Mutation and Gene Editing Notes

Mutation and Gene Editing

Genetics

  • D1.3 Mutations and gene editing
  • D2.1 Cell and nuclear division
  • D3.2 Inheritance

Guiding Questions

  • How do gene mutations occur?
  • What are the consequences of gene mutation?

Essential Understandings

  • D1.3.1 Gene mutations as structural changes to genes at the molecular level
  • D1.3.2 Consequences of base substitutions
  • D1.3.3 Consequences of insertions and deletions
  • D1.3.4 Causes of gene mutation
  • D1.3.5 Randomness in mutation
  • D1.3.6 Consequences of mutation in germ cells and somatic cells
  • D1.3.7 Mutation as a source of genetic variation

Nature of Science (NOS)

  • Commercial genetic tests can yield information about potential future health and disease risk.
  • Without expert interpretation, this information could be problematic.

What is a Genetic Mutation?

  • A genetic mutation is a permanent, random, and structural change in our DNA sequence.
  • It can occur in a coding or non-coding region of the DNA.

Gene Mutations as Structural Changes to Genes at the Molecular Level

Types of Mutations
  • Point Mutation
    • Changes to a single base in the DNA code.
    • May involve base substitutions, insertions, or deletions.
    • A single base pair may be changed, added, or deleted.
    • Example: A G (guanine) might be swapped for a C (cytosine).
  • Frameshift Mutations
    • Entire sections of DNA can be rearranged or lost.
    • This change will affect every codon beyond the point of mutation.
    • May dramatically change the amino acid sequence.

Consequences of Base Substitutions

  • Point Mutations
    • Silent: No change in the amino acid sequence.
    • Missense: Results in a different amino acid.
      • Conservative: Similar properties.
      • Non-conservative: Different properties.
    • Nonsense: Results in a stop codon.
  • DNA level Example
    • TTC, TTT, ATC, TCC, TGC
  • mRNA level Example
    • AAG, AAA, UAG, AGG, ACG
  • Protein level Example
    • Lys, Lys, STOP, Arg, Thr
Sickle Cell Anemia
  • Caused by a mutation in the gene that helps make hemoglobin.
  • Hemoglobin: A protein that carries oxygen in red blood cells.
  • Point mutation where A (adenine) is replaced by a T (thymine).
    • Valine is made instead of glutamic acid amino acid.
Single Nucleotide Polymorphism (SNPs)
  • When a single specific nucleotide is changed in a sufficiently large proportion of the population (typically >1%), it is called a single nucleotide polymorphism (SNP).
  • They can be used as genetic markers of diseases.
Crohn’s Disease
  • It is an immune-related chronic condition that causes inflammation in the digestive tract.
  • Single nucleotide polymorphisms - a single change in the DNA code

Consequences of Insertions and Deletions

Frameshift Mutations
  • Involves insertion or deletion of a base.
  • Changes the entire reading frame.
  • Often resulting in a polypeptide ceasing to function completely.
Huntington’s Disease
  • A neurodegenerative genetic disorder.
  • Caused by a frameshift mutation in the HTT gene on chromosome 4.
  • <27<27 repeats of CAG → ≥\geq35 repeats

Data-based questions: BRCA1 mutations

  • BRCA1 is a tumour suppressor gene.
  • There are over 500 variants of the gene that increase the risk of breast and ovarian cancer in women and the risk of some other cancers in both sexes. Most of these variants are extremely rare in human populations.
  • Splice mutations affect the editing out of introns, which are base sequences in genes that are transcribed but not translated.
Frameshift Mutations
  • Frameshift mutations change many/multiple amino acids in the polypeptide produced; protein structure much altered; protein unlikely to be able to carry out its function; tumour suppression impaired; errors in DNA that will result in oncogenic mutations not corrected; breast cancer frequently develops;
Calculations
  • 2.a. (11/54)x100 = 20%
  • Nonsense mutation is a change to a stop codon; truncated polypeptide produced; gene product of BRCA1 gene unlikely to function; increased chance of being a patient with breast cancer;
  • 3.a.eight patients in each group; much more diversity in mutations in African Americans; only frameshift mutations in Ashkenazi Jewish; frameshift, mis-sense, nonsense and splice mutations in African Americans;
  • B. much more genetic diversity among African Americans; more diverse origins; more different SNPs are in the African American gene pool; endogamy in Ashkenazi Jewish population/only marriages within the religion reduces genetic diversity;

Causes of Gene Mutation

  • Errors in DNA replication during cell division.
  • Exposure to mutagens (radioactive substances, x-rays, ultraviolet radiation and certain chemicals).
  • Viral infections and other infectious agents.
Proofreading Errors
  • Polymerase adds an incorrect nucleotide to the new strand of DNA.
  • Polymerase detects that bases are mispaired.
  • Polymerase uses 3'- 5' exonuclease activity to remove incorrect nucleotide.
  • The new DNA strand is cut, and the mispaired nucleotide and its neighbors are removed.
  • The missing patch is replaced with correct nucleotides by a DNA polymerase.
  • A DNA ligase seals the gap in the DNA backbone.
Mutagens
  • A mutagen is an agent that changes the genetic material of an organism
  • Physical
    • UV Radiation
    • X-Rays
  • Chemical
    • Cigarettes
    • Grilled Meat
  • Biological
    • Viruses (HPV)
    • Bacteria (H. Pylori)
Physical
  • Radiation increases the mutation rate.
Chemical
  • Mustard gas - induces chemical change in the DNA
  • Nitrous acid - converts C to U
Biological
  • HPV- human papilloma virus
    • HPV has a circular, double stranded DNA, protected by capsid proteins.
    • More than 100 HPV-types are known.
    • HPV16 and 18 cause 70% of all cervix cancers.
    • Infection by HPV: HPV infects epithelial cells in the cervical mucosa. HPV DNA integrates into the cellular genome when causing cancer.
  • Infection with H.Pylori

Randomness in Mutation

  • Some mutations do not cause any noticeable changes, while others can lead to genetic disorders or diseases.
  • Mutations can even give an organism an advantage in its environment, leading to new traits or abilities (species evolution).
  • Some base sequences may have higher probability of mutating.
  • Consequently, there is no natural mechanism for inducing a deliberate mutation in order to change a particular characteristic.

Consequences of Mutation in Germ Cells and Somatic Cells

  • Germ line (germ cells)
    • Haploid
    • 23 chromosomes (n) in human
    • Sperm & Ovum (egg)
  • Somatic cells
    • Diploid
    • 46 chromosomes (2n) in human
    • Skeletal and muscle cells, Blood cells , Stem cells, All other cells, Organ and tissue cells, Fat cells, Neuron cells
  • The mutation can be inherited and would occur in all body cells and show in the entire organism
  • The mutation will occur in certain areas of the body and will not be inherited by the next generation.

Mutation as a Source of Genetic Variation

  • Gene mutation is the original source of all genetic variation.
  • Most mutations are either harmful or neutral for an individual organism.
  • In a species they are, in the long term, essential for evolution by natural selection.

NOS: Commercial Genetic Tests

  • Can yield information about potential future health and disease risk.
  • One possible impact is that, without expert interpretation, this information could be problematic.