topic 8 - biology

• gene mutation:


  • the alteration of a base in the sequence of bases for one gene
  • this is likely to occur during DNA replication, which is during interohase in the cell cycle
• mutagenic agents:

  • the certain factors that increase the frequency of these gene mutations occuring
• examples of mutagenic agents:

  • high energy + ionising radiation - include radiation such as alpha and beta particles, x-rays and gamma rays. UV(ultraviolet) light is not ionising but it is still high enough so can cause damage and disrupt the structure of DNA
  • carcinogens - term given to chemicals that can alter the structure of DNA and interfere with transcription, these include chemicals in tobacco smoke, mustard gas and peroxides
• 6 different types of gene mutations:

  • addition
  • deletion
  • substitution
  • inversion
  • duplication
  • translocation
• addition mutation:


  • one extra base being added to the sequence
  • the impact of adding one base is that all subsequent codons are altered, known as a frame shift
  • this can be very harmful because all altered codons could potentially code for different amino acids and result in a very different sequence of amino acids resulting in a non-functioning protein
• deletion mutation:


  • the deletion of a base in a sequence
  • this causes a frame shift to the left, could result in a different polypeptide chain and a non-functioning protein
• substitution mutation:

  • one base has been changed for a different base but the number of bases remains the same so no frame shift
  • this results in only one codon changing and due to the genetic code being degenerate it may still code for the same amino acid and therefore have no impact
• inversion mutations:


  • a section of bases detach from the DNA sequence but when they rejoin they are inverted, so this section of code is back to front
  • this results in different amino acids being coded for in this region
• duplication mutation:


  • one or more bases are repeated
• translocation:


  • a sequence of bases is moved from one location in the genome to another
  • this could be movement, within the same chromosome or movement to a different chromosome
  • this is a substantial alteration and can cause significant impacts on gene expressions and therefore the resulting phenotype.
• what is a stem cell?

  undifferentiated cells that can divide indefinitely and turn into other specific cell types
• what is a totipotent stem cell?


  • can develop into any cell type including the placenta and embryo
  • during development totipotent cells translate only part of their DNA, resulting in cell specialisation
• what is a pluripotent stem cell?


  can develop into any cell type excluding the placenta and embryo

  • usually found in embryos & used in research
• what is a multipotent stem cell?

  can only develop into a limited number of different cell types

  • found in bone marrow
• what is a unipotent stem cell?


  can only develop into one type of cell
• what happens to totipotent cells during embryonic devleopment?


  certain parts of DNA are selectively translated so that only some genes are 'switched on' in order to differentiate the cell into a specific type and form tissues that make up the foetus
• unique feature of pluripotent cells and its use?

  they can divide in unlimited numbers and can therefore be used to repair or replace damaged tissue
• which types of stem cell are found in embyros?

  totipotent and pluripotent
• use of stem cells?

  • medical therapies eg bone marrow transplants, treating blood disorders
  • drug testing on artificially grown tissues
  • research eg on formation of organs and embryos
• how are induced pluripotent stem cells produced?

  • from mature, fully specialised(somatic) cells
  • the cell regains capacity to differentiate through the use of proteins, in particular transcription factors
• what is a transcription factor?

  a protein that controls the transcription of genes so that only certain parts of the DNA are expressed, eg in order to allow a cell to specialise.
• how do transcription factors work?

  • move from the cytoplasm into nucleus
  • bind to promotor region upstream of target gene
  • makes it easier or more difficult for RNA polymerase to bind to gene, this increases or decreases rate of transcription
• give an example of hormone that affects transcription and explain how it works

  • steroid hormone oestrogen diffuses through cell membrane
  • forms hormone-receptor complex with ER alpha receptor in the cytoplasm
  • complex enters the nucleus & acts as transcription factor to facilitate binding of RNA polymerase
• what is meant by epigenetics?

  a heritable change in gene funtion without change to the base sequence of DNA
• how does decreased acetylation of histone affect gene transcription?

  • positively-charged histones bind to negatively charged DNA
  • decreasing acetylation, increases positive charge of histones
  • binding becomes too tight and prevents transcription factors from accessing the DNA
  • therefore gene transcription is supressed
• how might epigenetic changes affect humans?

  they can cause disease, either by over activating a gene's function(such as in cancer) or by suppressing it
• give an application of epigenetics

  • treatments of various diseases
  • development of ways to reverse epigenetic changes
• describe the process of RNA interference, including the organisms in which it occurs

  RNA molecules act to inhibit gene expression, usually by destroying mRNA so that it cannot be translated

  • occurs in eukaryotes and some prokarytotes
• give some characteristics of a benign tumours?

  • slow growth
  • defined by a clear boundary due to cell adhesion molecules
  • cells retain function and normal shape
  • don't spread easily
  • easy to treat
• give some characteristics of a malignant tumours?

  • rapid, uncontrollable growth
  • ill-defined boundary (finger-like projections)
  • cells do not retain function and normal shape
  • spreads quickly and easily (metastasis)
  • difficult to treat
• describe the role of tumour-suppressor genes?


  codes for proteins that control cell division; in particular stopping the cell cycle when damage is detected

  • they are also involved in programming apoptosis ie 'self destruction' of the cell
• explain how tumour-suppressor genes can be involved in developing cancer?

  a mutation in the gene could code for non-functional protein.

  • increased methylation or decreased acetylation could prevent transcription
  • cells will divide uncontrollably resulting in a tumour
• describe the role of proto-oncogenes?

  controls cell division; in particular, code for proteins that stimulate cell division
• explain how proto-oncogenes can be involved in developing cancer?

  mutation in the gene could turn it into a permanently activated oncogene

  • decreased methylation or increased acetylation can cause excess transcription
  • this results in uncontrolled cell division and formation of a tumour
• explain how abnormal methylation of genes can cause cancer?

  hyper-methylation of tumour-suppressor genes or oncogenes can impair their function and cause the cell to divide uncontrollably
• explain how oestrogen can be involved in developing breast cancer?

  • steroid hormone that can initiate transcription
  • it does this by binding to a receptor site on the transcriptional factor
  • when it binds to the transcriptional factor it causes it to change shape slightly and this change in shape makes it complementary and able to bind to the DNA to initiate transcription
  • cell division can become uncontrollable
• what is the genome?


  the complete set of genetic information contained in the cells of an organism
• what is genome sequencing?

  identifying the DNA base sequence of an individual

  • this allows us to determine the amino acid sequence of the polypeptides coded for by that DNA
• what is the proteome?

  the complete set of proteins that can be produced by a cell
• can we directly translate the genome into the proteome?

  in simple organisms, yes

  but in complex organisms, due to the presence of non-coding DNA and regulatory genes it is much harder to obtain the proteome