1. PowerPoint to accompany
Foundations in Microbiology
Fifth Edition
Talaro
Chapter
9
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Microbial Genetics
Chapter 9 2

3. Genetics – the study of heredity
1. transmission of biological traits from
parent to offspring
2. expression & variation of those traits
3. structure & function of genetic material
4. how this material changes
3

4. Levels of genetic study
4

5. Levels of structure & function of the
genome
• genome – sum total of genetic material of an organism
(chromosomes + mitochondria/chloroplasts and/or
plasmids)
– genome of cells – DNA
– genome of viruses – DNA or RNA
• chromosome – length of DNA containing genes
• gene-fundamental unit of heredity responsible for
a given trait
– site on the chromosome that provides information for
a certain cell function
– segment of DNA that contains the necessary code to
make a protein or RNA molecule 5

6. Genomes vary in size
• smallest virus – 4-5 genes
• E. coli – single chromosome containing
4,288 genes; 1 mm; 1,000X longer than cell
• Human cell – 46 chromosomes containing
31,000 genes; 6 feet; 180,000X longer than
cell
6

7. 7

8. • Nucleic acids are made of nucleotides
similar to how proteins are made of amino
acids
• each nucleotide consists of 3 parts
– a 5 carbon sugar (deoxyribose or ribose)
– a phosphate group
– a nitrogenous base (adenine, thymine, cytosine,
guanine, and uracil)
8

9. DNA structure
• 2 strands twisted into a helix
• sugar -phosphate backbone
• nitrogenous bases form steps in ladder
– constancy of base pairing
– A binds to T with 2 hydrogen bonds
– G binds to C with 3 hydrogen bonds
• antiparallel strands 3’to 5’ and 5’to 3’
• each strand provides a template for the exact
copying of a new strand
• order of bases constitutes the DNA code 9

10. 10

11. Significance of DNA structure
1. Maintenance of code during reproduction.
Constancy of base pairing guarantees that
the code will be retained.
2. Providing variety. Order of bases
responsible for unique qualities of each
organism.
11

12. DNA replication is
semiconservative because each
chromosome ends up with one
new strand of DNA and one old
strand.

13. Semi-conservative replication of DNA
13

14. DNA replication
• Begins at an origin of replication
• Helicase unwinds and unzips the DNA double
helix
• An RNA primer is synthesized
• DNA polymerase III adds nucleotides in a 5’ to 3’
direction
• Leading strand – synthesized continuously in 5’
to 3’ direction
• Lagging strand – synthesized 5’ to 3’ in short
segments; overall direction is 3’ to 5’
14

15. Bacterial replicon
15

16. Flow of genetic information
16

17. • What are the products that genes encode?
– RNAs and proteins
• How are genes expressed?
– transcription and translation
17

18. Gene expression
• Transcription – DNA is used to synthesize
RNA
– RNA polymerase is the enzyme responsible
• Translation –making a protein using the
information provided by messenger RNA
– occurs on ribosomes
18

19. • Genotype - genes encoding all the potential
characteristics of an individual
• Phenotype -actual expressed genes of an
individual (its collection of proteins)
19

20. DNA-protein relationship
1. Each triplet of nucleotides (codon) specifies
a particular amino acid.
2. A protein’s primary structure determines its
shape & function.
3. Proteins determine phenotype. Living things
are what their proteins make them.
4. DNA is mainly a blueprint that tells the cell
which kinds of proteins to make and how to
make them.
20

21. DNA-protein relationship
21

22. 3 types of RNA
• messenger RNA (mRNA)
• transfer RNA (tRNA)
• ribosomal RNA (rRNA)
22

23. 23

24. DNA
Transcription
RNA polymerase
RNA
Translation
ribosomes
PROTEINS
24

25. Transcription
1. RNA polymerase binds to promoter region
upstream of the gene
2. RNA polymerase adds nucleotides
complementary to the template strand of a
segment of DNA in the 5’ to 3’ direction
3. Uracil is placed as adenine’s complement
4. At termination, RNA polymerase recognizes
signals and releases the transcript
• 100-1,200 bases long
25

