Chapter 18 Reading Quiz

1. Order small to large eukaryotic cell, bacteria, and virus

2. Which viral reproductive cycle destroys the host cell?

3. What are the harmless derivates that stimulate the immune system to create antibodies?

4. An infectious protein is called a…?5. An entire stretch of DNA that is required for

enzyme production is known as an …..?

1. Discuss the contributions of Mayer.

Mayer demonstrated that the stunting disease of tobacco plants was contagious thought it was caused by an unusually small bacteria- later findings demonstrated that the disease could not be bacteria-caused, but must be a particle much smaller (and unlike) a bacterium- the infectious particle was finally crystallized and observed and is now known as the tobacco mosaic virus (TMV)

2. List and describe the structural components of viruses, and explain why viruses are obligate parasites.

It is a genome enclosed in a protective coat

It is organized as single nucleic acid molecules

May have 4 to several hundred genes

Simple composition 1. Capsid – protein coat that encloses the viral genome2. Envelope – membrane that cloaks some viral capsids (head, sheath, DNA, tail fibers)

Viruses express their genes and reproduce only within a living cell

3. Briefly describe what happens when a virus infects a host cell.

A viral infection begins when the genome of a virus makes its way into a host cell

4. Distinguish between lytic and lysogenic reproductive cycles using phage T4 and phage as examples.

Lytic Cycle Viral replication cycle that

results in the death (or lysis) of the host cell

T4 phage attaches to cell surface, phage contracts sheath and injects DNA, hydrolytic enzymes destroy the host cell’s DNA, phage genome directs the host cell to make phage components and cell lyses and releases phage particles

Lysogenic Cycle A viral replication cycle that

involves the incorporation of the viral genome into the host cell genome

λ phage binds to the surface of ecoli and injects the DNA and inserts it by genetic recombination

5. Using viruses with envelopes and RNA as examples, describe variations in replication cycles of animal viruses.

Enveloped Viruses are characterized by:

1. Attachment2. Entry3. Uncoating4. Viral RNA & protein

synthesis5. Assembly and

release

RNA as viral genetic material:

- RNA viruses can be complicated (like retroviruses)

- mRNA or the strand that corresponds to mRNA is the + strand and it has the nucleotide sequence that codes for proteins

- The – strand is a template for synthesis of the + strand

6. Describe what vaccines are and how they are manufactured.

Vaccine a “harmless” variant or derivative of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen

7. Explain the role of reverse transcriptase in retroviruses.

It is the enzyme that transcribes DNA from an RNA template

Viral genomic RNA (reverse transcriptase)

Viral DNA

8. Describe how viruses recognize host cells.

They recognize their host cell by a complementary fit between external viral proteins and specific cell surface receptor sites

9. Describe several defenses bacteria have against phage infection.

Bacterial mutations can change receptor sites to avoid recognition- this in turn prevents infection

Restriction nucleases in bacteria recognize and cut up foreign DNA- self-destruction is avoided because bacterial DNA is chemically altered

10. Explain how viruses may cause disease symptoms, and describe some medical weapons used to fight viral infections.

Viruses damage or kill cells (viral infection lysosome releases hydrolytic enzymes)

They can be toxic or cause infected cells to produce toxins

Cause varying degrees of cell damage Immune system reacts, causing fever, aches,

inflammation Vaccines – harmless variants or derivatives of

pathogenic microbes that mobilize the immune system

Antiviral drugs – fight after the disease

11. List some viruses that have been implicated in human cancers, and explain how tumor viruses transform cells.

Retrovirus – adult leukemia Herpes virus – Epstein-Barr (mono) Burkitt’s lymphoma Papovavirus – human warts; cervical cancer Hepatitis B virus – chronic hepatitis; liver cancer Tumor viruses transform cells by inserting viral

nucleic acids into host cell DNA- this insertion is permanent as the provirus never excises

- insertion for DNA tumor viruses is straightforward

12. Distinguish between horizontal and vertical routes of viral transmission in plants.

Horizontal route of viral infection in which an organism receives the virus from an external source

Vertical route in which an organism inherits a viral infection from its parent

13. List some characteristics that viruses share with living organisms, and explain why viruses do not fit our usual definition of life.

CAN: mutate and evolve- have a genome with the same genetic code as living organisms

CANNOT: reproduce independently– Need a host cell for reproduction

14. Provide evidence that viruses probably evolved from fragments of cellular nucleic acid.

Genetic material of different viral families is more similar to host genomes than to that of other viral families

Some viral genes are identical to cellular genes Viruses of eukaryotes are more similar in

genomic structure to their cellular hosts than to bacterial viruses

Viral genomes are similar to cellular genetic elements like plasmids and transposons

15. Describe the structure of a bacterial chromosome.

It is composed of one double stranded, circular molecule of DNA

Structurally simpler and has fewer associated proteins than a eukaryotic chromosome

Found in the nucleoid region of the cell

16. Describe the process of binary fission in bacteria, and explain why replication of the bacterial chromosome is considered to be semiconservative.

