Explain why a diploid nucleus is used, rather than a nucleus from a gamete, when cloning a mammal by somatic cell nuclear transfer.

Pearson Edexcel International GCSE Biology (4BI1) — 5.4 Cloning · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Scientists cloned a sheep named Dolly by removing the nucleus from an unfertilised egg cell and replacing it with a nucleus taken from an udder cell of an adult sheep. The resulting clone was genetically identical to the adult sheep that donated the nucleus.

Model answer (4 marks)

A diploid nucleus contains two sets of chromosomes, which is required so that the clone has the full, correct number of chromosomes in every body cell. A gamete nucleus is haploid and contains only one set of chromosomes; using a haploid nucleus would give the embryo a reduced chromosome number, preventing normal development and making it genetically different from the donor.

Examiner tips

State that a diploid nucleus provides two chromosome sets
Explain that a haploid nucleus would give a reduced chromosome number and incorrect genetics
Link the chromosome number to normal development

Common mistakes

Confusing diploid with haploid, or vice versa
Failing to mention that the clone must have the same chromosome number as the donor
Not explaining the consequence of using a haploid nucleus

Mark scheme (4 marks)

A diploid nucleus contains two sets of chromosomes
This ensures the clone has the full / correct number of chromosomes (in every body cell)
A gamete nucleus is haploid and contains only one set of chromosomes
Using a haploid nucleus would not produce an organism genetically identical to the donor / the clone would not develop correctly

Key terms in this question

diploid nucleus · cloning