Hemoglobin is an oxygen-transport protein found in the red blood cells of all vertebrates. In mice, hemoglobin is a complex of four subunits, 2 alpha-globin chains and 2 beta-globin chains, with each subunit containing a heme molecule. The mouse beta-globin gene is approximately 1,500 nucleotides long, however, the cytoplasmic beta-globin mRNA is only about 750 nucleotides long. Select the best explanations for the difference in length. 1. The beta-globin gene is interrupted by one or more introns. The beta-globin mRNA is processed after being transcribed. 2. The beta-globin gene encodes both of the beta-globin subunits. The beginning of the beta-globin gene is not transcribed. 3. The beta-globin mRNA underwent splicing in the cytoplasm.
In the 1970s, scientists conducted experiments to determine the reason for the difference in length between the mouse beta-globin gene and the mature cytoplasmic beta-globin mRNA. In one type of experiment, beta-globin DNA was incubated with cytoplasmic beta-globin mRNA and the complexes formed were analyzed by electron microscopy. The microscope images had structures that were interpreted to be DNA/DNA duplexes, DNA/RNA duplexes, or unannealed strands. Choose the statement that correctly describes the typical finding from this type of experiment. You do not need to know the details of the experiment to answer this question. Use your knowledge of eukaryotic genes to identify the most logical answer.
1. No annealed sections present.
2. Two annealed sections of DNA/RNA duplex, with a loop of unannealed mRNA between them.
3. One uninterrupted annealed section of DNA/RNA duplex.
4. Two annealed sections of DNA/RNA duplex, with a loop of unannealed DNA between them.