Section 8 - 20 Gene Expression Flashcards
What is a mutation?
Any change to the quantity or the structure of the DNA of an organism.
What is a gene mutation?
Any change to one or more nucleotide bases, or any rearrangement of the bases, in DNA
What is a substitution mutation?
When a nucleotide in a section of DNA molecule is replaced by another nucleotide that has a different base.
What are three possible consequences of substitution mutation?
- formation of one of the three stop codons that mark the end of a polypeptide chain. - stopped prematurely
- Formation of a codon for different amino acid meaning structure of polypeptide produced would differ in single amino acid. - not correct shape or function
- formation of a different codon but one that produces a codon for the same amino acid - degenerate nature.
What is a deletion mutation?
The loss of a nucleotide base from a DNA sequence
What does a deletion mutation cause?
A frame shift
a reading frame that contains every three letters of the code has been shifted to the left by one letter.
What is addition mutation?
The extra base becomes inserted in the sequence.
Cause a frame shift to the right.
What is duplication mutation?
One or more bases are repeated.
Produces a frame shift to the right.
What is an inversion mutation?
Group of bases become separated from the DNA sequence and rejoin at the same position but in the inverse order.
Base sequence is therefore reversed and affects the amino acid sequence that results.
What is a translocation mutation?
Group of bases become separated from the DNA sequence on one chromosome and become inserted into the DNA sequence of a different chromosome.
Often having significant effects on gene expression leading to an abnormal phenotype.
These effects include the development of certain forms of cancer and also reduced fertility.
Typically how many natural mutations occur?
One or two mutations per 100000 genes per generation.
What two outside factors affect the basic mutation rate?
- mutagenic agents
- mutagens
What is included in the terms mutagenic agents or mutagens?
- high energy ionising radiation
- chemicals
What is cell differentiation?
The process why which each cell develops into a specialised structure suited to the role that it will carry out.
Another name for a fertilised egg
Zygote
What are totipotent cells?
A fertilised egg which has the ability to give rise to all types of cells.
What do controlling factors do?
ensure that genes for proteins are not expressed.
In what ways do genes prevented from expressing themselves?
- preventing transcription and so preventing the production of mRNA
- preventing translation
At what point can cells no longer specialise into another?
Once cells have matured and specialised they can no longer develop into other cells.
What are stem cells?
Undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced.
What are stem cells capable of?
They have the ability to divide to form an identical copy of themselves in a process called self-renewal.
Where do stem cells originate?
- embryonic stem cells
- umbilical cord blood stem cells
- placental stem cells
- adult stem cells
Where are totipotent stem cells found?
In the early embryo.
What are pluripotent stems cells?
Found in embryos
Can differentiate into almost any type of cell.
Examples of pluripotent stem cells
Embryonic stem cells
fetal stem cells
What are multipotent stem cells?
Found in adults
Differentiate into a limited number of specialised cells.
Devlop into cells of a particular type.
Give examples of multipotent stem cells
Adult stem cells
umbilical cord blood stem cells
What are unipotent stem cells?
Can only differentiate into a single type of cell.
Derived from multipotent stem cells and are made in adult tissues.
What are examples of unipotent stem cells?
Heart muscle cells that can divide to produce new heart tissue and so repair damage to heart muscle.
How can you produce induced pluripotent stem cells?
Type of pluripotent cell
produced by unipotent stem sells
How can you make induced pluripotent stem cells?
- Unipotent are almost any body cell
- these cells are genetically altered in a lab making them acquire characteristics of embryonic stem cells which are pluripoetnt
How do iPS and embryonic stem cells different?
iPS - capable of self-renewal - the potential to divide indefinitely to provide a limitless supply
How can pluripotent cells be used?
To regrow tissues that have been damaged in some way - accidents or disease
State the general principles involved in the control of gene expression by controlling transcription
- Genes switched on by specific molecule - transcriptional factors
- Each factor has a specific site to bind to a base sequence
- When bound transcription begins
- mRNA produced and info is translated to a polypeptide
- when genes did not express the site of factor is not active
- If not active then transcription and polypeptide synthesis can not occur.
What is a transcriptional factor?
Molecules that allow genes to be switched on.
This means transcription can occur.
These molecules move from the cytoplasm into the nucleus.
Give an example of a transcriptional factor
oestrogen
How can oestrogen affect genes?
Switch on a gene starting transcription.
