What gives the nucleus its shape is a mesh of intermediate filaments called the nuclear lamina. It forms an interface between the chromosomes and the inside of the nuclear envelope. If these large protein fibres were to be temporarily separated into their component nuclear lamina, the nuclear envelope would pull away from the chromosomes and withdraw into the endoplasmic reticulum. Problem solved.

I am confused by the last sentence starting with "If these large proteins..." Could someone explain this in another way? I would appreciate any response. Thanks!

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    $\begingroup$ Can you articulate what exactly confuses you about the sentence? $\endgroup$ – canadianer Mar 2 '15 at 3:08

This is a fair question, his text or source words it funny.

To keep it simple, the nuclear lamina is a network of filaments inside the nucleus and it includes the proteins associated with the nuclear membrane.

It's connected to the inner face of the nuclear lipid bilayer that makes up the nuclear envelope whereas the outer face is continuous with the endoplasmic reticulum.

Look at most any cartoon of a cell, it shows the nuc envelope attached to the ER, which it is. Makes up a big part of the cells endomembrane system.

cell showing nuc membrane connected to ER

I think in other words the author is trying to say if the lamina came apart, the envelope would essentially just be a continuation of the ER, as it's the lamina that essentially folds up that "portion of the ER membrane" up to create the nuc envelope.

  • $\begingroup$ This doesn't answer the question. $\endgroup$ – Ordinary Owl Mar 2 '15 at 9:42
  • $\begingroup$ How is that? @OminousOwl $\endgroup$ – rhill45 Mar 5 '15 at 0:56

Watch this animation and you'll understand the general process of mitosis:


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    $\begingroup$ This doesn't answer the question $\endgroup$ – rhill45 Mar 2 '15 at 3:53

Okay, so your nuclear lamina is composed like so,

enter image description here Source: Wikipedia

When cdc25 is upregulated between G2 and M, we see dephosphorylation of Cdk1 and it's subsequent association with cyclinB. Cdk1-Cyclin B activates the MPF, and the MPF initiates phosphorylation of the proteins called lamins. As the nuclear lamina depolymerizes, it's resorbed into the ER,

enter image description here Source: http://www.jbiomedsci.com/content/16/1/96

What we find is that lamins actually help anchor heterochromatin (tightly packed) to the nuclear lamina, so when we phosphorylate the lamins, the heterochromatin is shown to move from the periphery to the more central areas of the nucleus. So we couple this uprooting of the chromatin from the lamins, and the resorption of the lamina to the ER through phosphorylation of the lamins during mitosis to broadly answer the question.

  • $\begingroup$ This answer has more details than mine, good answer. $\endgroup$ – rhill45 Mar 3 '15 at 1:20

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