control of gene expression

• addition mutation - where one or more bases are added to the DNA sequence
• deletion mutation - where one or more bases are removed from the DNA sequence
• substitution mutation - where one or more bases are changed in the DNA sequence
• inversion mutation - where the order of bases in the DNA sequence is reversed
• duplication mutation - where one or more bases are duplicated in the DNA sequence
• translocation mutations - where a section of DNA sequence is moved to another section of the DNA sequence this can happen within or across chromosomes
• frameshift

  • caused when a base is inserted, removed or duplicated into the genome. this alters the number of bases in the DNA sequence
  • causes all amino acids that follow the mutation to be affected
• mutagenic agents - increase the rate at which mutations occur
• totipotent cells

  • exists for a very limited time during embryonic development
  • translate only part of their DNA
  • able to produce any type of body cell
• pluripotent cells

  • totipotent cells develop into pluripotent cells in embryos
  • pluripotent cells are able to divide in unlimited numbers and produce any type of cell that makes up the body
  • pluripotent cells can be used to treat human disorders
• multipotent cells

  • found in mature mammals
  • can develop into a limited number of cell types
• unipotent cells

  • found in mature mammals
  • can divide to produce new cells but can only produce one type of cell
• stem cells can be taken from: adult stem cells, embryonic stem cells and induced pluripotent stem cells
• stem cells can be used to reduce preventable deaths and to treat conditions that decrease the quality of life
• stem cells disadvantages

  • obtaining stem cells from embryos is a controversial issue for ethical reasons
  • some people believe using human embryos as a source of stem cells is depriving an embryo life
• use the diagram to explain what effect a mutation from CAA to CAG would have on a protein. how do you think this works on a molecular level?

  some amino acids are encoded for by more than one codon. this is said to make the genetic code degenerate. both CAA and CAG code for glutamine. this happens because there are multiple tRNA molecules for each amino acid. therefore the amino acid can be attached to any of these tRNA molecules. each tRNA has a different anticodon which can bind to different codon sequences.

  
• in what way does a pluirpotent stem cell differ from a totipotent stem cells?

  totipotent stem cells are present for a very short time at the very beginning of development. these are the cells of zyogte. ther are capable of developing into any tissue type.

  pluripotent cells are the cells of the early embryo. they are able to divide unlimited numbers and produce any body cell
• what is the advantage of using iPS cells rather than transplanting bone marrow stem cells from a donor?

  iPS cells can be generated from the cells of the patient. this means that they have the same molecular markers as any of the patients cells

  one of the most common risks of transplants is an immune response against the transplanted cells or tissue. this can cause the transplant to fail and can even be fatal
• transcription factors

  • proteins that control gene expression by stimulating or inhibiting the transcription of target genes
  • produced in the cytoplasm and move to the nucleus
  • in the nucleus they bind to a specific region of DNA to stimulate or inhibit the gene
• activators

  • transcription factors that stimulate gene expression
  • promote the transcription of genes by interacting with RNA polymerase and allowing it to bind to DNA
• repressors

  • transcription factors that inhibit gene expression
  • prevent the transcription of genes by stopping RNA polymerase from binding to DNA
• peptide hormones

  • bind to cell surface membrane and trigger a secondary messenger response
  • the secondary messenger will lead to the activation or inhibition of transcription of some genes
• lipid soluble steroid hormone

  • can pass through the phospholipid bilayer of the membrane
  • interact directly with DNA to promote or inhibit gene expression
• oestrogen in gene transcription

  1. enters the cytoplasm through the cell surface membrane and due to it being lipid soluble it can pass through the phospholipid bilayer
  2. binds to receptors on transcription factors in the cytoplasm, this causes the transcription factor to change shape and the transcription factor forms a receptor hormone complex that can enter the nucleus
  3. the receptor hormone complex binds to the promoter region of the DNA and this activates transcription and stimulates protein synthesis
• chromatin - combination of DNA and histones
• epigenome - a chemical layer that surrounds the chromatin
• the epigenome can either

  • make a chromatin more condensed which prevents transcription factors from binding to DNA so transcription is inhibited
  • make the chromatin less condensed which allows easier access to transcription factors so is promoting transcription
• epigenetic markers - groups that do not alter the base sequence but influence chromatin structure
• methylation

  • inhibits transcription
  • methyl groups bind to areas in the DNA where cytosine and guanine are together in the base sequence
  • methyl groups cause the chromatin to be more condensed and when they are more condensed then transcription factors cannot bind to DNA
  • methyl grouos are added to DNA to alter gene expression by an enzyme called methyl transferase
• acetylation

  • acetyl groups are removed from histone proteins
  • removal of acetyl groups increases the positive charge on histone proteins, this increases the attraction to phosphate groups on DNA
  • decreased acetylation causes the chromatin to be condensed so transcription factors cannot reach DNA
• inheritance

  • epigenetic markers can be inherited by offspring
  • inheritance of epigenetic control means that environmental factors experienced by an individual can influence the gene expression of their offspring
• abnormal methylation - methylation is not regulated properly and can affect the regulation of these important processes e.g cell division
• increased methylation

  • is methylation is increased too much it can decrease the gene expression of tumour suppressor genes more than normal
• tumour suppressor genes prevent cell division from taking place
• if the genes are underexpressed the cells divide uncontrollably and tumours are produced
• decreased methylation

  • if decreased too much it can increase the gene expression of pronto-oncogenes more than normal
• proto-oncogenes promote cell division
• if proto-oncogenes are overexpressed the cells will divide uncontrollably and tumours are produced
• epigenetic changes that cause disease are temporary and can be reversed