Binary fission is preceded by DNA replication, which begins at a single origin on the chromosome

It is an asexual process, producing clones 2 replication forks move bi-directionally

until they meet and replication is complete Bacteria can divide every 20 minutes

17. List and describe the three natural processes of genetic recombination in bacteria.

1. Transformation process of gene transfer during which a bacterial cell assimilates foreign DNA from the surroundings

2. Transduction gene transfer from one bacterium to another by a bacteriophage

3. Conjugation the direct transfer of genes between two cells that are temporarily joined

18. Distinguish between general transduction and specialized transduction.

Generalized transduction that occurs when random pieces of host cell DNA are packaged within a phage capsid during the lytic cycle of a phage

Specialized occurs when a prophage excises from the bacterial chromosome and carries with it only certain host genes adjacent to the excision site (AKA restricted transduction)

19. Explain how the F plasmid controls conjugation in bacteria.

The ability to form sex pili and to transfer DNA is conferred by genes in a plasmid called the F plasmid

20. Explain how bacterial conjugation differs from sexual reproduction in eukaryotic organisms.

Conjugation transfer of genes only ( no offspring)

Sexual reproduction donation of genetic material to an offspring

21. For donor and recipient bacterial cells, predict the consequences of conjugation between the following: 1) F+ and F- cell 2) Hfr and F- cell

1. The F factor (which is in the F+) replicates and one copy is tranferred to the F_

• Only some bacterial genes are donated• The recipient F- cell does not become F+ because only part of the F

factor transferred The recipient cell becomes a partial diploid

2. As the integrated F factor of the Hfr cell transfers to the F- cell, it pulls the bacterial chromosome behind its leading end the conjugation bridge usually breaks before the entire chromosome and tail end of the F factor can be transferred

As a result : Only some bacterial genes are donated Recombination occurs between the Hfr chromosomal fragment and the

F- cell = homologous strand exchange results in a recombinant F- cell

22. Define transposon, and describe two essential types of nucleotide sequences found in transposon DNA.

Transposons DNA sequences that can move from one chromosomal site to another

1. Insertion sequences the simplest, they contain only the genes necessary for the process of transposition (2 essential types)- nucleotide sequence coding for transposase (which catalyzes insertion of transposons into new chromosomal sites)- inverted repeats

23. Distinguish between an insertion sequence and a composite transposon.

Insertion the simplest transposons, they contain only the genes necessary for the process of transposition

Composite transposons which include additional genetic material besides that required for transposition; consist of one or more genes flanked by insertion sequences

24. Briefly describe two main strategies cells use to control metabolism.

1. Regulation of enzyme activity the catalytic activity of many enzymes increases or decreases in response to chemical cues

2. Regulation of gene expression enzyme concentrations may rise and fall in response to cellular metabolic changes that switch genes on or off

25. Explain why grouping genes into an operon can be advantageous.

Operon a regulated cluster of adjacent structural genes (gene that codes for a polypeptide) with related functions - common in bacteria and phages

26. Using the trp operon as an example, explain the concept of an operon and the function of the operator, repressor, and corepressor.

Mechanism for the control of gene expression Operator a DNA segment located within the

promoter or between the promoter and structural genes, which controls access of RNA polymerase to structural genes

Repressor specific protein that binds to an operator and blocks transcription of the operon

Corepressor a molecule, usually a metabolite, that binds to a repressor protein, causing the repressor to change into its active conformation

27. Distinguish between structural and regulatory genes.

Structural genes gene that codes for a polypeptide

Regulatory genes genes that code for repressor or regulators of other genes

28. Describe how the lac operon functions and explain the role of the inducer allolactose.

Operon that can be switched on or induced Lactose metabolism in Ecoli is programmed by the lac

operon – with 3 structural genes:1. Lac Z – codes for β galactosidase which hydrolyzes lactose2. Lac Y – codes for permease which transports lactose into the cell3. Lac A – codes for transacetylase which has no known role

Has a single promoter and operator Allolactose an isomer of lactose, acts as an inducer to turn

on the lac operon

29. Explain how repressible and inducible enzymes differ and how these differences reflect differences in the pathways they control.

Repressible Genes are switched on

until a specific metabolite activates the repressor

Generally function in anabolic pathways

Pathway end product switches off its own production by repressing enzyme synthesis

Inducible Their genes are switched

off until a specific metabolite inactivates the repressor

Function in catabolic pathways

Enzyme synthesis is switched on by the nutrient the path uses

30. Distinguish between positive and negative control, and give examples of each from the lac operon.

Negative Binding of active

repressor to an operator always turns off structural gene expression

Lac negative by repressor protein

Positive Occurs only if an

activator molecule interacts directly with the genome to turn on transcription

Lac positive by cAMP receptor protein (CRP)

31. Explain how cAMP is affected by glucose concentration.

When no glucose The cell accumulates cAMP, a nucleotide derived from ATP. - cAMP activates CRP so that it can bind to the lac promoter

When glucose concentration increases, glucose catabolism decreases the intracellular concentration of cAMP- thus, cAMP releases CRP

32. Describe how E. coli uses the negative and positive controls of the lac operon to economize on RNA and protein synthesis.

CRP is an activator of several different operons that program catabolic pathways

Glucose’s presence deactivates CRP, which slows synthesis of those enzymes a cell needs to use catabolites other than glucose

Ecoli prefers using glucose as its primary carbon and energy source and the enzymes for this are coded for by unregulated genes that are continuously transcribed the end