Chapter 8

Chapter 8

Structure and Function of the Genetic Material

·       8-1 Define genetics, genome, chromosome, gene, genetic code,

·       genotype, phenotype, and genomics.

·       8-2 Describe how DNA serves as genetic information.

·       8-3 Describe the process of DNA replication.

·       8-4 Describe protein synthesis, including transcription, RNA

·       processing, and translation.

·       8-5 Compare protein synthesis in prokaryotes and eukaryotes.

-          Genetics is the study of what genes are, how they carry information, how their information is expressed, and how they are replicated and passed to subsequent generations or other organisms.

-          DNA in cells exists as a double-stranded helix; the two strands are held together by hydrogen bonds between specific nitrogenous base pairs: AT and CG.

-          A gene is a sequence of nucleotides, that encodes a functional product, usually a protein.

-          The DNA in a cell is duplicated before the cell divides, so each offspring cell receives the same genetic information.

Genotype and Phenotype

-          Genotype is the genetic composition of an organism, its entire complement of DNA.

-          Phenotype is the expression of the genes: the proteins of the cell and the properties they confer on the organism.

DNA and Chromosomes

-          The DNA in a chromosome exists as one long double helix associated with various proteins that regulate genetic activity.

-          Genomics is the molecular characterization of genomes.

The Flow of Genetic Information

-          Following cell division, each offspring cell receives a chromosome that is virtually identical to the parent’s.

-          Information contained in the DNA is transcribed into RNA and translated into proteins.

DNA Replication

-          During DNA replication, the two strands of the double helix separate at the replication fork, and each strand is used as a template by DNA polymerases to synthesize two new strands of DNA according to the rules of complementary base pairing.

-          The result of DNA replication is two new strands of DNA, each having a base sequence complementary to one of the original strands.

-          Because each double-stranded DNA molecule contains one original and one new strand, the replication process is called semiconservative.

-          DNA is synthesized in one direction designated 5’ S 3’. At the replication fork, the leading strand is synthesized continuously and the lagging strand discontinuously.

-          DNA polymerase proofreads new molecules of DNA and removes mismatched bases before continuing DNA synthesis.

RNA and Protein Synthesis

-          During transcription, the enzyme RNA polymerase synthesizes a strand of RNA from one strand of double-stranded DNA, which serves as a template.

-          RNA is synthesized from nucleotides containing the bases A, C, G, and U, which pair with the bases of the DNA strand being transcribed.

-          RNA polymerase binds the promoter; transcription begins at AUG; the region of DNA that is the end point of transcription is the terminator; RNA is synthesized in the 5’ S 3’ direction.

-          Translation is the process in which the information in the nucleotide base sequence of mRNA is used to dictate the amino acid sequence of a protein.

-          The mRNA associates with ribosomes, which consist of rRNA and protein. 21. Three-base codons of mRNA specify amino acids.

-          The genetic code refers to the relationship among the nucleotide base sequence of DNA, the corresponding codons of mRNA, and the amino acids for which the codons code.

-          Specific amino acids are attached to molecules of tRNA. Another portion of the tRNA has a base triplet called an anticodon.

-          The base pairing of codon and anticodon at the ribosome results in specific amino acids being brought to the site of protein synthesis.

-          The ribosome moves along the mRNA strand as amino acids are joined to form a growing polypeptide; mRNA is read in the 5’ S 3’ direction.

-          Translation ends when the ribosome reaches a stop codon on the mRNA.

The Regulation of Bacterial Gene Expression

·       8-6 Define operon.

·       8-7 Explain pre-transcriptional regulation of gene expression in bacteria.

·       8-8 Explain post-transcriptional regulation of gene expression.

-          Regulating protein synthesis at the gene level is energy-efficient because proteins are synthesized only as they are needed.

-          Constitutive genes are expressed at a fixed rate. Examples are genes for the enzymes in glycolysis.

Pre-transcriptional Control

-          In bacteria, a group of coordinately regulated structural genes with related metabolic functions, plus the promoter and operator sites that control their transcription, is called an operon.

-          In the operon model for an inducible system, a regulatory gene codes for the repressor protein.

-          When the inducer is absent, the repressor binds to the operator, and no mRNA is synthesized.

-          When the inducer is present, it binds to the repressor so that it cannot bind to the operator; thus, mRNA is made, and enzyme synthesis is induced.

-          In repressible systems, the repressor requires a corepressor in order to bind to the operator site; thus, the corepressor controls enzyme synthesis.

