Question

The genetic defect responsible for \(\mathrm{HD}\) is a. A small sequence of DNA on chromosome 4 in which several base pairs are repeated many, many times. b. The normal gene has three DNA bases, composed of the sequence CAG. c. In people with \(\mathrm{HD}\), the sequence abnormally repeats itself dozens of times. d. Over time-and with each successive generations-the number of CAG diminishes. e. All and above f. a, b, and c only

Short answer

Answer: f. a, b, and c only

Step-by-step solution

Analyzing the statements

Let's examine each statement carefully to determine if it is true or false. a. A small sequence of DNA on chromosome 4 in which several base pairs are repeated many, many times. - This statement is true. HD is caused by a repeating DNA sequence on chromosome 4. b. The normal gene has three DNA bases, composed of the sequence CAG. - This statement is true. The sequence CAG constitutes a triplet that encodes for an amino acid (glutamine) in the normal gene. c. In people with HD, the sequence abnormally repeats itself dozens of times. - This statement is true. The CAG triplet repeats itself much more frequently in people with HD than in unaffected individuals. d. Over time-and with each successive generations-the number of CAG diminishes. - This statement is false. In HD, the number of CAG repeats generally increases as the disease is passed down from generation to generation. e. All and above - Since not all the statements are true, this option is incorrect. f. a, b, and c only - Since statements a, b, and c are the only true statements listed, this option is correct.

Identifying the correct answer

We have determined that statements a, b, and c are the true statements related to the genetic defect responsible for HD. Therefore, the correct answer is: f. a, b, and c only

Key concepts

DNA Sequences

DNA sequences are essentially instructions for building the proteins that make up every part of our body. Each sequence is a chain of nucleotides, which are the basic building blocks of DNA. These nucleotides are made up of four different bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The order of these bases dictates the function of the resulting protein.

In genetics, specific sequences of these bases, often referred to as 'triplets', function as coding units for amino acids. For example, the triplet CAG codes for the amino acid glutamine. This specific sequence is significant in various genetic contexts, including genetic disorders like Huntington's Disease (HD).

In people without HD, the CAG sequence appears a certain number of times within a gene. However, in individuals with HD, this sequence repeats far more frequently, leading to problems in protein synthesis and resulting in the symptoms associated with HD. Understanding DNA sequences and their exceptional patterns helps in diagnosing and providing insights into various genetic disorders.

Chromosome 4

Chromosome 4 is one of the 23 pairs of chromosomes found in humans. It carries vital genetic information that contributes to a variety of bodily functions and developmental processes.

Within chromosome 4 resides the gene known as HTT (huntingtin gene), which is of particular interest when it comes to Huntington's Disease. This gene contains the CAG sequence and its abnormal expansions that lead to HD symptoms.

Unlike average chromosomal mutations, which can occur anywhere along a chromosome, the issue with chromosome 4 concerning HD is specific to the expansion of the CAG repeat within the HTT gene. This distinction is crucial because targeting the expression of this specific gene becomes more practical, opening paths for treatments that can potentially manage or alleviate the symptoms of HD.

CAG Repeat Expansion

CAG repeat expansion is a crucial concept when studying Huntington’s Disease. This genetic anomaly involves the repeated occurrence of the CAG sequence within the huntingtin gene. Normally, humans have a set number of CAG repeats in this gene, which helps in the production of normal proteins.

However, in HD, this sequence undergoes significant expansion, meaning the triplet CAG repeats far more than usual, sometimes dozens or even hundreds of times. This increase in repeating sequences can lead to abnormal proteins, which disrupts normal cellular functions, ultimately resulting in the neurodegenerative symptoms observed in HD patients.

The number of CAG repeats dictates the onset and severity of the disease. Typically, a higher number of repeats correlates with earlier onset and more severe progression of symptoms. Understanding CAG repeat expansion is vital not only for diagnosing HD but also for exploring therapeutic interventions aimed at minimizing its harmful impact.