The diagram shows three stages in the development of a human embryo after fertilisation. A fertility nurse uses the diagram when discussing why cells in a v...
Question 1 Report
The diagram shows three stages in the development of a human embryo after fertilisation. A fertility nurse uses the diagram when discussing why cells in a very early embryo can be checked before an embryo is placed in the uterus during IVF treatment. At stage A there is one fertilised cell. At stage B there is a small ball of cells. At stage C the cells have formed an embryo with some different structures. All of the cells originally came from one sperm and one egg cell.
(a) Describe two changes shown between stages A and B. [2]
(b) Explain how stage B can form from one fertilised cell. [3]
(c) Explain why cells at stage C are becoming different from each other. [4]
Answer Details
(a) From stage A to B, the number of cells increases and the single cell becomes a ball or cluster of cells. [2]
(b) The fertilised egg divides repeatedly by mitosis. Before each division, DNA is copied. The daughter cells therefore have the same genetic information and chromosome number as the original fertilised cell. [3]
(c) Cells at stage C contain the same genes and DNA, but different genes are switched on in different cells. This causes different proteins to be made. Different proteins give cells different structures and functions, so the cells become specialised and form different embryo structures. [4]
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