Cell Specialisation (Edexcel International AS Biology): Revision Note

Differential Gene Expression

• Stem cells become specialised through differential gene expression

  • This means that only certain genes in the DNA of the stem cell are activated and get expressed

• Every nucleus within the stem cells of a multicellular organism contains the same genes, that is, all stem cells within an organism have an identical genome

• Despite the stem cells having the same genome, they are able to specialise into a diverse range of cell types because during differentiation certain genes are expressed ('switched' on)

• Controlling gene expression is the key to development as stem cells differentiate due to the different genes being expressed

• This differentiation occurs via the following basic steps:

  • Under certain conditions, some genes in a stem cell are activated, whilst others are inactivated

  • mRNA is transcribed from active genes only

  • This mRNA is then translated to form proteins

  • These proteins are responsible for modifying the cell (e.g. they help to determine the structure of the cell and the processes that occur within the cell)

  • As these proteins continue to modify the cell, the cell becomes increasingly specialised

  • The process of specialisation is irreversible (once differentiation has occurred, the cell remains in its specialised form)

Differential gene expression results in the differentiation of stem cells

Transcription factors control the expression of genes

• Eukaryotes use transcription factors to control gene expression

• A transcription factor is a protein that controls the transcription of genes by binding to a specific region of DNA

• They ensure that genes are being expressed in the correct cells, at the correct time and to the right level

• It is estimated that ~10% of human genes code for transcription factors

  • There are several types of transcription factors that have varying effects on gene expression

  • This is still a relatively young area of research and scientists are working hard to understand how all the different transcription factors function

  • Transcription factors allow organisms to respond to their environment

  • Some hormones achieve their effect via transcription factors

• Transcription factors that increase the rate of transcription are known as activators

  • Activators work by helping RNA polymerase to bind to the DNA at the start of a gene and to begin transcription of that gene

• Transcription factors that decrease the rate of transcription are known as repressors

  • Repressors work by stopping RNA polymerase from binding to the DNA at the start of a gene, inhibiting transcription of that gene

• Some transcription factors bind to the promoter region of a gene

  • This binding can either allow or prevent the transcription of the gene from taking place

  • Transcription factors interact with RNA polymerase, either by assisting RNA polymerase binding to the gene (to stimulate expression of the gene) or by preventing it from binding (to inhibit gene expression)

  • Therefore, the presence of a transcription factor will either increase or decrease the rate of transcription of a gene

In the example above, the transcription factor is an activator as it stimulates the transcription of the gene. Transcription factors, known as repressors, can also inhibit the transcription of genes