Questions on prokaryotic cells and viruses

Question 1

Describe the lytic cycle of lambda (λ) phage in a bacterial cell.

Accepted Answer

B1 attachment proteins on the phage bind to specific receptors on the surface of the bacterium B1 the phage injects its DNA into the cell, leaving the capsid outside B1 the viral DNA is replicated and viral proteins are made using the host's ribosomes, enzymes, nucleotides and ATP B1 new phage particles are assembled from the capsid proteins and copies of the DNA B1 a viral enzyme breaks down the cell wall so the cell bursts, or lyses, releasing the new phages

Question 2

Compare the genetic material and the ribosomes of a bacterial cell with those of a eukaryotic cell.

Accepted Answer

B1 a bacterium has one circular DNA molecule, whereas a eukaryote has several linear ones B1 bacterial DNA is not wound round histone proteins, whereas eukaryotic DNA is B1 bacterial DNA lies free in the cytoplasm in the nucleoid, whereas eukaryotic DNA is enclosed by a nuclear envelope B1 bacterial ribosomes are 70S and smaller, whereas the ribosomes in eukaryotic cytoplasm are 80S

Question 3

A student writes that a virus is simply a very small cell. Explain why this statement is wrong, referring to the structure of a virus in your answer.

Accepted Answer

B1 a virus is acellular: it has no cytoplasm and no cell-surface membrane of its own B1 it has no ribosomes, so it cannot make any protein for itself B1 it has no enzyme system for respiration, so it has no metabolism and cannot release energy from food B1 it consists only of DNA or RNA inside a protein capsid with attachment proteins outside, sometimes within a lipid envelope taken from a host cell

Question 4

A student argues that viruses are living because they reproduce and they evolve. Evaluate this argument, referring to the structure of a virus.

Accepted Answer

B1 viruses do replicate, and their genomes do mutate and undergo natural selection, which supports the claim B1 replication happens only inside a host cell, using the host's ribosomes and enzymes, because a virus has no cytoplasm, no ribosomes and no metabolism of its own B1 outside a host cell a virus particle is metabolically inert and can even be crystallised, unlike any accepted living organism A1 judgement: because a virus cannot replicate or release energy independently of a host, most biologists classify it as non-living or as lying at the boundary of life, so the claim is not well supported by its structure

Question 5

A culture is started with 150 bacteria, which divide by binary fission at a steady rate. Three hours later, with none of them having died, the culture contains 9600 bacteria. Calculate how many divisions have taken place, and calculate the time between one division and the next, in minutes.

Accepted Answer

M1 uses N = N₀ × 2ⁿ, so 2ⁿ = N ÷ N₀ = 9600 ÷ 150 = 64 A1 64 = 2⁶, so n = 6 divisions M1 time between divisions = time elapsed ÷ number of divisions, with three hours as 180 minutes: 180 ÷ 6 A1 30 minutes between one division and the next

Question 6

Lambda phage can remain latent in a population of bacteria for many generations and then destroy the cells. Explain how.

Accepted Answer

B1 the phage DNA is inserted into the bacterial chromosome, as a prophage B1 a repressor protein keeps the genes of the lytic cycle switched off, so no new particles are made and the cell survives B1 the prophage is replicated with the chromosome at each binary fission, so every daughter cell inherits it B1 DNA damage, for example by ultraviolet light, removes the repressor; the prophage is cut out and enters the lytic cycle, so the cells burst

Question 7

A culture is started with 200 bacteria which divide by binary fission every 30 minutes. Calculate how many bacteria are present after 5 hours, assuming that none of them die.

Accepted Answer

M1 number of divisions = 300 minutes divided by 30, giving 10 divisions M1 uses N = N₀ × 2ⁿ, so the calculation is 200 × 2¹⁰ A1 an answer of 204 800 bacteria

Question 8

A drug is designed to bind to the glycoprotein attachment proteins on the surface of HIV. Suggest why this drug would prevent HIV entering helper T cells, and suggest why it would have no effect on the influenza virus.

Accepted Answer

B1 the attachment proteins of HIV normally bind to the CD4 receptor found on helper T cells B1 with the drug bound to them the attachment proteins can no longer bind that receptor, so the virus cannot get into the cell B1 influenza carries different attachment proteins, which the drug is not complementary to, so it does not bind them

Question 9

Explain how penicillin kills a growing bacterium, and explain why the drug is harmless to the patient's own cells.