Combine with a receptor site on transcriptional factor. - change shape of DNA binding site
- Oestrogen into cell
- Binds to the active site of the receptor molecule of factor. Changes shape so complementary with DNA binding site.
- Factor enters the nucleus through the nuclear pore and binds to base sequence on DNA
- Stimulates transcription of the gene.
How does oestrogen diffuse easily through a phospholipid portion of cell-surface membranes?
It is a lipid soluble molecule
Who proposed double-helix model in 1953?
James Watson and Francis Crick
What is epigentics?
Provides an explanation as to how environmental influences such as diet, stress and toxins can subtly alter the genetic inheritance of an organism’s offspring.
What is a histone?
Proteins that DNA is wrapped around.
Both DNA and histones are covered in chemicals called tags.
What is an epigenome?
A second layer of chemical tags that cover DNA and histones.
What does epigenome determine?
The shape of the DNA -histone complex.
(Keeping genes that are inactive ina tightly packed arrangement so ensuring that they cannot be read) - epigenetic silencing
What is epigenteic silencing?
Keeping genes that are inactive in a tightly packed arrangement so ensuring that they cannot be read
We know that the DNA code is fixed. However, how do we know that the epigenome is flexible?
Chemical tags respond to environmental changes.
Eg. diet and stress - causing chemical tags to adjust the wrapping and unwrapping of the DNA and so switching genes on and off.
What is the epigenome of a cell?
The accumulation of the signals it has received during its lifetime.
Acts like a cellular memory.
What two things can be caused by the attachment of proteins on specific sequences of bases?
- acetylation of histone - leads to the activation or inhibition of genes.
- Methylation of DNA - attracting enzymes that can add or remove methyl groups.
What is a chromatin?
The DNA-Histone complex
What happens to the chromatin when the association of histones with DNA is weak?
The chromatin (DNA-histone complex) is less condensed - loosely packed.
What happens when the chromatin is less condensed?
The DNA is accessible by transcriptional factprs
This can initiate production of mRNA switching genes on.
What happens when the association between histone and DNA is strong?
DNA-histone complex (chromatin) is more condensed. - tightly packed
What happens when the chromatin is more condensed?
DNA is not accessible by transcriptional factors - can not initiate production of mRNA.
Genes are switched off.
In what two ways can the DNA-histone complex be condensed?
- decreased acetylation of histones
- Methylation of DNA.
What is acetylation?
- The process whereby an acetyl group is transferred to a molecule.
- The group donating the acetyl group is acetyl coenzyme A - link reaction
Deacetylation is the reverse where the acetyl group is removed.
What does decreased acetylation do?
- Increases the positive charges on histones.
- This increases their attraction to the phosphate groups of DNA.
- The association between DNA and histones is stronger and the DNA is not accessible to transcriptional factors.
- These factors cannot initiate mRNA production from DNA.
- Gene is switched off.
What is methylation?
The addition of a methyl group to a molecule.
In terms of chromatin this group is added to the cytosine bases of DNA.
Methylation normally inhibits the transcription of genes is what two ways?
- Prevents binding of transcriptional factors to the DNA.
- Attract proteins that condense the DNA-histone complex (inducing deacetylation of histones) - makes DNA inaccessible to transcription factors.
What is epigenetics?
Environmental factors that cause heritable changes without changing the order of the DNA code.
How does epigenetics link to cancer?
- diseased tissue from patients with colorectal cancer have less DNA methylation.
- This reduces the inhibition of transcription do more gene activity.
- Also areas of promotor regions that should be switched are not this means some genes are switched off.
What is the epigenome?
A layer of chemicals called tags that are wrapped around histones on the DNA and proteins.
What does the epigenome determine?
The shape of the DNA
- keeps inactive genes tightly packed (epigenetic silencing)
- Also, switch on genes by unwrapping so DNA is exposed and code can be transcribed.
How can the epigenome be adjusted?
As it is flexible and tags respond to environmental changes diet and stress have been shown to adjust.
How can you treat diseases using epigenetics?
- Use drugs to inhibit certain enzymes involved in either histone acetylation or DNA methylation.
- Also used in diagnostic tests to detect early stages of diseases. - identify level of methylation and acetylation
is heterochromatin?
A denser form of chromatin winding DNA tightly
DNA inaccessible.
What is euchromatin?
lightly packed form of chromotin
DNA looser.
