A-LevelBiologySelection and evolution9700/42 · May/June 2017 · Q414 Marks

Weeds reduce crop yields by competing with crop plants for space, light, water and minerals. The modes of action of three different types of herbicide are summarised in Table 4.1. Table 4.1 type of herbicide | mode of action | year of first widespread use --- | --- | --- photosystem II inhibitor | prevents photophosphorylation | 1960 ALS inhibitor | prevents synthesis of the amino acids isoleucine, leucine and valine | 1980 glyphosate | prevents synthesis of the amino acids phenylalanine, tryptophan and tyrosine | 1990 Fig. 4.1 shows the cumulative number of species of weeds that have become resistant to these three types of herbicide since 1960. [Figure 4.1]

(a)(i)Describe how the number of weed species resistant to herbicides has changed since 1960.[3]
(a)(ii)Explain how a weed species becomes resistant to a herbicide.[4]
(b)(i)ALS inhibitor herbicides work by binding to an enzyme present in chloroplasts called acetolactate synthetase (ALS). ALS is a globular protein consisting of four identical polypeptides each composed of 668 amino acids. The primary structure of the ALS polypeptide of each weed species resistant to ALS inhibitor herbicides has been sequenced. Amino acid substitutions at positions as far apart as position 122 and position 574 can result in resistance. The gene that codes for the ALS polypeptide does not contain any non-coding sections (introns). The first amino acid in the final polypeptide is methionine. State the number of base pairs in the gene that codes for an ALS polypeptide.[1]
(b)(ii)Explain why resistance to ALS inhibitor herbicide can result from substitutions of amino acids that are far apart in the primary sequence.[2]
(c)Genetic modification is one method used to develop herbicide resistance in crop plants. Other methods include: method 1: crossing a crop plant with a herbicide-resistant wild plant belonging to the same genus and then applying the herbicide method 2: causing mutations in the crop plants and then applying the herbicide. State two benefits of using method 1 and two benefits of using method 2 to develop herbicide resistance in crop plants.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[max 3] 1. no resistance to any herbicide at start of use ; 2. resistant to photosystem II inhibitors – increases, to 101–103 or from 1969 to 2013 ; 3. resistant to ALS inhibitors - increase to 153 - 155 or from 1981 to 2014; 4. resistant to glyphosate - increase to 32/33 or from 1993 - 1995 to 2014 ; 5. comparative point described ; e.g. ALS steepest gradient / ALS has highest number of species
[max 4] 1. random/ spontaneous, mutation; 2. herbicide is selection pressure; 3. mutant/resistant, individuals, survive / reproduce; ora 4. pass on, mutant / resistance, allele; ora 5. (mutant/resistance) allele increases in frequency (in population); ora 6. ref. to many generations ;
[1] (668 x 3) + 3 (stop codon) = 2007 bp or 668 x 3 = 2004 bp ;
[max 2] 1. after folding substituted amino acids are close together; 2. ref. to different bonding; 3. (substituted amino acids) causes change to protein, 3D / tertiary / quaternary / globular, structure ; 4. herbicide / inhibitor, unable to bind to, active / allosteric, site;
[max 4] method 1 benefits max 3 1. hybrid vigour / reduces inbreeding depression ; 2. increase in, genetic variation / gene pool / variety of alleles ; 3. increase in heterozygosity; ora 4. idea that low tech / easy to do / cheaper ; method 2 benefits 5. no need to find a suitable (wild) plant / can proceed even if no resistant (wild) plant exists; 6. will not introduce, unwanted alleles / poor characteristics, from (wild) plant ; 7. no chance of disease transfer;

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.