How development is controlled Flashcards
Do stem cells have the same genome? Are they able to specialise into a diverse range of cell types? What happens during differentiation?
- all stem cells within an organism have an identical genome
- despite having same genome, they are able to specialise into a diverse range of cell types because during differentiation certain genes are expressed /switched on
How do stem cells become specialised? What does this mean?
- stem cells become specialised through differential gene expression
- this means that only certain genes in the DNA of the stem cell are activated
What is an operon? Where are they found? What does it include?
- an operon (are only present in prokaryotes and archaea) is a section of DNA that includes:
- a cluster of structural genes that are transcribed together
- control elements, including a promoter region and an operator region
- some operons may include regulatory genes that code for activators or repressors
What is a promoter region? What is an operator region?
- promoter region = a DNA sequence that RNA polymerase initially binds to)
- operator region = (where transcription factors bind)
What does the lac operon contain?
- the lac operon contains genes that encode proteins involved in uptake and metabolism of a particular sugar lactose
What is a repressor protein?
- (is an RNA binding protein) that inhibits gene expression (by stopping RNA polymerase from binding to the DNA of a gene)
What is an activator protein?
- (is an RNA binding protein) that stimulates gene expression (by helping RNA polymerase from binding to the DNA of a gene)
What is apoptosis? How does this happen?
- controlled cell death (apoptosis)
- there is a small group of cell ‘suicide’ genes and when they are expressed this causes the cytoplasm and nucleus to fragment
What is the key to development and why?
- controlling gene expression is the key to development because stem cells differentiate due to the different genes being expressed
What does it mean if a cell becomes specialised?
- cell specialization means that cells change into specific cells
- when a cell becomes specialised it can carry out different (specific) functions
What are the basic steps that differentiation occurs in?
- under certain conditions, some genes in a stem cell are activated, whilst others are inactivated
- mRNA is transcribed from active genes only
- this mRNA is then translated to form proteins
- these proteins are responsible for modifying the cell, the cell becomes increasingly specialised
- the process of specialisation is irreversible (once differentiation has occurred, the cell remains in its specialised form)
What is a transcription factor? What do eukaryotes use this for? What does it ensure?
- a transcription factor is a protein that controls the transcription of genes by binding to a specific region of DNA
- eukaryotes use transcription factors to control gene expression
- it ensures that genes are being expressed in the correct cells, at the correct times and to the right level
What are activators? How do they work?
- activators are transcription factors that increase the rate of transcription
- activators work by helping RNA polymerase to bind to the DNA at the start of a gene and to begin transcription of that gene
What are repressors? How do they work?
- repressors are transcription factors that decrease the rate of transcription
- they work by stopping RNA polymerase from binding to the DNA at the start of a gene, inhibiting transcription of that gene
What happens if transcription factors bind to the promoter region of a gene?
- binding here can either allow or prevent the transcription of the gene taking place
- transcription factors interact with RNA polymerase, either by assisting RNA polymerase binding to the gene (stimulate gene expression) or by preventing it from binding (inhibiting gene expression)
- therefore, the presence of a transcription factor will either increase or decrease the rate of transcription of a gene
What does control of gene expression require in prokaryotes?
- in prokaryotes, control of gene expression requires the binding of transcription factors to operons
What is an operon? What does it include? What are its control elements?
- an operon is a section of DNA that includes:
- a cluster of structural genes that are transcribed together (these code for proteins e.g. enzymes)
- control elements, including a promoter region (a DNA sequence that RNA polymerase initially binds to) and an operator region (where transcription factors bind)
- some operons may include regulatory genes that code for activators or repressors
What is the basic definition of an operon?
- a group or cluster of genes that are controlled by the same promoter
What does the lac operon control? And what does this do?
- the lac operon controls the production of the enzyme lactase (aka beta galactosidase) and two other structural proteins
- lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell
Why is lactase known as an inducible enzyme?
- it is known as an inducible enzyme which means it is only synthesized when lactose is present
- this helps the bacteria avoid wasting energy and materials
What are the components of the lac operon?
in the following order:
- promoter for structural genes
- operator
- structural gene lacZ that codes for lactase
- structural gene lacY that codes for permease (allows lactose into cell)
- structural gene lacA that codes for transacetylase
What is epigenetics the control of? What does it involve?
- epigenetics is the control of gene expression by factors other than an individuals DNA sequence
- epigenetics involves the switching on and switching off of genes, but without changing the actual genetic code
In eukaryotic cells, what is DNA wrapped around?
- in eukaryotic cells, DNA is wrapped around proteins called histones to form chromatin
How can chromatin be chemically modified and what would that do? What are these modifications called?
- chromatin can be chemically modified to alter gene expression:
- by methylation of DNA (chemical addition of -CH3 groups)
- histone modification via acetylation of amino acid tails
- these modifications are called epigenetic tags
What is an epigenome?
- all the epigenetic tags in an organism is called the epigenome
What does the chemical modification of histones and DNA control?
- the chemical modification of histones and DNA controls how tightly the DNA is wound around them as the intermolecular bonding between the histones and DNA changes
What would happen to a section of DNA where the DNA is wound more tightly?
- If the DNA is wound more tightly in a certain area, the genes on this section of DNA are ‘switched off’ as the gene and promoter regions are more hidden from transcription factors and RNA polymerase
In basic terms, how does methylation cause the gene to be switched off?
- methyl group = attach to DNA (cytosine)
===> RNA polymerase cannot bind
===> mRNA not made
===> no transcription
===> switched off
How does methylation cause the inactivation of genes?
- methyl groups (-CH3) can attach directly to DNA to change the activity of a gene
- DNA methylation involves the addition of a methyl group to cytosine bases which influences gene expression
- methylation inhibits the binding of transcription factors and enzymes needed for transcription (e.g. RNA polymerase)
Why do cells use methylation?
- cells use this mechanism to lock genes in the ‘off’ position
- the gene is said to be repressed or inactivated
What are on histones that help DNA coil around the histone protein core?
- Lysine are amino acids on histone proteins
- lysine has a positively charged R group, this forms ionic bonds with the negatively charged phosphate backbone of DNA
- this helps DNA to coil tightly around the histone core
What happens when adding acetyl groups to lysine residues?
- adding acetyl groups (-COCH3) (acetylation) to lysine residues removes the positive ion and therefore removes a bond between the histone and DNA, this causes the DNA to be less tightly wrapped
What happens when DNA is less tightly wrapped? What is the gene said to be?
- when the DNA is less tightly wrapped, RNA polymerase and transcription factors can bind more easily and therefore gene expression can occur
- the gene is said to be activated
In summary, what does methylation do? What does acetylation do?
- methylation adds a methyl group to DNA at cytosine which inhibits transcription
- acetylation adds acetyl groups to lysine residues which removes the bond between lysine and phosphate backbone which causes DNA to be less tightly wound = gene expression can occur
What does deacetylation do?
- deacetylation removes acetyl groups and returns lysine to its positively charged state which has a stronger attraction to the DNA molecule and therefore inhibits transcription and once again stops the gene from being expressed
What does acetylation cause chromatin to become? What does this allow?
- acetylation causes chromatin to become less condensed, allowing genes to be transcribed
Is the epigenome heritable?
- like the genome, the epigenome is heritable and can be passed on in cell division