Accepted Answer

B1 penicillin blocks the cross-linking of the murein, or peptidoglycan, cell wall B1 a growing bacterium therefore builds a wall it cannot hold together, so the cell bursts B1 human cells have no cell wall at all, so there is nothing in them for the drug to act on

Question 10

Describe the stages of binary fission, in order, from a single bacterial cell to two daughter cells.

Accepted Answer

B1 the circular DNA molecule replicates, and any plasmids replicate separately and in variable numbers B1 the cell grows and the two DNA circles move apart to opposite ends of the cell B1 the cytoplasm divides and new cell wall material is laid down between the two daughter cells

Question 11

Describe the structure of a bacterial flagellum, and describe how it moves the cell.

Accepted Answer

B1 a flagellum is a long protein filament, anchored in the cell membrane B1 it is rotated by a molecular motor embedded in the membrane B1 the rotation, like a propeller, drives the bacterium forward through its surroundings

Question 12

Explain why plasmids, rather than mutations arising in the main chromosome, are responsible for antibiotic resistance spreading rapidly through a bacterial population.

Accepted Answer

B1 a plasmid carrying a resistance gene can be copied and passed directly from one bacterium to another, not only to daughter cells produced by division B1 this horizontal transfer spreads the gene through a population far faster than waiting for the same mutation to arise independently by chance in many separate cells B1 plasmids replicate independently of the main chromosome, so a cell can carry, and pass on, several copies at once

Question 13

Bacteria have been observed since the 1670s, but a virus was not observed until the electron microscope was developed in the 1930s. Suggest why viruses were not seen so much earlier.

Accepted Answer

B1 a virus particle is typically between about 20 and 300 nm across B1 that is smaller than the resolving limit of a light microscope, about 200 nm B1 so a virus could not be distinguished from its surroundings until an instrument with a shorter wavelength, and therefore finer resolution, became available

Question 14

Compare the cell wall of a bacterium with the cell wall of a plant cell, referring to the material each is built from and one consequence of the difference.

Accepted Answer

B1 a bacterial wall is built from murein (peptidoglycan), whereas a plant cell wall is built from cellulose B1 because the two materials are chemically different, an antibiotic such as penicillin, which blocks murein cross-linking, has no effect on a plant cell wall B1 both walls nonetheless resist the pressure created when water enters the cell by osmosis, stopping the cell bursting

Question 15

Explain why antiviral drugs must work by inhibiting virus replication.

Accepted Answer

B1 viruses are not living cells, so they have no metabolism, ribosomes or cell wall of their own for a drug to act on B1 the only processes that can be blocked are the stages of replication inside the host cell, such as entry, copying of the genome, assembly or release B1 the drug should target a molecule coded for by the virus, such as reverse transcriptase or a viral protease, so that the host cell's own processes are not harmed

Question 16

Name the substance from which a bacterial cell wall is built, and name the structure that carries genes such as antibiotic resistance and can be passed between bacteria.

Accepted Answer

A1 murein, also accepted as peptidoglycan A1 a plasmid, which is a small circle of DNA

Question 17

Some bacteria secrete a capsule of slime outside the cell wall. State two ways in which the capsule benefits the bacterium.

Accepted Answer

B1 any one of: it retains water, it helps the cell stick to surfaces or to other bacteria, or it makes phagocytosis by a white blood cell harder B1 any second, different benefit from the same list

Question 18

State two features that distinguish a plasmid from a bacterium's main chromosome.

Accepted Answer

B1 any one of: a plasmid is much smaller and carries far fewer genes, its genes are useful rather than essential, it replicates independently of the main chromosome, or it can be passed between bacteria B1 any second, different feature from the same list

Question 19

Aciclovir stops herpes simplex virus copying its DNA. A patient treated with aciclovir for a cold sore recovers, but the cold sore returns months later in the same place. Suggest why.

Accepted Answer

B1 the virus remains latent in the patient's sensory neurones, where its DNA is not being copied B1 aciclovir acts only on replicating virus, so it cannot remove the latent DNA, which is later reactivated

Question 20

In a model of phage infection, one phage infects one bacterium, and each lytic cycle releases 100 phages, every one of which infects a new bacterium. Assuming no phage is lost, calculate the number of phages released at the end of the third cycle.

Accepted Answer

M1 the number is multiplied by 100 at each cycle, so 100 × 100 × 100, or 100³ A1 1 000 000 phages

Question 21

New influenza particles stay attached to the surface of the infected cell until the viral enzyme neuraminidase cuts them free. Oseltamivir inhibits neuraminidase. Suggest how oseltamivir slows the spread of influenza in the body.

Accepted Answer

B1 new virus particles are not released from the infected cell B1 so fewer particles reach and infect other cells; the drug blocks the release stage of replication