By Johannes Buheitel, PhD
There comes a moment in every cell’s life, when it’s time to reproduce. For a mammalian cell, this moment usually comes at a ripe age of about 24 hours, at which it undergoes the complex process of mitosis. Mitosis is one of the two main chromosomal events of the cell cycle. But in contrast to S phase (and also to the other phases of the cell cycle) it’s the only phase that is initiated by a dramatic change in the cell’s morphology that, granted, you can’t see with your naked eye, but definitely under any half-decent microscope without requiring any sort of tricks (like fluorescent proteins): Mitotic cells become perfectly round. This transformation however, as remarkable as it may seem, is merely a herald for the main event, which is about to unfold inside the cell: An elegant choreography of chromosomes, which crescendoes into the perfect segregation of the cell’s genetic content and the birth of two new daughter cells.
To better understand the challenges behind this choreography, let’s start with some numbers: A human cell has 23 unique chromosomes (22 autosomes and 1 gonosome) but since we’re diploid (each chromosome has a homolog) that brings us to a total of 46 chromosomes that are present at any given time, in (nearly) every cell of our bodies. Before S phase, each chromosome consists of one continuous strand of DNA, which is called a chromatid. Then during S phase, a second “sister” chromatid is being synthesized as a prerequisite for later chromosome segregation in M phase. Therefore, a pre-mitotic cell contains 92 chromatids. That’s a lot! In fact, if you laid down all the genetic material of a human cell that fits into a 10 micrometer nucleus, end to end on a table, you would wind up having with a nucleic acid string of about 2 meters (around 6 feet)! The challenge for mitosis is to entangle this mess and ultimately divide it into the nascent daughter cells according to the following rules: 1) Each daughter gets exactly half of the chromatids. 2) Not just any chromatids! Each daughter cell requires one chromatid of each chromosome. No more, no less. And maybe the most important one, 3) Don’t. Break. Anything. Sounds easy? Far from it! Especially since the stakes are high: Because if you fail, you die (or are at least pretty messed up)…
To escape this dreadful fate, mitosis has evolved into this highly regulated process, which breaks down the high complexity of the task at hand into more sizable chunks that are then dealt with in a very precise spatiotemporal manner. One important feature of chromosomes is that its two copies – or sister chromatids – are being physically held together from the time of their generation in S phase until their segregation into the daughter cells in M phase. This is achieved by a ring complex called cohesin, which topologically embraces the two sisters in its lumen (we’ll look at this interesting complex in a separate blog post). This helps the cell to always know, which two copies of a chromosome belong together, thus essentially cutting the complexity of the whole system in half, and that before the cell even enters mitosis.
Actual mitosis is divided into five phases with distinct responsibilities: prophase, prometaphase, metaphase, anaphase and telophase (cytokinesis, the process of actually dividing the two daughter cells, is technically not a phase of mitosis, but still a part of M phase). In prophase, the nuclear envelope surrounding the cell’s genetic content is degraded and the chromosomes begin to condense, which means that each DNA double helix gets neatly wrapped up into a superstructure. Think of it like taking one of those old coiled telephone receiver cables (that’s your helix) and wrapping it around your arm. So ultimately, chromosome condensation makes the chromatids more easily manageable by turning them from really long seemingly entangled threads into a shorter (but thicker) package. At this point each chromatid is still connected to its sister by virtue of the cohesin complex (see above) at one specific point, which is called the centromere. It is this process of condensation of cohesed sister chromatids that is actually responsible for the transformation of chromosomes into their iconic mitotic butterfly shape that we all know and love. While our butterflies are forming, the two microtubule-organizing centers of the cell, the centrosomes, begin to split up and wander to the cell poles, beginning to nucleate microtubules. In prometaphase, chromosome condensation is complete and the centrosomes have reached their destination, still throwing out microtubules like it’s nobody’s business. During this whole time, their job is to probe the cytoplasm for chromosomes by dynamically extending and collapsing, trying to find something to hold on to amidst the darkness of the cytoplasm. This something is a protein structure, called the kinetochore, which sits on top of each sister chromatid’s centromere region. Once a microtubule has found a kinetochore, it binds to it and stabilizes. Not all microtubules will bind to kinetochores though, some of them will interact with the cell cortex or with each other to gradually form the infamous mitotic spindle, the scaffold tasked with directing the remainder of the chromosomal ballet. Chromsomes, which are attached to the spindle (via their kinetochores) will gradually move (driven by motor proteins like kinesins) towards the middle region of the mother cell and align on an axis, which lies perpendicular between the two spindle poles. This axis is called the metaphase plate and represents a visual mark for the eponymous phase. The transition from metaphase to anaphase is the pivotal moment of mitosis; the moment, when sister chromatids become separated (by proteolytic destruction of cohesin) and subsequently move along kinetochore-associated microtubules with the help of motor proteins towards cell poles. As such a critical moment, the metaphase-to-anaphase transition is tightly safeguarded by a checkpoint, the spindle assembly checkpoint (SAC), which ensures that every single chromatid is stably attached to the correct side of the spindle (we’ll go into some more details in another blog post). In the following telophase, the newly separated chromosomes begin to decondense, the nuclear envelope reforms and the cell membrane begins to restrict in anticipation of cytokinesis, when the two daughter cells become physically separated.
To recap, the process of correctly separating the 92 chromatids of a human cell into two daughter cells is a highly difficult endeavor, which, however, the cell cleverly deals with by (1) keeping sister chromatids bound to each other, (2) wrap them into smaller packets by condensation, (3) attach each of these packets to a scaffold a.k.a. mitotic spindle, (4) align the chromosomes along the division axis, so that each sister chromatid is facing opposite cell poles, and finally (5) move now separated sister chromatids along this rigid scaffold into the newly forming daughter cells. It’s a beautiful but at the same time dangerous choreography. While there are many mechanisms in place that protect the fidelity of mitosis, failure can have dire consequences, of which cell death isn’t the worst, as segregation defects can cause chromosomal instabilities, which are typical for tissues transforming into cancer. In future posts we will dive deeper into the intricacies of the chromosomal ballet, that is the centerpiece of the cell cycle, as well as the supporting acts that ensure the integrity of our life’s code.