3C

• cell differentiation

  process where a less specialised cell becomes more specialised for a particular function
• codominant alleles


  both alleles are expressed in the phenotype
• gene linkage

  when genes for 2 different characteristics are found close on the same chromosome, so they are less likely to be separated during crossing over, so they are linked and inherited together
• polygenic

  phenotypic characters determined by several interacting genes
• dihybrid inheritance

  inheritance of 2 pairs of contrasting characteristics at the same time
• In a dihybrid cross, 2 heterozygous parents are used to produce offspring in a 9:3:3:1 ratio of phenotypes (dominant-dominant, dominant-recessive, recessive-dominant, recessive-recessive), using 2 genes. However, due to gene linkage, the ratio can be closer to 3:1 (dominant-dominant, recessive-recessive), as the 2 genes are inherited together, and so are rarely separated.
• operon


  unit of linked genes that regulate other genes responsible for protein synthesis
• discontinuous variation


  phenotypic features that fall within specific categories, and are inherited from a small number of genes
• continuous variation


  phenotypic features that show a large range of values, that are polygenic and affected by environmental factors
• RNA splicing:

  1. pre-mRNA is transcribed from the DNA
  2. spliceosomes removes introns and join up exons
  3. exons can be joined in many different orders, producing different versions of mRNA
  4. different mRNA codes for different amino acids, and therefore, different proteins
  5. a single gene can produce several phenotypes
• Epigenetics is the study of genetic control by factors other than the base sequences on the DNA.
• Three intracellular systems that control genes in response to environmental factors:

  • DNA methylation
  • histone modification
  • non-coding RNA
• DNA methylation occurs when a methyl group is added to a cytosine. DNA methylation silences a gene or sequence of genes as it prevents transcription from occurring. DNA demethylation is when the methyl group is removed, which enables the gene to become active and be transcribed.
• heterochromatin


  densely coiled chromatin where the genes are not able to be transcribed to make proteins
• Histone acetylation is the addition of an acetyl group, which opens up the chromatin, making it active so genes can be transcribed. Removing the acetyl group produces heterochromatin.
• Histone methylation is the addition of a methyl group, which can cause the activation or inactivation of the chromatin, depending on the position of the group. However, methylation mostly silences a gene, or even a whole chromosome.
• Non-coding RNA makes up much of the human genome, and can silence genes or whole chromosomes by coating them. it can also act on histones to make DNA available or unavailable for transcription.
• Process of cell differentiation:

  1. chemical stimulus or transcription factor
  2. certain genes are activated

  • histone modification
  • DNA methylation
  • non-coding RNA
  • 3. mRNA is produced from these genes
  • 4. translation of mRNA to form a protein
  • 5. permanent modification of cell
• transcription factor

  protein that binds to the DNA in the nucleus and affects the process of transcribing DNA
• Transcription factors can bind to promoter sequences of DNA to stimulate transcription. They can also bind to enhancer sequences which changes the structure of the chromatin to become active or inactive.
• Early stages of development

  1. cleavage - repeated mitosis without interphase for growth between divisions
  2. morula - solid ball of totipotent cells (day 4)
  3. blastocyst - hollow ball of cells with an inner cell mass of pluripotent cells (day 5-6)
• totipotent


  undifferentiated cell that can form any of the different cell types needed for an entire new organism
• pluripotent


  undifferentiated cell that can form most of the cell types needed for an entire new organism
• multipotent


  a cell that can form a very limited range of differentiated cells within a mature organism
• therapeutic cloning:

  1. nucleus removed from normal cell
  2. transferred to human ovum cell without nucleus
  3. electric shock to trigger development
  4. mitosis produces ball of cells
  5. stem cells cultured to grow into organ or tissue
  6. organ or tissue transplanted into patient with no rejection problems
• Stem cell therapy problems:

  • cell differentiation has many unknowns
  • can cause cancer
  • organ transplants can be rejected, which can be fatal
  • immunosuppressant drugs to stop rejection increase the risk of infectious diseases
• Cures from stem cell therapy:

  • parkinson's disease
  • type 1 diabetes
  • damaged nerves
  • organs for transplants
• Ethical problems with stem cell therapy:

  • wrong to use embryonic cells according to some religions, as its stopping the possibility of human life
  • therapeutic cloning raises ethical concerns over when to stop the cloning