bio cancer and mutation

• definition of mutation

  change in nucleotide sequence of DNA, or in a structure of chromosome, or the number of chromosomes of an organism
• definition of gene mutation

  a mutation that involves a change in nucleotide sequence of a DNA molecule at a particular gene locus
• definition of chromosomal mutation

  mutation that involves a change in structure or number of chromosomes
• definition of substitution mutation

  replacement of one nucleotide pair with another pair of nucleotides
• definition of addition mutation

  insertion of one or more nuceotide pairs into a DNA sequence
• definition of deletion mutation


  a mutation in which one or more nucleotide pairs are removed from a DNA sequence
• effects of silent mutations

  1. No effect on amino acid sequence
  2. Due to degeneracy of genetic code
• effects of missense mutations (little effect on protein case)

  1. New amino acid has properties similar to those of the amino acid it replaces
  2. Replacement a.a is in the region of the protein which is not essential to protein's function
  3. Slight change in specific three dimensional conformation of protein leading to functional protein but at a lower capacity
• effects of missense mutations (protein activity severely altered case)

  1. substituted nucleotides occur in a crucial area in the protein, therefore alteration of single a.a significantly alters protein function
  2. Protein may not be as effective
• effects of nonsense mutations

  1. Codon for amino acid changed to stop codon
  2. Translation terminated prematurely
  3. Polypeptide formed shorter than normal, distorting unique 3-dimensional conformation of protein, leading to non-functional proteins
• effects of frameshift mutations due to addition or deletion of nucleotides not in multiples of 3

  1. Reading frame altered of nucleotide sequence is altered and nucleotides downstream are incorrectly grouped into codons
  2. Resulting in extensive change in sequence of amino acids. The change in amino acids may also result in premature termination.
• effect of frameshift mutation on protein formed

  1. Change in primary sequence of polypeptide causes complete change in three-dimensional structure of protein, causing non-functional protein
• effect of substitution of nucleotide on structure of haemoglobin

  1. Thymine in CTT substituted for adenine to form CAT
  2. Amino acid is changed from hydrophillic glutamate to hydrophobic valine.
  3. Mutated Hbs polymerise into long rigid chain when not bound to oxygen due to hydrophobic interactions between hydrophobic regions on on different Hb molecules
  4. Specific three-dimensional structure of haemoglobin is distorted as change in coiling and foiling of polypeptides
• effect of substitution of nucleotide on structure and function of haemoglobin

  Structure: The long rigid chains distort the membrane of the red blood cell, giving its distinct sickle shape

  Function: Decreased oxygen-carrying ability or clump and block small blood vessels leading to organ damage
• deletion of chromosomal fragment

  occurs when a chromosomal fragment is lost ; cri du chat syndroma
• duplication of chromosomal fragment

  occurs when a detached chromosomal fragment from a sister chromatid becomes attached as an extra fragment to a non-sister chromatid of a homologous chromosome
• inversion of chromosomal fragment

  occurs when a fragment of chromosome breaks off and reattaches to the original chromosome in reverse orientation
• translocation of chromosomal fragment

  occurs when a chromosomal fragment breaks and joins with a non-homologous chromosome ; chronic myelogenous leukemia
• Aneuploidy: extra chromosome or lacks a chromosome
• Non-disjuction occurs when:

  1. Homologous chromosomes fail to separate during Anaphase I of meiosis (all abberant gametes) OR
  2. Sister chromatids fail to separate during Anaphase II of meiosis (half normal gametes half abberant gametes)
• examples of aneuploidy:

  Down's syndrome/trisomy 21 ; extra chromosome 21 (3 chromosomes 21), total 47 chromosomes
• Polypoidy: more than 2 complete sets of chromosomes
• G1 checkpoint:

  1. Adequate cell size
  2. Sufficient nutrients
  3. Growth factors present
  4. Before cell cycle is allowed to transit into mitosis and meiosis
• G2 checkpoint (produce genetically identical cells):

  1. Adequate cell size
  2. Semi-conservative replication of DNA completed successfully (ensures errors in replication are repaired by DNA Polymerase)
• Metaphase checkpoint (produces genetically identical cells):

  1. All chromosomes attached to spindle fibres (prevents cell from entering next stage if there are errors, preventing mutation in daughter cells)
• Gain-of-function mutations:

  Occurs in proto-oncogenes which encode proteins to stimulate normal cell division, to form oncogene results in increase in amount of protein product synthesised or permanently activated proteins, resulting in uncontrolled cell division, leading to cancer

