In this chapter, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: What is the experimental basis for concluding that puffs in polytene chromosomes and loops in lampbrush chromosomes are areas of intense transcription of RNA?

Video transcript

Hello everyone. Let's look at our next problem. It says the active transcription sites in politan chromosomes are indicated by puffs and dipped brands. The largest puffin the politan chromosomes is called. Let's recall what politan chromosomes are. Quick definition. And these are those giant chromosomes formed of many chromosomes fused together um that are the result of many cycles of DNA replication without cytoplasmic division. So you just end up with this big, sprawling tentacle looking thing. Giant chromosome there. And on these big giant chromosomes you have areas you sort of go along here and we recall we have the kind of uh some smaller spellings here. And then you have a big old puff area here. And you see these little bands here which are alternating areas of more DNA versus more RNA in the chromosomes when they're stained. So this big area here is formed. That's the active transcription site. RNA is being transcribed there. So the double helix is being unwound. And as the chromosome is being uncoiled to allow DNA. Our RNA transcription. The sort of loops they have loops kind of spilling out um spilling out of it. Making this big puff. And the name of this big puff area is choice A. Which is ball Biyani rings. So that's the specific terminology for that choice. B is knob that's similar. The knob here are these sort of smaller um lumpy areas on our polity in chromosomes um where you have some activity going on. But it's not those big areas where everything is being uncoiled and chromosomes loops are spilling out everywhere. So that's why that's not correct. I see as bands. And again the bands are just those sort of stripey areas. We see alternate areas with the darker bands having more D. N. A. The lighter bands having more RNA. That we see in the regularly coiled up version of our polity in chromosome. So that's why that's not correct. And finally lateral loop. Choice D. This is a similar phenomenon but it occurs in a different type of chromosome called a lamp brush chromosome which are found in developing oocytes. Uh And there that lateral loop is an area where transcriptions going on. The chromosome is uncoiling. And you have kind of loops of chromosomes spilling out on the sides. So it's kind of a similar phenomenon indicating active transcription but for a different type of chromosome, not our polity in chromosomes. So again, the largest puffin, the politan chromosome in deep trans is choice A ball Vianney rings. See you in the next video