D1.3: Gene mutations

1.3.1- gene mutation as structural changes to genes at the molecular level

Mutation: a change in the nucleotide sequence of a section of DNA coding for a specific trait

These alternate forms may result in new functionalities (new alleles) or harmful activity (genetic disease)

3 types: Substitution and frameshift mutations (insertion and deletion)

Base substitution- one nucleotide is replaced by another

  • Silent base substitution: change to a triplet in the DNA, causing the same amino acid to be produced as originally (no effect on protein sequence due to degeneracy

  • Missense base substitution: expression of a different amino acid, leaving polypeptide functional or dysfunctional, depending on amino acid type (different amino acid)

  • Nonsense base substitution: shortened polypeptide is caused as substitution base is a stop codon, resulting in functional or dysfunctional polypeptide. (trunkate)

Consequences of base substitutions=

  • Sickle cell anaemia in RBC’s

Sickle cell anaemia is caused by a point mutation (base substitution mutation) of thymine by Adenine in the DNA antisense strand of the gene for B- . During transcription, the triplet mRNA codon is changed from GAG to GTG, which means that during translation, the negatively charged amino acid in the B-haemoglobin polypeptide glutamic acid by the amino acid valine. Since these two amino acids have different chemical properties from each other, the structure of the resulting protein is changed, and the red blood cells become sickle-shaped. This can cause blockages in blood vessels, pain in bones and joints, organ damage and shortened life of RBC’s, which leads to a condition called haemolytic anaemia

Frameshift- result of insertion or deletion

  • addition/deletion of a base changes every codon beyond the point of mutation, thus changing amino acid sequence.

(THE CAT SAW THE DOG RUN FAR)

  1. Insertion- one or more bases are added to the DNA sequence, causing a frameshift (THZ ECA TSA WTH EDO GRU NFA)

  • Huntington's disease is a neurodegenerative genetic disorder HTT gene on chromosome 4codes for the Huntington protein, which experiences a frameshift insertion mutation. The mutated gene shows one or several insertions of additional CAG sequences. The faulty protein results in neuronal degeneration in the brain and eventually death due to neurological dysfunctions.

  1. Deletion- one or more bases are removed from DNA sequence, also causing a frameshift. (THC ATS AWT HED OGR UNF AR)

  • CCR5 receptor protein found on the cell surface of WBC. CCR5 gene encodes the protein located on chromosome 3. Some populations have inherited a frameshift mutation (Delta 32) resulting in the deletion of a portion of CCR% gene. HIV binds to CCR5, homozygous carriers of this mutation are resistant to HIV infections.

Degeneracy- multiple codons can code for the same amino acid, helping to minimise the effects of base substitutions (single nucleotide changes)

Mutation in non-coding (98% of DNA)

Causes of gene mutation

  1. Proofreading errors during DNA replication

  2. Mutagenic agents (chemical, radiation)

Mutagen- an agent that induces permanent change to the genetic material of an organism. (increases the frequency of mutations above the natural background level)

  • Physical: radiation - UV radiation, x-rays

Increases the mutation rate if it has enough energy to cause chemical changes in DNA. Ionising radiation (gamma rays and alpha particles from radioactive isotopes)

  • Chemical: cigarette smoke, grilled meat, benzoyl peroxide.

Asbestos fibres, preservatives in fast food or ready-made meals such as nitrates, diesel exhausts, benzene, and tobacco smoke.

  • Biological: viruses, bacteria

Differences between transversions and transitions

  • Two possible transversions, but only one possible transition over base transition mutations are more likely than transversions

  • Substituting a single ring structure for another single ring structure is more likely than substituting a double ring for a single ring

  • ytransituonsare lkess likeyl result in amino acid substitutions and are therefore more likely to persist as ‘silent substitutions’ in populations

There is no natural mechanism for deliberately inducing a mutation to change a particular characteristic; humans are inventing some.

Consequence of mutation in germ cells and somatic cells

Somatic mutations

  • single body (e.g. skin cell, muscle cell)

  • always diploid, therefore not inherited

  • can lead to cancer

Germline mutations

  • in gametes

  • are always haploid and can be passed on to offspring

  • can increase cancer susceptibility

  • give rise to inheritable genetic diseases

Mutations as a source of genetic variation

A high level of genetic diversity allows species to survive environmental change and develop beneficial adaptations via natural selection.

Genetic variation is a defining feature of life- it enhances a species’ ability to adapt and survive in changing environments. Variation within a species results from:

  • mutations

-the source of all genetic variation, essential for evolution by natural selection and without variation, a population would decline.

  • gene flow (movement of genes between populations)

  • meiosis (egg and sperm formation shuffle chromosomes from mom/dad)

  • sexual reproduction

HL: Gene editing

D1.3.8: gene knockout as a technique for investigating the function of a gene by changing it to make it inoperative

D1.3.9: Use of CRISPR sequences and how the enzyme Cas9 helps its function

D1.3.1: Hypotheses

Suggest a possible mechanism

  • when a mutated exon is undergoing splicing

  • The pre-mRNA is checked for errors by a mechanism in the cell

  • The exon is cut out of pre-mRNA

  • The mRNA does not include the mutated code when it is translated, meaning that the polypeptide is not significantly truncated

Gene knockout: a genetic technique where a specific gene is deliberately inactivated or removed from an organism’s genome.

  • It is done to study the effects of the genes’ absence on the organism’s development, physiology, and behaviour.

  • often carried out on mice

CRISPR: sequence in DNA of bacteria that act like a memory bank

Cas9: enzyme that acts like molecular scissors. Can cut DNA at a sepcific place on DNA by another RNA

CRISPR: clustered regularly interspersed short palindromic repeats

Spacer sequences- bascteriua transcribe the spoacers into RNA which forms a complex with Cas9 enzyme and is used to search for viral base sequences in the raget DNA. This sequence is complementary via target `DNA

Repeat sequences- repeat sequences

The precise specifity of CRISPR system offers opportunities for gene editing, can come useful to treating genetic diseases.