3.8.2 GENE EXPRESSION

• Multicellular organisms made up from many different cell types specialised for their function.

  • Stem Cells - unspecialised cells that can develop into other types of cell. Divide to become new cells, which then become specialised.

  Found in all multicellular organisms. Found in embryo (become all specialised cells needed to form fetus) and in some adult tissues (become specialised cells that need replacing).
• • Totipotent cells - Stem cells that can divide and produce any type of body cell in an organism. Only present in mammals in early embryos. After this point embryonic stem cells become pluripotent. 
  • Pluripotent cells - Can specialise into any cell in body, except those that make up the placenta. Found in embryos. Can be used to treat human disorders. 
• The stem cells in adult mammals either multipotent or unipotent: 

  • Multipotent - able to differentiate into a few different types of cell. (eg. red and white blood cells can be formed from multipotent stem cells found in bone marrow).
  • Unipotent - can only differentiate into one type of cell (Cardiomyocytes - Heart muscle cells - make up a lot of the tissue in heart. Old or damaged cardiomyocytes can be replaced by new cardiomyocytes derived from a small supply of unipotent stem cells in the heart).
• Stem cells all contain the same genes — but during development they only transcribe + translate (express) part of their DNA - become specialised.

  Certain genes expressed and others switched off depending on differing conditions. 

  • Expressed genes get transcribed and translated into proteins. 
  • Proteins modify the cell — determine cell structure and control cell processes (including expression of more genes).
  • These changes to the cell cause the cell to become specialised. 

  These changes difficult to reverse, so once cell has specialised it stays specialised.
• Stem Cell Therapies - Replace cells damaged by illness or injury. 

  Existing - Bone marrow transplants -

  • Bone marrow contains stem cells that can specialise to form any type of blood cell.
  • Replace faulty bone marrow that produces abnormal blood cells - stem cells in transplanted bone marrow divide and specialise to produce healthy blood cells.
  • Treat leukaemia (a cancer of the blood/bone marrow) and lymphoma (a cancer of the lymphatic system). Also to treat some genetic disorders like sickle-cell anaemia.
• Future stem cell therapies: 

  • Spinal cord injuries — stem cells replace damaged nerve tissue.
  • Heart disease and damage caused by heart attacks — replace damaged heart tissue.
  • Bladder conditions — stem cells used to grow whole bladders, implanted in patients to replace diseased ones.
  • Respiratory diseases — donated windpipes stripped down to simple collagen structure, then covered with tissue generated by stem cells. Transplanted into patients.
  • Organ transplants — organs could be grown from stem cells to provide new organs.
• 3 main potential sources of human stem cells:

  • adult stem cells
  • embryonic stem cells
  • induced pluripotent stem cells (iPS cells)
• Adult Stem Cells -

  • Obtained from body tissues of adult. E.g. adult stem cells in bone marrow can be obtained in relatively simple operation — little risk involved, but lot of discomfort.
  • Adult stem cells can only specialise into a limited range of cells (multipotent). Scientists trying to find ways to make adult stem cells specialise into any cell type.
• Embryonic Stem Cells -

  • Obtained from embryos at an early stage of development.
  • Embryos created using IVF.
  • Once embryos are approx 4 to 5 days old, stem cells removed from them and rest of the embryo destroyed.
  • Embryonic stem cells are pluripotent.
• Induced Pluripotent Stem Cells (iPS cells)

  Created. ‘Reprogramming’ adult body cells so they become pluripotent.

  Adult cells made to express transcription factors associated with pluripotent stem cells. Transcription factors cause adult cells to express genes associated with pluripotency.

