The five kingdoms, species concepts and viruses questions

Question 1

The genome of tobacco mosaic virus is a single strand of RNA 6400 nucleotides long, and its capsid is built from 2130 identical protein subunits of 158 amino acids each. Calculate the number of nucleotides needed to code for one subunit, calculate that number as a percentage of the genome, and state what the percentage shows about how so small a genome can build a coat of 2130 proteins.

Accepted Answer

M1 three nucleotides code for one amino acid, so the number needed = 158 × 3 A1 474 nucleotides, accepting 477 where a stop codon is counted as well M1 percentage of the genome = 474 ÷ 6400 × 100 A1 7.4 per cent, to two significant figures B1 the coat protein is coded once and translated over and over, so a few per cent of the genome supplies every subunit of the capsid, which is why a particle built from many copies of one protein needs so little nucleic acid to specify it

Question 2

Explain why viruses are not placed in any of the five kingdoms, and describe how they are classified instead.

Accepted Answer

B1 the kingdoms are defined by features of cells, and a virus is not a cell: it has no cytoplasm, no ribosomes and no organelles B1 a virus carries out no metabolism of its own and can reproduce only inside a living host cell, so it has no nutrition or respiration to classify B1 viruses are classified instead by their genome, first as DNA viruses or RNA viruses, since no virus contains both B1 and then by whether that nucleic acid is double-stranded or single-stranded, giving four groups, with a named example such as single-stranded RNA for influenza or human immunodeficiency virus

Question 3

Identify the kingdom each of the four organisms in the table belongs to, and give the feature that decides each one.

Accepted Answer

B1 W is in the Prokaryotae, decided by the absence of a nucleus, and confirmed by the peptidoglycan wall, which is found in no other kingdom B1 X is in the Protoctista: it has a nucleus, so it is not a prokaryote, and it is a single cell with no wall that ingests its food, so it is neither a fungus nor a plant nor an animal, and Protoctista is the kingdom left B1 Y is in the Protoctista as well, not the Plantae: it photosynthesises and has cellulose walls, but it has no roots, stems, leaves or vascular tissue, which is what keeps the seaweeds out of the plant kingdom B1 Z is in the Animalia, decided by the absence of a cell wall together with ingestion of food into a gut, and confirmed by nervous coordination, which the other four kingdoms lack

Question 4

Evaluate the biological species concept as a definition of a species.

Accepted Answer

B1 in support: it names a real biological boundary rather than a judgement about appearance, because a group whose members breed with one another to give fertile offspring shares a gene pool, and members of two such groups do not B1 in support: it has two halves that are both creditable, interbreeding to produce fertile offspring and reproductive isolation from other such groups, so it says why the boundary matters and not only where it lies B1 against: the breeding test cannot be run on anything that reproduces asexually, such as a bacterium, nor on a fossil or a museum specimen, and it gives an awkward answer wherever two named species produce fertile hybrids B1 a judgement: it is the definition to give first wherever sexual reproduction can be observed, but it cannot be the only one, which is why the morphological concept is used for fossils and the ecological concept for organisms that never interbreed at all

Question 5

Compare the biological species concept with the ecological species concept, referring to what each one uses to define a species and to which organisms each one can be applied.

Accepted Answer

B1 the biological species concept defines a species by whether its members interbreed to produce fertile offspring and are reproductively isolated from other such groups, whereas the ecological species concept defines a species by the niche its members occupy B1 the biological concept can only be applied where interbreeding can actually be observed or tested, whereas the ecological concept can be applied to organisms that reproduce asexually, since a niche can be described whether or not anything interbreeds B1 the biological concept fails on fossils and on species that produce fertile hybrids, whereas the ecological concept fails where niches overlap, or where one species occupies different niches at different life stages, such as an insect with a larval stage B1 in practice the biological concept is given first wherever sexual reproduction can be observed, and the ecological concept is brought in specifically for the cases, such as bacteria, that the biological concept cannot handle at all

Question 6

Describe the structure of a virus particle, and describe why a virus is described as an obligate intracellular parasite.

Accepted Answer

B1 a virus particle consists of a length of nucleic acid, either DNA or RNA, enclosed within a protein coat called a capsid B1 in some viruses the capsid is further enclosed in an envelope derived from the membrane of the host cell in which the particle was assembled B1 a virus has no cytoplasm, no ribosomes and no organelles of its own, and carries out no metabolism, so it cannot respire, grow or respond to anything by itself B1 it can reproduce only inside a living host cell, using that cell's own ribosomes, enzymes and nucleotides, which is why it is described as an obligate intracellular parasite

Question 7

Four viruses are described. Influenza carries single-stranded RNA. Herpes simplex virus carries double-stranded DNA. Rotavirus carries double-stranded RNA. Canine parvovirus carries single-stranded DNA. Identify which one virus belongs to each of the four genome-based groups used to classify viruses.

Accepted Answer

B1 double-stranded DNA: herpes simplex virus B1 single-stranded DNA: canine parvovirus B1 double-stranded RNA: rotavirus B1 single-stranded RNA: influenza

Question 8

A student says that anything green and not moving is a plant, and anything growing in the soil that is not green is a fungus. Discuss whether these two rules correctly classify organisms.

