8A: Mutations and gene expression Flashcards
What is a gene mutation?
Change in the base sequence of DNA
When do gene mutations occur?
likely to occur during DNA replication, which is during interphase in the cell cycle
Deletion of bases
The loss of a nucleotide base from the DNA sequence
Effect of the deletion of bases
- Frame shift so each codon shifts one place to the left
- A whole new nucleotide base sequence is altered from the point downstream
- Most triplets will be different and code for different amino acids
- The polypeptide will be different and lead to the production of a nonfunctional protein that could considerably alter the phenotype
- One deleted base at the very start of a sequence could alter every triplet in the sequence
- A deleted base near the end of the sequence is likely to have a smaller impact.
Addition of bases
The addition of one or more bases inserted into the DNA nucleotide sequence
Effect of the addition of bases
- Usually, a frame shift and the whole sequence of triplets become altered
- The frame shift is to the right
- Changes base sequence downstream from the point of addition
- If 3 extra bases are added or any multiple of 3, then there will not be a frameshift
Duplication of bases
One or more bases are repeated in the base sequence
Effect of the duplication of bases
- Produces a frame shift to the right
- The frame shift is to the right
- Changes the sequence of bases downstream of the duplication
- If 3 extra bases are added, or any multiple of 3 bases, there will not be a frameshift
Inversion of bases
A section of bases becomes separated from the DNA sequence and rejoin at the same position but in the inverse order
Effect of the inversion of bases
- The base sequence of this portion is therefore reversed and affects the amino acid sequence that results
- New amino acids coded for
- Changes the tertiary structure of a protein
Translocation of bases
A section of bases is moved from one location in the genome to another
Effects of translocation of bases
- It can either be separated from the DNA sequence on one chromosome and become inserted into the DNA sequence of a different chromosome, or onto the same chromosome
- Leads to abnormal phenotype
- These effects include the development of certain forms of cancer and reduced fertility
Causes of gene mutation
Caused by mutagenic agents
- ionising radiation
- ultraviolet radiation
- carcinogens -> chemicals that can alter the structure of DNA and interfere with transcription
- viruses
What is cell differentiation?
process by which stem cells become specialised
What are stem cells?
Undifferentiated cells that continually divide and become specialised
Stem cells specialisation
- Stem cells contain the same genes, but during development, not all genes are transcribed and translated (expressed)
- Under the right conditions, some genes are expressed and others are switched off (Selective activation)
- mRNA is only transcribed from specific active genes
- The mRNA from these genes is translated into proteins
- These proteins modify the cell permanently and determine the cell structure
- Changes to the cell produced by these proteins cause the cell to become specialised
- Changes are difficult to reverse, so once specialised, stay specialised
4 different types of stem cells
. Totipotent
. Pluripotent
. Multipotent
. Unipotent
What are totipotent?
stem cells, which can mature (develop) into any type of body cell
What are pluripotent?
stem cells, which can develop into almost any type of body cell but not the cells of the placenta.
What are multipotent?
can divide and differentiate into a few different types of cells.
What are unipotent?
can divide and differentiate into one type of cell.
What is the difference between pluripotent embryonic stem cells and totipotent stem cells?
Pluripotent stem cells are found after the first few divisions, and totipotent are found before the first few divisions
What are umbilical cord blood stem cells?
multipotent
What are placental stem cells?
multipotent
What are induced pluripotent stem cells?
A type of pluripotent stem cell that is produced from adult somatic unipotent stem cells.
How are induced pluripotent stem cells produced?
- These body cells are genetically altered to make them acquire the characteristics of embryonic stem cells, which are pluripotent
- They do this by inducing genes and transcriptional factors within the cell to express themselves, meaning genes that were switched off to make the cell specialised are switched back on
Uses of stem cells in medicine
- bone marrow transplants
- regrow damaged tissues
- form beta cells in the pancreas in type 1 diabetes
- drug testing
- developmental biology research
Ethical concerns of stem cells
- destruction of an embryo created by IVF that could’ve been a fetus if it were in the womb
- Only use adult stem cells as they do not destroy an embryo however, they are limited in the range of cells they can specialise into
What is a transcriptional factor?
a protein that controls the transcription of genes by binding to a specific region of DNA
Where do transcription factors bind?
Each transcriptional factor has a site that binds to a specific base sequence of DNA in a promoter region
What are the 2 types of transcriptional factors?
. Activators
. Repressors
What are activators?
The site on the transcriptional factor that binds to DNA is active
. Stimulates and increases the rate of transcription, producing mRNA which is then translated into a polypeptide
. The gene is switched on - expressed
What are repressors?
