BIOLOGICAL MACROMOLECULES
NUCLEIC ACIDS
| Contents for this page | Related topics |  |
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Introduction Nucleosides and nucleotides: RNA DNA Biological function Additional questions |
Polysaccharides and proteins | |
| Learning Outcomes | ||
| After studying this section, you will understand what is meant by substitution, addition, and elimination reactions | ||
The nucleus and cytoplasm of cells contain organic macromolecules collectively known as NUCLEIC ACIDS. The molecules are of two main types, namely, RIBONUCLEIC ACID (abbreviated RNA), and DEOXYRIBONUCLEIC ACID (abbreviated RNA).
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These names are derived from the two simple sugars (monosaccharides, actually), RIBOSE and DEOXYRIBOSE. RNA contains ribose, while DNA contains deoxyribose (see the formulae on the left - can you spot the difference between the two?). DNA is found in the nucleus of the cells of animals and plants, while RNA is found in the cytoplasm, usually associated with particles called RIBOSOMES. Although both RNA and DNA had been known since the end of the 19th century, and DNA was suspected of being somehow involved with genetics of the cell, it was only in 1953 that Watson, Crick, and Wilkins established the structure of DNA , a discovery that earned them the Nobel Prize, and provided a molecular basis for the whole of Biology. |
Nucleic acids may be regarded as condensation polymers of NUCLEOTIDES. Before we look at these compounds, we must look at the polymers' basic stucture:
As seen in the diagram above, both DNA and RNA have a "backbone" consisting of repetitive sugar phosphate units, with certain bases attached to the sugar. This sugar is ribose in RNA and deoxyribose in DNA.
The bases are shown above. Thymine is found in DNA, and very occasionally in RNA. Uracil is only found in RNA. The others occur both inRNA and RNA.
The above bases can be linked covalently to the sugars, forming what are known as NUCLEOSIDES. We need not concern ourselves here with their names, but should bear in mind that they are abbreviated A, G, C, T and U, depending on the bases that are involved, that is, adenine, guanine, cytosine, thymine and uracil.
NUCLEOTIDES are phosphate esters of nucleosides, the phosphate group (which is ionised in aqueous media) being attached to ribose or deoxyribose, depending on whether we are dealing with RNA or DNA. Nucleic acids are thus POLYNUCLEOTIDES.
The major differences between DNA and RNA are summarised in the table below:
| DNA | RNA |
|---|---|
| Molecule is a double stranded helix | Molecule is single-stranded |
| Sugar is deoxyribose | Sugar is ribose |
| Contains thymine and no uracil | Contains uracil, and no thymine |
| Content of A = content of T and, content of G = content of C |
Content of A ≠ content of T and, content of G ≠ content of C |
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The figure on the right shown a short segment of DNA. The sugar-phosphate backbone is highlighted. The bases stick out from the sugars, and this has a profound influence on the stucture. It turns out that the bases have just the right disposition of atoms for them to form hydrogen-bonded pairs, A binding with T, and G binding with C. These pairings are shown in the figure below.
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It is important to note that the A-T and C-G base pairs result in the same separation distances (1.08 nm) between two sugar units in adjacent DNA chains. DNA strands are said to be COMPLEMENTARY if , for every A, T, C, or G at any position of the strand, there is a T, A, G or C on the other. Ultimately, two such DNA strands can link together to form a double, right-handed helix, as shown in the figure below.
The sequence of bases on a DNA strand is actually a message, coding for sequences of amino acids in proteins. The mechanism whereby proteins are built up in the cell is extremely complex, involving catalysis by numerous enzymes, but can be summarised as follows.
Unwinding of the double DNA helix gives two strands, one of which carries the coded message (the "coding strand"). This strand acts as a template to form a complementary strand of RNA, the process being known as TRANSCRIPTION. Now the RNA (appropriately called MESSENGER RNA) carries the message, which is "read" by the protein-building machinery in the cell three base-pairs at the time. Each triplet of bases, called a CODON corresponds to one amino acid in the sequence of the protein that is being made. The process is known as TRANSLATION:
The code was cracked by Khorana, Holly and Nierenberg, a remarkable achievement that earned them the Nobel Prize in 1968. The code is believed to be universal, applying to all cells capable of producing proteins, in all living organisms.
A segment of DNA that codes for a specific protein is called a GENE. It is estimated that the human DNA (called the HUMAN GENOME) contains between 30 000 and 40 000 genes. The complete sequence of bases in human DNA has been achieved.
As soon as the double-helix structure for DNA had been established, it became obvious that that structure explained how genetic characteristics, that is, the whole array of proteins that characterise a living species, could be transmitted through generations. It is that DNA can act as a template to make exact copies of itelf, in a process known as REPLICATION:
In this process, which takes place under catalaysis by a variety of enzymes, the DNA double helix first unwinds, then monomeric nucleotides pair with the correct bases on the single-helix regions, followed by the making of new phosphate-sugar links, thus elongating the sugar-phosphate backbones of the new strands.
