Genetics Discovery Lab - Cystic Fibrosis
One possible result of the Human Genome Project (HGP) is the ability to identify differences in the DNA of people
who have inherited genetic disorders. This is accomplished by sequencing the bases in DNA. DNA is a linear
molecule composed of four different bases adenine, cytosine, thymine, and guanine (abbreviated A, C, T, and G).
The information contained in the DNA molecule is encoded in these four bases.
These bases are “read” in groups of three, called codons. These codons code for specific amino acids, which form
specific proteins. Mutations in the base sequence can change the resulting protein structure. This can happen if
the mutation alters the amino acid specified by a codon or if it alters the sequence of bases in a codon by deleting
or adding a base. By sequencing the DNA bases from different people (some who have a disorder and some who
do not have a genetic disorder), geneticists can look for differences in the sequence that might cause the
disorder.
The DNA base sequence is the most detailed map of our
inheritable material. This map is similar to using a map of
the world (cell) to locate the united states (chromosome),
using a map of the U.S. to locate Wisconsin (chromosome
fragment), using a map of Wisconsin to locate Eau Claire
(gene), using a street map of Eau Claire to locate an address
where a specific family lives (the base sequence that
produced the protein). (see Figure 1)
WHAT IS CYSTIC FIBROSIS
Cystic fibrosis (CF) is an autosomal recessive, inherited genetic
disorder that affects mainly Caucasians. It is rare in people of
African descent and Asian descent. CF affects approximately
30,000 children and young adults in the U.S. and it occurs in
about one in every 2,500 newborns. Approximately 1,400
new cases are diagnosed each year, usually with the first
three years of life. The basic symptoms of CF include serious
digestive and respiratory problems and extremely salty
sweat. These symptoms result from the inability of the
exocrine glands to reabsorb chlorine and sodium. As a
result, thick, sticky secretions clog up the tubules in the
linings of the internal organs and cause irreversible
damage to various organs systems.
Earth
Cell
Chromosome
Country
State
Chromosome
fragment
gene
city
People
Nucleotide Base Pairs
Figure 1
Clogged bile ducts can cause cirrhosis of the liver and pancreatic tubules can rupture and spill out digestive
enzymes that can lead to the formation of fibrous tissue. Because the pancreatic enzymes cannot reach the
intestines, digestion and absorption of fats and proteins incomplete. The accumulation of thick mucus in the
airways of the lungs results in chromic coughing, impaired breathing, and increased (and often fatal)
susceptibility to bacterial infections. Because the ducts of the sweat and salivary glands do not reabsorb chorine
the sodium efficiently, individuals who have CF have extremely salty sweat – about five times as salty as that of
unaffected individuals.
Once researchers had the physical map of the region where the CF gene was located, they then examined the
base sequences of individual pieces of DNA for a difference that might cause the genetic disorder. At least 70% of
individuals who have CF share a common mutation in their sequences of bases. In this activity, you will examine
the DNA sequences of a small portion of the CF gene from six students and determine a few types of mutations
that can cause CF.
PROCEDURE
1. Geneticists use several methods to determine the base sequence
in a piece of DNA. In one common method, they use radioactivity
to mark different DNA bases and examine the radioactive bases
on a piece of photographic film. Each row across the film is a
position for a DNA base. A dark band in the column for a specific
base tell the geneticist which base is in that position. The is
similar to a map that tells you who (A,T,C, or G) lives at which
street address. Examine Figure 2 and read the positions from top
to bottom. What is the base sequence for the 10 positions in the
box area? ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ This
procedure allows geneticists to read many bases of DNA. They
also can compare the base sequences for the same gene in many
different people. (1 pt)
2. Figure 1.3 (on a separate full page) represents the bases present
in a small part of the gene for CF (from the maternal
chromosome of each individual) for six different high school
students. The entire CF gene contains about 250,000 bases – far
too many to list on this page. Use the “CF LAB WORKSHEETS” to
list the sequences of the maternal strand of DNA for the
individuals listed at the top of the worksheet. The corresponding
base sequences from the paternal chromosome are already
filled in under each individual’s name on the worksheet.
(Remember – chromosomes come in pairs – one from the
mother [maternal] and one from the father [paternal].) (6 pts)
3. Exchange worksheets with another class member and check each
other’s lists for accuracy. It is very important to have the correct
base sequences to work with.
Figure 2 An autoradiograph – a
method to determine base sequences.