  • One way transcription factors introduced to adult cells - infecting them with specially-modified virus. Virus has the genes coding for the transcription factors within its DNA. When virus infects adult cell, genes passed into adult cell’s DNA, meaning cell able to produce transcription factors.
• Benefits of Stem Cell Use

  • Could save many lives — e.g. many people waiting for organ transplants die before donor organ available. Stem cells could be used to grow organs.
  • Could make stem cells genetically identical to patient’s own cells - tissue/organ grown not rejected. 
  • Improve quality of life for many by replacing damaged cells.
• Ethical Considerations of Stem Cell Use

  • Destruction of embryo that could become baby. Pro-life - wrong to destroy embryos. 
  • Can obtain stem cells from unfertilised egg cells (artificially activated to start dividing).
  • Only using adult stem cells - can’t develop into all specialised cell types that embryonic stem cells can.
  • iPS cells have potential to be as flexible as embryonic stem cells, without ethical issues. Possible that iPS cells could be made from a patient’s own cells - genetically identical to patient’s cells so if used to grow new tissue or organ, wouldn’t be rejected. 
• Transcription Factors - control expression by controlling rate of transcription. 

  Eukaryotes - transcription factors move from cytoplasm to nucleus. Bind to specific DNA sites (promoters), found near start of their target genes (genes they control the expression of). 

  • Activators stimulate/increase rate of transcription — e.g. help RNA polymerase bind to the start of target gene and activate transcription. 
  • Repressors inhibit/decrease rate of transcription — e.g. bind to start of target gene, preventing RNA polymerase from binding, stopping transcription. 
• Oestrogen

  Also effects expression of genes. 

  Steroid hormone that can initiate transcription by binding to transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex.

  Complex moves from cytoplasm to nucleus where it binds to specific DNA sites near start of target gene. Act as activator of transcription.
• RNA Interference and Gene Expression -

  In eukaryotes and some prokaryotes the translation of mRNA produced by target genes can be inhibited by RNA interference, which breaks mRNA down before its coded info can be translated. 

  One type of small RNA molecule that may be involved is small interfering RNA (siRNA). 
• Gene Expression - siRNA

  • Enzyme cuts large double-stranded RNA molecules into smaller sections of siRNA.
  • One of the two siRNA strands combines with an enzyme.
  • siRNA molecule guides enzyme to an mRNA molecule by pairing up its bases with the complementary ones on a section of the mRNA.
  • Enzyme cuts mRNA into smaller sections
  • mRNA can no longer be translated - means gene not expressed (blocked)
• Epigenetic Control of Gene Expression in Eukaryotes

  Epigenetic control can determine whether a gene switched on or off — whether gene expressed (transcribed and translated) or not, without any changes to base sequence of DNA.

  Works through attachment or removal of chemical groups (epigenetic marks) to or from DNA/histone proteins. Epigenetic marks alter how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe the DNA.

  Epigenetic changes to gene expression can occur in response to changes in the environment.
• Epigenetics: heritable changes in gene function, without changes to the base sequence of DNA.
• Inheriting Epigenetic Changes

  • Organisms inherit their DNA base sequence from parents.
  • Most epigenetic marks on the DNA removed between generations, but some escape removal process and passed on.
  • Means expression of some genes in offspring can be affected by environmental changes that affected parents/grandparents.
• Epigenetic Mechanisms to Control Gene Expression (by Inhibiting Transcription):

  • Increased Methylation of DNA
  • Decreased Acetylation of Histones
• Increased Methylation of DNA

  Methylation - methyl group (an epigenetic mark) attached to DNA coding for a gene.

  Always attaches where a cytosine and guanine base are next to each other (CpG site).

  Increased methylation changes DNA structure so transcriptional machinery (enzymes etc.) can’t interact with gene — so gene not expressed (switched off).
• Decreased Acetylation of Histones -

  How condensed chromatin is affects accessibility of the DNA and whether or not it can be transcribed.

  Histones can be epigenetically modified by addition or removal of acetyl groups (an epigenetic mark).

  • Histones acetylated (acetyl groups added) - chromatin less condensed. Transcriptional machinery can access the DNA, allowing genes to be transcribed. 
  • Acetyl groups removed from histones - chromatin highly condensed. Genes can’t be transcribed as transcriptional machinery can’t access them. Acetyl groups removed by histone deacetylase (HDAC).
• Decreased acetylation of histones and increased methylation can lead to genes being switched off - can cause diseases. 

  Drugs designed to counteract the epigenetic changes that cause the diseases.

