4.4b DNA Profiling

26/02/2013 § 1 Comment

This is the introduction to my career as a forensic scientist. I can feel ittttt. There are ways scientists can use DNA to identify the individuals who own that particular genome. DNA profiling starts with the PCR, or the polymerase chain reaction. PCR is a technique that can amplify small quantities of DNA without having to use up the limited sample available to the scientist (like if I were only able to retrieve a portion of one strand of hair). The stages of PCR involve denaturing the DNA sample available with heat to separate the two strands. Next, a primer is allowed to bind to the DNA, creating multiple copies. Temperature is an important factor in PCR, starting off with a high temperature to denature the strands, then slowly cooling down to allow the primer to do its job.

Gel electrophoresis is used for DNA profiling. We did this during one of the labs (DNA!) and used a thin sheet of gel as a molecular sieve for the DNA fragments. An electric field is then applied to the gel and since the particles are either positively or negatively charged, they would naturally drift to either side of the gel. The size and charge of the fragments determine how far they go. Large and less charged fragments don’t travel far, unlike the small and highly charged fragments.

DNA profiling, as mentioned above, is used in a lot of forensic science, like in analyzing evidence obtained from crime scenes to solve a case, or determining who the father of a child is. Profiling works because all organisms have their own satellite DNA, which are short sequences of bases that are repeated many times. They vary between individuals in terms of how many repeats occur. The patterns on a gel would definitely show a difference between two samples.

Essay Questions

  1. Calculate and predict; genotypic and phenotypic ratios of offspring of dihybrid crosses involving unlinked autosomal genes.
  2. Identify which of the offspring in dihybrid crosses are recombinants.
  3. Describe the methods and aims of DNA profiling.
  4. Outline a technique for transferring genes between species.
  5. Describe the technique for the transfer of the insulin gene using E. coli.
  6. Discuss the potential benefits and possible harmful effects of genetic modification.
  7. Discuss the ethical arguments for and against the cloning of humans.
  8. Outline the ethical issues of cloning humans.



Page 168, PCR and Neanderthals

1. State the most common number of differences in base sequence between pairs of humans. —> 7

2. Humans and Neanderthals are both classified in the genus Homo and chimpanzees are classified in the genus Pan. Discuss whether this classification is supported by the data in the bar chart. —> This classification is supported in the data in the bar chart. It shows that humans and Neanderthals have the most common difference of base sequences is 26 differences. Humans and chimps have a most common difference of base sequences of 55 differences. The data shows that humans and neanderthals are most closely related than humans and chimps.

3. Suggest a limitation to drawing any conclusion from the human-Neanderthal comparison. —> The samples that determined the difference between human and Neanderthal are not so reliable because while the chart collected sequences from 994 humans and 16 chimpanzees, the chart contains DNA samples from one Neanderthal. We would need more data to be more confident about our deductions.

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§ One Response to 4.4b DNA Profiling

  • Dave Ferguson says:

    Great blog entry! I hope you do pursue a career in forensic science.

    IB 1-7

    Grade 7 A consistent and thorough understanding of the required knowledge and skills, and the ability to apply them almost faultlessly in a wide variety of situations.

    Data-based questions: PCR and Neanderthals page 168

    1. 7

    • data suggests Neanderthals more closely related to humans
    • because of the fewer differences in bases between humans and Neanderthals
    • Minimum difference in human-Neanderthal exceeds maximum human-human difference
    • therefore humans and Neanderthals not the same species

    3. Based on the bones of single Neanderthal/limited support

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