By Microbiologist Doctor-dr
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Table of Content |
Nucleic Acids
Human existence—and the survival of all other species—is essentially dependent on two types of nucleic acid molecules. Their truncated names, DNA and RNA, are well known, but their full names are less well known. They are deoxyribonucleic and ribonucleic acids, respectively. Nucleic acid molecules are polymers made up of millions of smaller molecules known as nucleotides—deoxyribonucleotides in DNA molecules and ribonucleotides in RNA molecules. A deoxyribonucleotide is made up of deoxyribose, a nitrogen base (either adenine, cytosine, guanine, or thymine), and a phosphate group (Figure 2-20). Ribonucleotides are identical to deoxyribonucleotides but include ribose instead of deoxyribose and uracil instead of thymine (Table 2-6.)
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Table 2-6 Comparison
of DNA and RNA Structure |
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DNA |
RNA |
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Polynucleotide
Strands |
2 |
1 |
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Sugar |
Deoxyribose |
Ribose |
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Base Pairs |
Adenine-Thymine Guaninie-Cytosine |
Adenine-Uracil Guanine-Cytosine |
Two of the bases in a deoxyribonucleotide, adenine and guanine, are known as purine bases since they are derived from purine. Pu rines are made up of two rings. Cytosine and thymine are pyrimidine bases since they are derived from pyrimidine. Pyrimidines are characterised by a single ring structure. Uracil, a pyramidine base, substitutes thymine in RNA.
DNA molecules, the body's biggest molecules, are extraordinarily massive polymers made up of many nucleotides. A single DNA molecule is made up of two long polynucleotide chains. The chains create a double helix by coiling around one another. A helix is a spiral form that resembles the shape of a spring wire. Figure 2-20 depicts a double helix DNA diagram.
In a DNA molecule, each helical chain has its phos phate-sugar backbone pointing outward and its bases pointing inward toward the bases of the other chain. Furthermore, each base in one chain is connected to a base in the other chain through hydrogen bonds to form a base pair. A DNA molecule's two polynucleotide chains are therefore kept together by hydrogen bonds formed between the two members of each base pair (Figure 2-20).
One crucial concept to remember is that DNA has just two types of base pairs. What exactly are they? They are represented by the letters A-T and G-C. Although a DNA molecule only includes only two types of base pairs, there are millions of them—over 100 million pairs in one human DNA molecule! Two more fascinating facts are that millions of base pairs appear in the same sequence in all the millions of DNA molecules in one individual's body but in a different sequence in all other persons' DNA. In summary, each person's base pair sequence in DNA is unique. For the time being, we will simply declare that DNA serves as the hereditary molecule. It bears the hefty burden of passing on the features of one generation to the next.
FIGURE 2-20 DNA is a molecule. The overall structure of a nucleotide and the two types of "base pairs": adenine (A) (blue) with thymine (T) (purple), and guanine (G) (green) with cytosine (C) (yellow). It is worth noting that the G-C base pair has three hydrogen bonds while the A-T base pair has two. Hydrogen bonds play a critical role in the structure of this molecule.
Metabolism
The word metabolism refers to all of the chemical events that take place in bodily cells. Nutrition and metabolism are discussed together because the sum of all chemical processes or metabolic activity occurring in cells is related to the usage of nutrients by the body after they have been digested, absorbed, and circulated to cells. The two primary forms of metabolic activity are described by the words catabolism and anabolism. Catabolism refers to chemical processes that break down big food molecules into smaller chemical units, releasing energy in the process. Anabolism entails a variety of chemical events, most notably dehydration synthesis reactions, which result in the formation of bigger and more complex chemical compounds from smaller subunits (Figure 2-21). Anabolic chemical processes need energy, namely adenosine triphosphate, or ATP.
FIGURE 2-21 The metabolic reactions Hydrolysis is a catabolic reaction that breaks down big molecules into smaller molecules, or subunits, by adding water. Dehydration synthesis is an anabolic process in which tiny molecules are synthesised into larger ones by eliminating water. Figures 2-12 and 2-17 provide detailed instances of dehydration synthesis.
The first is the pentose sugar ribose, which acts as the site of attachment for the nitrogen-containing molecule, adenine, and a unique grouping of three phosphate subunits (Figure 2-22). The "squiggle" lines represent covalent connections formed by the phosphate groups. These bonds are known as high-energy bonds because they release a considerable amount of energy when broken during catabolism-type chemical processes. The energy stored in ATP is utilised to perform bodily functions such as muscular contraction and movement, active transport, and biosynthesis.
FIGURE 2-22 Adenosine triphosphate (ATP) (ATP). A, ATP structure. Three phosphate groups are linked to a single adenosine group (A) (P). High-energy connections between phosphate groups can produce chemical energy that can be used to perform cellular function. B, A general diagram of the ATP energy cycle. The last high-energy phosphate bond in ATP stores energy. When that link is dissolved, energy is released to do cellular job. The resulting ADP and phosphate groups can be resynthesized into ATP, collecting more energy from food catabolism.
Catabolism
Catabolism refers to chemical processes that not only degrade relatively complex substances into simpler ones, but also liberate energy from them. This disintegration process is an example of a chemical reaction known as hydrolysis (see Figure 2-21). A water molecule is given to a bigger component during catabolism to break it down into smaller subunits as a result of hydrolysis. For example, hydrolysis of a fat molecule would break it down into its subunits-glycerol and fatty acid molecules; hydrolysis of a disaccharide such as sucrose would break it down into its monosaccharide subunits-glucose and fructose; and amino acids would be the components of protein hydrolysis.
Catabolism processes will eventually decompose these food molecule building blocks-glycerol, fatty acids, monosaccharides, and amino acids-into the end products carbon dioxide, water, and other waste products. Energy is released during this process. Some of the energy generated through catabolism is heat energy, which is responsible for keeping our bodies warm. However, more than half of the released energy is quickly recovered and stored as ATP.
Anabolism
Anabolism refers to chemical processes that combine simple molecules to generate more complex biomolecules, most notably carbohydrates, lipids, proteins, and nucleic acids. Thousands of anabolic responses occur in the body on a daily basis. Dehydration synthesis is the chemical process that is responsible for the combining of smaller units to produce bigger molecules (see Figure 2-21). It is an important response during anabolism. Water is eliminated as smaller subunits are fused together as a result of dehydration synthesis. The process necessitates the use of energy, which is provided by the breakdown of ATP. Anabolic processes connect monosaccharide units to generate bigger carbs, fuse amino acids into peptide chains, and form fat molecules from glycerol and fatty acid subunits.
QUICK CHECK
1. Name two important nucleic acids.
2. What is a nucleotide?
3. What is meant by the term "base pair"?
4. How is energy stored in the ATP molecule?
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