Control of Gene Expression

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1. Totipotency and Cell Specialisation

Stem Cells

  • Unspecialised cells that can differentiate into specific types of cells
  • Divide to become new cells, which then become more specialised, they are also capable of dividing to produce more stem cells--> self-renewal
  • Found in the embryo (embryonic stem cells), where they become the specialised cells needed to form a fetus, these stem cells are totipotent (develop to form any body cell)
  • Found in some adult tissues e.g. the inner lining of the small intestine, the skin and the bone marrow, where they become specialised cells that need to be replaced, these stem cells are multipotent (can only develop into a few types of cells)

Becoming Specialised

  • Become specialised during their development where they only transcribe and translate part of their DNA meaning that not all the genes are expressed
  • The genes expressed and turned off depend on the conditions
  • Genes that are expressed get transcribed into mRNA, which is then translated into proteins, these proteins modify the cell, changing its structure and determining the cell processes
  • This causes the cell to become specialised, this cannot be reversed
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2. Totipotency and Cell Specialisation

Stem cells in plants

  • Found in areas where the plant is growing e.g. the roots and shoots, these areas are called meristems
  • All stem cells in plants are totipotent (can mature into any cell type), this means they can be used to grow plant organs or whole new plants--> tissue culturing

Tissue Cultures:

1. A single totipotent cell is take from a meristem

2. The cell is placed in some growth medium that contains nutrients and growth factors (the growth medium is sterile, so microorganisms can't grow and compete with the plant cells

3. The plant stem cell will grow and divide into a mass of unspecialised cells which will mature to become specialised

4. The cells grow and specialise to form a plant organ or an entire plant depending on the growth factors used

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1. Regulation of Transcription and Translation

Transcription Factors

  • Protein molecules that control the transcription of genes
  • Move from the cytoplasm to the nucleus, within the nucleus thet bind to specific DNA sites called protomotors which are found near the start of their target genes (the genes that they control the transcription of)
  • Control expression by controlling the rate of transcription
  • Activators- increase the rate transcription e.g. they help RNA polymerase bind to the start of the target gene and activate transcription
  • Repressors- decrease the rate of transcription e.g. they bing to the start of the target gene, preventing RNA polymerase from binding, stopping transcription

Oestrogen

  • A hormone whose levels can affect transcription by binding to a transcription factor called an oestrogen receptor forming an oestrogen-oestrogen receptor complex
  • The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene, the complex can act as either an activator or a repressor
  • Whether it acts as an activator or repressor depends on the type of cell and target gene
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2. Regulation of Transcription and Translation

siRNA

  • A type of RNA called small interfering RNA
  • Short, double stranded RNA molecules that can interfere with the expression of specific genes
  • Their bases are complementary to specific sections of a target gene and the mRNA that's formed from it
  • Can interfere with both the transcription and translation of genes
  • Has a potential use in treating genetic disorders e.g. stopping a known harmful gene from being expressed

siRNA interferance

  • Affects translation through a mechanism called RNA interferance
  • In the cytoplasm, siRNA and associated proteins bind to the targer mRNA
  • The proteins cut up the mRNA into sections so it can no longer be translated
  • siRNA prevents the expression of the specific gene as its protein can no longer be made during translation
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