How do cells know what to grow into?

Chris Smith put this question to molecular biologist Diana Alexander from Cambridge Universty...

Diana - Exactly. All of the cells in our body pretty much have the same DNA, so the same chromosomes with the same sequences. But the reason why cells are different is because they have different genes that are expressed. So to kind of explain gene expression: if you imagine the genome as a big book with recipes for proteins, gene expression is where some of those recipes are copied into RNA, which is kind of like copies of pages of a book and those are used directly to produce the proteins. So different cell types have different compliments of proteins that are expressed and these determine their functions. For a retinal cell, you’d get production of photoreceptors so that gene for the photoreceptor is expressed. So what underlies these differences in gene expression is epigenetic mechanisms and those are the kind of mechanisms that regulate which genes are expressed by determining which of the pages in the book of the genome are used to make proteins.

Chris - I get it. So can I summarise then saying every cell in the body’s got a copy of the recipe book, but different cells in different tissues achieve their specialisation just by playing out or making some of those recipes but not all of them. So a brain cell is playing out a brain cell specific recipe of genes, whilst a hand sort of muscle cell, is expressing hand muscle genes and a skin cell knows it’s a skin cell. I suppose the key question is also how do those cells know to do that in the first place?

Diana - During the process of differentiation, which is cell specialisation, these epigenetic mechanisms are laid down and put in place. So transcription factors, which are these proteins that determine which genes are expressed will lay down these epigenetic marks and that will determine which genes will be expressed later on in the final form of the cell after it’s differentiated.