1. Microbiology: A Systems Approach
PowerPoint to accompany
Microbiology: A Systems Approach
Cowan/Talaro
Chapter 9
Microbial Genetics
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Chapter 9 Topics - Genetics - Flow of Genetics - Regulation - Mutation
- Recombination

3. Genetics
Genome
Chromosome
Gene
Protein
Genotype
Phenotype

4. The sum total of genetic material of a cell is referred to as the genome.
Fig. 9.2 The general location and forms of the genome

5. Chromosome Procaryotic Eucaryotic
Histonelike proteins condense DNA
Eucaryotic
Histone proteins condense DNA
Subdivided into basic informational packets called genes

6. Genes Three categories Genotype Phenotype
Structural – Code for proteins or peptides
Regulatory
Encode for RNA
Genotype
The actual gene types or genetic makeup
Phenotype
Expression of the genotypes ( measurable or observable)

7. Flow of Genetics Replication “Central Dogma”
DNA =>DNA
“Central Dogma”
DNA =>mRNA=>Protein
Transcription – DNA copied into RNA
Translation – mRNA used to build a protein or peptide chain

8. Representation of the flow of genetic information.
Fig. 9.9 Summary of the flow of genetic information in cell.

9. DNA
Structure
Replication

10. DNA is lengthy and occupies a small part of the cell by coiling up into a smaller package.
Fig. 9.3 An Escherichia coli cell disrupted to release its
DNA molecule.

11. Structure Nucleic Acid composed of nucleotides Nucleotide Phosphate
5 carbon sugar (Deoxyribose)
Nitrogenous base
Double stranded helix
Each strand has carbon-phosphate covalently linked to form the “backbone”
Nitrogenous bases stick out towards the middle
Right & left side are linked by H-bonds formed between the bases in the middle ( Complementary base pair bonding)
Every 10 nucleotides, the DNA twists – forms the helix
Antiparallel arrangement – strands have a 5’ end and a 3’ end. The two strands run in opposite directions.

12. Nitrogenous bases Purines Pyrimidines Adenine Guanine Thymine Cytosine
Uracil (only found in RNA – replaces thymine)

13. Purines and pyrimidines pair (A-T or G-C) and the sugars (backbone) are linked by a phosphate.
Fig. 9.4 Three views of DNA structure

14. DNA Structure

15. Replication Semiconservative Enzymes Leading strand Lagging strand
Okazaki fragments

16. Semiconservative Replication
New strands are synthesized in 5’ to 3’ direction
Each DNA strand acts as a template to recreate the opposite (complementary) strand.
When the two new DNA strands are formed, each has 1 original strand ( the parent or template strand) and one newly synthesized ( daughter) strand

17. Semiconservative replication of DNA synthesizes a new strand of DNA from a template strand.
Fig. 9.5 Simplified steps to show the semiconservative
replication of DNA

18. Enzymes Topoisomerases (Gyrase) – relieves supercoiling
Helicase – “unzips” – separates the two DNA strands
DNA polymerase III – Adds bases to the new DNA chain; proofreads for mistakes
DNA polymerase I – Removes the primer; closes gaps; repairs mismatches
Primase – makes a short chain of nucleic acids called a “primer”
Ligase – seals the Okazki fragments together; final binding of nicks in DNA during synthesis and repair

19. The function of important enzymes involved in DNA replication.
Table 9.1 Some enzymes involved in DNA replication

20. Leading strand
DNA polymerase can only add bases going in the 5’ to 3’ direction
RNA primer initiates the 5’ to 3’ synthesis of DNA in continuous manner
( the new DNA is made in one
continuous strand)

21. Lagging strand
On the lagging strand, the new area being exposed for copying is running in the opposite direction of the leading strand ( 3’ to 5’)
DNA pol can only add bases in the 5’ 3’ direction
Multiple primers are laid down
Short pieces of DNA called Okazaki fragments are synthesized
Okazaki fragments are ligated together to form one continuous strand (DNA ligase)

22. The steps associated with the DNA replication process.
Fig. 9.6 The bacterial replicon: a model for DNA
Synthesis

23. Refinements and Details of Replication

24. RNA
Transcription – the synthesis of RNA from a DNA template synthesize
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Ribozymes and spliceosomes (catalytic RNA)
Regulatory RNA
Codon – set of 3 RNA bases that code for an amino acid

25. mRNA Copy of a structural gene or genes of DNA
Can encode for multiple proteins on one message
Thymidine is replaced by uracil
Triplet Code -- the message is organized into codons (1 codon= three bases  codes for 1 amino acid)