Accepted Answer

B1 what is right: most plants are green, due to chlorophyll in their chloroplasts, and do not move from place to place, and many fungi do grow in soil and are not green, so the rules describe a common pattern B1 against: a seaweed is green and does not move but is classified in Protoctista rather than Plantae, since it lacks roots, stems, leaves and vascular tissue, so 'green and stationary' is not sufficient on its own B1 against: many soil organisms that are not green are not fungi at all; bacteria and various Protoctista also live in soil without photosynthesising, so 'in soil and not green' does not identify a fungus specifically either B1 judgement: a kingdom is defined by a combination of features, cellular organisation, cell wall material and method of nutrition among them, not by a single visible trait, so a rule based on colour and movement alone will misclassify organisms that share the trait without sharing the kingdom

Question 9

State three features that would place an organism in the kingdom Fungi rather than in the kingdom Plantae.

Accepted Answer

B1 the cell wall contains chitin rather than cellulose B1 it does not photosynthesise and has no chloroplasts or chlorophyll, feeding saprotrophically or parasitically instead B1 either of: the body is made of hyphae forming a mycelium, often with many nuclei in shared cytoplasm; or carbohydrate is stored as glycogen rather than as starch

Question 10

Explain why the presence or absence of a nucleus is used as the first division made when organisms are placed into kingdoms.

Accepted Answer

B1 it separates the Prokaryotae, whose DNA lies free in the cytoplasm as a circular molecule, from the four eukaryotic kingdoms whose DNA is enclosed in a nuclear envelope B1 the difference is not a single feature but a whole cell organisation: prokaryotes also lack membrane-bound organelles and have smaller 70S ribosomes B1 it is therefore the deepest division in the living world and reflects the oldest split in ancestry, so it is the division to make before any other

Question 11

Two populations of a fossil shellfish are known only from their shells. Suggest which species concept a palaeontologist must use to decide whether they are one species or two, and suggest one problem with the decision.

Accepted Answer

B1 the morphological species concept, which groups organisms by shared observable structural features, because the biological concept cannot be used B1 no breeding test is possible on fossils, so whether the two populations could have produced fertile offspring can never be established B1 structure can mislead: members of one species can differ in appearance, as males and females and different growth stages do, while two separate species can look nearly identical

Question 12

Compare the way a fungus obtains its food with the way an animal obtains its food.

Accepted Answer

B1 both are heterotrophic, since neither can make organic molecules from inorganic ones B1 a fungus secretes enzymes onto material outside its body and absorbs the soluble products, which is saprotrophic or parasitic nutrition, whereas an animal takes food into the body and digests it internally B1 the fungus therefore never handles solid food, which suits a body made of narrow hyphae, whereas an animal needs a gut and, in nearly all cases, the ability to move to its food

Question 13

Explain why a seaweed is classified in the kingdom Protoctista rather than in the kingdom Plantae, even though it photosynthesises and has cellulose cell walls.

Accepted Answer

B1 Protoctista is a kingdom defined by exclusion: it holds every eukaryote that is not a fungus, a plant or an animal, rather than by a single positive feature shared by all its members B1 a seaweed photosynthesises and has cellulose cell walls, both features shared with Plantae, but it lacks roots, stems, leaves and the vascular tissue that goes with them, which the Plantae in this scheme are defined as having B1 because it fails the specific requirements of the Plantae despite the resemblance in nutrition and wall material, a seaweed is placed in the kingdom left over, Protoctista, along with organisms that share almost nothing else in common with it

Question 14

Several species of mosquito look almost identical but differ in whether they are able to carry malaria. Suggest why relying on the morphological species concept alone would be a poor way to distinguish them, and suggest one additional kind of evidence that could separate them.

Accepted Answer

B1 the morphological species concept groups organisms by their observable structure, but two genuinely separate species, called cryptic species, can look nearly identical to each other despite being reproductively isolated B1 mosquito species that differ in a property as important as whether they transmit malaria could therefore be wrongly treated as a single species if only their appearance is compared, with serious practical consequences for disease control B1 comparing DNA sequence between the populations, or testing directly whether they interbreed to produce fertile offspring, could separate species that look alike but are not exchanging alleles

Question 15

Explain why an antibiotic prescribed for a viral infection has no effect on the virus itself, and explain one consequence of prescribing it anyway.

Accepted Answer

B1 antibiotics act on structures and processes that are specific to bacteria, such as bacterial ribosomes, bacterial cell walls and bacterial enzymes B1 a virus possesses none of these; it has no ribosomes, no cell wall and no metabolism of its own for an antibiotic to act on, so the antibiotic has no target inside it B1 the antibiotic instead reaches the bacteria the patient happens to be carrying elsewhere in or on the body, and any of those that survive by chance, because they carry a resistance allele, are more likely to breed and spread it, so unnecessary prescribing selects for antibiotic resistance without treating the viral infection at all

Organism	Nucleus	Cell wall	How it feeds	Body
W	absent	peptidoglycan	oxidises ammonium ions as its energy source	one cell about 2 µm long, dividing by binary fission
X	present	none	engulfs bacteria and digests them in vacuoles	one cell about 200 µm across, moving by flowing
Y	present	cellulose	photosynthesises	many cells, up to 2 m long, held to rock by a holdfast, with no roots, stems, leaves or vascular tissue
Z	present	none	takes food into a gut and digests it there	many cells, differentiated into tissues, with nervous coordination