Nucleic Acids (DNA and RNA)
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Nucleic acids are polymers utilised by cells as information molecules. The genetic information is stored in the form of deoxyribonucleic acid (DNA) providing instructions for the synthesis of proteins needed to build and maintain functioning cells, tissues, and organisms. Notably, some viruses employ ribonucleic acid (RNA) for storing genetic information. Within cells, RNA serves to carry the instructions from the DNA to the protein-synthesising machines, known as ribosomes.
The choice of nucleic acids as information molecules is determined by their ability to drive their self-replication. The replication of DNA before cell division enables each cell to have its copy of the genetic material.
Nucleic acids are often referred to as polynucleotides since they consist of monomers called nucleotides.
Each nucleotide comprises three components: a five-carbon sugar, a phosphate group and a nitrogenous base. In RNA, the sugar is ribose, while DNA incorporates deoxyribose. The nitrogenous bases are attributed to one of two families: purines and pyrimidines. Purines, distinguished by their larger size and two fused rings, include adenine (A) and guanine (G). Pyrimidines contain one ring and include cytosine (C), thymine (T) and uracil (U). Thymine is exclusive to DNA, while uracil is only found in RNA.
In a polynucleotide, nucleotides are joined by phosphodiester linkages formed through condensation reactions. This bonding creates the sugar-phosphate backbone, characterized by an alternating pattern of sugars and phosphates. The two ends of a polynucleotide molecule feature distinct chemical groups. The end with a phosphate group is termed the 5' end, while the end with a hydroxyl group is called the 3' end.
A DNA molecule is comprised of two polynucleotide strands that intertwine, forming a double helix. The strands are referred to as antiparallel as they run in opposite 5' to 3' directions. The nitrogenous bases are positioned inwardly towards the double helix from the sugar-phosphate backbone. Thus, the nitrogenous bases of the two strands face each other forming complementary base pairs. In DNA, adenine (A) always pairs with thymine (T), while guanine (G) pairs with cytosine (C). The complementary bases are held together by hydrogen bonds. There are two hydrogen bonds between A and T, and three hydrogen bonds between G and C.
The majority of RNA molecules are single-stranded. However, the complementary base-pairing between regions of the same RNA molecule can make it fold into a complex three-dimensional shape. A striking example is the 3D L-shape of transfer RNA (tRNA) molecules that serve to carry amino acids to the ribosome.
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