D1.3 mutations and gene editing Flashcards
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Describe the types of gene mutations
Substitution (a type of point mutation):
- one or more nucleotides in DNA is replaced by another nucleotide
Insertion (a type of frameshift mutation)
- one or more nucleotides added to DNA sequence
- all the codons after insertion are changed = many different amino acids produced = polypeptides unlikely to function as the shape is changed
Deletion (a type of frameshift mutation)
- one or more nucleotides removed from a DNA sequence
- as a result, all codons after deletion are affected = many different amino acids are produced = polypeptides unlikely to function as the shape is changed
What are single-nucleotide polymorphisms (SNPs)? + 1 example
Single-nucleotide polymorphisms are the result of a single-base substitution mutation, changing a single nucleotide base in a gene.
It may or may not result in a different amino acid in a protein, due to the DEGENERATE nature of the genetic code, where MULTIPLE codons can code for the same amino acid = the base substituition mutation may or may not change the amino acid produced
Sickle cell anaemia:
- the DNA sequence changes from GAG to GTG on the antisense strand
- the mRNA sequence changes from GAG to GUG at the 6th codon position
- the 6th amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine
= valine is hydrophobic, allowing the beta subunits to join together, causing haemoglobin to polymerise into insoluble fibrous strands. The insoluble haemoglobin cannot carry oxygen as effectively, making the individual feel constantly tired. The sickle shape of the cell also clogs the small capillaries
Describe frameshift mutation
when an INSERTION or DELETION occurs in non-multiples of 3, changing the code drastically and causing the code to stop making sense
Examples of mutagens causing DNA mutation (2)
Chemical mutagens:
- benzene (C6H6) used by industries to make other molecules used in our daily lives, like to produce acetone (for nail polish) = benzene is toxic and can cause LEUKAEMIA, so industries must carefully handle their employees and customers exposure to it, notably by wearing masks to prevent inhalation
- polystyrene and nylon fibres
Radioactive mutagens:
- High energy electromagnetic radiation ALSO known as ionising radiation (e.g. UV, X-ray, [alpha, beta, and gamma radiation from radioactive elements/nuclear weapons/power plant stations]) can BREAK BONDS btwn atoms including DNA, leading to mutagens
- Radiation-INDUCED chemical modifications to DNA bases can also cause mutations = these modifications alter the properties of pase pairing, leading to mispairing during DNA replication and the introduction of errors in replicated DNA
Describe mutation caused by error in DNA replication
- mutations do not always have a cause, and can simply be purely random
Causes:
Wrong nucleotide added to DNA strand:
- This error is mostly corrected by DNA polymerase III, where the code is checked and replaced with the correct base. However, even after verification and correction some mistakes can still happen
Break in DNA template strand:
- if a single-strand break occurs in the DNA template strand during DNA replication, it impedes the movement of the replication fork = disrupts the continuity of the template strand, resulting in replication errors and the halt of replication machinery
- a double-strand break is ever more severe, halts DNA replication completely
- If not properly repaired, these breaks can severely impact the integrity and stability of replicated DNA, leading to the loss of genetic material
Describe the randomness of mutations
mutations are completely random, and can also occur in mitochondrial DNA and RNA sequences.
Sites of DNA more prone to mutation:
- Cytosine ( CpG islands, where places with C are followed by G are more likely to mutate. When methylation happens, C can mutate to T.e). This is associated with colorectal cancer
Describe beneficial, neutral, detrimental, and silent mutations
when a mutation is passed on from one generation to the next, it forms a NEW allele
Beneficial (non-synonymous substitution):
- mutation that provides an indivual/species a higher chance of survival, good chance to be passed down to the next generation
Detrimental (non-synonymous substitution):
- Mutations that cause disease/death, less likely to be passed down to the next generation
Neutral (synonymous substitution)
- mutations occur in NON-CODING sections of DNA which do not have a function, OR due to the degeneracy of the genetic code (where multiple codons code for the same amino acid). DO NOT ALTER the function of genes produced
- no effect on the species health/chances of survival
Silent (synonymous substitution):
- Occur in the CODING sections of DNA but do not alter the amino acid and resulting protein due to the degeneracy of the genetic code
- no effect on the species health/chances of survival
What are non-coding sections of DNA?
