In humans, two closely related forms of apolipoprotein B (Apo B) are found in bl
ID: 94066 • Letter: I
Question
In humans, two closely related forms of apolipoprotein B (Apo B) are found in blood as constituents of the plasma lipoproteins. ApoB48, which has a molecular mass of 48 kilodaltons, is synthesized by the intestine and is a key component of chylomicrons, the large lipoprotein particles responsible for delivery of dietary triglycerides to adipose tissue for storage. ApoB100, which has a molecular mass of 100 kilodaltons, is synthesized in the liver for formation of the much smaller, very low-density lipoprotein particles used for the distribution of triglycerides to meet energy needs. A classic set of studies defined the surprising relationship between these two proteins. Sequences of cloned cDNA copies of the mRNAs from these two tissues revealed a single difference: cDNAs from intestinal cells had a T, as part of a stop codon, at a point where the cDNAs from liver cells had a C, as part of a glutamine codon (Figure 1). To verify the differences in the mRNAs and to search for corresponding differences in the genome, RNA and DNA were isolated from intestinal and liver cells and then subjected to PCR amplification, using oligonucleotides that flanked the region of interest. The amplified DNA segments from the four samples were tested for the presence of the T or C by hybridization to oligonucleotides containing either liver cDNA sequence (oligo-Q) or the intestinal cDNA sequence (oligo-stop). The results are shown in Table I. Are the two forms of ApoB produced by transcriptional control from two different genes, by a processing control of the RNA transcript from a single gene, or by differential cleavage of the protein product from a single gene? Explain your reasoning.
liver CDNAM ATGATACAATTTGAT Apo-B mRNA - intestine ATGATATAATTTGAT CDNA M I* Figure 1. Location of the sequence difference in cDNA clones from ApoB RNA isolated from liver and intestine. The encoded amino acid sequences, in one-letter code, are shown aligned with the cDNA sequences. The asterisk indicates a stop codon.Explanation / Answer
The Oligo-Q bind with the DNA of both intestinal and liver cells where as Oligo-Stop unable to bind with any of them. Means complementary sequence for Oligo-stop does not exist at DNA level. Thus the gene sequence for both the mRNA are same. But their mRNA sequence are different.Which means both mature mRNAs are produced from the same gene. After trnscription newly produced hnRNA are spliced differently to produce two splice varrient from which the two Apo lipoproteins are translated.