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Question 1 of 12
What is the lecture mainly about?
Lecturer: So, continuing our discussion of desert lakes, now I want to focus on what’s known as the “Empty Quarter”. The “Empty Quarter” is a huge area of sand that covers about a quarter of the Arabian Peninsula. Today it’s pretty desolate, barren and extremely hot. But there’ve been times in the past when monsoon rains soaked the Empty Quarter and turned it from a desert into grassland that was dotted with lakes and home to various animals. There were actually two periods of rain and lake formation: the first one began about 35000 years ago; and the second one dates from about 10000 years ago.
Student: Excuse me, Professor. But I’m confused. Why would lakes form in the desert? It’s just sand, after all.
Lecturer: Good question! We know from modern day desert lakes, like Lake Eyre, South Australia, that under the right conditions, lakes do form in the desert. But the Empty Quarter lakes disappeared thousands of years ago. They left behind their beds or basins as limestone formations that we can still see today. They look like low-lying, white or grey builds, long, narrow hills with flat tops, barely a meter high. A recent study of some of the formations presents some new theories about the area’s past. Keep in mind though that this study only looked at 19 formations. And about a thousand have been documented. So there’s a lot more work to be done.
According to the study, two factors were important for lake formation in the Empty Quarter: first the rains that fell there were torrential. So it would’ve been impossible for all the water to soak into the ground. Second, as you know, sand dunes contain other types of particles, besides sand, including clay and silt. Now, when the rain fell, water ran down the sides of the dunes, carrying clay and silt particles with it. And wherever these particles settled, they formed a pan, a layer that water couldn’t penetrate. Once this pan formed, further run-off collected, and formed a lake.
Now, the older lakes, about half the formations, the ones started forming 35000 years ago, the limestone formation we see, they’re up to a kilometer long, but only a few meters wide, and they’re scattered along the desert floor, in valleys between the dunes. So, the theory is, the lakes formed there, along the desert floor, in these long narrow valleys. And we know, because of what we know about similar ancient desert lakes, we know that the lakes didn’t last very long, from a few months to a few years on average. As for the more recent lakes, the ones from 10000 years ago, well, they seemed to have been smaller, and so may have dried up more quickly. Another difference, very important today for distinguishing between older lake beds and newer ones, is the location of the limestone formations. The more recent beds are high up in the dunes. Why these differences? Well, there are some ideas about that, and they have to do with the shapes of the sand dunes, when the lakes were formed. 37000 years ago, the dunes were probably nicely rounded at the top, so the water just ran right down their sides to the desert floor. But there were thousands of years of wind between the two rainy periods, reshaping the dunes. So, during the second rainy period, the dunes were kind of chopped up at the top, full of hollows and ridges, and these hollows would’ve captured the rain right there on the top.
Now, in grassland of Lake Ecosystem, we’d expect to find fossils from a variety of animals, and numerous fossils have been found at least at these particular sites. But, where did these animals come from? Well, the theory that has been suggested is that they migrated in from nearby habitats where they were already living. Then as the lakes dried up, they died out. The study makes a couple of interesting points about the fossils, which I hope will be looked at in future studies. At older lake sites, their fossil remains from hippopotamuses, water buffalo, animals that spend much of their lives standing in water, and also, fossils of cattle. However, at the sites of the more recent lakes, there’re only cattle fossils, additional evidence for geologists that these lakes were probably smaller, shallower, because cattle only use water for drinking. So they survive on much less. Interestingly, there are clams and snail shells; but, no fossils of fish. We’re not sure why. Maybe there is a problem with the water. Maybe it was too salty. That’s certainly true of other desert lakes.
Question 2 of 12
What is the professor’s opinion about the conclusions of the recent study of the limestone formations in the Empty Quarter?
Question 3 of 12
According to the professor, what feature of the sand dunes made the formation of the lakes possible?
Question 4 of 12
How is it possible to determine in which rainy period a lake was formed? Click on 2 answers.
