Draw the ultrastructure of a prokaryotic cell based on electron micrographs.

Outline what occurs in cells in the first division of meiosis.

Explain the role of cells in the defence against infectious disease.

a. cell wall;

b. plasma membrane; Clearly shown as a separate line under the cell wall or the inner line

c. cytoplasm AND 70S ribosomes; Do not allow (small) circles

d. nucleoid/naked DNA;

e. plasmid
OR
pili
OR
flagella/flagellum;

Structures correctly drawn and labelled.

Award [2 max] if any exclusively eukaryotic structures are shown.

Do not allow cilia as they are not found in prokaryotes.

a. halves the chromosome number/produces haploid cells;

b. at start of meiosis each chromosome consists of two sister chromatids attached by a centromere;

c. prophase (I): pairing of homologous chromosomes;

d. crossing over occurs;

e. chromosomes condense by supercoiling;

f. metaphase (I): pairs of homologous chromosomes/bivalents move to equator of spindle
OR
metaphase (I): orientation of pairs of homologous chromosomes (prior to separation) is random;

g. anaphase (I): centromeres do not divide
OR
anaphase (I): spindle fibre pulls chromosome/whole centromere with two sister chromatids to opposite poles;

h. telophase (I): arrival of centromere with sister chromatids at opposite poles;

Some of these can be awarded for correctly annotated diagrams.

No credit for events in meiosis II.

a. cells of skin provide a physical barrier/produce fatty acids/lactic acid/lysozyme which stops entry of microbes
OR
mucous membranes produce mucus to trap pathogens
OR
stomach cells produce hydrochloric acid which kills microbes;

b. platelets start the clotting process preventing access of pathogens;

c. (two types of) white blood cells fight infections in the body;

d. phagocytes ingest pathogens (by endocytosis/phagocytosis);

e. gives non-specific immunity to diseases / ingest any type of pathogen;

f. production of antibodies by lymphocytes/B cells;

g. in response to particular pathogens/antigens;

h. gives specific immunity;

i. lymphocyte/B cell makes only one type of antibody;

j. plasma cells produce large quantity of (one type of) antibody;

k. some lymphocytes act/remain as memory cells;

l. can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific antigen is re-encountered;

m. results in faster defence against second exposure to specific antigen/pathogen/disease;

In part (a) many candidates were penalised for drawing clear eukaryotic structures such as mitochondria. The general level of the drawings was better than in the past. However a prokaryote should not be represented as a square. Ribosomes must be shown as dots, not circles.

In part (b), those who had been well prepared easily gained the 5 marks, but there was a significant proportion who did not know the difference between mitosis and meiosis.

In part (c), defence against infection was well known. Poor candidates lost marks by using loose, non-biological expressions such as the ‘white blood cells eat the infection/disease’. However, most had a fair idea of the cellular response to infection.