With the exception of a few viruses, DNA forms the genetic material in all living organisms. In the prokaryotic cells DNA occurs in the cytoplasm. In the eukaryotic cells it is confined largely to the nucleus and forms the main component of chromosomes. A small amount of DNA is found in the cytoplasm in the organelles like mitochondria and plastids. It is called extra- nuclear DNA.
DNA is a polynucleotide of adenine, guanine, cytosine and thymine. Uracil nucleotides are absent. The DNA molecule is composed of two polynucleotide chains.
The structure of the DNA molecule and the arrangement of nitrogenous bases in the two polynucleotide chains, is very well explained by the double helix model proposed by Watson and Crick in 1953.
Watson and Crick
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Enzymes are complex biochemical catalysts, speeding up a particular reaction to produce an ordered, stable reaction system in which the products of any reaction are made when they are needed. A specific enzyme controls each reaction in a series of metabolic reactions. Enzymes also control cell metabolism by regulating how and when reactions occur.
They are made up of globular proteins that have complex tertiary or quaternary structure. Enzyme shape is maintained by hydrogen bonds and ionic forces and their function can be affected by changes in temperature and pH.
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The polarity of water
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an “electronegative” or electron “loving” atom compared with hydrogen.
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