gene expression Flashcards
What are stem cells?
*Undifferentiated / unspecialised cells capable of:
1. Dividing (by mitosis) to replace themselves indefinitely
2. Differentiating into other types of (specialised) cells
Describe how stem cells become specialised during development
*Stimuli lead to activation of some genes (due to transcription factors - see 8.2.2)
● So mRNA is transcribed only from these genes and then translated to form proteins
● These proteins modify cells permanently and determine cell structure / function
Describe totipotent cells
● Occur for a limited time in early mammalian embryos
● Can divide AND differentiate into any type of body cell (including extra-embryonic cells eg. placenta)
Describe pluripotent cells
● Found in mammalian embryos (after first few cell divisions)
● Can divide AND differentiate into most cell types (every cell type in the body but not placental cells)
Describe multipotent cells
● Found in mature mammals
● Can divide AND differentiate into a
limited number of cell types
Describe unipotent cells, using an example
● Found in mature mammals
● Can divide AND differentiate into just one cell type
Example: unipotent cells in the heart can divide and differentiate
into cardiomyocytes (cardiac muscle cells)
Explain how stem cells can be used in the treatment of human disorders
● Transplanted into patients to divide in unlimited numbers
● Then differentiate into required healthy cells (to replace faulty / damaged cells)
Explain how induced pluripotent stem (iPS) cells are produced
- Obtain adult somatic (body) cells (non-pluripotent cells or fibroblasts) from patient
- Add specific protein transcription factors associated with pluripotency to cells so express genes associated with pluripotency (reprogramming)
○ Transcription factors attach to promoter regions of DNA, stimulating or inhibiting transcription - Culture cells to allow them to divide by mitosis
Evaluate the use of stem cells in treating human disorders
For:
* can divide and differentiate into required healthy cells, so could relieve human suffering by saving lives and improving quality of life
- embryos are often left over from IVF and so would otherwise be destroyed.
- iPS cells unlikely to be rejected by patient’s immune system as made with patient’s own cells.
*iPS cells can be made without destruction of embryo and adult can give permission
Against:
*ethical issues with embryonic stem cells as obtaining them requires destruction of an embryo and potential life (embryo cannot consent)
*Immune system could reject cells and immunosuppressant drugs are required
*cells could divide out of control, leading to formation of tumours / cancer
Examples of treatments using stem cells
● Potential treatment of Type 1 diabetes by creating healthy islet cells that produce insulin
● Bone marrow stem cell transplant for sickle cell disease / blood cancers
1. Destroy patient’s bone marrow before treatment → so no faulty cells are produced
2. Transplant stem cells from healthy person → divide and differentiate into healthy cells
What are transcription factors?
● Proteins which regulate (stimulate or inhibit) transcription of specific target genes in eukaryotes
● By binding to a specific DNA base sequence on a promoter region
Describe how transcription can be regulated using transcription factors
- Transcription factors move from cytoplasm to nucleus
- Bind to DNA at a specific DNA base sequence on a promoter region (before / upstream of target gene)
- This stimulates or inhibits transcription (production of mRNA) of target gene(s) by helping or preventing RNA polymerase binding
Explain how oestrogen affects transcription
- Oestrogen is a lipid-soluble steroid hormone so diffuses into cell across the phospholipid bilayer.
- In cytoplasm, oestrogen binds to its receptor, an inactive transcription factor, forming an oestrogen-receptor complex
- This changes the shape of the inactive transcription factor, forming an active transcription factor
- The complex diffuses from cytoplasm into the nucleus
- Then binds to a specific DNA base sequence on the promoter region of a target gene
- Stimulating transcription of target genes forming mRNA by helping RNA polymerase to bind
Explain why oestrogen only affects target cells
Other cells do not have oestrogen receptors
What is RNA interference (RNAi)?
● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA
● This inhibits expression of (silencing) a target gene
