1. What you need to Know Plus Gene Regulation
Viruses and Bacteria
What you need to Know
Plus
Gene Regulation

2. Phage and Bacteria

3. Virus
Bacteria
Animal Cell

4. Structure of Viruses Viruses are not cells
Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

5. Capsids and Envelopes
A capsid is the protein shell that encloses the viral genome
A capsid can have various structures

6. Some viruses have structures have membranous envelopes that help them infect hosts
These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals
Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules

8. General Features of Viral Reproductive Cycles
Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell
Each virus has a host range, a limited number of host cells that it can infect
Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses
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9. Reproductive Cycles of Phages
Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

10. The Lytic Cycle
The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell
The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses
A phage that reproduces only by the lytic cycle is called a virulent phage
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

11. LE 18-6
Attachment
Entry of phage DNA
and degradation of
host DNA
Phage assembly
Release
Head
Tails
Tail fibers
Assembly
Synthesis of viral
genomes and proteins

12. The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome
This integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
Phages that use both the lytic and lysogenic cycles are called temperate phages
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13. LE 18-7 Phage DNA The phage attaches to a
host cell and injects its DNA.
Daughter cell
with prophage
Many cell divisions
produce a large
population of
bacteria infected with
the prophage.
Phage DNA
circularizes
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced or
Lysogenic cycle
is entered
Prophage
New phage DNA and proteins are
synthesized and assembled into phages.
Phage DNA integrates into the
bacterial chromosomes, becoming a
prophage.

14. Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version

15. LE 18-13
Original
prion
Prion
Many prions
New
prion
Normal
protein

16. The Bacterial Genome and Its Replication
The bacterial chromosome is usually a circular DNA molecule with few associated proteins
Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome
Bacterial cells divide by binary fission, which is preceded by replication of the chromosome

17. LE 18-14 Replication fork Origin of replication Termination

18. Mutation and Genetic Recombination as Sources of Genetic Variation
Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity
More genetic diversity arises by recombination of DNA from two different bacterial cells

19. Mechanisms of Gene Transfer and Genetic Recombination in Bacteria
Three processes bring bacterial DNA from different individuals together:
Transformation-Transformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment (Griffith)
Transduction -In the process known as transduction, phages carry bacterial genes from one host cell to another
Conjugation -Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined (Pili)

20. Transposition of Genetic Elements
The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome
Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria

21. Transposons
Transposable elements called transposons are longer and more complex than insertion sequences
In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance

22. Insertion sequence Insertion sequence
LE 18-19b
Transposing
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence 5¢ 3¢ 3¢ 5¢
Inverted repeat
Transposase gene

23. Repressible and Inducible Operons: Two Types of Negative Gene Regulation
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription
The trp operon is a repressible operon
An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription
The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose

24. LE 18-22a
Regulatory
gene
Promoter
Operator
DNA
lacl
lacZ No
RNA
made 3¢
mRNA
RNA
polymerase 5¢
Active
repressor
Protein
Lactose absent, repressor active, operon off

25. Lactose present, repressor inactive, operon on
LE 18-22b
lac operon
DNA
lacl
lacZ
lacY
lacA
RNA
polymerase 3¢
mRNA
mRNA 5¢ 5¢
Permease
Transacetylase
Protein
-Galactosidase
Inactive
repressor
Allolactose
(inducer)
Lactose present, repressor inactive, operon on

26. Inducible enzymes usually function in catabolic pathways
Repressible enzymes usually function in anabolic pathways
Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor

27. Positive Gene Regulation
Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP)
When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP
When glucose levels increase, CAP detaches from the lac operon, turning it off

28. Lactose present, glucose scarce (cAMP level high): abundant lac
LE 18-23a
Promoter
DNA
lacl
lacZ
RNA
polymerase
can bind
and transcribe
CAP-binding site
Operator
Active
CAP
cAMP
Inactive lac
repressor
Inactive
CAP
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized

29. Lactose present, glucose present (cAMP level low): little lac
LE 18-23b
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA
polymerase
can’t bind
Inactive
CAP
Inactive lac
repressor
Lactose present, glucose present (cAMP level low): little lac
mRNA synthesized

30. LE 19-2a 2 nm DNA double helix Histone His- tails tones Histone H1
Linker DNA
(“string”)
Nucleosome
(“bead”)
Nucleosomes (10-nm fiber)

31. LE 19-2b
30 nm
Nucleosome
30-nm fiber

32. LE 19-2c Protein scaffold Loops 300 nm Scaffold
Looped domains (300-nm fiber)

33. Concept 19.2: Gene expression can be regulated at any stage, but the key step is transcription
All organisms must regulate which genes are expressed at any given time
A multicellular organism’s cells undergo cell differentiation, specialization in form and function

34. Differential Gene Expression
Differences between cell types result from differential gene expression, the expression of different genes by cells within the same genome
In each type of differentiated cell, a unique subset of genes is expressed
Many key stages of gene expression can be regulated in eukaryotic cells

35. Regulation of Chromatin Structure
Genes within highly packed heterochromatin are usually not expressed
Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression

36. Histone Modification
In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails
This process seems to loosen chromatin structure, thereby promoting the initiation of transcription

37. LE 19-4 Histone tails DNA double helix Amino acids available
for chemical
modification
Histone tails protrude outward from a nucleosome
Unacetylated histones
Acetylated histones
Acetylation of histone tails promotes loose chromatin
structure that permits transcription

38. DNA Methylation
DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species
In some species, DNA methylation causes long- term inactivation of genes in cellular differentiation
In genomic imprinting, methylation turns off either the maternal or paternal alleles of certain genes at the start of development