I recently worked through the NSAA 2023 paper (the admissions test for those who wish to study Natural Sciences at the University of Cambridge) with my students and encountered the following question:
NSAA 2023 Section 1 Q80
Four blood cells were taken from a human:
- one that was about to start cytokinesis
- one actively producing antibodies
- two that cannot use oxygen for respiration
What is the total number of chromosomes present in all of the cells mentioned above?
(Ignore mitochondrial DNA.)
A 92
B 115
C 138
D 161
E 184
F 207
My Workings
Four blood cells were taken from a human:
- one that was about to start cytokinesis
[a diploid haematopoietic stem cells, with 2 copies of the 46 chromosomes] - one actively producing antibodies
[a diploid B-lymphocyte, with 1 copy of the 46 chromosomes] - two that cannot use oxygen for respiration
[two diploid anucleate red blood cells, each with 0 copies of the 46 chromosomes]
What is the total number of chromosomes present in all of the cells mentioned above?
(Ignore mitochondrial DNA.)
[1×(2×46)+1×(1×46)+2×(0×46) = 138 chromosomes]
A 92
B 115
C 138 ⟵ My Answer
D 161 ⟵ Answer Key
E 184
F 207
It turns out my answer does not match what is given in the answer key, but I cannot yet find a mistake in my workings. Since this is an important exam and the paper, as well as the answer key, probably went through multiple revisions before it became publicly available, I believe mistakes from the examiners' side are quite unlikely.
The fact that the given answer 161 (= 138+23) is an odd number suggests that one of the four cells could be haploid. However, this cannot be the case, for these are all somatic cells and not gametes.
Therefore, I wonder if I have missed anything here.
