Nucleic acids are macromolecules composed of monomeric units called nucleotides. These nucleotides constitute DNA and RNA.

Characteristics

Nucleic acids are the genetic material for all living cells. They play numerous important roles within the cell.

For example, they serve as energy stores for future use in phosphate transfer reactions, predominantly carried out by adenosine triphosphate (ATP).

They also form part of coenzymes, such as NAD, NADP, FAD and coenzyme A.  These coenzymes serve as mediators of important cellular processes and activated intermediates in numerous biosynthetic reactions.

Each nucleotide  consists of a nitrogenous heterocyclic base (purine or pyrimidine), a pentose sugar, and a phosphate group.

Nitrogenous bases are adenine, guanine, thymine, and cytosine in DNA, whereas, thymine is replaced by uracil in RNA (figure 29 and 30).

Figure 29. Purine
Figure 29. Purine

Figure 30. Pyrimidines
Figure 30. Pyrimidines
Deoxyribose sugar is present in DNA instead of ribose present in RNA. The sugars and phosphates in nucleic acids are connected to each other in an alternating chain.

They are linked by shared oxygens through a phosphodiester bond.

Deoxyribonucleic acid (DNA)

Deoxyribonucleic acid (DNA), the blueprint of life (Figure 31), is a nucleic acid that carries the genetic information in the cell and is capable of self-replication and synthesis of RNA.

Figure 31. Structure of DNA
Figure 31. Structure of DNA

Characteristics of DNA

DNA bases pair up with each other, A with T and C with G, to form units called base pairs.

Each base is also attached to a sugar molecule and a phosphate molecule. Together, they are called a nucleotide.

Nucleotides are arranged in two long strands that form a spiral ladder like structure called a double helix.

One of the most important properties of DNA is to replicate, i.e., making copy of itself (Figure 32).

Figure 32. DNA replication
Figure 32. DNA replication

Ribonucleic Acid (RNA)

Ribonucleic Acid (RNA) is a biologically important molecule that consists of a long chain of nucleotide units and is necessary for protein synthesis (mRNA, tRNA, and rRNA), and other cellular activities.

Characteristics of RNA

RNA, stands for ribonucleic acid, is a single stranded polymeric molecule made up of one or more nucleotides.

Each nucleotide is made up of a base (adenine, cytosine, guanine, and uracil, typically abbreviated as A, C, G and U), a ribose sugar, and a phosphate.

RNA is synthesized from DNA by an enzyme known as RNA polymerase during a process called transcription.

The new RNA sequences are complementary to their DNA template, rather than being identical copies of the template.

RNA is then translated into proteins by structures called ribosomes.

Types of RNA

There are three types of RNA involved in the translation process

  • messenger RNA (mRNA)
  • transfer RNA (tRNA)
  • ribosomal RNA (rRNA)

It is also the genetic material for some organisms that do not have DNA. Some viruses contain DNA; many only contain RNA.

RNAs are used in some cancer gene therapies to reduce the expression of cancer-causing genes. It is also used to suppress the expression of fruit ripening genes.

Differences between DNA and RNA

Structurally, DNA and RNA are almost identical. However, there are fundamental differences that account for the different functions of these two molecules.

Figure 33 Differences between DNA and RNA
Figure 33. Differences between DNA and RNA
Table showing differences between DNA and RNA

Features Deoxyribonucleic acid

Ribonucleic acid
Structure Double-stranded, consisting of phosphate group, sugar, and four nitrogen containing bases (adenine, thymine, cytosine, and guanine) Single stranded and consists of phosphate group, sugar, and four nitrogen containing bases (adenine, uracil, cytosine, and guanine)
Function Blueprint of biological system or life as it transmits genetic information from one generation to another Transfers genetic code needed for the creation of proteins from nucleus to the ribosome
Location DNA is found in the nucleus of a cell and in mitochondria Depending on the type of RNA, this molecule is found in a cell’s nucleus, its cytoplasm, and its ribosome
Stability Less reactive because of C-H bonds also stable in alkaline conditions More reactive because of C-OH bonds and not stable in alkaline conditions
Propagation Self-replicating Formed from the DNA by transcription
Base pairing Adenine links to thymine (A-T) and cytosine links to guanine (C-G) Adenine links to uracil (A-U) and cytosine links to guanine (C-G)
Effect of UV rays Can be damaged by exposure to ultra-violet rays More resistant to damage by ultra-violet rays than DNA