Lecture 20Outline of Last Lecture I. Fluid Mosaic ModelA. Mosaic Functions’II. Membrane FormationIII. Factors that Influence FluidityIV. Membrane ProteinsA. Integral Membrane ProteinsB. Peripheral Membrane Proteins Outline of Current Lecture I. Nucleic AcidsA. Nitrogenous BasesB. Base NumberingII. Base Linkages and PhosphorylationIII. Sugar Backbone of DNA and RNAIV. NucleosidesA. Nucleoside Uses in MedicineV. DNACurrent LectureThis lecture will switch to a new topic and focus on nucleic acids. The two most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These are the basic building blocks of genetics. They can be broken down into the most basic form, called nitrogenous bases. The bases are adenine, guanine, cytosine, thymine (in DNA), and uracil (in RNA). Adenine and guanine are in a category called purines. All the other bases are in the category called pyrimidines. The bases are in a cyclic structure and their carbons are numbered. Purines are numbered starting with the left hand Nitrogen and going counterclockwise around to 6.Starting where the 5th carbon is, the other linked carbons are number 7-9 going clockwise. The purines are simply numbered 1-6, starting with the bottom Nitrogen. The bases can be linked to sugars such as ribose or deoxyribose. These linked compounds are called nucleosides. Beta glycosidic bonds link the two. Nucleosides are named after their respective base. The names for each are: adenosine, thymidine, guanosine, cytidine, and uridine. Nucleosides are also numbered the same way as their bases, except using prime notation (1’, 2’, ect). When nucleosides are phosphorylated, when phosphate groups are added, they are called nucleotides.Phosphates are added to the 5’ end. Up to three phosphates can be, but don’t have to be, added. They are named as “’Nucleoside” 5’- mono(di, tri)phosphate. DNA and RNA are polymers are made from the same backbone. There are alternating ribose attached to phosphates, linked with diester bonds. There is always a free phosphate at the 5’ end and a free hydroxyl group at the 3’ end. Nucleotides are linked to the backbone and together with phosphodiester bonds. The BCHM 307 1nd Editionnucleotides are not part of the sugar backbone. The next nucleotide added to the strand is added to the 3’ end. Nucleosides have important medical uses for the treatment of cancer and viruses such as AIDS. Cancer and AIDS both causes cells to grow rapidly and synthesize DNA/RNA at a higher rate. A good tool to help treat these diseases is by disrupting this synthesis. Nucleotides are targets for drugs to disrupt the synthesis of DNA and RNA. Two drugs that can do this are AZT and DDI, which are nucleoside analogues. They are added to the growing strand like a normal nucleoside. They do not have a free hydroxyl group to attach the next nucleoside though. This disrupts the synthesis. The downside is that AZT and DDI are toxic to normal cells as well. Nitrogenous bases help to stabilize the helical structure through hydrogen bonding with each other. Guanine can form three bonds with cytosine. Adenine can only form two hydrogen bonds with thymine or uracil. Stronger interactions are formed through more hydrogen bonds. These hydrogen bonds are what form the double helix of DNA. DNA strands are complimentary and antiparallel. This also forms major and minor grooves in the structure.Next we will focus on DNA specifically. DNA has a half-life of around 521 years, making is very stable. Most organisms use DNA as their hereditary material. Viruses sometimes use RNA as their hereditary material. For prokaryotes their DNA is found in the cytoplasm, as they lack organelles. Their DNA is also circular. Eukaryotic DNA can be found in the nucleus, mitochondria, and in plant chloroplasts. Eukaryotic DNA is linear in chromosome form, found in the nucleus. It is circular in organelles though. All the DNA in a cell is called its
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