The nucleic acids play critical roles in information storage and protein synthesis. The two nucleic acid polymers, deoxyribonucleic acid [DNA] and ribonucleic acid [RNA] are made up of monomeric units called nucleotides. Each nucleotide monomer is made up of three subunits: a pentose sugar, a phosphate group, and a nitrogenous base.
The five nitrogenous bases belong to two categories: the smaller pyrimidines and the larger purines. Each pyrimidine is constructed of a ring of carbon and nitrogen atoms. The three pyrimidines are differentiated by the type and position of the short side chains on the ring. Cytosine, uracil, and thymine are the three pyrimidines. Note the side chains on each of these molecules. Which two of the pyrimidine molecules are structurally the most similar?
The two purines are built on a double ring structure. Again, it is the side groups that differentiate them. The two purines are adenine and guanine.
The two pentose sugars that are used in nucleotides are ribose and deoxyribose. Ribose is a typical 5-carbon monosaccharide with the formula C5H10O5. Deoxyribose, as the name implies, is lacking one oxygen -- from the hydroxyl group of the #2 carbon.
The phosphate group is represented by phosphoric acid.
The addition of a nitrogenous base to the #1 carbon of a pentose sugar produces a nucleoside. The further addition of a phosphate group to the #5 carbon of the sugar completes the nucleotide.
The nucleoetide shown here is adenosine monophosphate [AMP]. This monomer has a special role in the cell's energy banlance. Additional phosphates can be attached to the phosphate of AMP, to produce adenosine diphosphate [ADP] or adenosine triphosphate [ATP]. The phosphate-phosphate bonds are high energy bonds. ATP has been called the cells 'energy currency'. Other triphosphate nucleotides can also be produced and serve similar roles, storing and transporting small units of energy for short intervals.
Nucleotides can also be joined by dehydration synthesis inot long heteropolymer chains, the nucleic acids. The nucleotides ar joined by phosphodiester bonds between the #3 carbon of the sugar of one nucleotide and the phosphate group of the next nucleotide
One nucleic acid type is ribonucleic acid. RNa is always made up of nucleotides built on ribose sugar; only adenine, guanine, cytosine, and uracil [not thymine] bases are used; and the polymers are made of a single chain of nucleotides.
There are several specific categories of RNA, each filling a particular role in the cell. Ribosomal RNA [rRNA] is built into ribosomes; messenger RNA [mRNA] is transcribed from structural genes on the DNA and carries information from the nucleus to the ribosomes in the cytoplasm. Transfer RNA [tRNA] is the smallest of the group. each tRNA molecule recognizes and binds a particular amino acid that can be delivered [transported] to the site of protein synthesis on the ribosome.
Deoxyribonucleinc acid is built of nucleotides made from deoxyribose sugar and uses the bases adenine, cytosine, guanine, and thymine [not uracil]. DNA is a double helix. Two complementary and anti-parallel chains of nucleotides are held together by hydrogen bonds between the nitrogenous bases of their nucleotides.
The chains are described as complementary because the nucleotide sequence of one chain determines the sequence of the opposite chain. Adenine always pairs with thymine, and cytosine always pairs with gqunine. The size [large purine matched to small pyrimidine] and the number of hydrogen bonds possible [adenine and thymine have two hydrogen bonds and cytosien and guanine have three bonds] restricts pair combinations.
DNA is the cell's information storage molecule. The sequence of nucleotides on the DNA chain holds coded information determining the structure of proteins made by the cell. The DNA molecule is well-suited for its role as a code molecule. It has the potential for variability -- the nucleotides may be in any seequence. It has the potential for codability -- patterns of nucleotides can be translated, with each set of nucleotides 'matched' to a particular amino acid. In addition, the DNA molecule is stable [because of the strength of the covalent phosphodiester bonds joining the nucleotides] and it can be copied or replicated to provide new information sets to newly formed cells. The presence of the complementary chains of nucleotides is critical to the efficiency of the replication process.