard biochemical notation 

( Figure.2).     

     

               

                                            Carbons in ribose and deoxyribose are also numbered, but to distinguish them from the carbons in the bases, 

numbers for carbons in the sugars are given prime signs (see 

Figure.1 ).The sugar, phosphate group, and nitrogenous base are 

linked as shown in the generalized scheme for a nucleotide.When a nucleic acid 

polymer forms, the phosphate group of one nucleotide binds to 

the hydroxyl group from the pentose sugar of another, releasing 

water and forming a phosphodiester bond by a dehydration 

reaction.

                               In DNA the “backbone” of the molecule is built of phosphoric acid and deoxyribose ; to this backbone are attached the 

nitrogeneous bases ( Figure.3). 

          The 5′ end of the backbone has 

a free phosphate group on the 5′ carbon of the ribose, and the 

3′ end has a free hydroxyl group on the 3′ carbon. However, 

one of the most interesting and important discoveries about the 

nucleic acids is that DNA is not a single polynucleotide chain; 

it has two complementary chains that are precisely cross-linked 

by specific hydrogen bonding between purine and pyrimidine 

bases. The number of adenines equals the number of thymines, 

and the number of guanines equals the number of cytosines. 

This fact suggested a pairing of bases: adenine with thymine 

(AT) and guanine with cytosine (GC) (see Figure.4).

The result is a ladder structure ( Figure.5).

The upright 

portions are the sugar-phosphate backbones, and the connecting rungs are the paired nitrogenous bases, AT or GC. However, 

the ladder is twisted into a double helix with approximately 

10 base pairs for each complete turn of the helix .DNA molecules in organisms exist not as single chains 

folded into complex shapes, like proteins, but rather as two chains 

of nucleotides wrapped about each other—a long, linear molecule 

in eukaryotes and a circular molecule in most prokaryotes. The 

two nucleotide chains of a DNA polymer wind around each other 

like the outside and inside rails of a spiral staircase. Such a spiral 

shape is called a helix, and a helix composed of two chains is 

called a double helix. Each step of DNA’s helical staircase is composed of a base-pair. The pair consists of a base in one chain 

attracted by hydrogen bonds to a base opposite it on the other 

chain ( Figure.6).

The two DNA strands run in opposite directions (antiparallel),

and the 5′ end of one strand is opposite the 3′ end of the other 

( Figure.6 ). The two strands are also complementary—the sequence of bases along one strand specifies the sequence of 

bases along the other strand. 

The structure of DNA is widely considered the single most 

important biological discovery of the twentieth century. It was based on X-ray diffraction studies of Maurice H. F. Wilkins and 

Rosalind Franklin and on ingenious proposals of Francis H. C. 

Crick and James D. Watson published in 1953. Watson, Crick, 

and Wilkins were later awarded the Nobel Prize for Physiology 

or Medicine for their momentous work. Rosalind Franklin was 

not included because she died prior to the award. 

              RNA is similar to DNA in structure except that it consists 

of a single polynucleotide chain (except in some viruses), has 

ribose instead of deoxyribose, and has uracil instead of thymine.

                                   Ribosomal, transfer, and messenger RNAs are the most abundant 

and well-known types, but 

many structural and regulatory RNAs, such as micro RNAs, are 

known.

Roles of RNA

       RNA is similar to DNA, but with two major chemical differences. First, RNA molecules contain ribose sugars, in which the 

C-2 carbon is bonded to a hydroxyl group. (In DNA, a hydrogen 

atom replaces this hydroxyl group.) Second, RNA molecules use 

uracil in place of thymine. Uracil has a similar structure to thymine, except that one of its carbons lacks a methyl (—CH3) 

group. 

RNA is produced by transcription (copying) from DNA 

and is usually single-stranded (figure).

The role of RNA in 

cells is quite varied: It carries information in the form of messenger RNA (mRNA), it is part of the ribosome in the form of 

ribosomal RNA (rRNA), and it carries amino acids in the form of 

transfer RNA (tRNA). There has been a revolution of late in how we view RNA. 

Enzymes have been found where RNA, not protein, has catalytic 

activity. New roles for RNA are being discovered as we refine our 

view of cells at the molecular level. We now know that newly discovered forms of RNA, micro-RNA, and small interfering RNAs are 

involved in regulating gene expression and may even help protect the 

genome from invading viruses. 

All of this is changing how we view the role of RNA in cellular 

metabolism. We are even finding that much more of our own genome 

is being copied into RNA than is being used to make proteins.  

 Other Nucleotides are Vital Components of Energy Reactions

Nucleotides play other critical roles in the life of a cell. For example, adenosine triphosphate (ATP) is called the energy currency 

of the cell. The hydrolysis of ATP releases energy, which can be 

used to drive energetically unfavorable processes. Cells use the 

energy from ATP hydrolysis in a variety of transactions, the way 

we use money in society. ATP hydrolysis can provide energy for 

energetically unfavorable chemical reactions, for transport across 

membranes, or for the movement of cells.ATP is a ribonucleotide triphosphate (figure below), meaning 

it contains ribose, with three phosphate groups attached to the 5´ 

carbon of the ribose. The hydrolysis of ATP produces adenosine diphosphate (ADP) plus phosphate and releases energy. This reaction releases energy because the negatively charged phosphates 

are repelled by each other, making the overall molecule unstable. 

Like a coiled spring, they are poised to push apart. Thus, when a 

phosphate is removed, energy is released. This also means that 

cells must obtain energy from food molecules to synthesize ATP.

ATP. Adenosine triphosphate (ATP) contains 

adenine, a 5-carbon sugar, and three phosphate groups.
                                  Two other important nucleotide-containing molecules are 
nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD). These molecules function as electron carriers in a variety of cellular processes. You will see the action of 

these molecules in detail when we discuss respiration and photosynthesis in next posts.