20.1 - 20.6 Gene expression Flashcards
List some mutagenic agents
- X-rays
- Alpha particles
- Mustard gas
- Tobacco tar
What is a mutation?
Any change to the quantity or structure of DNA
What is a gene mutation?
Any change to one or more nucleotide bases or a change in the sequence of DNA bases
Describe how a change of one or more bases can produce sudden and distinct differences between individuals
- Change in nucleotide so different base
- Different DNA triplet, different mRNA codon
- May code for a different amino acid
- Different primary structure
- Hydrogen, ionic and disulphide bonds form in different places so different secondary structure
- Different tertiary structure
- Protein behaves differently, different phenotypes
Give the types of mutation
Gene mutation
Non-disjunction:
Failure of homologous chromosomes to separate
Polyploidy:
An extra set of chromosomes
List the 6 types of gene mutation
- Substitution
- Addition
- Deletion
- Duplication
- Inversion
- Translocation
Describe a base substitution
A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
Give the possible consequences of a base substitution
- Production of a stop codon
- Formation of a codon for a different amino acid
- Formation of a different codon but for the same amino acid
Describe the deletion of a base
A nucleotide is lost from the normal sequence of DNA. One deleted nucleotide causes all the triplets after the mutation to be different (frame shift).
Describe duplication
One or more bases repeated (frame shift)
Describe inversion
Group of bases separate and then rejoin in reverse order
Describe translocation
Group of bases from one chromosome insert into another
What causes mutations?
- Spontaneous
- However, rate of mutation can be increase by mutagenic agents
Define differentiation
The process by which each cell develops into a specialised cell
Define stem cell
An undifferentiated dividing cell that requires constant replacement and can differentiate to produce specialised cells
List the types of mammal stem cells
- Embryonic
- Umbilical cord blood stem cells
- Placental stem cells
- Adult stem cells
Describe embryonic stem cells
Differentiate into any type of cell in the initial cells
Define totipotent
Differentiate into any/all specialised cells and are only found in early embryos
Describe the negatives of using totipotent stem cells
- Ethical issues with using embryos
- Less abundant
Define pluripotent
Found in embryos and can differentiate into almost any type of cell e.g embryonic stem cells and fetal cells
Define multipotent
Found in adults and can differentiate into a limited number of specialised cells. Usually develop into cells of a particular type e.g stem cells in bone marrow can produce any type of blood cell
Define unipotent
Derived from multipotent stem cells and are made in adult tissue, Can only differentiate into a single type of cell e.g cardiomyocytes
Describe induced pluripotent stem cells (iPS)
- Pluripotent stem cells made from unipotent stem cells
- Unipotent body cells are genetically altered to make them aquire the characteristics of embryonic stem cells
- Done by inducing certain genes and trancription factors to express themselves
- Not an exact duplicate of embryonic stem cells
- Capable of self renewal
What’s the benefit of induced pluripotent stem cells?
- Less chance of rejection
- No ethical concerns
- More readily available
- Increased differentiation means it can be used to treat a greater number of conditions
Describe how and why embryonic stem cells are used in the treatment of disease
- Stem cells from the first few divisions of a fertilised egg have the greatest potential to treat disease
- Because they are totipotent
- Embryos are grown in-vitro and then cloned and induced to differentiate into required human tissue
Describe the advantages and disadvantages of embryonic stem cells
For:
* Totipotent
Against:
* Ethical concerns
* Small supply
* More chance of rejection
Describe the advantages and disadvantages of adult stem cells
For:
* Less chance of rejection
* No ethical concerns
* Greater supply
Against:
* Unipotent
Describe the growth of plant tissue cultures
- Mature plants maintain many totipotent stem cells
- Many plant cells can develop into any other cell
How do cells become specialised?
The result of which genes are expressed and which are not
Describe the process of gene expression
- Transcription factor binds to specific base sequence on the gene
- This triggers the process of trancription
- mRNA is produced and the info is processed into a polypeptide
When a gene is not expressed, this is because the transcription factor binding site is not active
Define transcription factor
A protein that can bind to a specific base sequence of DNA and stimulate transcription and expression of a gene
Describe how to activate a transcription factor
- Molecule binds to TF
- This causes the DNA binding site to change shape
- TF can now bind to DNA and trigger transcription
Describe the regulation of transcription by oestrogen
- Oestrogen is lipid soluble and diffuses throught the phospholipid cell membrane of target cells
- Oestrogen binds to a site on the receptor molecule of the transcription factor. The shape of the oestrogen and the receptor are complimentary.
- DNA binding site of the transcription factor changes shape and can now bind to DNA
- Transcription factor enters nucleus via nuclear pores and binds to a specific base sequence at the start of a gene
- Stimulates transcription of the gene so gene is now expressed
What is small interfering RNA? (siRNA)
A double stranded RNA molecule that has one strand that is complementrary to the bases on a specific mRNA molecule
Describe how siRNA works
- An enzyme cuts large double-stranded molecules of RNA into smaller sections called siRNA
- One of the two siRNA strands combines with an enzyme
- The siRNA binds with complementary bases on an mRNA molecule, bringing the enzyme with it
- The enzyme cuts the mRNA into smaller sections
- This means the mRNA can no longer be translated into a polypeptide. The gene has been blocked/is not expressed.
Define epigenetics
Environmental factors can cause heritable changes in gene function without changing the base sequence of DNA
Describe the epigenome
- DNA is wrapped around histones
- Both are covered in chemical tags that form the layer known as the epigenome
- The epigenome determines the shape of the DNA-histone complex
- The epigenome is flexible as the chemical tags can respond to environmental factors
Describe how the epigenome controls gene expression
- Acetylation of histones
- Methylation of DNA
Describe decreased acetylation/increased methylation
- Condensation of the DNA-histone complex
- Gene is tightly condensed
- Transcription factor cannot bind to promoter region
- No transcription, no translation, no protein
- Gene is not expressed
Describe increased acetylation/decreased methylation
- Less condensation of the DNA-histone complex
- Gene is not tightly condensed
- Transcription factor can bind to promoter region
- Transcription, translation, protein
- Gene is expressed
Define cancer
Rapid and uncontrolled cell division
Describe a benign tumour
Restricted to one tissue
Describe a malignant tumour
Can spread from one tissue to another
Describe a proto-oncogene
A gene which stimulates cell division when a growth factor binds to receptors on a cell surface membrane
Describe an oncogene
- A mutated proto-oncogene that becomes permanently activated/expressed
- Most mutations are acquired not inherited
Decribe how proto-oncogenes become oncogenes
- Receptor proteins on cell surface membrane is permanently activated
- Cell divison is expressed even in the abscence of the growth factor
- The oncogene codes for the production of excessive amounts of a growth factor, stimulating excessive cell division
Describe tumour supressor genes
- Slow down cell division
- Repair mistakes in DNA
- Initiate apoptosis
