Gene expression controls which genes will be transcribed and translated into proteins in a particular cell. This plays a vital role in cell differentiation during development of an embryo and in cell differentiation and activity during an organism’s life. Gene expression is controlled at multiple different levels:
- Chromatin packaging and histone modification
- Transcriptional control (RNA polymerase binding and transcription factors)
- RNA processing (alternate splicing)
- RNA transport and localisation
- Translation control
- Protein activity control
Part 1: Chromatin packaging
Chromatin is the name given to the highly condensed combination of DNA and histone proteins, the form in which all DNA exists except during DNA replication and transcription. The negatively charged DNA strands undergo supercoiling and wrap themselves round the positively charged histone proteins and then condense further to form chromatin. Histones are highly positively charged due to 1/4 of all their amino acid residues being either arginine or lysine, both of which are positively charged. Chromatin has repeating structures known as nucleosomes- each nucleosome consists of a 200bp length of DNA wrapped around eight histones- two copies of four histone proteins: H2A, H2B, H3 and H4. H1 is structurally different from the other four histones and is involved in binding to the ‘linker DNA’- the DNA between each nucleosome complex.
There are two types of chromatin found in cells- euchromatin and heterochromatin.
- Euchromatin- 90% of DNA in a cell is in this form during interphase. Euchromatin is less densely packed than heterochromatin and exists in either an ‘active’ or ‘inactive’ form. The active form can be transcribed while the inactive form cannot although euchromatin can easily be converted between active and inactive forms.
- Heterochromatin- 10% of DNA is in this form during interphase. Heterochromatin is highly condensed and cannot be transcribed. Thi is useful as cell differentiation depends on some genes being expressed while others are not. Heterochromatin is a more stable form than euchromatin.
[to be continued in part 2… sometime in the vaguely near future]
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