deoxyribose, and one of the four nitrogenous compounds: adenine (C H N ), cytosine (C H N O),

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1 Topic Page: DNA Definition: deoxyribonucleic acid (DNA) from Processing Water, Wastewater, Residuals, and Excreta for Health and Environmental Protection: An Encyclopedic Dictionary The double-stranded, helix-shaped macromolecule that contains the hereditary material vital to reproduction and transmitted between generations of cells. It consists of molecules of long unbranched chains of nucleotides, which are combinations of phosphoric acid (H PO ), the monosaccharide 3 4 deoxyribose, and one of the four nitrogenous compounds: adenine (C H N ), cytosine (C H N O), guanine (C H N O), and thymine (C H N O ). See also plasmid DNA, ribonucleic acid Summary Article: DNA from The Human Body Book: An Illustrated Guide to Its Structure, Function and Disorders Often referred to as the molecule of life, DNA (deoxyribonucleic acid) is found in almost all living things. It acts as a type of chemical code that contains instructions, known as genes, for how the body and all its different parts grow, develop, function, and maintain themselves. Image from: DNA (or In nearly all human cells, DNA is packaged into 46 X-shaped elements deoxyribonucleic acid) in called chromosomes, which are situated in the cell s nucleus. DNA s The Macmillan Encyclopedia enormous list of instructions takes the form of long, thin molecules, one per chromosome, each taking the shape of a double-helix. Each doublehelix has two long, corkscrew-like strands, which act as backbones for the molecule, twining around each other. These are held together by rungs, like a twisted ladder. The rungs are made of pairs of chemicals called bases: adenine (A), guanine (G), thymine (T), and cytosine (C). In each rung, A always pairs with T, and G with C. This structure gives DNA its two key features: the order of the bases contains the chromosome s genetic code, while the way the bases cross-link enables DNA to make exact copies of itself.

2 Double helix A DNA molecule is coiled and supercoiled in order to fit into the chromosomes (see section on Coils and supercoils). It also loops and twists. The molecule is accompanied by various proteins, particularly histones.

3 Base pairs The four bases can pair in only two configurations due to their chemical structures. Adenine and thymine each have two positions for forming hydrogen bonds and so fit together, while guanine and cytosine each have three hydrogenbond locations. How DNA works One of DNA s key functions is to provide the information to build proteins. Some proteins are the body s major structural molecules, others form enzymes, which control chemical reactions within the body. Manufacture of proteins occurs in two main phases, transcription and translation. In transcription, information is taken from the DNA and copied to an intermediate type of molecule called mrna (messenger ribonucleic acid). This is built from nucleotide units in a similar way to DNA. The mrna moves out of the cell s nucleus to protein assembly units called ribosomes. In the translation phase, the mrna acts as a template for the formation of units of protein, known as amino acids. There are about 20 different amino acids. Their order is specified by lengths of mrna three bases long, called triplet codons. The order of bases in each codon is the code for a particular amino acid (hence the term

4 genetic code). The mrna carries instructions to make a specific protein from a sequence of amino acids. 1. Transcription In the cell s nucleus, the DNA strands temporarily separate, with one acting as the template for the formation of mrna. Separate RNA nucleotides with the correct bases lock onto the exposed DNA bases in cross-linked fashion, thereby forming a mirror image of the DNA s information. 2. Translation In the cell s cytoplasm, the mrna attaches to a ribosome. Individual trna (transfer ribonucleic acid) molecules have specific amino acids attached. They can slot onto the mrna only if the order of their bases matches, ensuring they bring the correct amino acid. As the ribosome moves along the mrna, the trnas bring the correct sequence of amino acids, which fit together to construct a protein.

5 What are genes? A gene is generally regarded as a unit of DNA needed to construct one protein. It consists of all the sections of DNA that code for all the amino acids for that protein. These sections are not necessarily on the same strand of DNA or even on the same chromosome. There may be many strands of DNA, each containing the code for one portion of the protein. Typically, lengths of DNA called introns and exons are both transcribed (see illustration) to form immature mrna. The parts of mrna made from the introns are then stripped out by the cell s molecular machinery, leaving mature mrna for translation. There are also regulatory DNA sequences that code for their own proteins, affecting the gene transcription rate.

6 Eye colour Iris colour is affected by at least three genes, called bey 1 and bey 2 on chromosome 15, and gey on chromosome 19. Parts of a gene Regions called introns and exons both transcribe to form mrnas for different portions of a protein. The lengths made from introns are then spliced out chemically, to leave exon-only portions, which go on to make the protein.

7 Range of gene size Genes vary enormously in their size, which is usually measured in numbers of base pairs. Small genes may be just a few hundred base pairs long, while others are measured in millions of base pairs. The gene for beta-globin is one of the smallest. It codes for part of the haemoglobin molecule. It is compared, right, with a larger gene. Coils and supercoils DNA s multi-coiled structure allows an incredible length to be packed into a tiny space. The single length of DNA in a typical chromosome, if unwound, would stretch about 5cm (2in). There are 46 chromosomes in the nucleus of each cell. (A few cell types, such as mature red blood cells, lack DNA.) When cells are not dividing, the DNA is loosely coiled and winds around the nucleus, forming a tangled-looking structure called chromatin. This allows portions to be available for protein assembly and other functions. As a cell prepares to divide, its DNA coils into supercoils, which are shorter and denser, and visible as the typical chromosome X shapes.

8 Copyright 2009 Dorling Kindersley Limited

9 APA Dna. (2009). In S. Parker, The human body book: an illustrated guide to its structure, function and disorders. London, UK: Dorling Kindersley Publishing, Inc. Retrieved from Chicago "Dna." In The Human Body Book: An Illustrated Guide to Its Structure, Function and Disorders, by Steve Parker. Dorling Kindersley Publishing, Inc., Harvard Dna. (2009). In S. Parker, The human body book: an illustrated guide to its structure, function and disorders. [Online]. London: Dorling Kindersley Publishing, Inc. Available from: [Accessed 3 May 2018]. MLA "Dna." The Human Body Book: An Illustrated Guide to Its Structure, Function and Disorders, Steve Parker, Dorling Kindersley Publishing, Inc., 1st edition, Credo Reference,. Accessed 03 May 2018.