International A Level Biology / Development of organisms
How cells become specialised
Edexcel International A Level Biology (YBI11), chapter 9: Development of organisms, specification point 3.18. Free lesson.
Read the revision notes · International A Level Biology past papers
Every cell has the same genes, but different cell types switch on different genes and make different proteins. This differential gene expression explains how cells become specialised for different jobs.
How gene expression makes cells different
A gene is expressed when it is used to make its protein. All cells keep the full set of genes, but only some are transcribed and translated in each cell type. The proteins produced help control cell processes, such as enzyme-controlled reactions, or determine cell structure. A gene that is switched on is being transcribed. A switched-off gene is still present in the cell, but is not read to make its protein. Different proteins give different cell types the features they need for their jobs.
How a bone marrow cell makes haemoglobin
As a bone marrow cell develops into a red blood cell, it switches on the globin genes. The gene-to-protein sequence makes globin polypeptides, which combine with haem groups to form haemoglobin. The cell becomes specialised to carry oxygen. Other genes that the cell does not need remain unread. Haemoglobin fills the red blood cell and carries oxygen. Later, the cell loses its nucleus, leaving more room for haemoglobin. Link the protein to the cell's job: globin genes lead to haemoglobin, which enables oxygen transport.
How two plant cells use the same genes differently
A palisade mesophyll cell and a root hair cell both come from the same zygote by mitosis, so they have identical genes. They become different because they transcribe different genes and make different proteins. In a palisade cell, genes for chloroplast proteins are switched on. The cell makes photosynthesis enzymes and chlorophyll-binding proteins, and becomes packed with chloroplasts for photosynthesis. In a root hair cell, genes for extra ion carrier proteins are switched on. These proteins sit in the cell surface membrane and help the cell take up mineral ions by active transport. The root…
Why a muscle cell does not make insulin
A muscle cell does not lack the insulin gene. It came from the zygote by mitosis, which copies every gene, so it has the same insulin gene as a pancreas cell. The difference is that the insulin gene is switched off in the muscle cell. Because the gene is not transcribed, no insulin mRNA is made. Without insulin mRNA, the muscle cell does not make insulin.
How to explain specialisation in an exam
A strong explanation names the specific gene and protein, then links the protein to the named cell's function. Saying only that genes are switched on and different proteins are made is too general to explain a particular cell. For salivary gland cells, the amylase gene is switched on and transcribed. Active mRNA is translated at ribosomes, so amylase is made and secreted. Cheek cells have the amylase gene, but it is not transcribed there.