  GOF mutation results in a dominant allele, hence only one allele needs to be mutated to have the effect
• Gain-of-function mutations example:

  Ras gene coding for Ras protein, a G protein relaying signal from growth factor receptor on CSM to a cascade of protein kinases, resulting in normal cell division

  Ras oncogene codes for permanently activated Ras protein, which tiggers kinase cascade in absence of growth factor
• Loss-of-function mutation:

  Occurs in tumour supressor genes, which encode proteins to inhibit cell division or promote apoptosis, to form mutated tumor suppressor genes which lead to no protein product formed or decrease in amount of protein product or permanently deactivated proteins, resulting in uncontrolled cell division, leading to cancer.

  
  

  LOF mutation results in recessive allele hence two alleles of the gene needs to be mutated to have an effect
• Loss-of-function mutation example:

  p53 gene codes for p53 protein, a transcription factor that promotes synthesis of protein that triggers cell cycle arrest or promote apoptosis when DNA damage is detected

  Mutated p53 tumour suppressor gene result in no p53 being produced. Cells with damaged DNA continue to divide
• Properties of cancer cells

  1. Loss of anchorage dependence (anchor to surface before dividing)
  2. Lack of density-dependent inhibition (stop dividing upon contact with another cell)
• Causative factors of cancer

  Chemical:

  1. Chemical carcinogens
  2. Tar in cigarette smoke

  Genetic factor:

  1. Mutated oncogenes or TSG in gametes of parents

  Biological:

  1. Viruses

  Physical factors:

  1. Ultra-violet radiation
  2. Ionising radiation
• How do proto-oncogenes mutate?

  1. Movement of DNA: Translocated proto-oncogene ends up near an especially active promoter, increasing its transciption
  2. Point mutations to control element (promoter or enhancer) controlling proto-oncogene, increasing its expression
  3. Point mutation to coding sequence of oncogene for more active protein
• How do tumour supressor gene mutate?

  1. Under-expression of tumor supressor protein or inactivation of tumor supressor protein, through mutation in control element or coding sequence of tumour supressor gene
• Development of cancer as a multi-step process

  1. Accumulation of independent mutations in a single cell
  2. Loss-of-function of both alleles of tumor suppressor genes, leading to absence of tumor suppressor proteins and failure to inhibit cell cycle and trigger apoptosis
  3. Gain-of-function mutation to at least one allele of proto-oncogene produce excess or permanently activated protein, over-stimulating cell division
  4. Gene for telomerase activated, removing natural limit on number of times a cell can divide
  5. Uncontrolled division of cell, forming tumour
• Devt of cancer of multistep process (tumour)

  1. Tumour angiogenesis occurs, supplying tumour with nutrients and oxygen
  2. Cancer cells invade surrounding tissues
  3. Cancer cells enter circulatory system and metastasize to distant sites
• Explain why HbF reduces symptoms of sickle-cell

  1. Both HbF and HbS present in same cell
  2. Reduces formation of fibres / polymerisation into long rigid chains of HbS polypeptides
  3. Fewer red blood cells to block capillaries
• Evidence of quaternary structure in haemoglobin:

  1. Quaternary structure contains more than one polypeptide chains
  2. 4 polypeptide chain in haemoglobin
• Need for tight control of mitotic cell cycle

  1. Cell cycle regulated at mitotic checkpoints at G1, G2 and M, which determine if cell cycle can proceed
  2. Cell cycle regulated for normal cell growth and development
  3. Dysregulation of cell cycle and cell escaping cell cycle control mechanism leads to uncontrolled division of cells and cancer
• Why is it important for fetal haemoglobin HbF to be more saturated with oxygen

  1. Fetal haemoglobin binds more readily with oxygen
  2. Higher efficiency in binding to haemoglobin
  3. Ensure oxygen is transported to fetus at higher rate
  4. Fetal haemoglobin is better adapted to low oxygen concentration in the placeta
• Difference between cancer cells and normal germ line cells

  1. Cancer cells do not exhibit density dependent inhibition vs density dependent inhibition in normal cells
  2. Cancer cells lose anchorage dependence vs normal cells exhibit anchorage dependence
  3. Non functional cell cycle checkpoints in cancer cells vs functional cell cycle checkpoint in normal cells
  4. Uncontrolled cell division in cancer cells vs controlled cell division in normal cells
  5. No need for growth factor vs need for growth factor