Structure of DNA

DNA Structure

DNA is the molecule that holds the instructions for growth and development in every living thing.  Its structure is a double-stranded helix held together by complementary base pairs.  The basic units of DNA are nucleotides. These nucleotides consist of a deoxyribose sugar, phosphate and base.  

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Base pairing

The nucleotides are identical except for the base, which can be adenine, thymine, guanine or cytosine. There are chemical cross-links between the two strands in DNA, formed by pairs of bases held together by hydrogen bonds. They always pair up in a particular way, called complementary base pairing: Thymine pairs with Adenine (A - T) Guanine pairs with Cytosine (G - C) These basic units are linked together to form strands by string bonds between the deoxyribose sugar of one nucleotide. These strong bonds form a sugar-phosphate Backbone. The ends of the strand are called the 5' end at the phosphate end, and the 3' end at the deoxyribose end. The two strands of DNA are antiparallel. All the cells store their genetic information in the base sequance of DNA nd it is this base sequence which forms the genetic code. The Genotype is determined by the sequence of bases.

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Organisation of DNA - prokaryote

DNA is present in the cells of every living thing. However, the DNA is organised differently in different types of organisms. Bacteria are prokaryotes. They do not have a membrane-bound nucleus and DNA is free in the cytoplasm Bacteria have a singular circular chromosome in the centre of the cell that holds all the genes needed for that bacterium. Bacteria also have extra circles of DNA called plasmids These plasmids contain additional genes, such as for antibiotic resistance, which may increase a bacteriums chance of survival

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Organisation of DNA - Eukaryotes

Animals, plants and fungi are eukaryotes. They have a membrane-bolund nucleus and their chromosomes are linear rather than circular The DNA found in the linear chromosomes within the nucleus is tightly coiled and packaged around special proteins called histones Interestingly, circular chromosomes are also found in mitochondria and chloroplasts. They both use their own DNA to make some proteins needed for their function. This gives evidence for the theory that mitochondria and chloroplasts originated from prokaryotic cells that were engulfed by a mlarger cell 

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