DNA is accessible
Give two examples of epigenetic studies
- Rats - good care when young and later respond better to stess and nurture offspring better.
- Humans - gestational diabetes expose fetus to high glucose conc.
What happens when mRNA is interfered with?
- If mRNA is destroyed before translation, protein synthesis can not occur.
Explain the use of siRNA
- enzyme cutting double stranged RNA into small sections now siRNA
- 1 of the two siRNA strands combine with an enzyme (now single-stranded)
- siRNA guides enzyme go to mRNA and bind with complementary bases.
- The enzyme cuts mRNA into small sections
- mRNA can not be translated.
What is cancer?
Group of diseases caused by damage to the genes that regulate mitosis and the cell cycle.
What are the main two genes associated with cancer?
- tumour suppressant genes
- oncogenes
How does the nucleus vary between types of tumour?
- Benign - normal
- Malignant - larger - darker - more DNA
What is the cause of tumours?
Cancer cells are derived from a single mutant cell.
What is an oncogene?
Mutations of proto-oncogenes
What do proto-oncogenes do?
- Stimulate a cell to divide when growth factors attach to a protein receptor on its cell surface membrane.
- This activated genes causing DNA to replicate and the cell to divide.
What two reasons mean that genes are permanently activated due to oncogenes?
- receptor protein on cell-surface membrane permanently activated - cell division switched on even in absence of growth factors.
- Oncogene code for growth factor then produced in excessive amounts - simulates excessive cell division.
What is the result of oncogenes?
Cells divide too rapidly - out of control - tumour develops
What do tumour suppressor genes do?
slows down cell division so mistakes are repaired in DNA.
Also tells cell to die - apoptosis
What happens if a tumour suppressor gene is mutated?
Inactivated
This means it stops inhibiting cell division and cell grow out of control.
The cells are normally structurally or functionally different from the original cell.
Give three examples of forms of tumour suppressor gene
- TP53 - ovarian cancer
- BRCA1 - breast cancer
- BRCA2 - breast cancer
more than half of human cancers display abnormal TP53 genes which code for the p53 protein. - which is involved in apoptosis
What is the difference between oncogenes and tumour suppressor genes?
- Oncogenes - cause cancer when activated
- Tumour suppressor - cause cancer when inactivated
Explain the process by which hypermethylation may lead to cancer
- Hypermethylation in specific (promotor) region of tumour suppressor gene
- inactivate TSG
- Transcription of promotor region of TSG is inhibited
- TSG silenced
- Leads to increased cell division and formation of a tumour.
Give an example of a tumour suppressor gene involved in hypermethylation
- BRCA1 - development of breast cancer
What are the two forms of abnormal methylation?
- hypermethylation (increased methylation) - tumour supressor genes
- hypomethylation (reduced methylation) - oncogenes
Why is there an increased risk of breast cancer after menopause?
- Increased oestrogen concs.
- fat cells in breasts produce more oestrogen.
- Localised production triggers breast cancer.
- As the tumour is developed a further increase in oestrogen. conc. leads to increased development of tumour.
- Also, white blood cells are drawn into the tumour which increase oestrogen production
- So larger tumour develops.
How can oestrogen cause cancer to develop?
- Oestrogen activates a gene by binding to a gene which promotes transcription
- If the gene that’s bonded too in one that controls cell division and growth then it is activated
- This continues to divide and could produce a tumour.
How many years did it take to complete the genome?
13 years
- Helped by use of bioinformatics
What is bioinformatics?
Science of collecting and analysing complex biological data such as genetic codes.
using computers to read, store and organise biological data at a much faster rate than previously possible.
What technique is used to determine a complete DNA base sequence?
whole-genome shotgun sequencing
What is whole-genome shotgun sequencing?
- cutting DNA into small and easily sequenced sections
- Then computer algorithm aligns overlapping segments to assemble the entire genome.
- This allows us to identify potential medical problems and make early interventions to treat them.
What is proteome?
All the proteins produced in a given type of cell or organism at a given time, under specific conditions.
What does the human microbiome project do?
Sequencing the genomes of many prokaryotic and eukaryotic organisms.
Why is it relatively easy to determine the proteome of prokaryotic organisms?
- Vast majority of prokaryotes have one circular piece of DNA - with no histones.
- No non-coding portions of DNA
State one application of the knowledge of a proteome
- Identification of proteins that act as antigens on the surface of human pathogens
* These antigens can be used in vaccines