4. Examine Josina’s maternal and paternal sequences. Starting at position 1, go across and circle any
differences from the maternal and paternal positions on the DNA strands (if any). (1 pt)
5. If Josina’s maternal and paternal sequences are identical and she does not have CF, what does that tell
you about that sequence of bases that produce the protein? (1 pt)
6. Examine the maternal and paternal base sequences from Josina, Norma, and Karen, who do not have CF.
Starting at position 1, go across and circle any base that is not like Josina’s for each position. For example,
if in position 9 there is a C in Josina’s, but there is a T in Norma’s, circle the T in Norma’s. CIRCLE
DIFFERENCES IN BOTHT HE MATERNAL AND PATERNAL SEQUENCES THAT ARE DIFFERENT THAN
JOSINA’S! (3 pts)
7. Examine the base sequences for Leah, Martin and Richard (who HAVE CF). Compare each base sequence
(maternal and paternal) with that of Josina’s (who does not have CF) and circle any differences between
hers and each of the other individuals similar to what you did in step 6. (6 pts)
8. What is especially different about the DNA sequence for Richard? (Pay close attention to the last nine
bases on Richards sequence and the last nine bases on Josina’s in order to determine which bases are
actually missing.) (2 pts)
9. On your worksheet, translate the amino acid coded for each codon on the maternal and paternal strands
by putting the abbreviation for each amino acid on the half box next to the codon. Do this for each
person. (12 pts)
10. How can a person have a difference in his or her base sequence and yet not have a change in the amino
acid sequence and not have a genetic disorder? (1 pt)
11. Compare the maternal and paternal amino acid sequence formed by this portion of the base sequence of
DNA for Josina. What do you find? (1 pt)
12. Compare the maternal and paternal amino acid sequence formed by this portion of the base sequence of
DNA for Norma. What do you find? (1 pt)
13. Compare the maternal and paternal amino acid sequence formed by this portion of the base sequence of
DNA for Karen. What do you find? (1 pt)
14. How could Norma and Karen have a different amino acid sequences produced from the maternal
sequence of DNA than the paternal sequence of DNA and yet not have CF? (1 pt)
15. In genetics, we learned about individuals who have an allele from one parent that codes for a correct
protein and another allele that codes for an ineffective protein. What do you call this kind of individual
(i.e. Norma and Karen)? (1 pt)
16. Examine the base sequences for each individual. Place an “A” under each DNA base sequence that
produces the correct amino acid sequence for the protein and an “a” under each sequence that codes for
a different amino acid sequence and thus an ineffective protein. (12 pts)
17. Examine the amino acid sequence for the proteins of Leah, Martin, and Richard. SPECIFICALLY, what
causes CF in each these people. Be specific for each person. Is it an Addition, deletion, transposition, or
point mutation? Where? On which strand? (3 pts)
Leah:
Martin:
Richard:
18. Would a person who has a maternal sequence like Leah’s along with a maternal sequence like Norma’s
have CF? Explain….. (2 pts)
19. Assume that a person has one allele with the maternal sequence shown for Leah and a second allele with
the maternal sequence shown for Richard. Would the person have CF? Explain your answer.(2 pts)
20. A person can have the same sequence of DNA for the 24 bases as does Josina and still have CF. Propose a
hypothesis to explain this. How might you test your hypothesis? Remember that 24 bases of the DNA
sequence you have examined represent only a small fraction of the DNA(approximately 250,00 bases)
found in the CF gene. (1 pt)
MATERNAL SEQUENCES FROM INDIVIDUALS’ AUTORADIOGRAPH
NORMA
A C G T
KAREN
A C G T
JOSINA
A C G T
LEAH
A C G T
MARTIN
A C G T
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
23
23
23
24
24
24
RICHARD
A C G T
CF LAB WORKSHEETS
POS
NORMA
KAREN
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
POS
1
G
1
G
2
A
2
A
3
A
3
A
4
A
4
A
5
A
5
A
6
T
6
T
7
A
7
A
8
T
8
T
9
C
9
C
10
A
10
A
11
T
11
T
12
C
12
C
13
T
13
T
14
A
14
A
15
T
15
C
16
G
16
G
17
G
17
G
18
T
18
T
19
G
19
G
20
T
20
T
21
T
21
T
22
T
22
T
23
C
23
C
24
C
24
C
ALLELE LETTER -
ALLELE LETTER -
CF LAB WORKSHEETS
POS
JOSINA
LEAH
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
POS
1
G
1
G
2
A
2
A
3
A
3
A
4
A
4
A
5
A
5
A
6
T
6
T
7
A
7
A
8
T
8
T
9
C
9
C
10
A
10
A
11
T
11
T
12
C
12
C
13
T
13
T
14
A
14
C
15
C
15
T
16
G
16
G
17
G
17
G
18
T
18
T
19
G
19
G
20
T
20
T
21
T
21
T
22
T
22
T
23
C
23
C
24
C
24
C
ALLELE LETTER -
ALLELE LETTER -
CF LAB WORKSHEETS
POS
MARTIN
RICHARD
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
Chromos Amino
Chromos
Acid
Amino
ome 7
ome 7
Sequence
Acid
sequence
sequence
from the from the Sequence from the
mother Mother from the father
♀
♀
♂
Father ♂
POS
1
G
1
G
2
A
2
A
3
A
3
A
4
A
4
A
5
A
5
A
6
T
6
T
7
A
7
A
8
T
8
T
9
C
9
C
10
A
10
A
11
T
11
T
12
C
12
C
13
T
13
G
14
A
14
G
15
A
15
T
16
G
16
G
17
G
17
T
18
T
18
T
19
G
19
T
20
T
20
C
21
T
21
C
22
T
22
23
C
23
24
C
24
ALLELE LETTER -
ALLELE LETTER -