Bio exam 2 study questions revised

Bio exam 2 study questions revised - 1. Pulse-chase is a...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
1. Pulse-chase is a lab technique for marking a population of cells or molecules at a particular moment in time by means of a labeled molecule and then following their fate over time. Investigators performed a pulse-chase experiment to determine the length of S- phase. They used radioactive thymidine to label the cells. The labeled cells were found in M-phase after 4~5 hours. The researcher realized that the first labeled cells to enter mitosis must represent cells that just completed chromosome replication when it was labeled. Therefore 4~5 hour time lag corresponds to the time lag between the end of S- phase and beginning of M-phase. In conclusion, researchers called the time lag between S-phase and M-phase 'G2 phase', which lasts about 4~5 hours. By looking at the Figure 11.4, the labeled nuclei undergoing mitosis are observed over a period of about 6 to 8 hours and since all of these cells had to be in the S-phase when radioactive thymidine was available, it was logical to conclude that S-phase lasts about 6 to 8 hours. Pulse-chase experiment is used to provide experimental cells with a large concentration of a labeled molecule for a short time ( pulse ), followed by a chase which provides large amounts of unlabeled versions of the same molecule. The general idea of the pulse-chase experiment was to mark a population of molecules at a particular interval then follow their fate over a period of time. 2.) The experiments supporting the findings of moving G 2 cells into the M phase are the cell fusion experiment and the microinjection experiment. In the cell fusion experiment, researchers found that in the presence of certain chemicals, viruses, or an electric shock, the membranes of two cells can be made to fuse. The hybrid cell that results has two nuclei. Investigators fused cells that were in different stages of the cell cycle, and found that certain nuclei changed phases. For example, when a cell in M phase was fused with one in interphase, the nucleus of the interphase cell initiated M phase. The chromosomes in interphase cell started to condense, signaling the start of M phase. Through this, biologists were able to hypothesize that the cytoplasm of M-phase cells contains a regulatory molecule that induces interphase cells to enter M-phase. The second experiment was done on the South African claw-toed frog. As the eggs of these frogs mature, they change from a cell called an oocyte, which is arrested in a phase similar to G2, to a mature egg that has entered M phase. Due to the large size of the eggs, they were easy to purify large amounts of cytoplasm and use microsyringes to inject the eggs with cytoplasm from eggs in different stages of development. When biologists purified cytoplasm from M-phase frog eggs and injected it into the cytoplasm of frog oocytes arrested in the G2-like phase, the immature oocytes entered M phase. But when cytoplasm from interphase cells was injected into G2 oocytes, the cells remained in the G2-like phase. The researchers concluded that the cytoplasm of M-phase cells-but not
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 8

Bio exam 2 study questions revised - 1. Pulse-chase is a...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online