- regions that do not code for protein production
- however, mutations in non-coding regions that contain regulatory sequences still impact protein production by turning coding sequences on/off
- e.g. satellilte DNA in the centromere is used for structural purposes.
Describe mutation in germ and somatic cells
Germ cells:
- germ cells develop into gametes by meiosis (e.g. sperm, egg cells)
- are involved in passing on genetic information to offspring to make up the organisms germ line,
= if there is a mutated gene in a gamete, this allele WILL be present in the zygote after fertilisation.
Somatic cells:
- all cells which are not reproductive cells, reproduce by mitosis
- mutations in somatic cells are not passed on to the offspring, as somatic cells are not involved in sexual reproduction
- however mutations to proto-oncogenes in somatic cells result in cancer
Define natural selection
The differential survival due to the inheritance of triats that make an individual more likely to survive and reproduce
Explain mutation as a source of genetic variation
- gene mutation is the original source of genetic variation
- although most mutations are harmful or neutral, mutations are still essential for a species to evolve by natural selection
- new alleles are necessary for genetic variation for evolution
- without mutations, only exsisting genes would be passed on, with no new genetic variation
Describe gene knockout and its purpose
- gene knockout is a technique where a targeted gene is inactivated or removed from an organism, allowing scientists to study the impact of removing the gene on the organism
- there is a library of knockout organisms for research (e.g. mice, fruit fly, zebrafish)
- this library with various known knockouts for specific genes allow researchers to carry out tests on animals with a variety of genetic traits similar to those in humans, e.g. mice that have a genetic propensity for obesity, anxiety etc.
Describe procedure and use of CRISPR-Cas9 technology + examples of successful use of CRISPR-Cas9
CRISPR-Cas9 allows scientists to modify/delete genes in DNA. Cas9 is an endonuclease enzyme
Procedure:
1. DNA double helix with mutation isolated
2. Guide RNA created, matches with mutated DNA as it has complementary base pairing to the DNA segment
3. Cas9 injected into guide RNA mix
4. Cells injected into Cas9+gRNA mix
5. gRNA identifies the section of DNA and attaches
6. Cas9 cuts out the mutated sequence
7. Cas9 cleaves the double strands at where gRNA indicates the cleavage point
8. The faulty gene can now be replaced by the correct DNA OR the cell attempts to repair the break, effectively silencing the targeted gene sequence.
Examples of succesful CRISPR-Cas9 applications:
- Gene therapy: CRISPR-Cas9 is used to replace or repair a gene responsible for genetic diseases such as sickle cell anaemia (repair the haemoglobin blood gene removed from a patient, then stem cells are reinserted into patient to cure them)/huntingtons disease
- Genetic research: CRISPR-Cas9 is an effective method to knock out genes, allowing researchers to better understand the action of specific genes
- Malaria prevention: CRISPR-Cas9 can be used to modify the genome in mosquitoes to prevent the spread of malaria
- Agriculture: CRISPR-Cas9 can enhance desirable traits in plants, improving crop yield, nutritional content, and disease resistance
What are conserved/highly conserved sequences in genes?
Conserved sequence: highly identical/similar genes across a species or group of species
Highly conserved sequence: Identical/similar genes over a long period of evolution
Hypothesis for conserved/highly conserved sequences (2)
Functional constraint hypothesis:
- gene sequences are essential to the structure and function of a protein coded by a gene = any mutation to these conserved gene sequences are naturally selected against by necessity as the associated protein will not function
Slower rates of mutation hypothesis:
- highly conserved sequences are under selective pressure to maintain their function, resulting in a slower rate of mutation in these sequences
- higher mutation rates found in non-coding DNA (e.g. satellite DNA), as DNA polymerase proofreading and repair is less active
- DNA repair systems are more active in key genome zones where mutations will have serious consequences