-          Transcription of structural genes for catabolic enzymes (such as b-galactosidase) is induced by the absence of glucose. Cyclic AMP and CRP must bind to a promoter in the presence of an alternative carbohydrate.

-          Methylated nucleotides are not transcribed in epigenetic control.

Post-transcriptional Control

-          mRNA as a riboswitch regulates translation.

-          MicroRNAs combine with mRNA; the resulting double-stranded RNA is destroyed.

Changes in Genetic Material

·       8-9 Classify mutations by type.

·       8-10 Describe two ways mutations can be repaired.

·       8-11 Describe the effect of mutagens on the mutation rate.

·       8-12 Outline the methods of direct and indirect selection of mutants.

·       8-13 Identify the purpose of and outline the procedure for the Ames test.

-          Mutations and horizontal gene transfer can change a bacterium’s genotype.

Mutation

-          A mutation is a change in the nitrogenous base sequence of DNA; that change causes a change in the product coded for by the mutated gene.

-          Many mutations are neutral, some are disadvantageous, and others are beneficial.

Types of Mutations

-          A base substitution occurs when one base pair in DNA is replaced with a different base pair.

-          Alterations in DNA can result in missense mutations, frameshift, or nonsense mutations.

-          Spontaneous mutations occur without the presence of any mutagen.

Mutagens

-          Mutagens are agents in the environment that cause permanent changes in DNA.

-          Ionizing radiation causes the formation of ions and free radicals that react with DNA; base substitutions or breakage of the sugarphosphate backbone results.

-          Ultraviolet (UV) radiation is nonionizing; it causes bonding between adjacent thymines.

The Frequency of Mutation

-          Mutation rate is the probability that a gene will mutate when a cell divides; the rate is expressed as 10 to a negative power.

-          A low rate of spontaneous mutations is beneficial in providing the genetic diversity needed for evolution.

Identifying Mutants

-          Mutants can be detected by selecting or testing for an altered phenotype.

-          Positive selection involves the selection of mutant cells and the rejection of nonmutated cells.

-          Replica plating is used for negative selection—to detect, for example, auxotrophs that have nutritional requirements not possessed by the parent (nonmutated) cell.

Identifying Chemical Carcinogens

-          The Ames test is a relatively inexpensive and rapid test for identifying possible chemical carcinogens.

-          The test assumes that a mutant cell can revert to a normal cell in the presence of a mutagen and that many mutagens are carcinogens.

Genetic Transfer and Recombination.

8-14 Describe the functions of plasmids and transposons.

8-15 Differentiate horizontal and vertical gene transfer.

8-16 Compare the mechanisms of genetic recombination in bacteria.

-          Genetic recombination, the rearrangement of genes from separate groups of genes, usually involves DNA from different organisms; it contributes to genetic diversity.

-          In crossing over, genes from two chromosomes are recombined into one chromosome containing some genes from each original chromosome.

-          Vertical gene transfer occurs during reproduction when genes are passed from an organism to its offspring.

-          Horizontal gene transfer in bacteria involves a portion of the cell’s DNA being transferred from donor to recipient.

-          When some of the donor’s DNA has been integrated into the recipient’s DNA, the resultant cell is called a recombinant.

      Plasmids and Transposons

-          Plasmids are self-replicating circular molecules of DNA carrying genes that are not usually essential for the cell’s survival.

-          There are several types of plasmids, including conjugative plasmids, dissimilation plasmids, plasmids carrying genes for toxins or bacteriocins, and resistance factors.

-          Transposons are small segments of DNA that can move from one region to another region of the same chromosome or to a different chromosome or a plasmid.

-          Complex transposons can carry any type of gene, including antibiotic-resistance genes, and are thus a natural mechanism for moving genes from one chromosome to another.

Transformation in Bacteria

-          During this process, genes are transferred from one bacterium to another as “naked” DNA in solution.

Conjugation in Bacteria

-          This process requires contact between living cells.

-          One type of genetic donor cell is an F+; recipient cells are F-. F cells contain plasmids called F factors; these are transferred to the F cells during conjugation.

Transduction in Bacteria

-          In this process, DNA is passed from one bacterium to another in a bacteriophage and is then incorporated into the recipient’s DNA.

-          In generalized transduction, any bacterial genes can be transferred.

Genes and Evolution

8-17 Discuss how genetic mutation and recombination provide material for natural selection to act upon.

-          Diversity is the precondition for evolution.

-          Genetic mutation and recombination provide diversity of organisms, and the process of natural selection allows the growth of those best adapted to a given environment.