Chapter 5 The amino acid sequence of a polypeptide is programmed by a discrete unit of inheritance known as a gene Genes consist of DNA which belongs to the class of compounds called nucleic acids Nucleic acids are polymers made of monomers called nucleotides Roles of Nucleic Acids There are two types of nucleic acids DNA and RNA Both enable living organisms to reproduce their complex components from one generation to the next DNA provides directions for its own replication and also directs RNA synthesis and through RNA controls protein synthesis DNA is the genetic material that organisms inherit from their parents Each chromosome contains one long DNA molecule usually carrying several hundred or more genes When a cell reproduces itself by dividing its DNA molecules are copied and passed along from one generation of cells to the next Encoded in the structure of DNA is the information tat programs all the cell s activities Proteins are required to implement genetic programs The molecular hardware of the cell consist mostly of proteins Each gene along a DNA molecule directs synthesis of a type of RNA called messenger RNA mRNA The mRNA molecule interacts with the cell s protein synthesizing machinery to direct production of a polypeptide which folds into all or part of a protein DNA RNA protein flow of genetic information Ribosomes sites of protein synthesis Messenger RNA conveys genetic instructions for building proteins from the nucleus to the cytoplasm The Components of Nucleic Acids Nucleic acids are macromolecules that exist as polymers called polynucleotides Each polynucleotide consists of monomers called nucleotides A nucleotide in general is composed of three parts a nitrogenous base a five carbon sugar a pentose and one or more phosphate groups In a polynucleotide each monomer only has one phosphate group The portion of a nucleotide without any phosphate groups is called a nucleoside Nitrogenous base Each nitrogenous base has one or two rings that include nitrogen atoms They are called nitrogenous bases because the nitrogen atoms tend to take up H from solution thus acting as bases There are two types of nitrogenous bases Pyrimidine and Purine A pyrimidine has one six membered ring of carbon and nitrogen atoms The members of the pyrmidine family are cytosine C thymine T and uracil U to the rings Purines are larger with a six membered ring fused to a five membered ring The purines are adenine A and guanine G The specific pyrimidines and purines differ in the chemical groups attached Adenine guanine and cytosine are found in both DNA and RNA thymine is only found in DNA and uracil only in RNA Adenine A always pairs with thymine T and guanine G always pairs with cytosine C Sugar In DNA the sugar is deoxyribose in RNA it is ribose The only difference between these two sugars is that deoxyribose lacks an oxygen atom on the second carbon in the ring hence the name deoxyribose The sugar carbon numbers of a nucleoside or nucleotide have a prime after them Thus the second carbon in the sugar ring is 2 2 Prime carbon and the carbon that sticks up from the ring is called the 5 carbon Nitrogenous base sugar nucleoside Phosphate Group Adjacent nucleotides are joined by a phosphodiester linkage which consists of a phosphate group that links the sugars of two nucleotides This bonding results in a backbone with a repeating pattern of sugar phosphate units The nitrogenous bases are not part of the backbone One end has a phosphate attached to a 5 carbon and the other end has a hydroxyl group on a 3 carbon we refer to these as the 5 end and the 3 end All along this sugar phosphate backbone are appendages consisting of the nitrogenous bases The linear order of bases in a gene specifies the amino acid sequence the primary structure of a protein which in turn specifies that protein s three dimensional structure and its function in the cell The Structures of DNA and RNA Molecules RNA molecules usually exist as single polynucleotide chains DNA molecules have two polynucleotides or strands that spiral around an imaginary axis forming a double helix The two sugar phosphate backbones run in opposite 5 3 directions from each other this arrangement is referred to as antiparallel somewhat like a divided highway The sugar phosphate backbones are on the outside of the helix and the nitrogenous bases are paired in the interior of the helix The two strands are held together by hydrogen bonds between the paired bases Only certain bases in the double helix are compatible with each other A always pairs with T and G always with C The two strands of the double helix are complementary each the predictable counterpart of the other It is this feature of DNA that makes it possible to generate two identical copies of each DNA molecule in a cell that is preparing to divide When the cell divides the copies are distributed to the daughter cells making them genetically identical to the parent cell Thus the structure of DNA accounts for its function of transmitting genetic information whenever a cell reproduces Complementary base pairing can also occur between parts of two RNA molecules or even between two stretches of nucleotides in the same RNA molecule Base pairing within an RNA molecule allows it to take on the particular three dimensional shape necessary for its function Transfer RNA tRNA brings amino acids to the ribosome during the synthesis of a polypeptide In RNA adenine A pairs with uracil U thymine T is not present in RNA Another difference between RNA and DNA is that DNA almost always exists as a double helix whereas RNA molecules are more variable in shape This variability arises because the extent and location of complementary base pairing within an RNA molecule differs in different types of RNA 16 1 Chapter 16 DNA replication the process by which a DNA molecule is copied and how cells repair their DNA Transformation a change in genotype and phenotype due to the assimilation of external DNA by a cell bacterial cancerous cells Transforming agent in experiment by Frederick Griffith was DNA Viruses called bacteriophages bacteria eaters or phages for short A virus is a little more than DNA or sometimes RNA enclosed by a protective coat which is often simply protein To produce more viruses a virus must infect a cell and take over the cell s metabolic machinery Hershey and Chase concluded that the DNA injected by the phage must be the molecule carrying the genetic information that makes the cells produce new
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