7.2 - Factors affecting gene expression

What is a transcription factor?

A protein that controls the transcription of genes, so that only certain parts of the DNA are expressed (e.g. in order to allow a cell to specialise)

How do transcription factors work?

They move from the cytoplasm into the nucleus, bind to the DNA near their target gene & either increase or decrease the rate of transcription

Outline what is happening in this image

1. Oestrogen travels through the phospholipid bilayer by simple diffusion, as it is lipid-soluble & non-polar

2. Oestrogen then enters the nucleus through the nuclear envelope

3. Oestrogen binds to the oestrogen receptor & is held within the protein complex

4. Upon binding, the oestrogen receptor undergoes a confirmational shape change, causing it to change shape & leave the protein complex

5. The altered oestrogen receptor can now bind to the promoter, which attracts a cofactor. This complex allows RNA polymerase to bind tightly & transcription begins

Where are promoters found in DNA?

In the DNA sequence, before the beginning/in front of the gene

What is splicing?

In eukaryotic cells, mRNA contains introns (non-coding regions), which are removed & the exons (coding regions) are joined together by enzymes

How can splicing result in different products from a single gene?

There are a variety of ways that the exons can be joined back together by enzymes, meaning several different versions of the RNA can be produced

What is meant by epigenetics?

A heritable change in gene function, without changing the DNA base sequence:

• these changes are caused by changes in the environment (e.g. diet, stress & toxins) & can inhibit transcription

How can non-coding RNA affect gene expression?

There are many types of non-coding mRNA:

• some interfere with protein translation, some are involved in splicing & some chemically modify other RNAs

How can histone modification affect gene expression?

• Addition of an acetyl group (acetylation) activates chromatin, allowing transcription

  • opens up the packaging of the nucleosomes, meaning transcription factors can bind → greater gene expression

• Addition of a methyl group (methylation) can either activate or inactivate chromatin depending on the position of the lysine

How can DNA methylation affect gene expression?

Involves the addition of a methyl (CH₃) group to cytosine bases:

• silences a gene or sequence of genes → causes nucleosomes to pack more tightly together, meaning the gene is less accessible

• prevents transcription factors from binding therefore, gene transcription is suppressed

How are epigenetics involved in cell differentiation?

Epigenetics control the amount of a cell’s DNA that is transcribed, so ensures that only the proteins needed for that specific cell’s function are produced