Mitosis revealed in detail – ScienceDaily

Cell division ensures growth or renewal and is therefore vital for all organisms. However, the process differs somewhat in animals, bacteria, fungi, plants, and algae. Until now, little was known about how cell division occurs in algae. Researchers from the University of Bielefeld have used confocal laser scanning microscopy (CLSM) to capture the first-ever high-resolution three-dimensional images of cell division in living cells of the microalgae Volvox carteri, and have identified novel cellular structures involved in the process. Professor Dr. Armin Hallmann from the Faculty of Biology is leading the study. The results have now been published open access in the journal The plant cell.

The cell is the smallest organizational unit of life. It contains the building blocks necessary for life in a compact form and is where vital biochemical reactions take place. With the help of enzymes, transformations of substance and energy take place, processes also called metabolism. The interior of the cell is separated and therefore protected from the environment by the cell membrane. Genetic material, the cell’s information store, is often located in the cell’s nucleus in the form of DNA. When a cell divides by mitosis, it first divides its nucleus into two identical daughter nuclei with the same genetic material. Then the rest of the cell divides and two identical daughter cells are produced. The complex and genetically determined process of mitosis in particular must take place very precisely: all the genetic material, divided into chromosomes, must be separated precisely in the two nuclei of the daughter cells.

Cell division of the alga Volvox carteri combines animal and plant characteristics

“Cell division is one of the most fundamental processes in living organisms. It has essentially been preserved over countless millions of years of evolution and can be found in all organisms,” says Professor Armin Hallmann, head of the Plant Cellular and Developmental Biology research group at the university. from Bielefeld. Yet the mechanisms of cell division in animals, fungi, plants, and algae each have characteristic features. The multicellular green algae Volvox carteri is a particularly interesting example. “It exhibits both animal and plant characteristics in mitosis,” says Hallmann. The researchers have now been able to clarify this phenomenon in their study. “Until now, researchers knew very little about the exact process of mitosis in this green algae.”

Mitosis in the microalgae Volvox carteri

Thanks to their analyses, the scientists were able to identify five crucial characteristics for mitosis in the microalgae Volvox carteri.

The first two characteristics relate to the envelope of the core of the microalgae. “The nuclear envelope does not disintegrate at the start of mitosis, as is often the case, but remains in place until shortly before the end of nuclear division,” explains Armin Hallmann. “Instead, it becomes porous and permeable, so cellular components are exchanged between the interior of the cell nucleus and the cytosol – a fluid that surrounds the cell nucleus. Thus, for a time, the cell nucleus loses its typical property of confined reaction space, although the nuclear envelope is still present.

The third characteristic is related to the centrosomes of the cell. These are cellular structures that play a central role here in the organization of the mitotic spindle. The mitotic spindle organizes the chromosomes in such a way that they can be precisely separated into the two newly formed cell nuclei. “We were able to show that the centrosomes play a crucial role in the mitosis of Volvox carteri even though they are located outside the nuclear envelope. They form the basic structure to organize the precise division of genetic material using the nuclear division spindle within the nuclear envelope. Until now, we only knew of spindle organization by centrosomes arising from cell division in animals,” says Hallmann.

A fourth characteristic is the formation of a specific filamentous structure, the phycoplast, at the end of mitosis. Once the cell nucleus has divided, the rest of the cell must also divide so that the newly formed cells can finally separate from each other. The dynamic phycoplast is the basis for the formation of a cleavage furrow that eventually divides the cell, while plants form a different structure that ultimately leads to the formation of a strong dividing cell wall. “The particularity of algae is that the phycoplast is formed directly by recycling the nuclear division spindle, which is then no longer necessary”, explains the scientist.

Finally, the researchers were able to detect enormous dynamics of the entire internal architecture of the cell as well as of the nuclear envelope during cell division.

Making molecular processes visible

The researchers were able to record cell division processes by producing fluorescent proteins (proteins that glow when exposed to light) and tracking them within the cell using confocal laser scanning microscopy (CLSM). For the first time, scientists have managed to image the mitosis of microalgae in three dimensions in living cells and to characterize it in detail, taking the example of Volvox carteri.

“The question we asked ourselves was: how exactly does cell division work in green algae? What structures are involved in mitosis and what role do they play in the process? says first author Dr. Eva Laura von der Heyde. She previously conducted research in Hallmann’s research group as a doctoral student and is now a postdoc. In order to be able to locate important proteins involved in cell division in the cell, their genes are linked to the gene for a fluorescent protein using molecular biology techniques. The proteins involved in cell division thus become fluorescent, which distinguishes them from all other proteins in the cell. “We used a special laser to excite different fluorescent proteins to glow. Using a confocal laser scanning microscope, we were able to detect the yellow-green glow of microstructures formed by proteins in living cells,” explains Eva Laura von der Heyde.

The researchers also recorded on video how proteins move during cell division, how they form microstructures and how these structures are rebuilt. In a time-lapse video, which condenses 30 minutes of mitosis into nine seconds and shows it simultaneously in ten optical cross-sectional planes, it becomes clear how centrosomes organize the formation of the nuclear division spindle and how the nuclear division spindle ultimately transforms into phycoplast after the separation of chromosomes.

Evolution overview

In the long term, Armin Hallmann and Eva Laura von der Heyde hope to be able to use these new discoveries to learn more about the evolution of cell division. How did the different variants of cell division found today in animals, fungi, plants and algae originate? “During evolution, the first land plants developed from primordial green algae. This is why the green algae Volvox carteri also has properties that it has in common with land plants that grow today. However, it is striking that Volvox carteri also has properties found in animals living today. Other of their characteristics are again found only in green algae. These special characteristics also make this model organism so important for our understanding of the evolution of cell division,” says Hallmann.

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