CONSTRUCTION OF A HEMOGLOBIN GENE
1993 Woodrow Wilson Biology Institute
College Prep. Juniors and Senior.
- To see how eight pairs of triplets
are equivalent to part of a gene and
control a part of heredity.
- To understand
how red blood corpuscles manufacture normal
- Transparencies of two colors to represent
- nuclear sap,
- magic markers for coding the triplets,
- complimentary triplets made of plastic book
- list of complementary triplets:
Genes control the
synthesis of all the chemicals needed by organisms. Watson and
Crick looked for a code in the molecules of the nucleic acid DNA.
They found that DNA ( deoxyribonucleic acid) is a chemical consisting
of four kinds of units called nucleotides. Each nucleotide is
made up of three components: phosphoric acid, a single sugar,
and a nitrogen base. The only difference among thee four nucleotides
is the nitrogen base, which can be one of four: adenine(A), cytosine(C),
guanine(G), and thymine(T). DNA is seen as a long "double
helix" made of two complementary strands. One strand carries
the "DNA Codons". A codon is like a letter in an alphabet.
One codon equals three bases. Each codon is designed to locate
an amino acid in the right place in the manufacture of giant molecules
such as protein.
There are about 20 million triplet
pairs in a single human chromosome. Contained in a chromosome
are genes. A gene is a unit of heredity with a sequence of nucleotides.
They direct the sequence of amino acids during protein synthesis.
In this activity we will use a specific triplet pair involved
in the synthesis of the two beta chains of the hemoglobin (Hb)
molecules. To demonstrate how a gene is constructed (This activity
is only one of a number of possible such sequences that could
produce the normal amino acid sequence).
Hemoglobin is an enzyme that carries
oxygen in red blood corpuscles. Many diseases are related to
(caused by) hereditary defects in enzymes. Sickle cell anemia
is one such defect. Change in a single nucleotide in the Hb gene
can produce a change in one place in the Hb acid chain. This
minute change is sufficient to cause a serious defect in the molecule
and cause sickle cell disease.
- 1 Codon = 1 Amino Acid
- 146 Amino Acids = 1 Beta Chain
- 141 Amino Acids = 1 Alpha Chain
- 21 Alpha Chains + 2 Beta Chains = 1 Hb Molecule.
(For Teacher) Eight (8) tRNA's
attached to amino acids.
- Valine (GUA)
- Histidine (CAU)
- Threonine (ACU)
- Proline (CCU)
- Glutamic acid (GAG)
- Obtain plastic book divider tabs.
Leave one long divider as an uncut complementary strand of
- Make 9 complimentary triplets
using magic markers ( GTA, CAT, TTA, ACT, CCT, GAG, GAG, AAA, TAA).
- Scatter the nine triplet pieces
over a colored transparency marked as nucleus on one covor over the projector. This
represents triplets floating in the nuclear sap.
- Ask volunteers to come and match
the floating triplet pieces by joining the complementary triplets to the
single strand to make a double helix.
- The codon pair TAA/ATT is a "Terminator"
signaling the end of a gene. ( We are adding it to the end of our
host chain of eight codons for simplicity). It should result in transparency #2 when rightly
PART B: MUTATION/ SUBSTITUTION:
- Remove the the protective paper
that helps the dividers to attach. Take nine small strips of oval shaped paper
write 'tRNA ' on all the nine pieces to demonstrate mutation/substitution.
- Write the name of each triplet
and correspond them with their triplets on the transparency. Then swith the
names at the sixth position on the double helix to change the normal sequence to
show substitution/mutation. Connect them length wise to the corresponding triplets.
Switch the sixth position trplet.
- Now the sixth position on the
new mutated amino acid chain contains Valine instead of normal glutamic acid.
- The above substitution changes
the electrical structure of the Hb molecule and the red cells
"collapse" into the sickle shape when there is little
oxygen present. (Sickle cell anemia is a disease related to
heredity defects in enzymes.
- Give a set of the same material
to each student or student group to construct the entire hemoglobin molecule.
Then let them read the code to check defects. Connect each triplet to the complementary
strand and run two cords through them on either sides to make one large
chromosome. Twist the two cords to show the twisted double helix!
- Ask questions following outcomes
outline after the demonstration.
- Distribute student worksheets as
a follow up activity/ or extension of the activity.
- One strand of the double helix
contains the codon triplets.
- The other strand is made of complementary
- The gene here has 146 triplet
- Normal Hb can be made only if enough
proper codon is in its proper place in the gene.
- In this nucleic acid, eight pairs
of triplets are equal to part of a gene that controls
a part of heredity.
STUDENT WORK SHEET
- Here are 8 codons of the gene code
for Hb. The 9th codon is a terminator codon. Fill in the complementary triplets in
the proper place.
C A T G T A A A T T G A G G A C T C C T T T A T T
- What do the codons represent?
- What do the lines represent?
- How many bonds are there
between G & C-------- , T & ------ , A & U ---
- Why is the number of bonds important in nucleic acids?
- Measure the length of your paper
strip "gene". How long would it be if it had
- If the gene with 146 triplets is
part of a chromosome with 20,000,000 triplets, how long would the paper chromosome
- The DNA in cell nuclei is shortened
when it is shaped into (knots) (helices) (coils of helices). What is your choice?
for Dna Made Easy A bio-LOGIC Kit
by Herbert A. Nestler.
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281