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Pack 14 - Control of gene expression
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Revision resource for gene expression
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biology
a level
gene expression
biology
pack 14 - the control of gene expression
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Jacob Shepherd
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Jacob Shepherd
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Resource summary
Pack 14 - Control of gene expression
Gene mutations
Usually bad but allows for variation
Caused by mutagenic agents
caused by:
Chemicals
High energy ionising radiation
Can be random and spontaneous
Different types:
Addition
Addition of one nitrogen containing base.
Deletion
Removal of one nitrogen containing base.
Causes frame shift mutation
Substitution
Nonsense mutation
Base change results in formation of one of the three stop codons
Mis-sense mutation
Where the base change results in a different amino acid being coded for
Silent mutation
DNA degeneracy means it may still code for the same amino acid
Inversion
Piece of DNA flips over
Duplication
Whole codon or base duplicated
Stem Cells and Totipotency
Differentiation:
1. All cells contain the same genes
2. Some genes are permanently expressed in all cells (mitochondria)
3. Some genes are permanently not expressed (no insulin in small intestine
4. Other genes are switched on and off when needed
5. This will lead to differentiation as different proteins are expressed in various cells.
Stem Cells
Undifferentiated cells
Need to be constantly replaced
Ability to divide to form copies of themselves
In mammals stem cells are found in the:
(for embryonic stem cells)
Differentiate into al types of cells
Can be used to cure things like heart damage, diabetes and blindness etc.
(Umbilical cord stem cells)
Blood similar to adult stem cells
(placental cells)
Placenta differentiate into specific cells
(adult stem cells)
Found in body tissue of foetus
...potent
Totipotent = differentiate into all cells
Zygotes
Pluripotent = differentiate into most cells
Embryos
Multipotent = differentiate into limited cell
Adults
Unipotent = differentiate into single cell type
e.g. retina
Induced pluripotent stem cells
A type of pluripotent cell produced from a unipotent cell, genetically altered in a lab with transcription factors
Advantages:
Can obtain stem cells from adults
Produce all cell types from adult cells
Able to provide original cells from patient, so no immune response
Plant tissue culture
Features of plant growth regulators:
Wide range of effects on plant tissue
Effects on a particular tissue depends upon the conc. of growth factor
Same conc. affects different tissues in different ways
Effect of one growth can be modified by the presence of another
Growth regulators stimulate plant growth, use of in vitro development
In plants, many cells remain totipotent
Xylem cells cannot differentiate because they have no nucleus
Cells form tissue culture share same genetic material and so are clones
Regulation of transcription and translation
For cells to differentiate it must be switched on whilst the others are off.
Effects of oestrogen on transcription:
A transcription factor is
A protein that activates or inhibits ignition of eukaryote transcription, that goes form the cytoplasm to the nucleus
Transcription factors:
Each transcription factor moves from cytoplasm to the nucleus
It has a specific site which binds to a specific base sequence of DNA called a promoter
The binding of the transcription factor allows the RNA polymerase to bind which allows transcription to occur
This causes the mRNA to be produced which is translated to produce increased amounts of protein
The gene is said to be switched on
When the gene is switched off the site of the DNA where the transcription factor binds is inactive
No transcription can occur so no polypeptide synthesis
Oestrogen:
Sex hormone
Lipid soluble
Allows it to enter the cell directly
Stimulation of transcription factor:
Oestrogen diffuses through bilayer
Binds to receptor attached to transcription factor
TF changes shape
TF able to enter the nucleus and bind to DNA and initiate transcription
Epigenetic control of gene expression
Epigenetics is the process by which environmental factors can causes heritable changes in gene function without changing the base sequence of DNA
Features of a genome:
DNA sequences do not change
DNA wrapped around histones
Chemicals attached to histones
Chemicals determine shape of DNA - histone complex
Inactive genes kept tightly packed (epigenetic silencing)
Unwrapped regions of active genes for transcription
Factors that influence the epigenome in the:
Foetus
Signals within cells
Nutrition from Mother
Following birth
Environment
Signals within body (e.g. hormones)
Controlled by increased methylation of DNA
Or, decreased acetylation of associated histones
Acetylation
Transfer of acetyl group to a molecule
Acetyl co-enzyme A is the donor
(de-acetylation is the reverse process)
How decreased acetylation of histones reduces transcription:
1. Acetylation increases +ve charge on histones
2. This increases their attraction to phosphate groups of DNA
3. Stronger association between DNA and histones
4. Transcription factors unable to bind
5. No transcription mRNA
Methylation
Addition of methyl group (CH3) to molecule
It is added to the cytosine base of DNA
How increased methylation of DNA can reduce transcription:
1. Addition of methyl group to molecule
2. Added to cytosine base of DNA
3. Prevents binding of transcription factors
4. Attracts proteins that condense DNA-histone complex making DNA inaccessible to transcription factors
How does DNA-histone complex association affect transcription?
