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Summary Unit 8 - The control of gene expression

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Whole of Unit/Module 8, control of gene expression for AQA A level Biology based on class notes, specification, textbooks, websites, videos and revision sessions. Summarised into bullet point notes that are quick and to the point with just information you need for the exam.

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Module 8 (Genes) Revision Notes

What is a Stem Cell?

 a unspecialised/undifferentiated cell
 potential to form different types of cells

How does a stem cell be come a specialised cell?

 differentiation
 3 changes: cell shape, number of organelles, new content
 occurs by controlling gene expression (some gene are activated, other genes are
inhibited)

Stem Cell in Animals/Mammals/Humans?

 Totipotent = Zygote
 Pluripotent = Embryonic Stem Cells
 Multipotent = Bone Marrow Stem Cell
 Unipotent = Tissues

What are Induced Pluripotent Stem Cells (iPS Cells)?

turning unipotent body cells into pluripotent cells (like embryonic stem cells), involves
activating certain deactivated genes using transcription factors

Stem Cell Therapy in Humans?

 2 uses,
 use stem cells to produce tissues/organs for transplant
 use stem cells to treat irreversible diseases e.g. heart disease, type 1 diabetes, paralysis
(inject stem cells at site of disorder – will differentiate to become local specialised cells
e.g. heart muscle cells, beta cells of pancreas, neurones)

Stem Cell in Plants?

 In embryo = Zygote/Embryonic Stem Cells
 In adult = Meristem Cells in Stem/Shoot/Root

Uses of Stem Cells from Plants?

 traditionally cuttings were taken from plants (stem/shoot/root) and used to grow
genetically identical plants – possible due to presence of meristem cells
 tissue culture (micro propagation) = large scale application of cuttings
 process,
 take cutting from shoot/stem/root (called explant)
 place explant in nutrient rich medium so meristem cells divide by mitosis
 produces a mass of meristem cells (called callus)
 take each meristem cell and grow in plant growth factor medium to promote
differentiation and formation of shoot/root
 transfer plant to soil and greenhouse
 then transfer to field

, What is Controlling Gene Expression?

 either Activating or Inhibiting a Gene
 activating gene = protein made
 inhibiting gene = protein not made

Example of activating genes?

 using oestrogen
 oestrogen can enter a cell by simple diffusion and bind to receptors on the transcriptional
factor
 causes transcriptional factor to change shape
 so transcriptional factor can now enter nucleus and bind to promoters on the DNA to
activate transcription
= activated genes (protein to be made)

Example of inhibiting genes?

 using siRNA (small interfering RNA)
 making siRNA = double stranded RNA cut down into small sections, made single
stranded, then attaches to an enzyme
 siRNA will bind to complementary sections on mRNA = the enzyme will cut the mRNA
so translation cannot occur = gene inhibited (protein not made)

What is Epigenetics?

 Heritable changes in gene function without changes to base sequence of DNA
 Changes may due to lifestyle, stress, diet
 Chromatin (DNA-Histone Complex) is surrounded by an Epigenome (chemical layer)
 Epigenome can either cause the Chromatin to become more condensed or more loose
 Chromatin becoming more condensed means transcription factors cannot reach the DNA
and the gene will be inactivated
 Chromatin becoming more loose means transcription factors can reach the DNA and the
gene will be activated
 These changes may be brought about by Acetylation or Methylation

How does Methylation and Acetylation affect the Genome?

 Increased Methylation = adding methyl groups, this attracts proteins which condense the
DNA-Histone Complex so transciption factors cannot gain access (gene inhibited)
 Decreased Acetylation = removing acetyl groups, increases positive charges on the
Histone which increases the attraction to the phosphate groups on DNA which condense
the DNA-Histone Complex so transciption factors cannot gain access (gene inhibited)




What is a Gene Mutation?

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