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AQA A level Biology Topic 8 – The control of gene expression - Notes (For 2023)
AQA A2 level Biology Topic 8 – The control of gene expression - Notes (For 2023)
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Stem cells and totipotencyStem cells: retain the ability to divide and differentiate into a range of specialised cell
types
- Given the correct chemical and genetic signals stem cells can differentiate into
highly specialised cells
- There are two broad categories of stems cell:
- Embryonic stem cells
- Adult stem cells
- Keeps one self renewal stem cell, while creating specialised cells to continue
replicating
Timeline of stem cells
1. Embryonic stem cells: totipotent stem cells, which can mature into any body
cell
- only present in mammals in the first few divisions of embryo
- Blastocyst: a zygote which has undergone mitosis for ⅚ days creating a
ball of cells
- Cells on the inner surface are pluripotent
2. Umbilical cord blood stem cells: derived from umbilical cord blood and are
similar to adult stem cells
, 3. Placental stem cells: found in the placenta and develop into specific types of
cells
4. Tissue stem cells: stem cells in foetus, baby and adults
- specific to a particular tissue or organ within which they produce the cells
to maintain and repair tissues throughout an organism’s life
- unipotent stem cells
Types of stem cells (in differentiating order)
1. Totipotent cells: can differentiate into any type of cell
- found in early embryo
- all 220 types of cells
- As the zygote divides and matures, its cells develop into slightly more
specialised cells called pluripotent stem cells
2. Pluripotent stem cells: found in embryos and can differentiate into almost any
type of cell
- any of the three germ layers
3. Multipotent stem cells: found in adults and can differentiate into a limited
number of specialised cells
4. Unipotent stem cells: can only differentiate into a single type of cell
- derived from multipotent stem cells
- made in adult tissue
,
, 1. Differentiation and specialisation of totipotent cells to shoot/root cells
2. Micropropagation in vitro
3. Clone and Mitosis
4. High IAA and 2,3-D
5. IAA affects shoot, like shown in 3. Cytokinin affects root development.
Becoming specialised
Regulation of differentiated cells: During development of a cell not all of the DNA will
be transcribed and translated. So, only certain genes are expressed (turned on) in any
one cell at any one time for its function:
1. Some genes are permanently expressed in all cells
2. Some are permanently switched off
3. Some are switched on only when needed
- This depends on signals from the environment, like hormones and growth
factors
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