Chapter 6: Bacteria and Viruses

Question 1

Bacteria belong to a class of organisms known as

Answer 1

Prokaryotes

Question 2

Concepts of Bacteria (4)

Answer 2

1. DNA arranged in a long series on a chromosome
2. The genome of most bacteria is a single molecule of double-stranded DNA in the form of a closed circle
3. contain extra autonomous DNA elements called plasmids
4. reproduce asexually by cell growth and division

Question 3

bacteriophages/ phages

Answer 3

1. genetic material can be DNA or RNA, constituting a short “chromosome
   2.

Question 4

What arethe four ways bacterial DNA can be transformed?

Answer 4

1. conjugation with plasmid transfer
2. conjugation with partial genome transfer
3. transformation
4. transduction.

Question 5

Difference in DNA union from eukaryotes and bacteria

Answer 5

In bacteria, rarely are two complete chromosomes brought together; usually, the union is of one complete chromosome plus a fragment of another.

Question 6

conjugation

Answer 6

The contact and fusion of two different bacterial cells. After fusion, one cell, called a donor, sometimes transfers genomic DNA to the other cell.

Question 7

What is transferred during conjugation and what happens after entry? (3)

Answer 7

• transferred DNA may be part or (rarely) all of the bacterial genome
• In some cases, one or more plasmids, if present, are transferred. Some plasmids are capable of carrying genomic DNA into the recipient cell.

Any genomic frag- ment, transferred by whatever route, may recombine with the recipient’s chromosome after entry.

Question 8

transformation

Answer 8

When a bacterial cell take up a piece of DNA from the external environment and incorporate this DNA into its own chromosome

Question 9

transduction

Answer 9

When certain phages pick up a piece of DNA from one bacterial cell and inject it into another, where it can be incorporated into the chromosome

Question 10

horizontal transmission+ ex (2)

Answer 10

a type of gene transmission without the need for cell division

ex: DNA transfer by conjugation, transformation, or transduction

Question 11

vertical transmission

Answer 11

the passage of DNA down thorough the bacterial generations

Question 12

For bacteria, why is horizontal transmission a powerful method?

Answer 12

They can adapt rapidly to changing environmental conditions.

Question 13

Cell clones

Answer 13

Members of a colony that have a single genetic ancestor

Question 14

prototrophic

Answer 14

can grow and divide on minimal medium

Question 15

auxotrophic

Answer 15

cells that will not grow unless the medium contains one or more specific cellular building blocks such as adenine, threonine, or biotin.

Question 16

bio-

Answer 16

Requires biotin added as a supplement to minimal medium

Question 17

arg−

Answer 17

Requires arginine added as a supplement to minimal medium

Question 18

met−

Answer 18

Requires methionine added as a supplement to minimal medium

Question 19

lac−

Answer 19

Cannot utilize lactose as a carbon source

Question 20

gal−

Answer 20

Cannot utilize galactose as a carbon source

Question 21

str^r

Answer 21

Resistant to the antibiotic streptomycin

Question 22

str^s

Answer 22

Sensitive to the antibiotic streptomycin

Question 23

The transfer of genetic material in E. coli conjugation is not

Answer 23

reciprocal

Question 24

F+ in conjugation

Answer 24

Strains that carry F and can donate

Question 25

Fertility factor (F)

Answer 25

• a plasmid
• directs the synthesis of pili (sing., pilus), projections that initiate contact with a recipient and draw it closer.

Question 26

The DNA transferred from donor cell to
recipient in conjuagation is (2)

Answer 26

Usually the Fertility factor and is single stranded

Question 27

Explain how the fertility factor is transferred

Answer 27

The F plasmid DNA in the donor cell makes a single-stranded version of itself in a peculiar mechanism called rolling circle replication. The circular plasmid “rolls,” and as it turns, it reels out a newly synthesized, single-stranded “fishing line.” This single strand of DNA passes through a pore into the recipient cell, where the other DNA strand is synthesized, forming a double helix.

Question 28

Explain conjugation in Hfr

Answer 28

During conjugation, the F factor inserts itself in the chromosome and the Hfr bacterium would attempt to transfer its entire DNA to the F–bacterium. However, since the mating bridge is structurally fragile the genetic transfer is often incomplete. Hence, the recipient bacterium may only receive a portion of, and rarely the entire, bacterial genome of the donor Hfr bacterium. The chromosomal fragment can then engage in recombination with the recipient chromosome.

Question 29

Does an Hfr cell die after donating its chromosomal material to an F− cell?

Answer 29

The answer is no. Just like the F plasmid, the Hfr chromosome replicates and transfers a single strand to the F− cell during conjugation. The replication of the chromosome ensures a complete chromosome for the donor cell after mating.

Question 30

What happens after Hfr donates partial dna and its in the recipient cell now?

