1.5 gene expression

Phenotypic expression of genes

Gene pairs of homologous chromosomes control phenotypic characteristics (physical factors) expressed from genes

Genes of corresponding pairs are called alleles – alternative forms of the same gene

Epigenetics

Study of how the environment influences gene expression

DNA accumulates chemical tags collectively called the epigenome – additional level of coding on top of the DNA base sequence

Chemical DNA changes how cells read and express DNA

Types of chemical tags:

Methylation of cytosine

Acetylation of histone proteins

Epigenetic tags can be inherited from parents through sperm and eggs

Epigenome of cells can be influenced by environmental factors

Transcription factors

Genes can be switched on, causing transcription to take place, and switched off, preventing transcription

A promoter sequence is a region on the DNA that plays a role in transcription

Promoter sequence is where RNA polymerase binds to initiate transcription

Molecules called transcription factors can also bind to specific base sequences and assist in binding of RNA polymerase.

Activators bind to DNA and increase the rate of transcription

Repressors decrease the rate of transcription

Methylation of cytosine bases

Process in which a methyl group is added to a cytosine base

Does not alter DNA sequence but can be influence gene expression

Inhibits transcription

Genes retain their methyl groups so patterns of methylation can be passed down to daughter cells

Histone modification

Acetylation of histone proteins changes RNA polymerase accessibility to genes

Acetylation causes chromatin to decondense, causing loose packing of histones, allowing RNA polymerase to bind to DNA so genes can be expressed

Deacetylation causes chromatin to condense limiting accessibility for RNA polymerase, meaning transcription can not occur

Epigenetics and diseases

Environmental factors may change patterns of DNA methylation

Cancer- caused by uncontrolled cell division

Epigenetic factors can affect the expression of the genes that control cell division

If changes lead to uncontrolled cell division, a tumour will start to grow

Types of gene affected by epigenetic changes to cell division:

• Proto-onco genes (code for proteins involved in cell division

• Tumour suppression genes

• DNA repair genes

Cell differentiation

Differentiation is the process in which cells with identical chromosomes and genes become specialised in structure and function.