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ANNA TREOHAN
EVANS HIGH SCHOOL
EVANS, GA 30809.

CONSTRUCTION OF A HEMOGLOBIN GENE


TARGET:

College Prep. Juniors and Senior.

TIME:

50 minutes

OBJECTIVES:

  1. To see how eight pairs of triplets are equivalent to part of a gene and control a part of heredity.
  2. To understand how red blood corpuscles manufacture normal hemoglobin.

MATERIALS:

  1. Transparencies of two colors to represent
  2. nuclear sap,
  3. magic markers for coding the triplets,
  4. complimentary triplets made of plastic book
  5. dividers,
  6. list of complementary triplets:
    • GTA
    • CAT
    • TTA
    • ACT
    • CCT
    • GAG
    • GAG
    • AAA
    • TAA

BACKGROUNG INFORMATION

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.

RELATIONSHIP:

  • 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.

PART B

(For Teacher) Eight (8) tRNA's attached to amino acids.

  • Valine (GUA)
  • Histidine (CAU)
  • Lencine(UUA)
  • Threonine (ACU)
  • Proline (CCU)
  • Glutamic acid (GAG)
  • Lysine(AAA)


Transparency #1


Transparency #2


Transparency #3


PROCEDURE/ DEMONSTRATION:

  1. Obtain plastic book divider tabs. Leave one long divider as an uncut complementary strand of DNA.
  2. Make 9 complimentary triplets using magic markers ( GTA, CAT, TTA, ACT, CCT, GAG, GAG, AAA, TAA).
  3. 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.
  4. Ask volunteers to come and match the floating triplet pieces by joining the complementary triplets to the single strand to make a double helix.
  5. 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 matched.

PART B: MUTATION/ SUBSTITUTION:

  1. 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.
  2. 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.
  3. Now the sixth position on the new mutated amino acid chain contains Valine instead of normal glutamic acid.
  4. 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.
  5. 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!

OUTCOMES:

  1. Ask questions following outcomes outline after the demonstration.
  2. Distribute student worksheets as a follow up activity/ or extension of the activity.
    1. One strand of the double helix contains the codon triplets.
    2. The other strand is made of complementary triplets
    3. The gene here has 146 triplet pairs.
    4. Normal Hb can be made only if enough proper codon is in its proper place in the gene.
    5. In this nucleic acid, eight pairs of triplets are equal to part of a gene that controls a part of heredity.

STUDENT WORK SHEET

  1. 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


    1. What do the codons represent?
    2. What do the lines represent?
    3. How many bonds are there between G & C-------- , T & ------ , A & U ---
  2. Why is the number of bonds important in nucleic acids?
  3. Measure the length of your paper strip "gene". How long would it be if it had 146 triplets?
  4. If the gene with 146 triplets is part of a chromosome with 20,000,000 triplets, how long would the paper chromosome be?
  5. The DNA in cell nuclei is shortened when it is shaped into (knots) (helices) (coils of helices). What is your choice?

Ref: (Worksheet)

Teacher's guide for Dna Made Easy A bio-LOGIC Kit by Herbert A. Nestler.



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