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Chapter 23: Problem 14

Explain the cause of the genetic disease sickle cell anemia.

Short Answer

Expert verified
Sickle cell anemia is caused by a point mutation in the HBB gene that changes the shape of hemoglobin, leading to abnormally shaped red blood cells, which can block blood flow.

Step by step solution

01

Understand the Basic Cause

Sickle cell anemia is caused by a mutation in the HBB gene, which provides instructions for making a protein called beta-globin, a component of hemoglobin found in red blood cells.
02

Describe the Mutation

The specific mutation is a point mutation where the amino acid glutamic acid is replaced by valine at the sixth position of the beta-globin chain. This is known as a hemoglobin S (HbS) mutation.
03

Explain the Structural Change

The substitution of amino acids causes hemoglobin molecules to stick together and form long fibers. This changes the shape of red blood cells from a normal disc shape to a rigid, sickle shape.
04

Discuss the Impact on Red Blood Cells

Sickle-shaped cells are less flexible and can get stuck in small blood vessels, blocking blood flow and leading to pain and organ damage.
05

Address Inheritance Patterns

Sickle cell anemia is inherited in an autosomal recessive pattern, meaning that both copies of the HBB gene in each cell have mutations. The parents of an individual with sickle cell anemia each carry one copy of the mutated gene.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

HBB gene mutation
Sickle cell anemia is caused by a specific change in the HBB gene. This gene provides the instructions for creating a protein called beta-globin, which is an important part of hemoglobin. Hemoglobin is the molecule in red blood cells responsible for carrying oxygen throughout the body.
Simplifying this, think of the HBB gene as a recipe, and beta-globin as one ingredient in the final dish, which is hemoglobin.
When this 'recipe' has a mistake, it leads to the wrong ingredient being used.
beta-globin protein
The beta-globin protein is crucial for the proper functioning of hemoglobin. Hemoglobin is composed of four protein subunits: two alpha-globin and two beta-globin chains. When these subunits come together, they form a complete hemoglobin molecule. This molecule is vital as it transports oxygen from the lungs to tissues throughout the body.
  • Normal Functioning: When beta-globin functions normally, red blood cells maintain their usual round shape and are flexible enough to move easily through blood vessels.
  • Defective Beta-Globin: In sickle cell anemia, the beta-globin subunit is altered due to a mutation, leading to the production of abnormal hemoglobin.
When the beta-globin protein is defective, it not only affects red blood cells' shape but also impacts their ability to flow smoothly through blood vessels.
hemoglobin S (HbS) mutation
The hemoglobin S (HbS) mutation involves the replacement of one amino acid in the beta-globin chain: glutamic acid is replaced by valine at the sixth position. This single change in the amino acid sequence has significant consequences for hemoglobin's structure and function.
  • Structural Changes: The alteration causes hemoglobin molecules to stick together and form long fibers inside red blood cells, especially when they release oxygen.
  • Cell Shape Impact: These fibers distort red blood cells into a rigid, sickle shape, instead of the usual flexible, round shape.
Because of this mutated hemoglobin, or HbS, the red blood cells become less efficient in carrying oxygen and more prone to blockages in small blood vessels.
autosomal recessive inheritance
Sickle cell anemia is inherited in an autosomal recessive pattern. This means that for an individual to express the symptoms of sickle cell anemia, they must inherit two copies of the mutated HBB gene—one from each parent.
  • Carriers: Individuals with only one copy of the mutated gene are called carriers, or they have sickle cell trait. They usually do not show symptoms of the disease but can pass the mutated gene to their offspring.
  • Affected Individuals: If both parents are carriers, each of their children has a 25% chance of having sickle cell anemia, a 50% chance of being a carrier, and a 25% chance of not inheriting the mutation at all.
This inheritance pattern explains why sickle cell anemia can appear in families even if parents do not show symptoms.

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