Lecture 3

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University of Toronto Scarborough
Biological Sciences
Clare Hasenkampf

BGYA01H Lecture 3 September 17, 2007 Please note- there was a slight mistake on the lecture schedule and relevant textbook readings. For lectures 11 and 12, the relevant textbook chapter is Chapter 10. NUCLEIC ACIDS Now I would like to begin our study of the last category of macromolecules, the nucleic acids. There two types of nucleic acids are DNA and RNA. Both DNA and RNA are examples of polymers. Therefore they are both made by adding monomer units together via condensation reactions. Nucleic acid polymers are very different in their characteristics from the polypeptides and complex carbohydrates because they have different monomer units. For both nucleic acids, the monomer starting units, are molecules called nucleotides. Each nucleotide, is itself, made of three different chemical groups. Each nucleotide has 1) A 5-carbon, ring sugar (each carbon of the sugar molecule is numbered). Figure 3.14, page 50 Middle section) 2) a phosphate group, Figure 3.23, page 58. 3) and a nitrogen-containing molecule known as a nitrogenous base. (Figure 3.23 is very diagrammatic and does not show what the phosphate groups and sugars really look like. The purpose of this figure is to show how the components are connected.) The nitrogenous base is attached to the first carbon of the sugar, and the phosphate is attached to the fifth carbon of the sugar. Please be aware that all of the bonds connecting the elements within the nucleotide are covalent (strong) bonds. The monomers for RNA and DNA are slightly different from each other, but they share many structural features. (These similarities and differences are summarized in Table 3.3, page 459. The monomer units for RNA are called ribonucleotides. Ribonucleotides each have the sugar ribose. Each ribonucleotide has one of the 4 bases either adenine, uracil, guanine or cytosine. The last component of the ribonucleotide is the phosphate group. The monomer units for DNA are called deoxyribonucleotides. Deoxyribonucleotides consist of the sugar deoxyribose. Each Deoxyribonucleotide has one of the 4 bases : adenine, thymine, guanine or cytosine. The last component of the deoxyribonucleotide is the phosphate group. In their active form deoxyribonucleotides and ribonucleotides occur as nucleotide triphosphates. There is a lot of energy stored in the pyrophosphate bond. Figure 6.5, page 124 www.notesolution.comWe will mainly be looking at nucleotide triphosphates as the monomer units for making larger nucleic acids. But guanine nucleotide triphosphates (GTP) and adenine nucleotide triphosphates are important energy molecules in the cell. Often if enzymes need energy to make or break a covalent bond, it gets the energy from ATP. As I have already mentioned, RNA and DNA are both examples of polymers; so we can expect that the monomer units of each will be put together by condensation reactions to make the polymers. Lets look at a small polymer of RNA Overhead of Figure 3.24, page 58, left side, page 47. Here we can see what the sugar ribose looks like and can also see what the phosphate groups structure is. The individual ribonucleotides (the monomer units) of RNA are held together to form a polymer by covalent bonds known as phosphodiester linkages. All of the bonds holding the ribonucleotides together are strong, covalent bonds. We call one polymer of RNA a strand of RNA. If we look at a strand of RNA we can see it has a polarity(here polarity means the two ends of the molecule are different). One end has a free phosphate group; this end is called the 5 end (because the phosphate comes off the 5 carbon). The other end is called the 3 end; it has a free OH group (that comes off the third carbon). This polarity is important. Now lets think about DNA. DNA is the hereditary material for the cell. INDEPENDENT STUDY p233-237 (left side) Figures 11.2, 11.3, 11.4 and 11.5 The experiments described under the general heading What is the evidence that the gene is DNA. C HARACTERISTICS OF THE HEREDITARY MATERIAL The hereditary material must contain an organisms plan for where and when it will synthesize its needed proteins, carbohydrates and lipids, and in doing it is determining the structures within cells, the pattern of growth of the organism and how the organism will cope with its environment; the plan will even need to have details on when to make a copy of itself. So The hereditary material must be able to be copied faithfully so that every time a cell divides into two cells each cell gets a complete copy of the DNA. Also, as the hereditary material DNA must be able to direct cellular activity (implement the plan). So as we look at the structure of DNA be on the look out for how DNAs structure can allow it to function as the hereditary material. Now lets look at the structure of DNA. Overhead of Figure 3.24 right side, page 58. but a better figure can be seen in Figure 11.9, page 240. Two neighboring deoxyribonucleotides (the monomers) of a larger DNA molecule are held together by phosphodiester bonds. Therefore all of the bonds holding together the polydeoxyribonucleotide (of a single polymer) are strong bonds. An individual polymer of DNA is also called a strand of DNA. www.notesolution.com
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