Differentiation & Variation

• Stem cell


  A cell that can divide (by mitosis) an unlimited number of times
• Potency


  The ability of stem cells to differentiate into more specialised cell types

  
• Types of potency


  • Totipotency
  • Pluripotency
  • Multipotency
• Totipotent stem cells


  Stem cells that can differentiate into any type of cell including those that make up the placenta

  
• Pluripotent stem cells


  Embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into cells forming the placenta and umbilical cord

  
• Multipotent stem cells


  Adult stem cells that can differentiate into a limited range of cell types.
• Plants also contain stem cells in areas of growth, such as their shoots and roots
• Stem cell therapy


  The introduction of adult stem cells into damaged tissue to treat diseases and injuries
• Sources of human stem cells

  • Embryonic stem cells
  • Adult stem cells

  
• Embryonic stem cells


  • Have huge potential in the therapeutic treatment of many diseases
  • Use is banned or tightly regulated in many countries due to ethical concerns
• Adult stem cells


  • Less controversial than embryonic stem cells as the donor is able to give permission
  • If donated from one person to another, they need to be a close match to avoid rejection by the patient's immune system
• Tasks of official regulatory authorities

  • Reviewing proposals for scientific research that uses stem cells and deciding if this research should be allowed to go ahead
  • Licensing and monitoring of research centres that are involved in stem cell research
  • Providing guidelines and codes of practice for stem cell researchers to ensure they are working to the same high standards
  • Monitoring developments in scientific research into stem cell therapies
  • Providing governments and other professional bodies with correct, up-to-date advice and information on stem cell research
• Differential gene expression


  The process by which stem cells become specialised, where only certain genes in the DNA of the stem cell are activated and expressed
• Transcription factor


  A protein that controls the transcription of genes by binding to a specific region of DNA
• Transcription factors


  • They ensure that genes are being expressed in the correct cells, at the correct time and to the right level
  • Some act as activators to increase the rate of transcription, others act as repressors to decrease the rate of transcription
• Operon


  A section of DNA in prokaryotes that includes a cluster of structural genes that are transcribed together and control elements like a promoter and operator region
• Lac operon


  An operon in some bacteria that controls the production of the enzyme lactase and two other structural proteins
• Lac operon when lactose is absent


  1. Regulatory gene is transcribed and translated to produce lac repressor protein

  2. Lac repressor protein binds to the operator region

  3. RNA polymerase is unable to bind to the promoter region

  4. Transcription of the structural genes does not take place

  5. No lactase enzyme is synthesized
• Lac operon when lactose is present


  1. Uptake of lactose by the bacterium

  2. Lactose binds to the repressor protein, causing it to change shape so cannot bind to the operator region

  3. RNA polymerase is able to bind to the promoter region and transcription takes place

  4. mRNA from all three structural genes is translated

  5. Enzyme lactase is produced and lactose can be broken down and used for energy
• An organism's internal or external environment can influence gene expression patterns
• The binding of lactose to the repressor protein frees up the operator region of the lac operon so RNA polymerase can bind and begin transcription of the structural genes
• Epigenetics


  The control of gene expression by factors other than an individual's DNA sequence
• Epigenetics


  • Involves the switching-on and switching-off of genes, but without changing the actual genetic code
  • In eukaryotic cells, nuclear DNA is wrapped around proteins called histones to form chromatin
  • Chromatin can be chemically modified in different ways to alter gene expression
• Epigenetic modifications


  • Methylation of DNA (chemical addition of -CH3 groups)
  • Histone modification via acetylation of amino acid tails
• Epigenome


  All the epigenetic tags in an organism
• Environmental factors


  Can cause epigenetic changes
• DNA methylation


  Methyl groups are attached to DNA preventing transcription of gene.

  Transcription factors cannot bind to DNA so gene is inactive/ switched off.
• Acetylation of histones


  Addition of acetyl groups causes DNA to be less tightly wrapped around histone.

  Transcription factors can bind more easily to DNA so Genes are expressed/ switched on.
• Epigenetic changes can be passed on following cell division
• Phenotype


  The observable characteristics of an organism
• Phenotypic variation


  The difference in phenotypes between organisms of the same species
• Factors that can explain phenotypic variation

  • Genetic factors
  • Environmental factors
  • Combination of genetic and environmental factors
• Genotype


  The genetic makeup of an organism
• Genetic variation


  The small differences in DNA base sequences between individual organisms within a species population
• Types of characteristics based on number of genes

  • Monogenic (controlled by a single gene)
  • Polygenic (controlled by several genes)
• Environment

  Can affect how organisms grow and develop
• Discontinuous variation


  Qualitative differences in the phenotypes of individuals within a population that fall into discrete and distinguishable categories
• Continuous variation


  Quantitative differences in the phenotypes of individuals within a population that do not fall into discrete categories
• Phenotypes affected by multiple genes or multiple alleles for the same gene at many different loci, as well as the environment, often give rise to continuous variation
• Polygenes


  Genes that have a combined effect on a phenotype