Mutations and gene expression

• iPCs: pluripotent cells that have been produced from unipotent cells by addition of transcription factors.
• Cancer: mutations in genes causing uncontrolled cell growth.

  
  

  Tumour suppressor genes normally prevent cells dividing when they shouldn't but mutation allows cell division without reason.
• Proto-oncogenes: stimulate normal cell division.

  Mutation of these forms oncogenes that stimulate division.
• substitution: a nucleotide with a different base replaces the origional nucleotide in the sequence.

  This could cause a STOP codon, shortening the polypeptide chain, or an amino acid might change forming a different shaped protein and altering its function.

  Or no change may occur because genetic code is degenerate.
• Deletion: a nucleotide base is lost from the sequence causing a frame shift to the left.
• Addition: an extra base is inserted into the sequence causing a frame shift to the right.
• Inversion: a group of nucleotides become seperated from the DNA sequence and re-join but in reverse order.

  This could affect one or multiple amino acids based on how many nucleotides are reversed.
• Duplication: one or more nucleotides are repeated causing a frame shift to the right.
• Translocation: a group of nucleotides are separated from the DNA sequence of one chromosome and are inserted into the DNA sequence of a different chromosome.
• A gene mutation is a change to the structure of DNA producing a different allele of a gene.
• In differentiated cells, segments of the DNA that aren't required for the cells specialised function are shut down and wrapped tightly around histone proteins.

  
  

  However, stem cells retain use of all the genes by having DNA wrapped loosley around histone proteins.
• Transcription factors have a specific site which can bind to a specific base sequence on DNA in the nucleus.

  This enables RNA polymerase to bind and initiate transcription.
• Some molecules act as activators or repressors which bind to the transcription factor to change the shape of its DNA binding site.

  Activators allow it to bind (switching genes on)

  Repressors prevent it from binding (switching genes off)
• Oestrogen:

  1. Oestrogen is lipid soluble so diffuses through the phospholipid bilayer.
  2. It binds to a receptor on the transcription factor.
  3. This changes the shape of the DNA binding site of the transcription factor.
  4. The complex enters the nucleus through nuclear pores & binds to specific base sequences of the DNA.
  5. Causing RNA polymerase to bind to DNA & begin transcription.
• the epigenome is a series of molecular tags that regulate gene transcription and can be dynamically (over time) altered by environmental factors.
• Epigenetics is heritable change in gene function without changes to base sequences.
• Epigenetic tags include:

  Methylation of DNA

  Acetylation of histone proteins
• DNA methylation involves adding a methyl group (-CH3). This alters the structure of the DNA, making it less accessible to enzymes involved in transcription. Methylation usually occurs at cytosine bases next to guanines. The pattern of methylation is passed down during cell division.
• Transcription is when mRNA is synthesized using one of the DNA strands as a template. It takes place in the nucleus.
• Histones are positively charged proteins with negatively charged DNA wrapped around them.

  They contain amino acids with carboxyl groups which can be modiied by addition or removal of acetyl groups.

  These modifications can alter the charge ofthe protein, changing how tightly the DNA can wrap around the histones, making it more or less accessible.

  Increased acetylation makes the DNA more accessible to transcription factors.
• Increased acetlyation decreases the positive charge of the histone protein so DNA is less attracted.

  DNA is less tightly wound around histones.

  Genes are switched on.
• Decreasing acetlyation increases the positive charge of the histone protein so DNA is more attracted.

  DNA is wound more tightly around the histones.

  Genes are switched off.
• Increased methylation prevents binding of transcription factors so reduces gene transcription.

  • This is called hypermethylation
• Decreased methylation allows binding of transcription factors so increasing gene transcription.

  • This is called hypomethylation.
• RNA interference is when RNA moleculesinhibit gene expression by causing destruction of specific mRNA molecules, thus preventing translation.
• Types of RNA interference:

  miRNA (microRNA)

  siRNA (small interfering RNAs)
• siRNA mechanism:

  1. an enzyme cuts large double stranded RNA into small sections of siRNA.
  2. one of the two strands of siRNA combines with an enzyme.
  3. the siRNA guides the enzyme to an mRNA by binding complementary sequences of mRNA.
  4. the enzyme cuts the mRNA into small sections.
  5. the mRNA is no longer translated into a polypeptide and gene expression is blocked.
• siRNA are short double stranded RNA molecules
• miRNA molecules fold back on themselves forming a hairpin structure.
• miRNA mechanism:

  1. miRNA arent fully complementary to a specific mRNA so may target more than one mRNA molecule.
  2. miRNA blocks translation rather than cutting mRNA into fragments.
  3. the mRNA is moved to a processing body where it is either egraded or stored for later use.
• Benign tumour: a mass of abnormal cells that develop when there is no need for growth or repair.
• Malignant tumour: a tumour that tends to spread to other parts of the body via blood or lymph systems.
• Metastasis: the process of a tumour spreading & secondary tumours are formed called metastases.
• Features of benign tumours:

  • can grow large in size
  • slow growth
  • cell nucleus looks normal
  • cells are well differentiated
  • cells produce adhesion molecules so stick together
  • surrounded by a capsule so tissue is compact
  • localised effects on the body
• features of malignant tumours:

  • can grow large in size
  • grow rapidly
  • nucleus looks larger & darker due to lots of DNA
  • cells are unspecialised (de-differentiated)
  • cells don't produce adhesion molecules so can spread
  • no capsule so grows finger like projections
  • Have systemic (whole body) effects eg. weight loss
• Benign tumour treatment:

  • can usually be removed by just surgery
  • rarely reoccur after treatment
• Malignant tumour treatment:

  • removed with radiotherapy and/or chemotherapy as well as surgery
  • more likely to reoccur after treatment
• What are the two types of gene mutations leading to cancer?

  tumour supressor gene mutation

  proto-oncogene mutation
• tumour supressor genes:

  • if DNA damage is detected or a mistake in the cell cycle occurs then the cell cycle will stop.
  • The stop is controlled by tumour suppressor genes
• Mutation of tumour suppressor genes will allow the cell cycle to continue even in the presence of mutated DNA.