| Date | May 2012 | Marks available | 2 | Reference code | 12M.3.SL.TZ2.17 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 17 | Adapted from | N/A |
Question
Explain the reasons for the reclassification of Prokaryotes and Eukaryotes into Eubacteria, Archaea and Eukaryota.
Markscheme
studies of (base sequences of) rRNA provided evidence for three domain classification;
differences in cell walls / Archaea and Eukaryotes have no peptidoglycan in cell wall, Eubacteria do have peptidoglycan in cell wall;
differences in membrane bonding compared to Eubacteria and Eukaryotes / Archaea have ether bonds in lipid membranes whereas others do not;
presence or absence of histone proteins / histone proteins present in all Eukaryotes, present in some Archaea, none in Eubacteria;
Examiners report
The answers to this question were very vague. Most candidates only mentioned that studies of rRNA provided evidence for a new classification.