Weak association:
Complex less condensed
Easier access for transcription factors
Increased transcription
Strong association:
Complex more condensed
No access for transcription factors
Prevented transcription
Twin Studies:
Twins have been used to study influence of either the genome or the environment to humans
Identical twins may have different gene expression due to epigenetics (e.g. methylation)
Epigenetics and Inheritance
Epigenetics and disease:
How can epigenetics trigger cancer formation:
In healthy cells normally there is no methylation near promoters
Regions are highly methylated in cancer cells
Genes which should be active, are therefore switched off.
Occurs in early development cancer
People with inherited cancer have increased methylation in this type of gene
Epigenetic Therapy:
Treatments:
Drugs inhibit enzymes involve in histone acetylation or DNA methylation - can re-activate genes
Drugs need to be carefully targeted - other wise risk of cancer
Diagnostics
Identify level of DNA methylation and histone acetylation at early stage of disease
Effect of RNA interference on gene expression
In eukaryotes and some prokaryotes, the translation of mRNA can be inhibited by breaking down the mRNA before translation can occur.
The mechanism of siRNA:
1. An enzyme cuts large strands of double stranded RNA into small sections called siRNA
2. One of the two siRNA combines with an enzyme
3. The siRNA molecule guides the enzyme to the mRNA by pairing up bases with complementary ones in section of the RNA molecule
4. Once in position the enzyme cuts mRNA into small sections
5. The mRNA is no longer capable if being translated into a polypeptide
6. Gene expression now no longer occurs
Epigenetics and Cancer
What is cancer?
Group of diseases caused by damage to the genes that regulate mitosis and the cell cycle
Generally derived from a single cell
Unrestrained growth of cells
Group of abnormal cells develops and continuously grows in size
Benign = Stays in one location
Malignant = can break off and cause additional damage
Steps in mutation that create a cancer cell:
Initiation mutation:
Creation of uncontrolled mitosis
Mutation in descendant cell changes subsequent cells to be different from normal
Genetic control of cell division:
Proto-oncogenes:
Stimulate cell divison
How a proto-oncogene can become permanently expressed (form oncogene):
1. Gene for growth factor permanently expressed, stimulating cell division
2. Receptor protein on cell surface membrane permanently activated, even in absence of growth factor, stimulating cell division
The result of this produces oncogenes which affect cell division in two ways:
The receptor protein can be permanently activated so that cell division is on even in the absence of growth factors
The oncogene may code for excessive amounts of the growth factor, cell division is stimulated
Tumour suppressor genes:
Slow down cell division, repair mistakes in DNA and initiate cell death
If it becomes mutated it will be inactivated so cell division will increase, forming a tumour
3 features of a tumour
Divide continuously
Originates from a single cell
Cells continue to divide and not die
Genome Project
Bioinformatics:
Science of collecting and analysing complex biological data such as genomes via use of algorithms
Electrophoresis:
DNA fragments of different sizes have an electric charge (DNA=-ve) so it moves to the +ve, further fragment = smallest
DNA Sequencing:
Whole Genome Shotgun:
1. Cut DNA into small sequences
2. Use computer algorithms to align overlapping segments
3. Assemble the entire genome
Advantages:
Allows terminators and other components to be run in one pot instead of 4
Use colours to distinguish between different terminators
Less labour intensive
Faster turnaround
Original sequencing process was Sanger sequencing
Role of Dideoxyribose nucleotides:
Different shapes to deoxyribose nucleotides and so cause the DNA chain to terminate
Role of primer:
Short piece of DNA that gives DNA polymerase something to attach DNA nucleotides to
Single Nucleotide polymorphisms: (SNP's)
Single base variations in the genome associated with disease and other disorders
Genome = Genetic material of an organism
Proteome = All of proteins produced in a given cell by a genome
Human Microbiome project (bacteria)
Help cure disease - gives insight to metabolism of organism
Find useful genes - clean up pollutants and make biofuels
Determining the proteome in prokaryotes is easier than in eukaryotes because:
Only one circular piece of DNA, not associated to histones
None of the non-coding portion of DNA that are found in eukaryotes
Determining the genome and proteome in more complex organisms is difficult because:
Many non-coding genes
Many genes involved in regulating other genes
Slight variation in DNA sequences between individuals of same species
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