Viruses And Bacteria

Viral and Bacterial Genetics

0.5 µm

Size Virus

• Eukaryotic cells – Bacterium Animal cell

~100 microns

• Prokaryotic cells – ~1 micron

• Viruses – ~0.25 microns

Animal cell nucleus 0.25 µm

14.1

Viral Structure

RNA Capsomere of capsid

Membranous envelope Membranous Head envelope Capsid Tail Head Capsid sheath RNA

DNA

RNA

Capsomere

DNA

DNA DNA

RNATail fiber Glycoprotein

Glycoprotein

Glycoprotein

Glycoprotein

18 × 250 mm

20 nm (a) Tobacco mosaic virus

70–90 nm (diameter)

80–200 nm (diameter)

50 nm

50 nm

(b) Adenoviruses

(c) Influenza viruses

80 × 225 nm

50 nm (d) Bacteriophage T4

Viral Reproduction • Viruses can not reproduce on their own • A host cell is needed to copy a virus • Host recognition – Protein interaction at the cell membrane

14.2

Lytic vs. Lysogenic Cycles • Lytic Cycle – Results in lysis or destruction of the host cell

• Lysogenic Cycle

14.3

– Viral genes are inserted into the host genome – The cell divides passing the viral genes to new cells – Eventually new viruses are made and released

Retroviruses (HIV)

• Contain RNA – RNA is transcribed to DNA by the enzyme reverse transcriptase – Reverse transcriptase is error prone and causes mutation – HIV uses a lysogenic cycle of reproduction initially 14.4

HIV HIV

1

The virus fuses with the cell’s plasma membrane.

White blood cell

2 Reverse transcriptase

synthesizes DNA strand. HOST CELL Reverse transcriptase

Viral RNA

3

Reverse transcriptase synthesizes a second DNA strand

RNA-DNA hybrid

0.25 µm HIV entering a cell

4

The new DNA is incorporated into the cell’s DNA.

DNA NUCLEUS Provirus Chromosomal DNA

RNA genome for the next viral generation

mRNA

5

Genes are transcribed and serve as mRNAs for translation into viral proteins. 6 The viral proteins

include capsid proteins and reverse transcriptase

14.4 New HIV leaving a cell

Capsids are assembled around viral genomes. 8

9

New viruses bud off from the host cell.

Vesicles transport the proteins from the ER to the cell’s plasma membrane. 7

Viroids and Prions • Viroids – Free RNA fragments that can insert themselves into a genome

• Prions – Misfolded proteins that cause neurologic disease

14.5

• Prokaryotic cells have a variety of shapes – The three most common of which are spheres (cocci), rods (bacilli), and spirals

Figure 27.2a–c 1 µm (a) Spherical (cocci)

2 µm (b) Rod-shaped (bacilli)

(c) Spiral

5 µm

Cell Surface Structures • One of the most important features of nearly all prokaryotic cells – Is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment

14.6

• Using a technique called the Gram stain – Scientists can classify many bacterial species into two groups based on cell wall composition, Gram-positive and Gramnegative Lipopolysaccharide Peptidoglycan layer

Cell wall

Cell wall

Outer membrane

Peptidoglycan layer Plasma membrane

Plasma membrane

Protein

Protein Grampositive bacteria

Gramnegative bacteria 20 µm

(a) Gram-positive. Gram-positive bacteria have a cell wall with a large amount of peptidoglycan that traps the violet dye in the cytoplasm. The alcohol rinse does not remove the violet dye, which masks the added red dye.

14.6

(b) Gram-negative. Gram-negative bacteria have less peptidoglycan, and it is located in a layer between the plasma membrane and an outer membrane. The violet dye is easily rinsed from the cytoplasm, and the cell appears pink or red after the red dye is added.

• The cell wall of many prokaryotes – Is covered by a capsule, a sticky layer of polysaccharide or protein 200 nm

Capsule Figure 27.4

• Some prokaryotes have fimbriae – Which allow them to stick to their substrate or other individuals in a colony

Fimbriae 200 nm Figure 27.5

Motility • Most motile bacteria propel themselves by flagella Flagellum Filament 50 nm Cell wall

Hook Basal apparatus

Plasma membrane

Bacterial DNA • Chromosomal DNA is circular and found condensed in an area of the cytoplasm

• Plasmid DNA is a small circular piece of DNA containing only a few genes

14.8

Bacteria Transfer DNA Easily • Transformation is the alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment.

• Transduction occurs when a phage carries bacterial genes from one host cell to another as a result of aberrations in the phage reproductive cycle.

• Conjugation transfers genetic material between two bacterial cells that are temporarily joined. 14.7

Bacterial Transfer Drawings

Regulation of metabolic pathways (a) Regulation of enzyme activity Precursor Feedback inhibition

Enzyme 1

Enzyme 2

Enzyme 3

(b) Regulation of enzyme production

Gene 1

Gene 2

Regulation of gene expression

Gene 3

– Enzyme 4

Gene 4 –

Enzyme 5

14.10

Tryptophan

Gene 5

Turning on the operon

14.9

The lac operon: regulated synthesis of enzymes

14.9