26. Transcription
In a gene there are 2 strands of DNA – the coding strand which contains the information sequence to build the protein and the opposite, complementary template strand.
In transcription, only 1 strand of DNA is copied, not both. The coding strand has the information to build the protein but it is not copied directly.
Copying a nucleic acid occurs by reading the template strand and synthesizing a complementary strand. This new mRNA strand will have the same information as the coding strand of the DNA

27. Transcription
RNA polymerase catalyzes the reaction -- reads the template strand and makes a complementary strand of mRNA.
Synthesis occurs in 5’ to 3’ direction
You don’t copy the entire strand of DNA – only the sequence that is the gene. So you need to know where to start copying and where to end.
The promoter region is a short sequence of bases that occurs right before the start of the gene and signals to RNA pol where to start copying.
Termination sequences occur at the end of the gene and signal to RNA pol to stop copying.

28. 3 Stages of Transcription
Initiation - RNA pol binds to the promoter region on the template strand.
Promoter regions contain typical repeat sequences that are recognized as the start of a gene. The promoter region itself does not get copied.
Elongation – The RNA pol moves down the template strand of DNA in the 3’ 5’ direction, unwinding the DNA. It reads the strand and builds the complementary RNA strand in the 5’  3’ direction. The section behind the RNA pol that has already been copied winds back together.
Termination – When the termination sequences are reached, the RNA pol falls of the DNA and the mRNA transcript is released.

29. The synthesis of mRNA from DNA.
Fig The major events in mRNA synthesis

30. Transcription

31. Transcription

32. Transcription

33. tRNA Complimentary sequences form hairpin loops
Amino acid attachment site
Anticodon – complementary to the codon; makes the tRNA specific to carry only 1 amino acid.
Participates in translation (protein synthesis)

34. Important structural characteristics for tRNA and mRNA.
Fig Characteristics of transfer and message RNA

35. rRNA Consist of two subunits (70S)
A subunit is composed of rRNA and protein
Participates in translation

36. Ribosomes bind to the mRNA, enabling tRNAs to bind, followed by protein synthesis.
Fig. 9.9 Summary of the flow of genetics

37. Codons
Triplet code – A set of 3 mRNA bases that specifies a given amino acid
The genetic code is redundant. There are 64 codons but only 20 amino acids:
some amino acids - more than one codon can specify the same amino acid.
64 codons
61 code for amino acids
Start codon – AUG is the start codon and it also codes for methionine
3 Stop codons (Nonsense codons) – do not code for any amino acids but stop translation

38. The codons from mRNA specify a given amino acid.
Fig The Genetic code

39. Genetic Code Redundancy:
Certain amino acids are represented by multiple codons
Allows for the insertion of correct amino acids even when mistakes occur in the DNA sequence
Wobble:
Only the first two nucleotides are required to encode the correct amino acid
The third nucleotide does not change its sense
Permits some variation or mutation without altering the message
Universal
The genetic code is translated almost identically in all organisms

40. Representation of the codons and their corresponding amino acids.
Fig Interpreting the DNA code

41. Protein Translation Protein synthesis have the following participants
mRNA
tRNA with attached amino acid
Ribosome

42. Participants involved in the translation process.
Fig The “players” in translation

43. Translation
Ribosomes bind mRNA at the 5’ end, near the start codon (AUG)
tRNA anticodon with attached amino acid binds to the start codon
Ribosomes move to the next codon, allowing a new tRNA to bind and add another amino acid
Series of amino acids form peptide bonds
Stop codon terminates translation

44. The process of translation.
Fig The events in protein synthesis

45. For procaryotes, translation can occur at multiple sites on the mRNA while the message is still being transcribed.
Fig Speeding up the protein assembly line in bacteria

46. Transcription and translation in eucaryotes
Similar to procaryotes except
AUG encodes for a different form of methionine
Transcription and translation are not simultaneous
Pre-mRNA is modified
Introns – (non-coding sequences) removed
Exons – (coding sequences) spliced together
5’ Guanine cap added ( helps to place the ribosome on the 5’ end of the mRNA)
Poly-A tail added at 3’ end helps keep the mRNA from being degraded by enzymes in the cytoplasm

47. The processing of pre-mRNA into mRNA involves the removal of introns.
Fig The split gene of eucaryotes

48. Proteins are modified after translation
Posttranslational modifications:
Proteins begin to fold upon themselves to achieve their tertiary conformation even before the peptide chain is released
methionine may be clipped off
Cofactors may be added
Some join with other proteins to form a quaternary structure