Topic 8 : the control of gene expression Flashcards
What is a gene mutation?
A change in the DNA base sequence which arises spontaneously during DNA replication.
What is a mutagenic agent?
A factor which increases the rate of mutation, e.g. UV light, alpha particles.
What are substitution mutations?
A base is replaced by a different base in DNA
What are deletion mutations?
1 or more DNA bases are lost from the DNA base sequence
What are addition mutations?
1 or more DNA bases are added to the DNA base sequence
What are inversion mutations?
A sequence of DNA bases detaches from the DNA sequence and then rejoins at the same position in the reverse order.
What are duplication mutations?
A sequence of DNA bases is repeated.
What is frameshift
- Occurs when mutations change the number of nucleotides by a number not divisible by 3
- This shifts the way the genetic code is read so all the DNA triplets downstream from the mutation change so significant effects on the encoded polypeptide.
How does a gene mutation lead to the production of nonfunctional protein
- Changes the base triplets in DNA so changes the sequence of codons on mRNA
- So changes sequence of amino acids in the polypeptide (primary structure changed)
- So position of hydrogen/ionic/disulphide bridges between R groups changes
- So different tertiary structure of the protein
- For enzymes, the active site changes shape so it is no longer complementary to substrate so substrate cannot bind, so enzyme-substrate complexes can’t form
What are stem cells?
Stem cells are undifferentiated cells capable of dividing by mitosis to clone and replace themselves, and differentiating into other types of specialised cells.
How do stem cells become specialised
Stimuli lead to the activation of some genes due to transcription factors so mRNA is transcribed only from these genes and then translated to form proteins. These proteins modify cells permanently and determine the cell structure or function.
What are totipotent cells
Stem cells that can divide and differentiate into any type of body cell (including extra-embryonic cells, e.g. placenta). These occur for a limited time in early mammalian embryos.
What are pluripotent cells
Stem cells that can divide and differentiate into most cells (every cell types in the body except placental cells). Found in mammalian embryos after first few cell divisions.
What are multipotent cells
Stem cells that can divide and differentiate into a limited number of cell types. They are found in mature mammals.
Example of multipotent cells
Bone marrow cells- divide and differentiate into different types of blood cells.
What are unipotent cells
Stem cells that can divide and differentiate into just one cell type. They are found in mature mammals.
Example of unipotent cell
Unipotent cells in the heart can divide and differentiate into cardiomyocytes (cardiac muscle cells)
How are induced pluripotent stem (iPS) cells produced
- Obtain adult somatic (body) cells from patient
- Add specific protein transcription factors associated with pluripotency to cells so they express genes associated with pluripotency. [The transcription factors attach to promoter regions, stimulating or inhibiting transcription.]
- Culture cells to allow them to divide by mitosis.
Once they are made, iPS cells can divide and differentiate into healthy cells to be transplanted into the same patient.
Evaluate the use of stem cells
FOR:
1. Can divide and differentiate into required healthy cells so could relieve human suffering by saving lives and improving quality of life
2. Embryos are often left over from IVF and so would otherwise be destroyed.
3. iPS cells unlikely to be rejected by the patient’s immune system as they are made with the patient’s own cells.
4. iPS cells can be made without the destruction of embryos and the adult can give permission
AGAINST:
1. Ethical issues with using embryonic stem cells as obtaining them requires the destruction of embryos and potential life.
2. Immune system could reject cells and immunosuppressant drugs are required
3. Cells could divide out of control, leading to the formation of tumours
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.
How does oestrogen initiate transcription
- Oestrogen is a lipid-soluble hormone so it can diffuse into the cell across the phospholipid bilayer
- In the 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
- This complex diffuses from the cytoplasm into the nucleus and binds to a specific DNA bae sequence on the promoter region of a target gene
- This stimulates the transcription of target genes, forming mRNA by helping RNA polymerase to bind.
What are epigenetics?
Heritable changes in gene function/expression without changes to the base sequence of DNA, caused by changes in the environment
What is epigenome?
All chemical modification of DNA and histone proteins- methyl groups on DNA and acetyl groups on histone proteins.
To inhibit transcription…
Increased methylation of DNA and decreased acetylation of histones
