Human cytomegalovirus (HCMV) is a common virus affecting humans. In people with a fully functioning immune system, infection by HCMV usually causes no, or only mild, symptoms. Fig. 2.1A is a diagram of a section through HCMV. In Fig. 2.1B, only the outer part of HCMV is sectioned. [Figure 2.1A] [Figure 2.1B] [Figure 2.1] The viral DNA shown in Fig. 2.1 contains genes that code for proteins important in viral replication and viral structure, including viral DNA polymerase and proteins known as tegument proteins. Viruses can only replicate in host cells as they need to use processes and contents of the host cell. Complete viral particles that are released from the host cell are known as virions.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [1] S = protein ; A amino acid ; R protein coat ; I capsomere
2. [1] T = capsid ; A protein coat ; R capsomere
3. [1] 170 (nm)
4. [1] replication / synthesis / AW, of (viral) DNA ; I replication of genes ; A stated function of DNA polymerase I unwinding of DNA ; e.g. forms phosphodiester bonds ; joins / AW, (adjacent DNA) nucleotides ; R joins complementary nucleotides ; I joins sugar phosphate backbone
5. [1] presence of (viral), envelope / (outer) membrane / phospholipid bilayer
6. [1] ref. fluid / flexible
7. [1] ref. to preparation (for electron microscopy); e.g. distortion / squashed / artefact ; I ref. to use of the microscope e.g. electron beam affects
8. [1] AVP; e.g. suggestion of variation in tegument protein, number / arrangement / organisation / AW
9. [1] similarity: both have DNA (double stranded, naked / no histones) ; A DNA plus neutral feature e.g. not surrounded by a nuclear envelope ; I both have genes ; I components of DNA e.g. deoxyribose / nucleotides / phosphates ; R both have, single stranded / circular, DNA ; R both have, DNA free in the cytoplasm / DNA and RNA
10. [1] difference: bacterial DNA, circular / closed loop (v HCMV linear) ; I ref. to naked DNA ; R if comparison for HCMV is incorrect e.g. multilayered OR bacteria (can also) have, plasmid DNA / plasmids ; R if implied that bacterial DNA is only plasmid DNA OR bacterial DNA (also) has genes for, metabolism / AW ora OR bacterial DNA free in cytoplasm but HCMV DNA surrounded by, capsid / protein coat / T
11. [1] no, S phase / G2 phase / mitosis / cytokinesis ; A remains in (G1) interphase ; A synthesis phase for S phase ; I named mitotic stages ; look for CON e.g. 'no S-phase, so mitosis only has one chromatid'
12. [1] (semi-conservative) DNA, replication / synthesis, does not occur; A sister/identical, chromatids, do not form ; R if a phase is stated and it is not given as the S-phase
13. [1] failure to pass checkpoints / does not reach checkpoints; A ref. to no error checking occurs (in context of checkpoints) ; I proofreading / error checking, (by DNA polymerase) during DNA replication
14. [1] cell does not, grow / increase in size / increase cytoplasm ; (context of preparing for mitosis / cytokinesis) A grows to normal (non-dividing) size
15. [1] idea that the cell does not, duplicate / synthesise, organelles / extra materials (context is for, cytokinesis/new cells) ; A centrioles do not replicate ; A microtubules not synthesised ; R if stated as occurring in S phase (even if e.g. G2 given as well)
16. [1] ref. to time delay in immune response (compared to normal) ; e.g. takes a longer time for, immune system to respond ; slower (secondary) immune response ; immune response similar to primary response
17. [1] more time / easier / AW, for HMCV to infect cells / replicate / spread (so cause damage) ; AW ; R ref. to HCMV as, cells that reproduce / viruses that carry out mitosis
18. [1] few(er), immune system cells / named, (in circulation) ; I fewer white blood cells ; e.g. fewer, macrophages / neutrophils ; A phagocytes ; fewer (B- and T-) memory cells ; A no memory cells (no longer present from primary response) ; fewer, (B/T,-) lymphocytes / T-helper cells / T-killer (or cytotoxic) cells ; context for lymphocytes could be reduced number circulating at time of viral activation or owing to poor secondary response(compared to normal) ; fewer/no, plasma cells
19. [2] consequence of fewer immune system cells ;; e.g. less chance of/AW, encounter with / recognition of / activation by, (non-self / foreign) antigen / HMCV / virus / pathogen / infected cell ; (fewer memory cells) ref. reduced clonal expansion / AW (so fewer lymphocytes / plasma cells)