  • HDAC inhibitor drugs inhibit HDAC enzymes (remove acetyl groups), so genes remain acetylated and proteins can be transcribed. 
  • Drugs that stop DNA methylation eg. by physically blocking enzymes involved in process.

  Problem with drugs that counteract epigenetic changes - specificness. Changes occur normally in lots of cells. 
• Twin Studies - Evaluate data on relative influences of genes and environment on phenotype. 

  Twins genetically identical, so any differences in phenotype due to environmental factors. 

  • If a characteristic is very similar, genetics probably plays more important role.
  • If a characteristic is different, the environment must have larger influence.

  Larger sample size better for drawing valid conclusions - more representative of population.
• If a cell divides uncontrollably the result is a tumour — a mass of abnormal cells. 

  Cancers = tumours that invade and destroy surrounding tissue.

  • Malignant Tumours - Cancers. Grow rapidly, invade and destroy surrounding tissues. Cells can break off the tumours and spread to other parts of the body in the bloodstream or lymphatic system.
  • Benign Tumours - Not cancerous. Grow slower, often covered in fibrous tissue that stops cells invading other tissues. Often harmless, but can cause blockages and put pressure on organs. Some benign tumours can become malignant.
• Tumour Cells

  Differ from normal cells:

  • Larger and darker nucleus. Can have more than one.
  • Irregular shape.
  • Don’t produce all proteins needed to function correctly.
  • Different antigens.
  • Don’t respond to growth regulating processes.
  • Divide (by mitosis) more frequently.
• 2 types of gene that control cell division — tumour suppressor genes and proto-oncogenes. Mutations in these genes can cause cancer, as it can result in uncontrolled cell division.
• Tumour Suppressor Genes

  • Functioning normally - slow cell division by producing proteins that stop cells dividing/cause them to self-destruct (apoptosis).
  • Mutation in gene - inactivated. Protein it codes for not produced and cells divide uncontrollably (rate of division increases) resulting in a tumour.
• Proto-oncogenes

  • Functioning normally - stimulate cell division by producing proteins that make cells divide.
  • Mutation in gene - overactive. Stimulates cells to divide uncontrollably (rate of division increases) resulting in a tumour.

  Mutated proto-oncogene = oncogene.
• Methylation means adding a methyl (–CH3) group onto something.

  When happens too much (hypermethylation) or too little (hypomethylation) - becomes a problem. Growth of tumours can be caused by abnormal methylation of certain cancer-related genes.

  • Tumour suppressor genes hypermethylated - genes not transcribed, so proteins produced to slow cell division not made = uncontrolled cell division.
  • Proto-oncogenes hypomethylated - act as oncogenes, increasing production of proteins that encourage cell division = cells stimulated to divide uncontrollably.
• Increased exposure to oestrogen thought to increase risk of developing breast cancer.

  Oestrogen can stimulate certain breast cells to divide and replicate. 

  • More cell divisions taking place naturally increases chance of mutations occurring, so increases chance of cells becoming cancerous.
  • If cells do become cancerous, rapid replication could be further assisted by oestrogen, helping tumours form quickly.
  • Oestrogen can introduce mutations directly into DNA of certain breast cells, increasing chance of them becoming cancerous.
• Risk Factors for Cancer

  Increase a person’s chance of getting cancer. Genetic or environmental.

  Genetic - Some cancers linked with specific inherited alleles. If you inherit that allele you’re more likely to get that type of cancer.

  Environmental - Exposure to radiation, lifestyle choices (eg. smoking, increased alcohol consumption, high-fat diet) linked to increased chance of developing some cancers.
• Preventing Cancer

  If a specific cancer-causing mutation known, possible to screen for mutation in a person’s DNA.

  Knowing about increased risk means preventative steps can be taken to reduce it.

  Knowing about specific mutations means more sensitive tests can be developed - lead to earlier and more accurate diagnoses.
• Treating and Curing Cancer

  Treatment for cancer different for different mutations, so knowing how specific mutations cause cancer useful for developing drugs to target them.

  Gene therapy may be able to treat cancer caused by some mutations. Currently only been used in trials.