Part I – Molecular Biology of Sickle Cell Anemia.
Recent research in biology has connected known “disease genes” first to specific sites on specific chromosomes, and then to the altered proteins these genes specify in the organism. Hemoglobin is a prime example; it is the well known protein that carries oxygen in the red blood cell. The hemoglobin protein is made of four polypeptide chains: 2 alpha chains (141 amino acids long and 2 beta chains (146 amino acids long). Hence there is a gene for the alpha globin peptide chain and another gene for the beta globin peptide chain. There are many known mutations in the HBB gene (beta globin gene) leading to a variety of inherited diseases. The genetic disease sickle cell anemia is one example: the genetic code is altered such that the amino acid in position six is replaced by another amino acid.
Find out on which chromosome in the genome this gene is located: visit the NCBI (National Center for Biotechnology Information) home page at www.ncbi.nlm.nih.gov and on the right side of home page under “Hot Spots,” click on the Genes and Disease part of the “Coffee Break, Genes and Disease, NCBI Handbook”, then click on Anemia, sickle cell to find out the chromosome number ______ and location/position________ of this disease gene. From this page you can link to lots of other information about sickle cell anemia.
Making Normal beta-chain of Hemoglobin:
Here is the first part of the DNA sequence for the beta chain of normal hemoglobin; fill in the complementary DNA strand, using the base-pairing rules for making DNA:
Opposite adenine (A) put thymine (T) and vice versa
Opposite guanine (G) put cytosine (C) and vice versa
1
GTG CAC CTG ACT CCT GAG GAG
_________________________________
Now make the messenger RNA from this second strand of DNA, using the base-pairing rules for making RNA:
Opposite adenine (A) put uracil (U), opposite thymine (T) put adenine (A)
Opposite guanine (G) put cytosine (C) and vice versa
__________________________________
Now use the accompanying genetic code, and translate this messenger RNA into a sequence of amino acids.
This table shows the 64 mRNA codons and the amino acid each codon codes for.
|
|||||
|
2nd base |
||||
U |
C |
A |
G |
||
1st |
U |
UUU (Phe/F) |
UCU (Ser/S) |
UAU (Tyr/Y) |
UGU (Cys/C) |
C |
CUU (Leu/L) |
CCU (Pro/P) |
CAU (His/H) |
CGU (Arg/R) |
|
A |
AUU (Ile/I) |
ACU (Thr/T) |
AAU (Asn/N) |
AGU (Ser/S) |
|
G |
GUU (Val/V) |
GCU (Ala/A) |
GAU (Asp/D) |
GGU (Gly/G) |
|
Making Sickle Cell Hemoglobin:
In sickle cell anemia, there is a mutation at the 17th nucleotide of DNA in this gene; the nucleotide (base) is changed from A to T; fill in the second strand:
1
GTG CAC CTG ACT CCT GTG GAG
__________________________________
Now make the messenger RNA from this second mutated strand:
__________________________________
Now, using the genetic code, translate this new messenger RNA into a sequence of amino acids:
____________________________________
You can see that in normal hemoglobin, amino acid #6 is _________ and in sickle cell hemoglobin amino acid #6 is __________. Observe the structural formulae for these two amino acids:
Characterize the amino acid change in terms of polar versus nonpolar amino acids, or alternatively, which amino acid above carries an overall charge:
Although the altered hemoglobin has only one amino acid changed out of the total of 146, it’s a crucial amino acid. When this new amino acid is at position #6 instead of the correct amino acid, the hemoglobin molecule is altered so that it becomes more hydrophobic. As a result, when the altered hemoglobin chains fold into their three-dimensional shape, they tend to stick to each other, forming long insoluble fibers of hemoglobin within the red blood cell. The red blood cell is deformed by this altered hemoglobin. It becomes more fragile, rupturing easily in tiny capillaries and clogging them up. Eventually, as the damaged red cells break down at an increased rate, the body experiences anemia along with other characteristic symptoms.
Check your understanding:
Tell how sickle cell hemoglobin differs from normal hemoglobin at the primary level of protein structure:
At the tertiary level of protein structure:
What is the effect on the red cell which contains this altered hemoglobin?
Credits: From Mulvihill, C. (1996). Making the Chromosome-Gene-Protein Connection. The American Biology Teacher, 58 (6), 364-368. With permission from the National Association of Biology Teachers.
|
|
Professional Development |