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Viruses and Bacteria. What you need to Know Plus Gene Regulation. Phage and Bacteria. Virus. Bacteria. Animal Cell. Structure of Viruses. Viruses are not cells

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viruses and bacteria

Viruses and Bacteria

What you need to Know

Plus

Gene Regulation

slide3

Virus

Bacteria

Animal Cell

structure of viruses
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
capsids and envelopes
Capsids and Envelopes
  • A capsid is the protein shell that encloses the viral genome
  • A capsid can have various structures
slide6
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
general features of viral reproductive cycles
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
  • go to video
reproductive cycles of phages
Reproductive Cycles of Phages
  • Phages are the best understood of all viruses
  • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
the lytic cycle
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
le 18 6

Attachment

LE 18-6

Entryof phage DNA

and degradation of

host DNA

Phage assembly

Release

Head

Tail fibers

Tails

Synthesis of viral

genomesand proteins

Assembly

the lysogenic cycle
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
  • Go to video
le 18 7

Phage

DNA

The phage attaches to a

host cell and injects its DNA.

Daughter cell

with prophage

LE 18-7

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

The bacterium reproduces

normally, copying the prophage

and transmitting it to daughter cells.

Certain factors

determine whether

The cell lyses, releasing phages.

Lytic cycle

is induced

Lysogenic cycle

is entered

or

Prophage

Phage DNA integrates into the

bacterial chromosomes, becoming a

prophage.

New phage DNA and proteins are

synthesized and assembled into phages.

viroids and prions the simplest infectious agents
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
le 18 13

LE 18-13

Original

prion

Prion

Manyprions

New

prion

Normal

protein

the bacterial genome and its replication
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
le 18 14

Replication fork

Origin of

replication

LE 18-14

Termination

of replication

mutation and genetic recombination as sources of genetic variation
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
mechanisms of gene transfer and genetic recombination in bacteria
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)
transposition of genetic elements
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
transposons
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
le 18 19b

LE 18-19b

Transposing

Insertion

sequence

Insertion

sequence

Antibiotic

resistance gene

Transposase gene

Inverted repeat

repressible and inducible operons two types of negative gene regulation
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
le 18 22a

Promoter

Regulatory

gene

Operator

LE 18-22a

lacl

lacZ

DNA

No

RNA

made

mRNA

RNA

polymerase

Active

repressor

Protein

Lactose absent, repressor active, operonoff

le 18 22b

LE 18-22b

lac operon

lacA

DNA

lacl

lacY

lacZ

RNA

polymerase

mRNA

mRNA 5¢

Transacetylase

Permease

-Galactosidase

Protein

Inactive

repressor

Allolactose

(inducer)

Lactose present, repressor inactive, operon on

slide26
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
positive gene regulation
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
le 18 23a

Promoter

LE 18-23a

DNA

lacl

lacZ

RNA

polymerase

can bind

and transcribe

Operator

CAP-binding site

Active

CAP

cAMP

Inactive lac

repressor

Inactive

CAP

Lactose present, glucose scarce (cAMP level high): abundant lac

mRNA synthesized

le 18 23b

Promoter

LE 18-23b

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

le 19 2a

LE 19-2a

2 nm

DNA double helix

Histone

tails

His-

tones

Histone H1

10 nm

Nucleosome

(“bead”)

Linker DNA

(“string”)

Nucleosomes (10-nm fiber)

le 19 2b

LE 19-2b

30 nm

Nucleosome

30-nm fiber

le 19 2c

LE 19-2c

Protein scaffold

Loops

300 nm

Scaffold

Looped domains (300-nm fiber)

concept 19 2 gene expression can be regulated at any stage but the key step is transcription
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
differential gene expression
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
regulation of chromatin structure
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
histone modification
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
le 19 4

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

dna methylation
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