Use the information below, as well as the previous passages, to answer this question.
DNA sequences lead to the production of proteins through a two-step process. The first step is called transcription. During transcription, the DNA strands are copied into a piece of RNA one base at a time. This copy travels out of the nucleus to the cytoplasm. Here the second step will begin, called translation. During translation, the RNA will put the building blocks of proteins, called amino acids, together in the right order so that a proper protein can be produced.
Another line of research has focused on a protein called Apolipoprotein E (APOE). The gene for APOE is found on human chromosome #19. Sequence analysis shows that the APOE protein comes in several different forms, which geneticists call alleles. Three alleles occur in humans: APOE 2, APOE 3, and APOE 4. This protein, in its normal form, is known to clean cholesterol and other types of fat away from neurons, helping them to stay healthy and function properly. The APOE proteins are under study at this point to see if there is a link between any of them and early and late onset Alzheimer’s disease. For this study, the research team looked at gene sequences from 1000 individuals. A summary of their findings is in the chart below:
|Allele||# in Population||# with Early Onset||Average Age of Early Onset Diagnosis||# with Late Onset||Average Age of Late Onset Diagnosis|
Alzheimer’s disease is an irreversible, progressive brain disease that is characterized by the development of brain lesions (known as plaques) and tangles in the delicate fibers of the nerve tissue. This causes the loss of connections between neurons, the specialized cells of the nervous system, and the eventual death of these cells. Scientists have long known that neurons are highly specialized cells. They have a streamlined design, giving them the ability to quickly conduct electrical messages as the body responds to changes in its environment. This specialization comes at a price, however, as these neurons can only make repairs to themselves if the damage is slight. Once damaged beyond repair, or dead, these cells can’t regenerate, leaving the victim lacking a sufficient number of vital neurons.
There are two types of Alzheimer’s disease: early onset, and late onset. Early onset occurs in people age 30 to 60. It is the rarer form of the disease, representing less than 5% of the people who contract it. Most cases of Alzheimer’s are of the late onset form, which develops after age 60. The causes of both diseases are still being worked out. Much research has been done, including a hunt for a possible genetic connection that may be the root cause of the disease. Scientists are looking at genes, sequences of DNA which code for the making of proteins, as possible contributors to both varieties of Alzheimer’s. Researchers first asked if perhaps victims have faulty DNA which could be causing their condition. Scientists pinpointed a specific neuron protein on Chromosome #21 that may lead to Alzheimer’s. It is called amyloid precursor protein (APP).
Researchers studied APP DNA sequences, making comparisons between people who have both types of Alzheimer’s, as well as people who don’t. The results of the sequence analysis are shown in the table below:
|Patient Type||Age||Partial AAP Gene Sequence From Chromosome #21|
Which statement best describes how a bad APOE allele could lead to the development of late onset Alzheimer’s disease?