8.1 Structure and Function of Genetic Material

• Genetics is the science of heredity.

  • It includes the study of genes: how they carry information, how they replicate and pass to subsequent generations of cells or between organisms, and how the expression of their information within an organism determines its characteristics.

• The genetic information in a cell is called the genome.

  • A cell’s genome includes its chromosomes and plasmids.

• Chromosomes are structures containing DNA that physically carry hereditary information; the chromosomes contain the genes.

• Genes are segments of DNA (except in some viruses, in which they are made of RNA) that code for functional products.

• The base pairs 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.

• The genetic code, the set of rules that determines how a nucleotide sequence is converted into the amino acid sequence of a protein.

• When the ultimate molecule for which a gene codes (a protein, for example) has been produced, we say that the gene has been expressed.

• The flow of genetic information can be shown as flowing from DNA to RNA to proteins, as follows:

  • DNA —> RNA —> Protein

  • This theory was called the central dogma by Francis Crick in 1956, when he first proposed that the sequence of nucleotides in DNA determines the sequence of amino acids in a protein.

• The genotype of an organism is its genetic makeup—all its DNA—the information that codes for all the particular characteristics of the organism.

• Phenotype refers to actual, expressed properties, such as the organism’s ability to perform a particular chemical reaction.

• Noncoding regions called short tandem repeats (STRs) occur in most genomes, including that of E. coli.

  • STRs are repeating sequences of two- to five-base sequences. These are used in DNA fingerprinting.

• Transcription is the synthesis of a complementary strand of RNA from a DNA template.

• Ribosomal RNA (rRNA) forms an integral part of ribosomes, the cellular machinery for protein synthesis.

• Messenger RNA (mRNA) carries the coded information for making specific proteins from DNA to ribosomes, where proteins are synthesized.

• Transcription begins when RNA polymerase binds to the DNA at a site called the promoter.

• RNA synthesis continues until RNA polymerase reaches a site on the DNA called the terminator.

• Protein synthesis is called translation because it involves decoding the “language” of nucleic acids and converting it into the “language” of proteins.

• The site of translation is the ribosome, and transfer RNA (tRNA) molecules both recognize the specific codons and transport the required amino acids.

• Each tRNA molecule has an anticodon, a sequence of three bases that is complementary to a codon.

• Thus, eukaryotic genes are composed of exons, the regions of DNA expressed, and introns, the intervening regions of DNA that do not encode protein.

• Particles called small nuclear ribonucleoproteins, abbreviated snRNPs and pronounced “snurps,” remove the introns and splice the exons together.

8.2 The Regulation of Bacterial Gene Expression

• An operon is a group of genes that are transcribed together and controlled by one promoter.

• The operator, which is like a traffic light that acts as a go or stop signal for transcription of the structural genes.

• In repressible operons, the structural genes are transcribed until they are turned off.

• When excess tryptophan is present, the tryptophan acts as a corepressor binding to the repressor protein.

• Regulation of the lactose operon also depends on the level of glucose in the medium, which in turn controls the intracellular level of the small molecule cyclic AMP (cAMP), a substance derived from ATP that serves as a cellular alarm signal.

• A part of an mRNA molecule, called a riboswitch, that binds to a substrate can change the mRNA structure.

8.3 Changes in Genetic Material

• The survival of new genotypes is called natural selection.

• A mutation is a permanent change in the base sequence of DNA. Such a change will sometimes cause a change in the product encoded by that gene.

• The most common type of mutation involving single base pairs is base substitution (or point mutation), in which a single base at one point in the DNA sequence is replaced with a different base.

• A base substitution resulting in a nonsense codon is thus called a nonsense mutation.

• Besides base-pair mutations, there are also changes in DNA called frameshift mutations, in which one or a few nucleotide pairs are deleted or inserted in the DNA.

• These spontaneous mutations apparently occur in the absence of any mutation-causing agents.

• Agents in the environment, such as certain chemicals and radiation, that directly or indirectly bring about mutations are called mutagens.

• Photolyases, also known as light-repair enzymes, use visible light energy to separate the dimer back to the original two thymines.

• Nucleotide excision repair is not restricted to UV-induced damage; it can repair mutations from other causes as well.

• Positive (direct) selection involves the detection of mutant cells by rejection of the unmutated parent cells.

• Any mutant microorganism having a nutritional requirement that is absent in the parent is known as an auxotroph.

• Many known mutagens have been found to be carcinogens, substances that cause cancer in animals, including humans.

8.4 Genetic Transfer and Recombination

• Genetic recombination refers to the exchange of genes between two DNA molecules to form new combinations of genes on a chromosome.

• If a cell picks up foreign DNA (called donor DNA in the figure), some of it could insert into the cell’s chromosome—a process called crossing over—and some of the genes carried by the chromosomes are shuffled.

• Vertical gene transfer occurs when genes are passed from an organism to its offspring.

• In all of the mechanisms, the transfer involves a donor cell that gives a portion of its total DNA to a recipient cell.

• During the process of transformation, genes are transferred from one bacterium to another as “naked” DNA in solution.