Answer 30

The transferred strand is converted into a double helix in the recipient cell, and donor genes may become incorporated in the recipient’s chromosome through crossovers, creating a recombinant cell. If there is no recombination, the transferred fragments of DNA are simply lost in the course of cell division.

Question 31

exconjugants

Answer 31

recipient with Hfr cell conjugation

Question 32

Explain interrupted-mating conjugation experiment

Answer 32

At specific times after mixing Hfr with F-, they removed samples, which were each put in a kitchen blender for a few seconds to separate the mating cell pairs. This procedure is called interrupted mating. Then it was plated on a medium containing streptomycin to kill the Hfr donor cells, which bore the sensitivity allele strs. The surviving strr cells then were tested for the presence of alleles from the donor Hfr genome.

Question 33

In the conjugating Hfr, single-stranded DNA transfer begins

Answer 33

point O which is known to be the site at which the F plasmid is inserted

Question 34

The farther a gene is from O, the_____ it is transferred to the F−. The transfer process will generally ____ before the _____ genes are transferred, and, as a result, these genes are included in ____ exconju- gants.

Answer 34

1. later
2. stop
3. farthermost
4. fewer

Question 35

How can we explain the second unusual property of Hfr crosses, that F− exconjugants are rarely converted into Hfr or F+?

Answer 35

the inserted F was transmitted as the last element of the lin- ear chromosome

Question 36

Terminus

Answer 36

last to enter

Question 37

endogenote + ex (2)

Answer 37

complete genome F- recipient cell

Question 38

exogenote +ex (2)

Answer 38

incomplete one, derived from the Hfr donor cell

Question 39

merozygote +stage in conjugation+ what it contains+ what the cell is (3)

Answer 39

The cell once donor Hfr dna come sin F- but no crossing over occurred. The cell at this stage has two copies of one segment of DNA: one copy is part of the endogenote and the other copy is part of the exogenote. Thus, at this stage, the cell is a partial diploid.

Question 40

In Hfr and F- crosses, there must be… and produces…

Answer 40

there must be an even number of crossovers. An even number of crossovers produces a circular, intact chromosome and a fragment.

Question 41

Talk abt this:

Answer 41

In a cross such as the one just described, one type of potential recombinants of genotype leu+ arg− met+ requires four crossovers instead of two (see Figure 6-17d). These recombinants are rarely recovered because their frequency is very low compared with that of the other types of recombinants.

Question 42

The transforming DNA is incorporated into the bac- terial chromosome by

Answer 42

a process analogous to the double- recombination events observed in Hfr×F− crosses.

Question 43

What happens during a phage infection?

Answer 43

A phage attaches to a bacterium and injects its genetic material into the bacterial cytoplasm. The phage genetic information then takes over the machinery of the bacterial cell by turning off the synthesis of bacterial components and redirecting the bacterial synthetic machinery to make phage components. Newly made phage heads are individually stuffed with replicates of the phage chromosome. Ultimately, many phage descendants are made and are released when the bacterial cell wall breaks open.

Question 44

How can we study inheritance in phages when they are so small that they are visible only under the electron microscope?

Answer 44

After lysis, the progeny phages infect neighboring bacteria. This cycle is repeated through progressive rounds of infection, and, as these cycles repeat, the number of lysed cells increases exponentially. Within 15 hours after one single phage particle infects a single bacterial cell, the effects are visible to the naked eye as a clear area, or plaque, in the opaque lawn of bacteria cov- ering the surface of a plate of solid medium (Figure 6-26). Such plaques can be large or small, fuzzy or sharp, and so forth, depending on the phage genotype. Thus, plaque morphology is a phage character that can be analyzed at the genetic level.

Question 45

Virulent phages

Answer 45

immediately lyse and kill the host

Question 46

Temperate phages+ what happens to their dna (2)

Answer 46

Temperate phages can remain within the host cell for a period without killing it. Their DNA either integrates into the host chromosome, to replicate with it, or replicates separately in the cytoplasm, as does a plasmid.

Question 47

Generalized transduction is caused by

Answer 47

Faulty head stuffing. When a bacterial cell harboring an inserted phage occasionally lyses, some of the phage progeny carry fragments of bacterial DNA, and these phages can transform the genotypes of recipient bacterial cells.That bacterial DNA fragment can then be incorporated into the recipient cell’s chromosome by recombination.

Question 48

the larger the co-transduction frequency, the____ together the genes are

Answer 48

closer

Question 49

Co-transduction frequency

Answer 49

measured as the percentage of cells that have inherited both genes, rather than the selected first gene by itself.

Question 50

Specialized transduction is carried only by

Answer 50

temperate bacteriophage which undergoes lysogenic cycle in donor cell

Question 51

In specialized transduction what genes are transferred to recipient?

Answer 51

only those restricted gene are situated on the side of integrated viral genome have a chance to enter into recipient cell.