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September 12, 2017

MAKING SENSE OF THE CELL CYCLE

Biology is the only science in which multiplication means the same thing as division. - Anonymous


REFERENCES:

An Introduction to Molecular Biology/Nucleus [https://en.wikibooks.org/wiki/An_Introduction_to_Molecular_Biology/Nucleus]
Molecular Cell Biology, Lodish et al., Chapter 13
Chapter 3 Cell Division. pp 25- 31 in Singh, R.J. (1993) Plant Cytogenetics CRC Press
Introduction to Genetic Analysis, Griffiths et al. Chapter 3
The Cell, A Molecular Approach, G.M. Cooper, Chapters 8, 14
Glover, D. (June, 1993) The centrosome.  Sci. Am. 268:62-68.



RATIONALE: My purpose today is not just to give you another bunch of mechanisms to memorize. Rather, it is intended to explain why the events of mitosis that we see under the microscope occur. Understanding what's happening at the molecular level should provide a rationale for the entire process of mitosis.

Learning Checklist:

1. The cell cycle separates gene expression, DNA replication and cell division into discrete phases. Be able to describe the main events in G1, S, G2 and M.
2. The cell spends most of its time in interphase, during which chromosomes are uncoiled, allowing gene expression to occur. Know the main components and functions of the interphase nucleus.

3. Be able to describe the main events in mitosis, and be able to identify which stage of mitosis a particular cell is in:

a. Prophase - condensation of chromosomes; disassembly of nuclear envelope into vesicles, attachment of spindle fibers.
b. Metaphase - Centrosomes pull spindle fibers taut, causing chromosomes to migrate to center of cell.
c. Anaphase - poleward movement of chromosomes through shortening of kinetochore spindles, after which poles are pushed apart by polar spindles.
d. Telophase - laminar vesicles reassemble into nuclear envelope; chromosomes decondense; cytokinesis
4. Be able to describe the functions of  polar and kinetochore spindle fibers.  What evidence is there that the anaphase migration of chromosomes is driven by depolymerization of kinetochore spindle fibers?
5. Be able to describe the nuclear envelope cycle.
6. Understand the experiments that demonstate the how chromosomes are oriented in two daughter cells after mitosis.
7. Understand the data which demonstrate that the duration of the cell cycle is not dependent on chromosome number.


I. THE CELL CYCLE


Example of a eukaryotic cell cycle. Actual times for each phase depend on the species, the tissue, and the physiological state.

A. The cell cycle is defined with respect to DNA replication.

Under the microscope,  somatic cells seem to spend most of their time doing nothing. These long periods of inactivity are punctuated by relatively short periods during which DNA synthesis and mitosis take place. Cells in culture can be made to grow synchronously,  such that all  cells divide at the same time. When you feed radioactive nucleotides to a synchronous culture, the incorporation of  radioactive nucleotides occurs at a precisely-defined period, several hours prior to mitosis.


1) G 1 ("gap in DNA synthesis") - most  GENE EXPRESSION occurs here. In non-dividing or terminally differentiated cells, G1 persists indefinitely, and is referred to as G 0. Chromatin is dispersed in nucleus.
2) S - (DNA synthesis); The cell requires a discrete signal to begin a round of DNA replication. eg. if you fuse a cell in S phase with a cell in G 1, the nucleus from G 1 will begin DNA synthesis.
3) G 2 ("second gap in DNA synthesis") The nucleus is reorganizing in preparation for mitosis. Some chromatin condensation occurs here, but not enough to be visible under light microscopy.
4) M - ("mitosis or meiosis") - chromosomes partitioned between two daughter cells.

II. THE INTERPHASE CELL

Interphase, which includes G 1 or G 0, S and G 2, is where the cell spends most of its time. Interphase is where most gene expression occurs. Put another way, most of what the cell does, other than dividing, occurs during interphase.

Ref: http://neurosci.utmb.edu/cellbio/nuclear_envelope.htm
 

A. The nucleus is an organelle whose purpose is to carry out the processes of gene expression and DNA replication.

B. The nuclear envelope can be considered to be a differentiated form of the Rough Endoplasmic Reticulum.

This makes sense, because the purpose of the nucleus is to make mRNA, rRNA and tRNA which are all used for translation of proteins. Ribosomes are attached to the ER. Therefore,  RNA molecules will be in immediate contact with the ER as they emerge from the nucleus. The nuclear membrane, therefore, is continuous with the RER.

C. The nuclear envelope consists of outer membrane and inner membranes enclosing the perinuclear space.

1. Outer membrane serves to define the shape of the nucleus and organize the nuclear pore complexes.
2. Inner membrane has chromosome attachment  sites.

D. Nuclear pore complexes regulate molecular traffic into and out of the nucleus.

1.Eight pore proteins are circularly arranged around a pore of ~ 40 nm inner diameter.
2. Out: rRNA's, tRNA's, mRNAs
3. In: Nuclear proteins (eg. histones, HMG's, topoisomerases, polymerases


VIDEO: Into the Nucleus - animation of import into nucleus via nuclear pore http://youtu.be/UyhqLpjicZg


Displayed from http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/Articleimage/2012/SM/c2sm25777g/c2sm25777g-f13.gif




 
It is interesting to speculate about how evolution might have arrived at the modern nucleus. One possible set of steps would be:
  1. Attachment of ribosomes to membranes in some ancestor of eukaryotes
  2. Attachment of chromosomes to the ER. This allows direct transport of RNA along ER to ribosomes.
  3. Mechanisms for organizing the RER into a discrete nuclear envelope.
3D Reconstruction of a section of the nuclear membrane from Dictyostelium, using cryoelectron tomography.
(NPC - nuclear pore complex; INM - inner nuclear membrane; ONM - outer nuclear membrane.)

from Alexander Rigort, Felix J. B. Bäuerlein, Elizabeth Villa, Matthias Eibauer, Tim Laugks, Wolfgang Baumeister, and Jürgen M. Plitzko (2011) Focused ion beam micromachining of eukaryotic cells for cryoelectron tomography PNAS 109 no. 12 4449-4454
Open Access



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