Fig. 4.1 is drawn at a magnification of $\times 32000$. Calculate the real width $X\text{--}Y$ in Fig. 4.1 in nanometres (nm) and state your answer to the nearest $10\,\text{nm}$. Complete Fig. 4.2 to show the equation you will use for the calculation.
State the term for disease-causing organisms, such as the bacterium V. cholerae.
Outline one way in which an uninfected person may become infected by V. cholerae.
Choleragen is made after V. cholerae has penetrated (passed through) the mucus layer and attached to intestinal epithelial cells. It has two subunits: subunit A is made of one polypeptide; subunit B is made of five identical polypeptides; the polypeptide in subunit A is different from the polypeptides in subunit B. Two genes, ctxA and ctxB, are required to produce choleragen. Only one strand of the DNA that makes up gene ctxA is involved in making subunit A. Only one strand of the DNA that makes up gene ctxB is involved in making subunit B. Explain why only one strand of the DNA of each gene is involved in the production of the subunits.
Laboratory tests were carried out using two different monoclonal antibodies that had been designed and produced to act against the LPS of bacterial cultures of V. cholerae O1: mAb 2D6 acts against the O-polysaccharide; mAb ZAC-3 acts against the core polysaccharide and lipid A components. Explain why the mAb ZAC-3 produced against the core polysaccharide and lipid A components will not act against the O-polysaccharide of the LPS molecules.
The test results showed that both mAbs were effective at causing agglutination (clumping) of bacteria and at preventing their motility. This suggests that they could be useful for preventing cholera and for treating the disease. Discuss whether mAb 2D6 and mAb ZAC-3 may be useful for preventing cholera and for treating the disease.