Why is cell division necessary 1

The restoration of the cell nucleus after cell division

Research report 2011 - Friedrich Miescher Laboratory for biological working groups in the Max Planck Society

Friedrich Miescher Laboratory for biological working groups in the Max Planck Society, Tübingen
The nucleus, the control center of the eukaryotic cell, is separated from the cell plasma by the nuclear envelope. At the beginning of cell division, the nuclear envelope falls apart and the DNA in the cell nucleus condenses into chromosomes. After the chromosomes have been evenly distributed to the daughter cells that are created, they decondense again and are enclosed by a new nuclear envelope. The regeneration of the nuclear envelope is a complicated interplay between cellular membranes and proteins, the rules of which scientists at the Friedrich Miescher Laboratory in Tübingen want to understand.

introduction

In 1835, the Tübingen botanist Hugo von Mohl observed the division of a plant cell under the microscope for the first time. Since then, numerous researchers have microscopically examined cells from different organisms to describe the processes that take place during the division of the cell and the cell nucleus (mitosis). In the last few decades genetic and molecular biological methods have been developed which make it possible that many processes in the first half of the cell cycle in particular are well understood, even at the level of molecular details. However, the processes that lead to the rebuilding of a functioning cell nucleus at the end of mitosis have been less well studied. Scientists at the Friedrich Miescher Laboratory in Tübingen are investigating this question.

The cell nucleus in the interphase

The cell nucleus is the organelle of the eukaryotic cell that is easiest to see under the light microscope. In mammalian cells, the nucleus is about 5 to 15 µm in diameter. It contains the largest part of the genetic material, the DNA. The cell nucleus is bounded by the nuclear envelope (Fig. 1), which consists of two biological membranes, the inner and outer nuclear membrane [1]. Biological membranes are made up of a double layer of lipids and, due to their water-repellent nature, are impermeable to many substances. The nuclear envelope thus represents a physical barrier between the interior of the cell nucleus and the cell plasma; it prevents an uncontrolled exchange of substances between these two cell areas. Nevertheless, an efficient and regulated transport of molecules must take place between them so that the cell can convert the genetic information stored in the cell nucleus. This transport takes place through nuclear pore complexes that are embedded in the two membranes of the nuclear envelope. They mediate the exchange of molecules between the nucleus and the cytoplasm [2].

In order to produce proteins in the cell, the information stored in the DNA must be transcribed into a so-called messenger RNA (mRNA) in the cell nucleus. This process is known as transcription. The mRNA is required in the cell plasma as a construction guide for protein production and must therefore be transported from the cell nucleus through the nuclear pore complexes. Since protein production can only take place in the cytoplasm, proteins, which in turn are required in the cell nucleus, have to get from the cytoplasm to the nucleus - in this case the nuclear pore complexes have to be passed again. This is why there is a constant, efficient transport flow through the nuclear pores, which is estimated at a few hundred molecules per nuclear pore and second.

Most of the time, cells are in what is called the interphase. Your cell nuclei are then surrounded by a complete nuclear envelope and there is an active exchange of substances between the cell plasma and the interior of the cell nucleus. However, the structure of the cell and, above all, the appearance of the cell nucleus change dramatically as soon as cells and the cell nucleus begin to divide. This process is called mitosis, during which it must be ensured that the two daughter cells that are created receive all of the same genetic information. To ensure this, the DNA must first be copied in the cell nucleus - this process is known as replication. Then the DNA is distributed to the two daughter cells that are created. This is a considerable logistical undertaking that takes place in perfectly controlled processes during mitosis (Fig. 2).

In a first step, the individual strands of DNA condense into chromosomes, the transport form of DNA, and in multicellular cells the nuclear envelope falls apart at the beginning of mitosis. This means that the membranes of the nuclear envelope are broken down into smaller pieces of membrane. The nuclear pore complexes in turn break down into their individual protein building blocks, the nucleoporins - these are distributed throughout the cell plasma. Finally, the chromosomes are collected in the middle of the cell and then distributed evenly between the two daughter cells. This is done by the mitotic spindle apparatus, which is made up of long protein polymers called microtubules.

At the end of mitosis, a nuclear envelope must then form again around the DNA in the daughter cells. To do this, the DNA first decondenses and pieces of membrane bind to the DNA surface and finally fuse with one another. At the same time, nuclear pore complexes have to build up again to ensure transport through the newly created nuclear envelope. These impressive, light microscopic processes of mitosis, the condensation and decondensation of DNA as well as the breakdown and structure of the nuclear envelope have long fascinated cell biologists [3]. But despite the enormous increase in knowledge in recent years, many aspects of cell division, especially the processes of late mitosis, are not or only insufficiently understood at the molecular level.

Structure of the cell nucleus in the test tube

When it comes to the reconstruction of the cell nucleus at the end of mitosis, it is primarily a question of how the compact mitotic chromatin decondenses, how the double membrane is formed from the inner and outer nuclear envelope and encloses the decondensed chromatin and how the nuclear pore complexes are built up. Cell biologists are always enthusiastic to watch how the gigantic molecular structures of the nuclear pores - they are 15 times the mass of a ribosome (the "factories" of protein production in the cytoplasm) - are built up from their individual components, the nucleoporins, and into the double membrane integrate.

In extracts from eggs of the South African clawed frog Xenopus laevis (Fig. 3) many processes such as DNA replication, import of proteins into the cell nucleus, structure of the mitotic spindle and also the structure of the cell nucleus at the end of mitosis can be simulated and examined [4]. Chromatin decondensation can be induced in egg extracts by incubation of mitotic chromatin and cell nuclei can be produced in the test tube. During such a reaction the chromosome structure first dissolves, so that the chromosomes seem to merge with one another and form a more or less homogeneous structure (Fig. 4). At the same time, membrane vesicles and some of the nucleoporins bind to the DNA surface. These nucleoporins, which bind first, act as crystallization points for the further development of the nuclear pores. The membrane vesicles then fuse with one another, creating a closed inner and outer nuclear membrane. At the same time, other nucleoporins bind to the DNA and form intact nuclear pore complexes. It is assumed that the decondensation of chromatin and the structure of the nuclear membranes and the nuclear pores are not only coordinated in time, but also mechanically linked: To prevent, for example, a nuclear envelope without nuclear pores from forming around the DNA and thus giving the cell access to it loses its genetic information, the nuclear membranes can only bind to decondensed chromatin and only develop if nuclear pores are formed at the same time [5].

Since the reaction takes place in the test tube, it can be manipulated quite easily. For example, it can be stopped at certain points or individual proteins can be removed from the cell extract and the effect of such a loss can be studied. Such experiments have already made it possible to determine for some factors and proteins in which of the steps they are involved.

Chromatin, membranes and proteins