Topic 3.3 Flashcards
Stem cells
Unspecialised cells that divide by mitosis to become new cells which specialise via differentiation
How can stem cells differentiate into cells?
By potency, there’s two types:
- Totipotency: ability to produce any specialised cell including extra embryonic cells
- Pluripotency: ability to produce any specialised cells except extra embryonic cells
Differential gene expression
Stem cells are specialised via different gene expression where genes in their DNA become active and get expressed
1. Certain chemicals cause a gene to become activated and so its transcribed producing mRNA
2. mRNA from the active genes is translated into proteins
3. These proteins modify the cell determining the cells structure and function
4. Changes to the cell by these proteins cause the cell to become specialised
Transcription factors
Gene expression can be controlled by altering the rate of transcription of genes via transcription factors which are proteins that bind to DNA and activate or deactivate genes determining whether transcription of the genes occurs
Transcription factors: activators
Factors that increase rate of transcription
-> allow RNA polymerase to bind to DNA and so transcription takes place
Transcription factors: repressors
Factors that decrease rate of transcription
-> prevent RNA polymerase from binding to DNA and so stops transcription
Transcription factors: eukaryotes
Transcription factors in eukaryotes bind to specific DNA sites near the start of their target genes
Transcription factors: prokaryotes
Transcription factors in prokaryotes bind to operants which are sections of DNA made up of three sections
. Regulatory gene: codes for a transcription factor
. Control elements:
- promoter -> where RNA polymerase binds to
- operator -< where transcription factors bind to
Structural genes: code for useful proteins
Lac Operon in E.coli
E.coli is a bacterium that can use lactose to respire if glucose isn’t available
-> the structural genes that produce enzymes needed to respire lactose are found on an operon called lac operon
Lac Operon in E.coli: lactose not present
Regulatory gene produces lac repressor which binds to operator when no lactose is present preventing transcription of structural genes as it blocks the binding of RNA polymerase to the promotor
Lac Operon in E.coli: lactose present
Lac repressor produced by regulatory gene binds to lactose present which changes the repressor’s shape so it can no longer bind to the operator allowing RNA polymerase to bind to the promoter so transcription of structural genes occurs
Obtaining stem cells: adult stem cells
Obtained from body tissues of an adult
. Simple operation with little risk involved but quite a lot of discomfort
. Adult stem cells are pluripotent so they aren’t as flexible as embryonic stem cells
Obtaining stem cells: embryonic stem cells
Obtained from early embryos created by in vitro fertilisation (IVF)
. When embryos 4 to 5 days old stem cells are removed and the rest of the embryo is destroyed
. Embryonic cells are totipotent so very flexible
. People disagree with the use of embryos as they believe at the moment of fertilisation life begins si its wrong to destroy embryos
Society decisions on use of stem cells
Regulatory authorities have been established to consider the benefits and ethical issues surrounding embryonic stem research and they work by:
. Looking at proposals of research by taking ethical issues into account to make sure research is done for a good reason and to prevent repetitions
. Licensing and monitoring centres involving embryonic stem cell research to ensure only fully trained staff carry out research and to avoid unregulated research
. Producing guidelines and codes for practice so all scientists work in a similar manner using acceptable sources and controllable methods
. Monitoring developments in research to ensure changes in field are regulated and guidelines are up to date
. Providing guidance to government and professionals to promote the science involved in embryo research and help society understand its importance
Stem cells in medicine
. Scientists believe they can help replace damaged tissues in various diseases
Eg. Treatment like lukemia, where bone marrow transplants replace destroyed stem cells
. Scientists are exploring stem cell treatments for:
- spinal cord injures -> repair damaged nerve tissue
- hearts disease & heart attack damage -> replace damaged heart tissue
Potential benefits of use of stem cells in medicine
. Save lives -> can grow organs, reducing wait times for transplants
. Improve quality of life -> may help restore vision for the blind
Continuous variation
Individuals in a population vary within a range
-> no distinct categories
Discontinuos variation.
Individuals in a population fall into discrete categories
-> no intermediates
Variation in phenotype
Most phenotypes are a result of an interaction between genotype and environment
Variation in phenotype: variation due to genotype -> myogenic
Phenotype controlled by ONE gene
-> gives rise to discontinuous variation in phenotype
Variation in phenotype: variation due to genotype -> polygenic
Phenotype controlled by a number of genes at different loci
-> gives rise to continuous variation in phenotype
Epigenetic control
In eukaryotes epigenetic control can determine whether certain genes are expressed, altering the phenotype
-> Epigenetic control doesn’t change the base sequence of DNA it works by attaching or removing chemical groups from DNA affecting how easily proteins needed for transcription interact with transcribe genes
Methylation of DNA
. Methyl group (-CH3) is attached to the DNA coding for a gene at a CpG site (cytosine and guanine bases are next to each other)
. Methylation of DNA stops the transcription of the affected gene by preventing binding of transcription factors and enzymes needed for transcription (eg. RNA polymerase)
-> the gene is repressed (not expresses)
Modification of histones
. Histones -> proteins around which DNA wraps around chromatin
-> how condensed chromatin is affects the accessibility of the DNA and so whether proteins and enzymes needed for transcription can bind to it
. When acetyl groups are added to histones, chromatin gets less condensed and so proteins involved in transcription can bind to DNA allowing genes to be transcribed (genes are activated)
. When acetyl groups are removed from histones, chromatin becomes highly condensed and so genes in the DNA can’t be transcribed because transcription proteins can bind to them (genes are repressed)
Inheritance of epigenetic changes
When cells divide and replicate, epigenetic changes to its gene expression may be passed to its daughter cells so that certain genes that were activated/deactivated in the daughter cells
- If the epigenetic change occurred in original cell in response to a change in the environment the daughter cell will be equipped to deal with the changed environment as the original cell was able to