26. Transcription
26

27. Translation
• Ribosomes assemble on the 5’ end of a
mRNA transcript
• Ribosome scans the mRNA until it reaches
the start codon, usually AUG
• A tRNA molecule with the complementary
anticodon and methionine amino acid enters
the P site of the ribosome & binds to the
mRNA
27

28. Translation
28

29. 29

30. Interpreting the DNA code
30

31. Translation elongation
• A second tRNA with the complementary
anticodon fills the A site
• A peptide bond is formed
• The first tRNA is released and the ribosome slides
down to the next codon.
• Another tRNA fills the A site & a peptide bond is
formed.
• This process continues until a stop codon is
encountered.
31

32. 32

33. Translation termination
• Termination codons – UAA, UAG, and
UGA – are codons for which there is no
corresponding tRNA.
• When this codon is reached, the ribosome
falls off and the last tRNA is removed from
the polypeptide.
33

34. Polyribosomal complex
34

35. Eucaryotic transcription &
translation differs from procaryotic
1. Do not occur simultaneously. Transcription
occurs in the nucleus and translation occurs in
the cytoplasm.
2. Eucaryotic start codon is AUG, but it does not
use formyl-methionine.
3. Eucaryotic mRNA encodes a single protein,
unlike bacterial mRNA which encodes many.
4. Eucaryotic DNA contains introns – intervening
sequences of noncoding DNA- which have to be
spliced out of the final mRNA transcript.
35

36. Split gene of eucaryotes
36

37. Multiplication of dsDNA viruses
37

38. Multiplication of +ssRNA
38

39. Regulation of protein synthesis &
metabolism

40. Operons
• a coordinated set of genes, all of which are
regulated as a single unit.
• 2 types
– inducible – operon is turned ON by substrate:
catabolic operons- enzymes needed to metabolize
a nutrient are produced when needed
– repressible – genes in a series are turned OFF by
the product synthesized; anabolic operon –
enzymes used to synthesize an amino acid stop
being produced when they are not needed
40

41. Lactose operon: inducible operon
Made of 3 segments:
• Regulator- gene that codes for repressor
• Control locus- composed of promoter and
operator
• Structural locus- made of 3 genes each coding
for an enzyme needed to catabolize lactose –
β-galactosidase – hydolyzes lactose
permease - brings lactose across cell membrane
β-galactosidase transacetylase – uncertain function
41

42. Lac operon
• Normally off
– In the absence of lactose the repressor binds
with the operator locus and blocks transcription
of downstream structural genes
• Lactose turns the operon on
– Binding of lactose to the repressor protein
changes its shape and causes it to fall off the
operator. RNA polymerase can bind to the
promoter. Structural genes are transcribed.
42

43. Lactose operon
43

44. Arginine operon: repressible
• Normally on and will be turned off when
nutrient is no longer needed.
• When excess arginine is present, it binds to
the repressor and changes it. Then the
repressor binds to the operator and blocks
arginine synthesis.
44

45. Repressible operon
45

46. Antibiotics that affect gene
expression
• Rifamycin – binds to RNA polymerase
• Actinomycin D - binds to DNA & halts mRNA
chain elongation
• Erythromycin & spectinomycin – interfere with
attachment of mRNA to ribosomes
• Chloramphenicol, linomycin & tetracycline-bind
to ribosome and block elongation
• Streptomycin – inhibits peptide initiation &
elongation
46

47. Mutations – changes in the DNA
• Point mutation – addition, deletion or
substitution of a few bases
• Missense mutation – causes change in a
single amino acid
• Nonsense mutation – changes a normal
codon into a stop codon
• Silent mutation – alters a base but does not
change the amino acid
47

48. Excision repair
48

49. Ames Test
49

50. Types of intermicrobial exchange
conjugation requires the attachment of two
related species & formation of a
bridge that can transport DNA
transformation transfer of naked DNA
transduction DNA transfer mediated by
bacterial virus
50

51. conjugation
51

52. transformation
52

53. Generalized transduction
53

54. Specialized transduction
54

55. Transposons –DNA segments that shift
from one part of the genome to another
55