DNA Portfolio Entry

To demonstrate your expertise in DNA analysis, answer the following questions and include the
information in your portfolio.

1. Read the resource document section titled “DNA” for a brief review of DNA concepts and
answer the following questions.

a. Name the 4 nucleic acid bases (also known as nitrogenous bases).

Adenine, Cytosince, Guanine, Thymine
b. Describe how nucleic acid bases pair up.
A-T, C-G
c. How many chromosomes does a human have?
46, or 23 pairs
d. The two variants of a gene are known as:
alleles
2. Read the resource document section titled “Electrophoresis” for a brief overview of the
Gel Electrophoresis technique and answer the following questions.

a. Summarize the electrophoresis procedure.

- Measure each strand
- count the number of repeated sections by pushing DNA through gel like material
- The DNA is negatively charged, and one side of the gel is positively charged pushing the
DNA strand forward
- Based on the strands length is how far it will go

b. What is the purpose of electrophoresis in DNA Analysis?

It is used to help separate and identify fragments of DNA

c. Why does DNA move through the gel?

Because the DNA has a negative charge and one side of the gel has a
positive charge, pulling it towards the positive

d. How is DNA “measured” in this technique?

The longer the strand, the farther it will go toward the positive charge

3. Read the resource document section titled “VNTR” for a brief overview of how Variable
Number of Tandem Repeats is used in DNA Analysis and answer the following
questions.

a. What about genes is generally the same from person to person?

Repeated sequences are the same person to person.
 b. What tends to vary?
The amount of times it repeats.

c. How can the number of repeats be gauged?

The number of repeats is gauged by dividing the entire molecular weight of a given
Variable Number of Tandem Repeats (VNTRs) by the molecular weight of the repeated
sequence

d. How does VNTR differ from STR?

STRs are found more often in DNA than VNTRs.

4. Read the resource document section titled “STR” for a brief overview of how Short
Tandem Repeats are used in DNA Analysis and answer the following questions.

a. What is a Short Tandem Repeat?

A short sequence of nucleotides repeated multiple times

b. Why is evaluating a person’s STRs useful in DNA analysis?

Because they are found more often and therefore easier to use when
trying to compare two samples and match them

c. How long are STRs?

Around 2-10 base pairs long

d. Why are STRs favored over VNTRs by forensic labs?

They are favored because they appear more often and are shorter,
making them easier to use in comparisons

e. What are the odds that two people have matching STRs in all 13 sites used in
CODIS?
One in ten trillion

5. Read the resource document section titled “PCR” for a brief overview of how PCR is
used in DNA Analysis and answer the following questions.

a. What is the main reason that forensic scientists use PCR in DNA Analysis?
It can copy small sections of DNA

b. Summarize the PCR procedure.

- Extract small amount of DNA from sample
- Mixed with chemical cocktail (natural enzymes, primers, and the four
nucleotide bases)
- After mixed, they are put into tiny plastic tubes
 - Put into a machine that heats and cools them to specific temperatures
which:
- Denature the DNA
- Annealing: Primers bind to the single strands that have denatured
- Enzymes copy the code of the DNA

6. Read the resource document section titled “RFLP” for a brief overview of how Restriction
Fragment Length Polymorphism is used in DNA Analysis and answer the following
questions.

a. What does RFLP measure?

RFLP measures DNA containing short sequences that
vary from person to person (aka VNTRs)

b. What does it mean if 2 different samples show VNTRs of different lengths?

The samples did not come from the same person.

c. What does it mean if 2 different samples show VNTRs of the same length?
They could possibly be from the same person.

d. Is this conclusive? Why?

No, it is never conclusive, only possible. Because some people have
similar sequences of DNA just like fingerprint comparisons.

e. Why has RFLP been mostly replaced with newer techniques?

It takes weeks to complete, unlike newer techniques.

7. Read the resource document section titled “SNP” for a brief overview of how Single
Nucleotide Polymorphism is used in DNA Analysis and answer the following questions.

a. What are SNPs?

Single Nucleotide Polymorphisms

b. What happens when SNPs occur inside a gene?

They create different alleles.

c. How is SNP different from how STR or VNTR is used in DNA Analysis?
It is the sequence itself that is used, not the length of the sequence.

d. Why do SNPs tend to be passed unchanged across generations?

Because mutations in the SNP are extremely rare.
 e. Why is SNP analysis rarely used in forensics?
SNP sequences are common in every person, to distinguish between
dozens of people would take too long.

8. Read the resource document section titled “mtDNA” for a brief overview of how
Mitochondrial DNA is used in forensics and answer the following questions.

a. Describe mtDNA and where it is found.

mtDNA is found in a cell’s many mitochondria, separate from the DNA
found in the nucleus.

b. Why can mtDNA usually be extracted from old or degraded samples in which
nuclear DNA was unable to be extracted?
Because there are so many copies of the mtDNA in the cell, there is much
more to extract than the DNA solely in the nucleus.

c. Why is mtDNA of an unidentified victim often compared to a possible maternal

relative?
mtDNA is inherited only from the mother.

d. What happens to the mtDNA in sperm during fertilization?

The mtDNA of the sperm is left behind.

9. Use the links to famous DNA analysis cases in the table to complete the table.

Colin Pitchfork Michael Blassie Kirk Bloodsworth

Summary Alec Jefferys and An unknown Kirk Bloodsworth

colleagues created soldier, killed in the was charged and
the RFLP way of Vietnam War, was convicted of the
determining buried in the Tomb rape and murder of
someone from their of The Unknown. a 9 year old child.
DNS fingerprint. The Blassie family He was sentenced
petitioned years to death. With the
later to have the help of his attorney,
remains DNA he was able to get
tested because DNA testing to
they lost their son compare his DNA
in the war and to the DNA found at
wanted to know if the scene.
those were his
remains. mtDNA
was taken from the
remains and it was
determined that it
was Micheal
 Blassie, their
missing son.

What was the Police asked mtDNA helped Bloodsworth was

outcome using Jefferys for help, return someone’s exonerated
DNA analysis? and when they remains back to because his DNA
compared crime their family. did not match.
scene blood with
their prime suspect
it exonerated him
and led them to the
actual culprit, Colin
Pitchfork.

What is the It helped serve It helped bring Bloodsworth was

significance of the justice, and get an peace to a family, the first death row
case? innocent person out they were able to inmate to be
of prison. properly bury their exonnerated by
son. post-conviction
DNA testing.