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Lecture 20

lecture 20

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Department
Biology
Course
BIO120H1
Professor
Jennifer Harris
Semester
Winter

Description
www.notesolution.com cell shape and function. - Imp o rtantl, the nucleus is compartmental z i d. Here, separat ni g the cytoplasm from the nucleus, there is a nuclear membrane shown in the slide, the nuclear envelope. - Prophase is the first stage of mitosis and there are two main things that start to happen, two main changes that occur. - One is that sister chromatids start to condense, they align and they form microtubule attachment sites. That is shown in the nucleus outlined in the slide. Here in brown we see the DNA, these DNA double helices have been replicated so now we have two copies of these so they were replicated during interphase. Now we see these two pieces of DNA, each of them is called a chromatid. The two chromatids are starting to connect together and form one unit. Additionally, they form attachment sites for microtubules at these red marks called kinetochore. - At the same time this is happening, in the cytoplasm the centrosomes begin to organize this bipolar microtubule spindle. Importantly, notice that the microtubules and the DNA, they are still separated by a nuclear envelope so theyre kept away from one another at this stage. Packaged and tagged to interact with microtubules - Now to look at how these two steps happen. - So we can thin k of this stage as preparin g the DN A for mi tosis an d preparing the microtubules for mitosis. - For the DNA, there are basically 2 steps. One step is packaging them up so instead of being very loose strands of DNA filling the nucleus, they are packaged into solid packages that can be transported within the cell in an effective way. So theyre packaged up and the other thing that is done to them is that there is a tag put on them that is recognized by microtubules. - There is basically 2 steps here, theyre packaged and tagged to interact with microtubules. Individually condensed by condensins Glued together by complexes called cohesins - This packaging occurs when all the DNA is duplicated in S phase, there are 2 copies of each chromosomes and these remain associated as sister chromatids. - During prophase, each chromatid is individually condensed by condensins and that is shown in this picture here, this is one DNA coil, one of these sister chromatids. This DNA in interphase would be very loose and filling the cell but here, these condensins are basically coiling this up and packaging it together sort of like how you would coil up a hose in your backyard. It connects them together making them a tighter package. - In this picture right here, we are looking at 2 different sister chromatids. The chromatids remain attached here called the centromere so that is an important part of the chromosomes there, where those 2 chromatids remain attached. We can see that they are each individually condensed by these condensins & here in pink we can see the condensins running along each sister chromatid. They are separately compacted by these condensins. These condensed sister chromatids are glued together by complexes called cohesions. - This cartoon up here, these are the two sister chromatids. These cohesins are connecting these condensed chromatids together into one large package. If we fit this into this cartoon right here, the cohesins would be running down the center attaching one sister chromatid to the other sister chromatid so theyre all one large unit. www.notesolution.com Microtubules Kinetochores - So that is packing these chromosomes, now theyre also tagged to interact with microtubules. This tag for the microtubule attachment, this forms at the centromere region of the chromosomes. This is where the two sister chromatids remain attached to one another and these sites are specifically called kinetochore so that is shown here in this diagram so this is a protein complex that binds to these chromosomes and can also bind to the microtubules. So this tags them to interact with microtubules and these microtubules as well see later will drag these chromosomes apart. - That is organizin g the DN A in the nucleus an d as that DN A is bein g organized in the nucleus, the microtubules in the cytoplasm are being organized to form the mitotic spindle. - If we go outside into the cytoplasm, the mitotic spindle is being organized by the centrosomes. So weve learned about the centrosomes before, here is the centrosomes structure right here. Deep inside the centrosomes there are 2 centrioles & then surrounding those centrioles are hundreds of different proteins. The key proteins on the surface of these centrosomes are the gamma tubulin ring complexes. These gamma tubulin ring complexes act as templates to nucleate microtubule growth so theyll start to nucleate microtubule growth & MTs will now shoot out from these centrosomes. - This is an important place & another thing we learned about microtubules is that at these plus ends of these microtubules here that are emanating out from the centrosomes, we know that plus ends of microtub
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