A fertilized egg produces many cells by mitosis, after which the cells become different through differentiation. Different cells express different genes, producing the proteins that give each cell its specialized structure and function.
The Mechanism
The fertilized egg, or zygote, first undergoes repeated mitotic divisions. Mitosis generally produces daughter cells containing the same genetic information, so nearly all developing cells contain the same genome.
The cells nevertheless experience different conditions. For example, signalling chemicals can form concentration gradients across an early embryo. Cells in different positions receive different signals, which alter their patterns of gene expression, the use of genetic information to produce functional products, usually proteins.
Some genes are transcribed and translated in one cell but not another. The proteins produced may act as enzymes, receptors, channels or structural components. These proteins change the cell’s metabolism, shape and behaviour, causing it to develop a specialized function.
For example, a developing muscle cell expresses genes for contractile proteins, whereas a neuron expresses genes needed for electrical signalling. The cells have essentially the same DNA but different active genes and therefore different proteomes.
During development, cells also lose developmental potential:
Cell potency | Developmental potential |
|---|---|
Totipotent | Can differentiate into all cell types required to form an organism |
Pluripotent | Can differentiate into many body cell types |
Multipotent | Can differentiate into a limited range of related cell types |
A common misconception is that specialized cells contain different genes. Usually, they contain the same genes but express different combinations of them.
Exam Technique
This is assessed in IB Biology B2.3 Cell specialization at both SL and HL. For an “explain” question, link the stages clearly: mitosis produces cells with the same genome → positional signals affect gene expression → different proteins are produced → cells develop specialized structures and functions.