D2.2 Gene Expression

• Gene expression is the mechanism by which genetic information in a gene is expressed as a functional protein
• gene expression involves transcription of a gene to produce mRNA and the translation of the mRNA to create a functional protein
• functional proteins have an effect on the phenotype of an organism
• phenotype of organisms are the observable traits of an organism
• genotype is the combination of alleles inherited by an organism
• transcription is regulated by promotor regions, enhancers and transcription factors
• enhancers are regulatory sequences of DNA located away from the gene
• transcription factors are proteins that bind to enhancer and promotor regions of DNA
• promotors, enhancers and regulatory proteins allow a cell to respond to internal and external factors and control the genes transcribed
• translation can be controlled and regulated by the degradation of mRNA by nuclease enzymes (nuclease breaks down mRNA to RNA nucleotides that can be recycled to produce more genes)
• in human cells, mRNA can be present for minutes to days until the nuclease enzyme is able to break it down
• epigenesis is the process by which multicellular organisms are developed from a zygote (by differentiation) and it results from the interactions of genes to their environments as they develop
• epigenetics are the studies of organisms caused by modification of gene expression. the phenotype is changed but the genotype stays the same
• epigenetic tags are chemical markers that attach to DNA or histone protein and influence the transcription of genes
• the pattern of gene expression in a cell determines how it differentiates
• the genome includes all the mitochondrial, chloroplasts and chromosome DNA in a cell
• no cell expresses all of the genes in its genome
• a transcriptome refers to all of the RNA molecules transcribed at a specific time within a cell or organism, it represent the active genes
• a proteome is the complete set of proteins present in a cell or organism at a given time
• proteomes are larger than genomes due to the alternative splicing of mRNA exons that produce multiple proteins and the modifications to those proteins after translation
• methyl groups can act as epigenetic tags by attaching to the cysteine in the promotor region in DNA or histone proteins.
• epigenetic tags often remain in place during mitosis and meiosis. the daughter cells inherit phenotypic changes without changes to the DNA sequence
• most epigenetic tags are erased during fertilization and are not passed on to future generations. this process is called reprogramming
• environment affects epigenomes to a huge extent, these include:

  diet - of the individual or mother during pregnancy

  cigarette smoke - may lead to lung cancer

  air pollution - such as diesel fumes can add or remove methyl groups changing the genes that are expressed
• the reprogramming of epigenomes of a zygote produces stem cells
• monozygotic twins are identical twins resulting from the fertilization of one egg
• monozygotic twins are clones and have the same genome
• throughout life, epigenomes of identical twins become different due to environment, this results in different gene expressions
• gene expression can be impacted by factors outside of the cells such as:

  presence of biochemical molecules i.e. E. coli bacteria, which is impacted by the presence or absence of the sugar lactate

  hormones from hormone-receptor complexes in the cytoplasm of the cell