Question

The genetic defect responsible for 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 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

Understand the concepts

Understand the terminology related to the genetic defect of Huntington's Disease (HD). It's important to know the role of DNA sequences, genes, and base pairs.

Analyze each statement

Going through each statement to understand and analyze its correctness: 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 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

Evaluate each statement

Evaluate the correctness of each statement in relation to the genetic defect for HD: a. This statement is correct - the defect is a small sequence of DNA on chromosome 4 with repeated base pairs. b. This statement is correct - the normal gene has three DNA bases with the sequence CAG. c. This statement is also correct - HD is associated with the abnormal repetition of the CAG sequence. d. This statement is incorrect - the number of CAG repeats does not decrease, but rather increases, over generations.

Identify the correct answer

Based on the evaluation of the statements, we can determine that the correct answer is the option that includes statements a, b, and c: f. a, b, and c only

Key concepts

DNA Base Pairs

Deoxyribonucleic acid (DNA) is the blueprint of life, containing the instructions needed for an organism to grow, develop, and reproduce. DNA strands are composed of molecules called nucleotides, each containing a phosphate group, a sugar group, and a nitrogen base. There are four types of nitrogen bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair up with each other to form base pairs, A with T and C with G, creating the rungs of the DNA double helix ladder.
In the context of Huntington's Disease (HD), the focus is on the sequence of three bases, CAG, which normally occurs several times within the DNA on chromosome 4. This trinucleotide sequence is a critical part of the genetic code that, when functioning correctly, contributes to normal cell operations. However, HD is the result of the excessive repetition of this CAG sequence, which leads to the production of abnormal proteins that are harmful to neurons.

CAG Repeat Expansion

Huntington's Disease (HD) is a prime example of a genetic disorder caused by the expansion of a trinucleotide repeat, specifically the CAG sequence. In individuals without HD, this sequence repeats about 10 to 35 times within the gene known as HTT. During DNA replication, the DNA polymerase may slip, and as a result, the number of CAG repeats can increase. In individuals with HD, these repeats occur much more frequently, typically ranging from 36 to more than 120 times.
This phenomenon of CAG repeat expansion is not static; it can worsen over generations, a concept known as anticipation. With each successive generation, the number of repeats can increase, leading to an earlier onset and more severe progression of the disease. Contrary to the belief that these repeats diminish over time, they actually tend to increase, which can exacerbate the condition in the affected family line.

Genetic Defect Analysis

Genetic defect analysis is the process of examining DNA to identify anomalies that may lead to genetic disorders, such as HD. Scientists use various techniques to analyze genes and their sequences, looking specifically for mutations like the CAG repeat expansion. In the case of HD, molecular genetic testing can measure the number of CAG repeats. This type of testing is crucial for diagnosing the disease, predicting its onset, and providing at-risk individuals with information regarding their genetic status.
Through genetic counseling, individuals can discuss the implications of genetic testing, such as the probability of developing HD and the risks of passing it on to offspring. Genetic defect analysis not only facilitates the understanding of the specific mechanics behind HD but also has profound implications for finding potential treatments and preventative measures for this and similar genetic disorders.