Factors Affecting Gene Expression

Temperature can influence gene expression as many enzymes are temperature dependent. High or low temperatures can slow down enzyme activity, affecting transcription and translation.
Exposure to certain chemicals or radiations can alter the DNA structure or the epigenome, affecting gene regulation mechanisms and thus gene expression.

DNA methylation can inhibit gene expression by preventing transcription factors from binding to the DNA. Hypermethylation of a gene promoter region often leads to gene silencing.
Histone modification can change the structure of the chromatin, making it either more or less compact. A more compact chromatin structure is less accessible and hence reduces gene expression, whereas a less compact chromatin increases gene expression.

Transcription factors bind to promoter and enhancer regions of genes influencing the rate of transcription.
Regulators of translation can influence the process of protein synthesis at the post-transcriptional level, for example, by affecting mRNA stability or by regulating the initiation and elongation steps of translation.
Post-translational modifications of proteins, like phosphorylation, can activate or deactivate proteins, affecting their function and ultimately influencing gene expression.

Gene mutations could cause diseases. In sickle cell anaemia, for example, a mutation in the gene producing haemoglobin leads to the production of an abnormal form of the protein.
Certain variations in the genome, such as single nucleotide polymorphisms (SNPs) can influence gene expression by altering the binding sites for transcription factors or by causing amino acid changes.
Copy number variations (CNVs) involve duplications or deletions of large sections of DNA, which can also significantly affect gene expression.

Both Micro RNAs (miRNAs) and small interfering RNAs (siRNAs) can prevent the translation of specific mRNAs, hence reducing gene expression.
They do so by binding to the mRNA, either degrading it or otherwise preventing it from being used as a template for translation.