Gene Control

• Some genes are always expressed such as those involved in respiration, mitosis, TRNA production
• Differentiation of stem cells leads to the inability to redifferentiate
• Differentiation can change the size, shape and metabolism of a cell
• Stem cells are from animals and meristem cells are found in plants
• Stem cells are able to self renew and differentiate into specialised cells
• Totipotency is a cell which can differentiate into any other cell, these are found in a very early embryo
• Pluripotent cells are cells able to form any cell in the body except the placenta
• Multipotent cells can differentiate into many types of cell depending on their type, eg, skin stem cells can differentiate into any type of skin cell
• Unipotent cells can only make one type of cell by differentiation
• Differentiation is reversible only in plants
• Embryonic stem cells can be grown in cell culture and differentiated into useful tissues
• Stem cells can be obtained from the umbilical cord, the embyro, the placenta and from adults - often as a skin stem cell
• Induced pluripotent stem cells are reprogrammed somatic cells that have been exposed to factors able to alter potency, the chromosomes are unwound and methylation is removed, switching on all genes which may have been switched off
• IPS stem cell therapy is an effective method as it uses the patients own cells so there is no chance of rejection, no embryos are destroyed
• Transcription factors are specific proteins with a complimentary tertiary structure to the promotor region on a gene, they allow RNA polymerase to bind
• Activator transcription factors will promote RNA polymerase whereas repressor factors will inhibit RNA polymerase action
• Oestrogen is lipid soluble so will diffuse into the cell and bind to a receptor on a transcription factor in the cytoplasm
• Oestrogen binding to a transcription factor will cause it to undergo a conformational change of shape, exposing the DNA biding site
• The oestrogen bound transcription factor will diffuse into the nucleus via a nuclear pore where it will bind to the promoter on the gene, this causes the production of MRNA and a protein, expressing the gene
• Epigenetics is the process of methylation of DNA and acetylation of histones leading to alteration of gene expression
• DNA is packaged around histones in complexes of 8 histones
• DNA methylation is the addition of a methyl group to a cytosine, this causes the gene to be tightly coiled, preventing transcription factors from binding to the promotor region, switching the gene off as no MRNA can be produced
• Histone acetylation is where an acetyl group is added to a histone, causing the histones to spread out, exposing the promotor region for transcription factors to bind
• Drugs can be used to increase and reduce methylation and acetylation of genes, allowing control of gene expression
• Epigenetic modifications can be carried between generations
• Methylation of tumour suppressor gene will prevent it from being expressed, leading to uncontrolled cell division
• Acetylation of protooncogene will switch the gene on, allowing production of proteins which promote cell division
• Methylation of oncogene will switch off the mutated gene, preventing uncontrolled cell division
• Heterochromatin is involved in switched off genes
• Euchromatin is involved in switched on genes
• Small interfering RNA is involved in RNA interference, this breaks down MRNA in the cytoplasm, preventing translation to regulate gene expression
• Double stranded RNA is produced and hydrolysed into small interfering RNA, these form protein complexes and unravel into single stranded RNA using ATP
• Single stranded small interfering RNA will bind to MRNA with a complimentary target sequence in the cytoplasm forming hydrogen bonds between complimentary base pairs
• The association between small interfering RNA and MRNA prevents ribosome binding so no protein can be translated, it attracts an enzyme to hydrolyse the complex
• RNA interference could be used to prevent expression of mutated tumour suppressor gene or oncogene by intercepting MRNA, preventing uncontrolled cell division
• RNA interference could be used to prevent expression of viral DNA in host cells
• Hetero OFF
• Eu ON