Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (ssaupe@csbsju.edu); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321

Molecular Biology of Some Blood Disorders

Pre-Class Assignment

  1. Print a copy of this exercise, the Fig 1 - nucleotide sequence and Fig 2: Introns & exons diagrams.  Bring all to class.

  2. Type a response to the following and bring to class:

  • Briefly describe the human hemoglobin molecule and its function

  • Define transcription, transcription, exon, intron, codon, mature mRNA, pre-mRNA

  • What is sickle-cell anemia?

  • What is β-thalassemia?

Post-Class Assignment

  1. On a separate sheet of paper, number 1 to 29.  Answer all questions.  No need to type.

Transcription:
      Human β -globin is a protein that is found in human hemoglobin.  The DNA sequence for this protein is shown in CAPITAL LETTERS in Figure 1 (you will need to print a copy).

  1. On Figure 1, mark the beginning and the end of the human β-globin gene with brackets ([ ]).   Remember, the nucleotide sequence of the gene is represented by CAPITAL LETTERS.

  2. Approximately how many nucleotides make up this gene? ______________. 

  3. Based on the number reported in the question above, how many amino acids do you expect to find in b-globin protein?  ___________.   Explain your answer.

      The entire β -globin gene is transcribed into mRNA (called pre-mRNA), but not all of it is translated into protein (i.e., exon vs. intron regions).  Exon regions of the β -globin gene are indicated in Figure 2 by the larger CAPITAL LETTERS, and the intron region are represented by the smaller CAPITAL LETTERS.  

  1. Separate the exon and introns with a slash ( / ).

  2. How many exon regions are there? ______

  3. How many introns regions are there?  ______

Figure 1.  Nucleotide sequence for the anti-sense strand of β -globin DNA.  (from Vigue, 1987).  Click on thumbnail (pdf). Figure 2.  Diagrammatic representation of exon and intron regions (from Vigue, 1987).  Click on thumbnail (pdf).

    After the pre-mRNA is transcribed, the introns are "snipped out" by enzymes to form "mature mRNA" that will be translated into protein.

  1. Approximately how many nucleotides are found in the exon regions of the β -globin gene? ______

  2. What percent of the total nucleotides for the β -globin gene do the exon regions represent? _______

  3. Based on the number of nucleotides reported in the question above, how many amino acids do you expect to find in the β -globin protein?  Explain your answer.

  4. How many total amino acids actually make up the primary sequence of β -globin protein?  _______

  5. How many different amino acids make up the human b-globin protein? ________ (note:  each amino acid has a standardized three-letter abbreviation)

    Note that there are sequences of several nucleotides at the beginning and the end of the gene that are used for punctuation; these tell the cell where the gene begins and ends. 

     Find the promoter region (underlined, line 1).  This is where RNA polymerase binds to the DNA. 

  1. What is the nucleotide sequence of the promoter region on the sense strand of the DNA? ____________.

  2. Note that RNA polymerase doesn't begin to transcribe the gene for several nucleotides (lower case letters).  How many nucleotides are "downstream" of the promoter region that aren't transcribed? _______    

  3. Note in the second line (CAPITAL LETTERS before the number 1) that there is a string of nucleotides that are transcribed but not translated?  Approx. how many nucleotides make up this section?  _____________ .

  4. TAC is the DNA sense-strand codon that initiates translation.  What is the DNA anti-sense codon that initiates translation?  __________.  Now, circle this codon.  

  5. What amino acid does this codon normally specify? ________________

  6. Offer an explanation why this amino acid is not written above the DNA like most of  the other amino acids.

    Locate the transcription termination signal (underlined, 5th line from bottom).  This is the sequence of nucleotides that tells the RNA polymerase to release from the DNA.

  1. What is the nucleotide sequence of the termination signal on the sense-strand of the DNA? ____________.

  2. How many nucleotides did RNA polymerase transcribe after reaching the transcription termination sequence? _______________

  3. There are three codons that specify the end of translation.  Identify the DNA sense strand sequence of these three codons __________, __________, __________.

  4. Which of these codons serves as the translation termination codon for the β -globin gene? ______

  5. Circle the termination codon.

  6. Once the termination codon is reached, how many nucleotides were transcribed but not translated before reaching the transcripton termination signal?  __________

Translation.

  1. What is the first amino acid in the primary sequence of β -globin protein? ________________

  2. What is the last amino acid the primary sequence of β -globin protein? _______________

  3. Using a codon table, identify the amino acids that will result from the translation of nucleotides in the areas between the arrow (�  � ).

Mutations.
    Numerous mutations of the human β -globin gene are known.  Sickle cell anemia is a disease that results form a single mutation.  the adenine indicated by an asterisk (*) is replaced by a thymine (T) in individuals with this condition.  

  1. The mutation of the normal codon GAG (at the asterisk) to GTG results in the replacement of the normal amino acid glutamic acid (abbreviated glu) with the amino acid _________________ in the β -globin protein of an individual with sickle cell anemia.

    There are several causes of β -thalassemia.  One cause is the insertion of an "A" (adenine) between codons 71 and 72 so that the mutated sequence of nucleotides is:   ...TTT A AGT...

  1. In the normal b-globin gene, what amino acids do codons 72 and 73 specify?  __________________  and ___________________.

  2. In an individual with b-thalassemia caused by an adenine insertion, which amino acids do the two codons following codon 71 specify?  _______________ and  __________________.


Reference:
    This exercise is adapted from an article by Vigue, CL (1987) Murphy's law and the human beta-globin gene.  American Biology Teacher 49: 76.

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Last updated: July 14, 2009     � Copyright by SG Saupe