Unit 8

• A gene mutation is the change in the base sequence of DNA.

  • It’s most likely to occur during DNA replication.
• Mutagenic agents: factors that increase the rate of gene mutations.
• Examples of mutagens:

  • Sunlight
  • Ionising radiation - x-rays and gamma
  • Chemicals
  • Carcinogens - tobacco, mustard gas
  • Nitrites
• Substitution mutation:

  • 1 base is replaced with another
  • Only one codon changes - 1 amino acid changes
  • No change to amino acid sequence - degenerates nature of genetic code so changed codon might still code for same amino acid
• Addition mutation:

  • Frameshift mutation - addition of 1 or more bases to base sequence
  • Shifts to the right
  • Triplet/codons change the amino acid sequence - may result in non-functioning protein
• Deletion mutation:

  • Frameshift mutation - deletion of 1 or more bases in base sequence
  • shifts to the left
  • Triplets/codons change the amino acid sequence - result in different polypeptide chain and non-functioning protein
• Inversion mutation:

  • A group of bases are separated from DNA sequence and re-join same position but inverted (back to front)
  • No Frameshift —> number of bases stays the same
  • Triplet/codons in inverted section change - different amino acid coded for in this region
• Duplication mutation:

  • A sequence of bases is inserted twice or multiple times
  • Frameshift - triplets/codons change downstream of mutation
  • Changes amino acid sequence
• Translocation mutation:

  • A group of bases detach from one chromosome and attach to another DNA sequence of another chromosome.
  • Causes significant impact on gene expression leading to abnormal phenotype
• Stem cells are undifferentiated cells that can divide and become specialised.
• Totipotent stem cell:

  • Can divide and differentiate into any type of body cell (including embryonic cells)
  • only occur for limited time in early mammalian embryos
  • During development, totipotent cells translate part of DNA
• Pluripotent stem cells:

  • Can divide and differentiate into most cell types
  • Differentiate into all cell types except cells of placenta
  • found in embryos
• Multipotent stem cells:

  • Can divide and differentiate into limited number of cell types
  • Found in mature mammals
• Unipotent stem cells:

  • Can divide and differentiate into one type of cell
  • Example - cardiomyocytes (cardiac muscle cell) can be made from unipotent cells.
• Embryonic stem cells:

  • Blastocyst = very early embryo
  • taken from inner cell mass from blastocyst
  • Embryos up to 16 days after fertilisation contain pluripotent stem cells.
• Sources of stem cells:

  1. Embryonic stem cells
  2. Placenta
  3. Tissue stem cells
• Induces pluripotent stem cells (iPS cells):

  1. Produced from adult somatic (body) cells
  2. Specific protein transcription factors associated with pluripotency put into cells, causing cell to express genes associated with pluripotency (reprogrammed)
  3. Cells cultured
• Cancer is the result of mutations in genes that regulate mitosis.

  • Uncontrolled cell division = tumour
• Benign tumour = non-cancerous and does not spread
• Malignant tumours = cancerous and spread throughout the body via metastasis
• Benign tumours:

  • Non-cancerous
  • Grow slowly
  • Produce adhesion molecules sticking them together to a particular tissue.
  • Surrounded by a capsule - remain compact and within tissues
  • Normal/ regular nuclei - well differentiated
  • Usually harmless
  • Easily removed by surgery and rarely return
• Malignant tumours:

  • Cancerous
  • Grow larger quickly and uncontrollably
  • Larger nucleus - cells become unspecialised
  • Do not produce adhesive and not capsulated - cells break off and spread
  • Tumour grows projections into surrounding tissues + develops its own blood supply
  • Removed by chemotherapy + radiotherapy and surgery - more likely to return
• Tumour suppressor gene:

  • Genes that produce proteins to slow down cell division and to stop a cell dividing if DNA is damaged.
  • If mutation causes tumour suppressor gene to not produce proteins - cell division continues uncontrollably
• Tumour suppressor gene:

  When cell division occurs too quickly - proteins ca cause the cell to self destruct (apoptosis)
• Proto-oncogenes:

  • Genes that produce proteins to stimulate cell division
  • When they are mutated - genes become overactive and produce lots of proteins. —> process permanently activated and uncontrollable cell division
  • Mutated proto-oncogene is called an oncogene
• Abnormal methylation in tumour suppressor genes:

  • Can be hypermethylated - increased number of methyl groups attached to it
  • Results in gene being inactivated / turned off
  • Proteins that slow down mitosis are not produced
• Abnormal methylation in oncogenes:

  • Oncogenes can be hypomethylated - reducing number of methyl groups attached to it
  • Results in gene permanently switched on
  • Many proteins are produced to stimulate cell division
• Oestrogen concentration in development of breast cancer:

  • Oestrogen binds to oestrogen receptor (transcription factor)
  • Stimulates expression of genes
  • Many of these genes are linked female secondary sexual characteristics via promotion of cell division.
• Oestrogen concentration in development of breast cancer:

  • A cholesterol derived hormone - lipid soluble
  • After menopause, fat cells/ adipose tissues in breasts start producing high concentration of oestrogen.
  • Causes breast cancer in women post-menopause
• How growth of cancer in breast can be minimised:

  • Growth minimised by drugs (Tamoxifen)
  • Binds to oestrogen receptor
  • Blocks oestrogen from binding
  • No release of inhibitory complex
  • No expression of genes
• Regulation of transcription:

  • They move from cytoplasm to nucleus - bind to DNA base sequence on the promoter region
  • Promoters are found near the start of a target gene
• Transcription factors enable RNA polymerase to attach to start of a gene and begin transcription.
• Regulation of transcription - activators and repressors:

  • Activators - transcription factor helps RNA polymerase to attach/bind to start of gene and activate transcription.
  • Repressors - transcription factor decrease rate of transcription by preventing RNA polymerase from binding.
• Oestrogen and transcription:

  • Oestrogen is a steroid hormone - lipid soluble
  • Oestrogen receptor is the transcription factor
• Role of oestrogen in initiating transcription:

  1. Diffuses across plasma membrane into cytoplasm
  2. In cytoplasm, it binds to complementary oestrogen receptor (inactive transcription factor) forming hormone-receptor complex.
  3. Transcription factor changes shape - makes it complementary to DNA + TF becomes activated
  4. Activated TF diffuses through nuclear pores into nucleus and binds to specific DNA base sequence on promoter region.
  5. RNA polymerase can attach to make mRNA - transcription begins
• RNA interference (RNAi) - RNA molecules inhibit translation of mRNA produced by transcription.
• Effect of RNA interference - (siRNA):

  1. Double stranded RNA associates with an enzyme (DICER) - cuts and unwinds to form 2 single strands of RNA and siRNA.
  2. one of the single strands associates with proteins. siRNA binds with an enzyme - RISC and forms RNA induced silencing complex.
  3. siRNA binds to mRNA by complementary base pairing.

  • Enzyme cuts up mRNA
  • cut up mRNA cannot be translated - gene is switched off
• Effect of RNA interference -(miRNA):

  1. When miRNA first transcribed it’s a long folded hair-pin shape - miRNA associates with enzyme (DICER) and unwinds into 2 single strands of RNA (miRNA)
  2. one single strand associates with proteins. miRNA has specific base sequence and not fully complementary to target mRNA.
  3. mRNA protein complex binds to mRNA by complementary base pairing and blocks translation.
• RNA can be moderated until 2 types of RNA:

  1. Small interfering RNA - siRNA
  2. Micro interfering RNA - miRNA
• Epigenetics: Heritable changes in gene expression without changes to DNA base sequence.

  • These changes are caused by changes in the environment and can inhibit transcription