Gene expression Flashcards
Cell differentiation
When a cell develops into a more specialised cell suited to its function
4 classes of stem cells
Totipotent
Pluripotent
Multi potent
Unipotent
Totipotent stem cells
In an early embryo, after the very first few cell divisions
Can differentiate into any cell
Diploid zygote cell
Early cells of a fertilised egg
Pluripotent stem cells
After more cell divisions after the first few cell divisions
Can spécialisé into any type of cell except placenta and umbilical cells
Can self renew (and differentiate into any of the three germ layers)
Embryonic stem cells
Induced pluripotent stem cells
Multipotent stem cells
Found in mature mammals only
Can only differentiate and divide into a limited number of cell types
Hematopoietic stem cells - differentiate into cells of the blood and immune system
Neural stem cells - differentiate into cells of the neurons and glia etc
Unipotent stem cells
Found only in mature mammals
Can only differentiate into a single type of cell
Skin stem cells
Muscle stem cells
Cardiomyocyte stem cells
What are induced pluripotent stem cells?
Derived from unipotent cells (eg. skin or blood cells) and are reprogrammed back to an embryonic-like pluripotent state
How are induced pluripotent stem cells made?
Adult cells made to express transcription factors normally associated with pluripotent stem cells so it causes the adult cells to express genes associated with pluripotency
Transcription factors are introduced into adult cell by infecting with a modified virus that has genes coding for the transcription factors in its DNA. When the virus infects the cell these genes are passed to the adult cells DNA
What are adult stem cells specific to?
Specific to the particular organ or tissue they are in. Produce cells to maintain and repair specifically those tissues.
Two ways genes are prevented from expressing themselves?
Prevent transcription (production of mRNA) Prevent translation
Talk about xylem vessels and red blood cells
Xylem vessels and red blood cells are so specialises that they lose their nuclei once they mature so they cannot develop into other cells
Embryonic stems cells ethical issues
Whether an embryo 14< should have same respect as a fétus or adult
Using embryos undermines respect for human life
One step closer to cloning
Embryos just a ball of cells, nothing like a human
Wrong to allow human suffering to continue when there is the possibility of alleviating it
Embryos already used in fertility treatments so no sense in destroying spare ones when they could be used for research
What are transcriptional factors?
Specific molecule that move from the cytoplasm to the nucleus to switch a gene on
Usually protein molecules
What do transcriptional factors have?
A site that binds to a specific base sequence on the DNA
What do transcriptional factors do?
Control the rate of transcription of genes
If a gene is not expressed what has happened to the transcriptional factor?
The binding site on the transcriptional factor (that binds to the DNA of the gene it will activate) is INACTIVE
Explain how oestrogen aid a transcription factor
Oestrogen binds to a specific complimentary receptor on the transcription factor forming and oestrogen-oestrogen receptor complex
This causes the transcription factor’s binding site to change shape and become complimentary to the specific base sequence of the gene it will activate
Transcription factors can be both
Repressors and activators
How do transcription factors which activate work?
Stimulate/increase rate of transcription
Help RNA polymerase bind to start of target gene
Activate transcription
How to transcription factors that repress work?
Inhibit/decrease rate of transcription
Bond to start of target gene, preventing RNA polymerase from binding
Stop transcription
Explain how siRNA in mammals and miRNA in plants inhibits translation of mRNA (5)
1) mRNA leaves cytoplasm
2) double stranded siRNA in cytoplasm associates with proteins and unwinds
3) single strand bonds to target mRNA (base sequence of siRNA is complimentary to base sequence of mRNA)
4) proteins associated with the siRNA strand cut the bound mRNA into fragments
5) fragments move into a processing body which degrades them
Explain how miRNA works in mammals to prevent translation (4)
1) miRNA is NOT fully complimentary to target mRNA and it is less specific than siRNA so can target more than one mRNA molecule
2) associates with proteins and binds to the target mRNA in the cytoplasm
3) miRNA-protein complex blocks translation of the mRNA (but doesn’t cut the mRNA into fragments)
4) mRNA moved into a processing body where it is stored/degraded. If it is stored it can be returned and translated another time.
Explain how E.coli digests lactose.
What happens when no lactose vs yes lactose
1) E. Coli can digest glucose and lactose (if there is no glucose).
2) However to digest lactose it needs an enzyme. The gene for this enzyme is usually switched off to prevent energy being wasted so in the presence of lactose the gene is switched on.
No lactose:
lac repress or binds to gene, stopping transcription because enzyme not needed and don’t want to waste energy
Yes lactose:
lactose binds to lac repressor, stopping it from binding to DNA so gene is transcribed, enzyme made and lactose digested.
Define epigenetics
Study of heritable phenotype changes that don’t involve alterations to the DNA base sequence
Inactive gene (table - (5))
Decreased acetylation Increased methylation DNA-histone complex is more condensed Heterochromatin Transcription factors have no access Gene is INACTIVE
Active gene (table - (5))
Increased acetylation Decreased methylation DNA-histone complex is less condensed Euchromatin Transcription factors have access Gene is ACTIVE
Explain how increased methylation can inactivate a gene
Methyl groups (chemical tags) are added to CpG sites on DNA
These attract proteins that condense the DNA-histone complex to form heterochromatin
DNA inaccessible to transcription factors
Cannot initiate mRNA transcription
Gene INACTIVE
Explain how decreased acetylation can make a gene inactive
Decreased acetylation increases the positive charge on the histones
This increases attraction to the negative phosphate groups DNA
Stronger associations between DNA and histones so more condensed and forms heterochromatin structure
DNA inaccessible to transcription factors
Cannot initiate mRNA production
Gene INACTIVE
What is acetylation?
An acetyl group is transferred to a molecule (donated from an acetylcoenzyme A in this case)
For a gene to be inactive, there will be _______ methylation and _________ acetylation.
Increased methylation
Decreased acetylation
For a gene to be active, there will be _______ methylation and _________ acetylation.
Decreased methylation
Increased acetylation
How environmental changes affect epigenetics?
Diet and stress can cause DNA to condense/uncondense and switch genes on/off
How are the symptoms of Fragile X syndrome caused? (4)
Caused by duplication mutation where CGG is repeated multiple times
Increases CpG sites and so increases methylation
Gene switches off and protein it codes for is not produced
Lack of this protein causes symptoms of this disease
Cancer genes switching on and off stuff
Caused by activation of normally inactive genes
Active genes that usually repair DNA are switched off
Normal cells: sections near promoter region of gene have no methylation
Cancer cells: these areas become highly methylated and genes that should be active are switched off
Epigenetics and inheritance
Mother has gestational diabetes
Fétus exposés to high concentration of glucose
Causes epigenetics changes to daughters DNA
Increases likelihood of her getting gestational diabetes
What does aza…ine drug do?
AZACITIDINE
used in chemotherapy for cancer caused by increased methylation of tumour suppressor genes
What does HDAC stand for?
HDAC (histone deacetylase enzymes) inhibitor drugs
Give and example of an HDAC drug and what it does.
ROMIDEPSIN
- disease caused by decreased acetylation so histone deacetylase enzymes are inhibited with this drug.
Genes remain acetylated
Transcription can occur
How are drugs in cancer therapies designed?
Very specific and on only target rapidly dividing cells
Epigenetics therapy
- drugs that inhibit enzymes that cause DNA methylation can reactivate genes that were silence by methylation
- drugs to inhibit certain enzymes involved in histone acetylation or DNA methylation
- Diagnostic test: detect early stages of cancer/brain disorders/arthritis by identifying the level of DNA methylation or histone acetylation
