Grade 12 Biology - Genetic Aberrations and DNA Technology

Mutations

• A mutation is any alteration in the genetic makeup (genetic code) of an organism.
• Mutations cause genetic aberrations (out of the norm).
• Factors that lead to genetic changes (sequence of nucleotides) during a lifetime may be caused by:
  • Nucleotides being damaged
  • Nucleotides being lost by chance
  • Breakdown of DNA by mutagens.

Causes of Nucleotide Damage or Loss

• One or more nucleotides being damaged or lost by chance:
  • Crossing over of paternal and maternal chromosomes in meiosis.
  • Replication of DNA.
  • Transcription of DNA to RNA.
• Breakdown of DNA by mutagens:
  • Environmental factors such as sunlight, radiation, and smoking.
  • Mutagenic chemicals (formaldehyde, benzene, carbon tetrachloride).
  • Viruses and micro-organisms

Gene Mutations

• Small, localized changes in the structure of DNA strands.
• Changes that involve a single nucleotide are called point mutations.
• They occur by:
  • Substitution
  • Insertions
  • Deletions

Application of DNA Technology – DNA Fingerprinting

• Each person has unique DNA (except identical twins).
• Although 99.9% of human DNA is identical, the differences occur in the highly variable, non-coding part of DNA.
• DNA profiling involves extracting and identifying the highly variable regions of a person’s DNA that contain repeating sequences of base-pairs called STRs (short tandem repeats).
• At the same point in the DNA of different people, the number of repeated sequences of base pairs varies considerably, thus distinguishing one DNA profile from another.
• From 13-20 different sites on DNA molecules are investigated, which is enough to show that an individual’s profile is unique.
• Scientists can use these repeated sequences that vary to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products.

DNA Profile

• The cells are treated with chemicals to extract the DNA.
• Restriction enzymes are used to cut at the beginning and end of each repeated sequence, resulting in fragments of different lengths.
• Through a complicated process known as PCR:
  • A large number of these fragments are made to provide a substantial amount of DNA to work with.
• The DNA fragments that result are then separated and detected, using different techniques such as electrophoresis.
• A pattern is obtained that reflects different numbers of base pair repeats in different individuals.
• The length of a particular DNA fragment depends on the number of repeats present.
• These separated DNA fragments are represented as dark bands on a piece of film.
• This is DNA fingerprinting.
• Each of our cells carries an identical set of this unique DNA that differs from that of any other person (except identical twins).
• If two genetic profiles show identical banding patterns, it is virtually certain that they come from the same person.
• In related people, some parts may be similar, but no-one else will have exactly the same sequences in every part of their DNA.
• DNA is a non-reactive, chemically inert molecule, which is why it can be recovered from patches of long-dried blood or semen in murder investigations and even extracted from bones of ancient Neanderthals.

Disadvantages of DNA profiling

• Violation of privacy:
  • The information regarding genetic traits could lead to health insurers denying coverage or claims.
• Issues on accuracy
  • Possible errors could be made: equipment, personnel, experience
• Manipulation
  • Tampering,
  • Irresponsible handling
  • Manipulation of data in genetic profiling

Uses of DNA profiling

1. Forensics
2. Diagnosing inherited disorders
3. Identifying casualties
4. Paternity testing
5. Fight illegal trading