Microbial Genetics Structure and Function of the Genetic Material

MI1330 Chapter 8 Microbial Genetics Structure and Function of the Genetic Material: Genome – sum of all the DNA in the cell; includes chromosomes and plasmids Chromosome – DNA containing genes Genes – segments of DNA that codes for a protein Base pairs – A-T, C-G Genetic Code - codons code for amino acids DNA and Chromosomes – - Bacteria typically have a single circular chromosome (Fig. 8.1) - Looped and folded and attached to the plasma membrane Figure 8.1 A prokaryotic chromosome. uper iled-how it s in e cell Chromosome Chromosome Figure 8.2 The Flow of Genetic Information. Parent cell DNA Genetic information is used within a cell to produce the proteins needed for the cell to function. Transcription Translation Cell metabolizes and grows Genetic information can be transferred horizontally between cells of the same generation. Genetic information can be transferred vertically to the next generation of cells. V tr New combinations of genes Horizontal gene transfer Recombinant cell Offspring cells Figure 8.5 A summary of events at the DNA replication fork. REPLICATION Proteins stabilize the unwound parental DNA. The leading strand is synthesized continuously by DNA polymerase. 3' DNA polymerase 5' Enzymes unwind the parental double helix. Replication fork RNA primer Primase 5' DNA polymerase 3' Parental strand Okazaki fragment DNA ligase 3' DNA polymerase 5' The lagging strand is synthesized discontinuously. Primase, an RNA polymerase, synthesizes a short RNA primer, which is then extended by DNA polymerase. DNA polymerase digests RNA primer and replaces it with DNA. DNA ligase joins the discontinuous fragments of the lagging strand. Figure 8.6 Replication of bacterial DNA. Replication fork An E. coli chromosome in the process of replicating REPLICATION Origin of replication Parental strand Daughter strands Replication fork Replication fork Replication fork Termination of replication Bidirectional replication of a circular bacterial DNA molecule Dna replication happening bi-directionally Figure 8.7 The process of transcription. TRANSCRIPTION DNA RNA polymerase mRNA Protein RNA polymerase RNA nucleotides DNA RNA polymerase bound to DNA RNA Template strand of DNA Promoter (gene begins) RNA synthesis RNA polymerase RNA RNA polymerase binds to the promoter, and DNA unwinds at the beginning of a gene. RNA is synthesized by complementary base pairing of free nucleotides with the nucleotide bases on the template strand of DNA. The site of synthesis moves along DNA; DNA that has been transcribed rewinds. Terminator (gene ends) Transcription reaches the terminator. Complete RNA strand RNA and RNA polymerase are released, and the DNA helix re-forms. Figure 8.8 The genetic code. Second position Phe Tyr Degeneracy = more than one codon codes for the same amino acids Cys Ser Leu His Pro Arg Gln Ile Asn Ser Lys Arg Thr Asp Val Ala Gly Glu Third position First position Leu Figure 8.9 The process of translation (1 of 4). Ribosome P Site Ribosomal subunit tRNA Anticodon Ribosomal subunit Components needed to begin translation come together. mRNA Start codon Second codon mRNA On the assembled ribosome, a tRNA carrying the first amino acid is paired with the start codon on the mRNA. The place where this first tRNA sits is called the P site. A tRNA carrying the second amino acid approaches. Figure 8.9 The process of translation (2 of 4). Peptide bond forms A site E site mRNA The second codon of the mRNA pairs with a tRNA carrying the second amino acid at the A site. The first amino acid joins to the second by a peptide bond. This attaches the polypeptide to the tRNA in the P site. mRNA Ribosome moves along mRNA The ribosome moves along the mRNA until the second tRNA is in the P site. The next codon to be translated is brought into the A site. The first tRNA now occupies the E site. Figure 8.9 The process of translation (3 of 4). Growing polypeptide chain tRNA released mRNA mRNA The second amino acid joins to the third by another peptide bond, and the first tRNA is released from the E site. The ribosome continues to move along the mRNA, and new amino acids are added to the polypeptide. Figure 8.9 The process of translation (4 of 4). Polypeptide released mRNA New protein mRNA Stop codon When the ribosome reaches a stop codon, the polypeptide is released. Finally, the last tRNA is released, and the ribosome comes apart. The released polypeptide forms a new protein. Figure 8.10 Simultaneous transcription and translation in bacteria. TRANSLATION RNA polymerase DNA Me t DNA mRNA Protein RNA polymerase DNA Direction of transcription 5' Peptide Polyribosome Ribosome mRNA Direction of translation Me t Mutation : - A change in the genetic material. Fig. 8.16 - May cause altered proteins. - Mutations may be neutral, harmful . benefical , or - Responsible for antibiotic resistance and new infectious strains. Figure 8.17 Base substitutions. Parental DNA Replication During DNA replication, a thymine is incorporated opposite guanine by mistake. Daughter DNA Daughter DNA Daughter DNA If not corrected, in the next round of replication, adenine pairs with the new thymine, yielding an AT pair in place of the original GC pair. Replication Granddaughter DNA Transcription When mRNA is transcribed from the DNA containing this substitution, a codon is produced that, during translation, encodes a different amino acid: tyrosine instead of cysteine. mRNA Translation Amino acids Cysteine Tyrosine Cysteine Cysteine Genetic Transfer and Recombination: Genetic Recombination– - Bacteria can incorporate foreign DNA into their chromosome Horizontal Gene transfer – - Transferred between cells. - Bacteria acquire new DNA sequences that may be beneficial Transformation- Bacteria take up “naked” DNA in solution Fig. 8.28 Conjugation - Transfer of DNA between cells using sex pilus. - May involve chromosome Only some can do this Direct contact • Conjugation may also involve a plasmid called an F factor and are called F+ cells. (Fig. 8.30) • Hfr cells contain the F factor on the chromosome. F facto fertitlit first fac discove Transduction (Fig. 8.32) – - Viruses that infect bacteria (phage) carry DNA sequences. - The phage DNA may code for toxins. Conjugative plasmids - R factors - encode antibiotic resistance (Fig. 8.25). Pilus and conjugation proteins Origin of transfer Mercury resistance Sulfonamide resistance Streptomycin resistance erminant r-det Plasmids– - Circular self replicating DNA sequences containing genes. - Code for toxins, enzymes or antibiotic resistance. Origin of replication RTF Very small compared to a chromosome Chloramphenicol resistance Tetracycline resistance