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2. Nucleic Acids.pdf

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Biology (Sci)
BIOL 200
Richard Roy

Naveen Sooknanan McGill Fall 2011 Nucleic Acids: Nucleic acids are active polymers located mainly I the nucleus of the cell, although some important exceptions can apply (mRNA and tRNA in protein synthesis). These compounds were discovered by the Swiss physician Friedrich Miescher in 1869.  This was done by isolating the nuclei of white blood cells found in pus from discarded surgical bandages  These compounds were found to be acidic, thus coined the term nucleic acids Nucleic acids are crucial in protein synthesis as single stranded RNA and double stranded DNA make up the first two steps of protein synthesis. RNA, ribonucleic acids, are shorter and less stable than DNA and thus have a shorter life span  They are approximately 100s to 1000s of nucleotides in length  RNA can come in various shaped, including tRNA, mRNA and rRNA, which will be discussed later DNA, deoxyribonucleic acid, is a double stranded double helix nucleic acid  Some DNA strands can be hundreds of millions of nucleotides in length Both DNA and RNA are built from monomers called nucleotides. Although these nucleotides differ slightly between RNA and DNA, they all contain backbone made from a pentose (5 carbon sugar) and a phosphate group.  Ribose is the sugar found in RNA whereas 2-deoxyribose is the sugar found in DNA o In DNA, an OH on carbon 2 replaces the hydrogen found in ribose  The pentose is covalently linked to a phosphate group which provides the acidic quality of the nucleic acid o This covalent bond is called a phosphodiester bond The nucleotides also contain bases, of which there are 5: adenine, guanine, uracil, thymine and cytosine. They can be further subdivided into 2 categories:  Adenine (A) and guanine (G) are purines and consist of two fused rings  Uracil (U, found in RNA), thymine (T, found in DNA) and cytosine (C) are pyrimidine and have a single ring structure 1Naveen Sooknanan McGill Fall 2011 A nucleoside refers to a nucleotide (A, T, U, C or G) which is missing a phosphate group  They are named adenosine, guanosine, cytidine, thymidine and uridine  A nucleoside monophosphate has 1 phosphate group  A nucleoside diphosphate has 2 phosphate groups  A nucleoside triphosphate has 3 phosphate groups o Adenosine triphosphate (ATP) is the energy currency of the cell due to high energy phosphate bonds DNA and RNA are biased; one end, called the 5’ end, has a free phosphate group whereas the other end, called the 3’ end, has a free hydroxyl group  One side of the nucleic acid strand is made up of alternating phosphate and pentose units. This is commonly referred to as the sugar phosphate backbone  The other side contains exposed bases which can be used as information for protein synthesis  These strands are usually written in abbreviated structures such as 5’ ATTACCGG 3’ for simplicity, naming only the base initials  The phosphate and sugar of two adjacent nucleotides form a phosphodiester bond DNA, which contains genetic information responsible for all activities of the cell, were analyzed in the 1940s and 50s in order to determine their shape  William Atsbury performed the first X-ray diffraction of DNA in 1947 which determined that the space between bases was 0.36 nm  J.M. Gulland discovered in the same year that hydrogen bonding was involved between base pairs in double stranded DNA molecules  Erwin Chargoff found that the ratio of purines to pyrimidines in DNA was 1:1in 1948 o This is known as Chargoff’s ratio  In 1953, James Watson and Francis Crick determined by X-ray diffraction and careful modeling that DNA formed a double helix structure made of two polynucleotide strands o They were awarded the Nobel Prize in medicine and physiology in 1962 o Rosalind Franklin, the woman who supplied the X-ray diffraction patterns for this
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