STRUCTURE AND FUNCTION OF THE GENETIC MATERIAL LEARNING OBJECTIVES Define genetic, genome, chromosome, gene, genetic code, genotype, phenotype, and genomics. Describe how DNA serves as genetic information. Chromosomes - Genetics is the science of heredity. - Genome is the genetic information in a cell. A cell's genome includes its chromosomes and plasmids. - The sequencing and molecular characterization of genomes is called genomics. - Chromosomes are structures containing DNA that physically carry hereditary information; the chromosomes contain the genes. Genes - Genes are segments of DNA in a chromosome (except in some viruses, in which they are made of RNA) that code for functional products. - DNA (deoxyribonucleic acid) is a macromolecule composed of repeating units called nucleotides. Each nucleotide consists of a nitrogenous base (adenine, thymine, cytosine, or guanine), deoxyribose (a pentose sugar), and a phosphate group. The base pairs (two nucleotides on opposite complementary DNA strands that are connected via hydrogen bonds) always occur in a specific way: adenine always pairs with thymine, and cytosine always pairs with guanine. Because of this specific base pairing, the base sequence of one DNA strand determines the base sequence of the other strand. The two strands of DNA are thus complementary. You can think of these complementary DNA sequences as being like a positive photograph and its negative. - The complementary structure allows for the precise duplication of Nucleotide DNA during cell division. Again, think of the photograph analogy: if you have a negative, you can always make another copy of the positive print. Likewise with DNA: if you know the sequence of one strand, you also know the sequence of the complementary strand. - The genetic code is the set of rules that determines how a nucleotide sequence is converted into the amino acid sequence of a DNA protein. Genetic information is encoded by the sequence of bases along a strand of DNA. Therefore, 1000 of these four bases, the number contained in an average-sized gene, can be arranged in 41000 different ways. This astronomically large number explains how genes can be varied enough to provide all the information a cell needs to grow and perform its functions. GENOTYPE AND PHENOTYPE The genotype of an organism is its genetic makeup, the information that codes for all the particular characteristics of the organism. The genotype represents possible properties, but not the properties themselves. Phenotype refers to actual, expressed properties such as the organism's ability to perform a particular chemical reaction. Phenotype, then, is the manifestation of genotype. DNA AND CHROMOSOMES Bacteria typically have a single circular chromosome consist- ing of a single circular molecule of DNA with associated proteins. DNA REPLICATION LEARNING OBJECTIVE Describe the process of DNA replication. DNA Replication in Bacteria Before looking at DNA replication in more detail, let's take a closer look at the structure of DNA. It is important to understand the concept that the paired DNA strands are oriented in opposite directions relative to each other. In order for the paired bases to be next to each other, the sugar components in one strand are upside-down relative to the other. The end with the hydroxyl attached to the 3' carbon is called the 3' end of the DNA strand; the end having a phosphate attached to the 5' carbon is called the 5' end. The way in which the two strands fit together dictates that the 5' 3' direction of one strand runs counter to the '5' 3' direction of the other strand. This structure of DNA affects the replication process because DNA polymerases (enzymes which catalyze the formation of new DNA or RNA) can add new nucleotides to the 3' end only. First of all, the overall process of replication is as follows: DNA is replicated by uncoiling of the helix, strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by complementary base pairing. A more detailed description of the process of replication is as follows: Replication begins at a specific site in the DNA called the origin of replication (a particular sequence in a genome at which replication is initiated). DNA replication is bidirectional from the origin of replication. DNA replication To begin DNA replication, unwinding enzymes (arrows) occurs in both directions called DNA helicases (enzymes that unwind the two from the origin of replication in the circular DNA found in most bacteria. Helix uncoiling complementary parent DNA strands during DNA and strand separation replication) cause the two parent DNA strands to unwind and separate from one another at the origin of replication to form two "Y"-shaped replication forks. These replication forks are the actual site of DNA copying. As the strands continue to unwind and separate in both directions around the entire DNA molecule, new complementary strands are produced by the hydrogen bonding of free DNA nucleotides with those on each parent strand. DNA Polymerase Complementary base pairing As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds.. In the end, each parent strand serves as a template to synthesize a complementary copy of itself, resulting in the formation of two identical DNA molecules. Therefore, each new DNA molecule has one strand from the old DNA and one strand form the new. As the cell elongates, the two DNA molecules are physically separated. There is a great deal of genetic information in the bacterial chromosome. For example Escherichia coli, the most studied of all bacteria, has a genome containing 4,639,221 base pairs, which code for at least 4288 proteins.