Question 5 of 12
What does the professor imply about the lack of water buffalo and hippopotamus fossils in the more recent lakes?
Question 6 of 12
What possible explanation does the professor give for the apparent absence of fish in the most ancient lakes?
Question 7 of 12
What does the professor mainly discuss?
Narrator: Listen to part of a lecture in a Biology Class. Professor: As we learn more about the DNA in human cells and how it controls the growth and development of cells, then maybe we can explain a very important observation, that when we try to grow most human cells in a laboratory, they seem programmed to divide only a certain number of times before they die. Now this differs with the type of cell. Some cells, like nerve cells, only divide seven to nine times in their total life. Others, like skin cells, will divide many, many more times. But finally the cells stop renewing themselves and they die. And in the cells of the human body itself, in the cells of every organ, of almost every type of tissue in the body, the same thing will happen eventually. OK, you know that all of persons’ genetic information is contained on very long pieces of DNA called Chromosomes, 46 of them in the human cells. That’s 23 pairs of these Chromosomes of various length and sizes. Now if you look at this rough drawing of one of them, one Chromosome about to divide into two. You’ll see that it sort of looks like, well actually, it’s much more complex than this, but it reminds us of a couple of springs linked together to coil up pieces of DNA. And if you stretch them out you will find they contain certain genes, certain sequences of DNA that help to determine how the cells of the body will develop. When researchers looked really carefully at the DNA in Chromosomes though, they were amazed, we all were, to find that only a fraction of it, maybe 20-30%, converts into meaningfulgenetic information. It’s incredible; at least it was to me. But if you took away all the DNA that codes for genes, you still have maybe 70% of the DNA left over. That’s the so-called JUNK DNA. Though the word junk is used sort of tongue in cheek.The assumption is that even if these DNA doesn’t make up any of the genes, it must serve some other purpose. Anyway, if we examine these ends of these coils of DNA, we will find a sequence of DNA at each end of every human Chromosome, called a telomere. Now a telomere is a highly repetitious and genetically meaningless sequence of DNA, what we were calling JUNK DNA. But it does have an important purpose: it is sort of like the plastic tip on each end of shoelace. It may not help you tie your shoe but that little plastic tip keeps the rest of the shoelace, the shoe string from unraveling into weak and useless threads. Well, the telomere at the end of Chromosomes seems to do about the same thing – protect the genes the geneticallyfunctional parts of the Chromosome from being damaged. Every time the Chromosome divides, every time one cell divides into two, pieces of the ends of the Chromosome, the telomeres, get broken off. So after each division, the telomeres get shorter and one of the things that may happen after a while is that pieces of the genes themselves get broken off the Chromosomes. So the Chromosome is now losing important genetic information and is no longer functional. But as long as the telomeres are at certain length, they keep this from happening. So it seems that, when the, by looking at the length of the telomeres on specific Chromosomes, we can actually predictpretty much how long certain cells can successfully go on dividing. Now, there are some cells that just seem to keep on dividing regardless, which may not be always a good thing if it gets out of control. But when we analyze the cells chemically, we find something very interesting, a chemical in them, and an enzyme called telomerase. As bits of the telomere break off from the end of Chromosome, this chemical, this telomerase can rebuild it, can help reassemble the protective DNA, the telomere that the Chromosome is lost. Someday we may be able to take any cell and keep it alive functioning and reproducing itself essentially forever through the use of telomerase. And in the future, we may have virtually immortal nerve cells and immortal skin cells of whatever because these chemical, telomerase can keep the telomere on the ends of Chromosomes from getting any shorter.
Question 8 of 12
The professor discusses research about the percentage of a chromosome’s DNA that contains genetic information. How did she feel about this research?
Question 9 of 12
What does the professor say about the DNA in a telomere?
Question 10 of 12
Why does the professor mention shoelaces?
Question 11 of 12
What does the professor imply about the length of the telomeres on a cell’s chromosomes?
Question 12 of 12
According to the professor, how is the chemical telomerase related to the telomere?