B20 gene expression

• mutations are changes in the sequence of nucleotides in DNA molecules
• the types of gene mutation are:

  • addition
  • deletion
  • duplication
  • inversion
  • substitution
  • translocation
• in an addition mutation, one or more bases are added to the sequence
• in a deletion mutation, one or more bases are removed from the sequence and not replaced
• in a duplication mutation, one or more bases are repeated
• in an inversion mutation, a group of bases are separated and then joined at the same position but backwards
• in a translocation mutation, a group of bases are separated from one chromosome and inserted into the sequence of another chromosome
• in a substitution mutation, one or more bases are changed for a different one or more bases
• mutations are caused by chance, mutagenic agents increase that chance
• mutagenic agents include:

  • chemical mutagens such as alcohol, benzene, tar, asbestos
  • high energy ionising radiation such as alpha/beta radiation, UV and X-rays
  • spontaneous errors in DNA replication
• not all mutations are harmful, or even have an effect at all
• stem cells are undifferentiated cells which can keep dividing to produce other cells
• there are different types of stem cell:

  • totipotent
  • pluripotent
  • multipotent
  • unipotent
• totipotent stem cells can differentiate into every type of specialised cell
• pluripotent stem cells can differentiate into almost every type of specialised cell, except extraembryonic and placental
• multipotent stem cells can differentiate into other limited types of specialised cell
• unipotent stem cells can differentiate into one type of specialised cell
• cells differentiate because transcription factors cause different genes to be switched on and off, causing the different cells to specialise in different ways
• pluripotent stem cells have a number of different uses in repairing damaged tissue
• induced pluripotent stem cells can be produced from unipotent stem cells
• transcription factors are molecules which bind to a specific site on DNA to begin transcription, they have a complementary base sequence to the specific site
• oestrogen has the ability to start transcription
• the process of oestrogen starting transcription is:

  • oestrogen is lipid-soluble so freely diffuses across the cell membrane into the cytoplasm
  • oestrogen binds to a receptor site on a transcription factor
  • this causes a conformational change in the shape of the DNA binding site on the transcription factor, making it able to bind to the DNA
  • the transcription factor enters the nucleus via a nuclear pore, and binds to DNA
  • this stimulates transcription
• small-interfering RNA is used to switch off genes short-term
• the process of siRNA switching off genes short-term is:

  • siRNA binds to complementary sequence of mRNA
  • mRNA should be single-stranded but the cell detects this sequence as double-stranded
  • the sequence is labelled as abnormal and broken down by enzymes
  • this prevents translation
• epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA
• epigenetics show that environmental factors can make changes to the function of genes which can be inherited
• the epigenome is the second layer of chemical tags which covers DNA and histones
• the epigenome determines the shape of the DNA histone-complex by keeping genes that should be inactive tightly packed together so they cannot be read, and by unwrapping genes that should be active so they are exposed and can be easily read
• an organism’s epigenome changes throughout its lifetime, as more signals are accumulated, it is originally from the mother and cells within the foetus, but after birth it can be affected by environmental factors as well as hormones
• once a message from a hormone or an environmental factor is passed to a specific protein, attached to a specific base sequence, there are two possible effects:

  • acetylation of histones
  • methylation of DNA
• when acetyl groups attach to histones, DNA is packed more loosely, so genes are more likely to be expressed as they can be easily read
• when methyl groups attach to cytosine, DNA is packed more tightly, so genes are less likely to be expressed as they cannot be easily read
• cancer can arise as a result of a mutation, where uncontrolled cell division leads to the formation of a tumour
• there are two types of tumour, benign and malignant
• benign tumours can grow to a large size

  malignant tumours can grow to a large size
• benign tumours grow very slowly

  malignant tumours grow very rapidly
• benign tumours appear relatively normal

  malignant tumours appear to have larger and darker nuclei
• benign tumours are usually made up of specialised cells

  malignant tumours are usually made up of unspecialised cells
• benign tumours produce adhesion molecules

  malignant tumours do not produce adhesion molecules