The site on the transcriptional factor that binds to DNA is not active
. Inhibits and decreases the rate of transcription, preventing RNA polymerase from binding, so transcription stops and the polypeptide is not synthesised
. Gene is switched off - not expressed
What is oestrogen?
steroid hormone that can initiate transcription by switching on a gene (activator)
What is the role of Oestrogen in initiating transcription?
1) Oestrogen is lipid soluble, so it diffuses through the phospholipid bilayer into the cytoplasm
2) It binds to a receptor site on an inactive transcriptional factor (oestrogen receptor)
3) The shape of the receptor and the oestrogen are complementary so they form a hormone-receptor complex
4) As oestrogen binds, the DNA binding site changes shape on the inactive transcriptional factor, which becomes activated
5) The transcriptional factor can now diffuse into the nucleus and bind to a specific base sequence on the promoter region of DNA
6) This complex acts as an activator and stimulates transcription by helping RNA polymerase to bind
What is epigenetics?
The heritable changes in gene function, without changes to the base sequence of DNA
What does the epigenome determine?
The shape of the DNA-histone complex
What is the effect of increased methylation?
1) The methyl group is added to the CpG site where a phosphodiester bond next to each other links a cytosine and guanine base
2) Methylation inhibits the transcription of genes by preventing the binding of transcriptional factors to DNA and inducing acetylation
3) This makes the DNA-histone complex pack more tightly together, so DNA is inaccessible to transcription factors
4) So RNA polymerase cannot interact with the gene
5) The gene is not transcribed and is switched off and not expressed
What is the effect of deacetylation?
1) Acetyl groups are removed by histone deacetylase enzymes
2) Decreased acetylation increases the positive charge on the histones
3) This increases their attraction to the negative phosphate groups on the DNA
4) The association between DNA and histones is stronger, and the chromatin becomes highly condensed
5) DNA is not accessible to transcription factors, which cannot initiate mRNA production from DNA
6) Gene is switched off, not expressed
What does RNAi interference do?
Translation of the mRNA produced from target genes can be inhibited by RNAi
What is the mechanism of RNAi interference?
1) Once mRNA has been transcribed, it leaves the nucleus for the cytoplasm
2) Dicer cuts the large double-stranded molecules of mRNA into small interfering RNA
3) The siRNA molecule connects to an enzyme and activates it, forming a RISC
4) The siRNA unwinds, and the strands separate, which exposes the nucleotide bases
5) The siRNA-enzyme complex molecule guides the enzyme to the mRNA molecule
6) The single-stranded siRNA binds to the target mRNA through complementary base pairing
7) The enzymes hydrolyses the mRNA into smaller fragments
8) The mRNA is no longer capable of being translated into a polypeptide
9)The gene is not expressed
What are the types of tumours?
- Malignant (cancerous)
- Benign (not cancerous)
What are the 2 types of genes that control cell division and mutations?
- Proto-oncogenes
- Tumour supressor genes
How to identify tumour cells?
- The nucleus is larger and darker than normal cells
- There may be more than one nucleus
- They have an irregular shape
- They don’t produce all the proteins they need to function properly
- They have different antigens on their surface
- They divide more frequently than normal cells
What is the development of a tumour due to?
- a gene mutation in either the tumour suppressor gene
- the abnormal methylation of tumour suppressor genes and oncogenes
- increased oestrogen concentrations
What is the normal function of tumour suppressor genes?
- Maintains normal rates of cell division and so prevents the formation of tumours
- Code for proteins involved in control of cell division to
. slow down cell division
. repair mistakes in DNA
. stops the cell cycle when DNA damage
detected, resulting in apoptosis
The role of tumour suppressor genes in tumour development
- Mutation occurs in the DNA base sequence of the tumour suppressor gene, causing it to become inactive
- This changes the amino acid sequence and tertiary structure of the protein it produces
- So the protein is non-functional and cannot inhibit cell division
- Resulting in uncontrolled cell division
- Forming a tumour
What is the normal function of oncogenes?
. Proto-oncogenes stimulate a cell to divide by producing a protein that stimulates cells to divide when they attach to a protein receptor on its cell surface membrane
. This then activates genes that cause DNA to replicate and the cell to divide
What is the role of oncogenes in tumour development?
- Mutations occur in a proto-oncogene, which forms an oncogene
- The effect of the proto-oncogene is increased, and the gene is overactive, and more proteins are produced
- Cell division is permanently activated in 2 ways
a) The receptor protein on the cell surface membrane can be permanently activated, so that cell division is switched on even in the absence of growth factors
b) the oncogene may code for a growth factor that is then produced in excessive amounts - Resulting in rapid, uncontrolled cell division
- forming a tumour
