Lecture 27: Cell Cycle and Cell Division
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Introduction to cell division
An evolutionary goal of all living systems is to reproduce. Since the basic unit of life is a cell, and we know - thanks at least in part to Francesco Reid - that life begets new life - this means that there must be a process by which to create new cells from parental cells. We also know intuitively that multicellular organisms must somehow increase their number of cells during their growth by creating copies of existing cells. The process by which one cell creates one or more new cells, for both single and multi-celled organisms, requires a parental cell to divide and is called cell division.
From the standpoint of the Design Challenge framework we can stipulate that the big problem of cell division is to make a copy of a cell. If a condition for success requires that the daughter cells be viable then a number of subproblems can be defined:
1. The cell must replicate its DNA so that at least two cells have a functional copy after cell division is complete - we have discussed this process already.
2. The cell must make sufficient copies of the rest of the cellular content so that daughter cells are viable or it must find a way to ensure that the copied DNA (even without a full replica of cellular content) is viable.
3. The cell must divide the replicated cell content and DNA between at least two independently bounded compartments.
4. To ensure success, the process must be happen in an evolutionarily competitive time and be accomplished with an evolutionary selection-friendly amount of biochemical resources.
While it is not a strict requirement that this process happen in a coordinated manner, Nature has selected for systems in which all of the steps in the process happen in a highly coordinated way. This helps cells meet requirement number 4 in the list above. The coordinated process and the mechanisms of control are generally referred to as the cell cycle. This term can be used to describe the coordinate process used by any cell that is undergoing cell division. When we observe Nature we find that it has evolved two major modes of reproduction: sexual and asexual. Within each of these modes of reproduction we find several major modes of cell division that occur frequently across all domains of life. We consider three of these modes: binary fission (used primarily by single celled bacteria and archaea), mitosis (used often by eukaryotes in processes of cell division NOT associated with sexual reproduction) and meiosis (a process of cell division tightly linked to sexual reproduction). We discuss these processes in the sections that follow.
Cell division in the bacteria and archaea
Bacteria and Archaea
Like all other life forms, bacteria and archaea have one key evolutionary driver: to make more of themselves. Typically, bacterial and archaeal cells grow, duplicate all major cellular constituents, like DNA, ribosomes, etc., distribute this content and then divide into two nearly identical daughter cells. This process is called binary fission and is shown mid-process in the figure below. While some bacterial species are known to use several alternative reproductive strategies including making multiple offspring or budding - and all alternative mechanisms still meet the requirements for cell division stipulated above - binary fission is the most commonly laboratory-observed mechanisms for cell division the bacteria and archaea so we limit our discussion to this mechanisms alone.
(Aside: Those who want to read more about alternatives to binary fission in bacteria should check this link out.)
Binary Fission in bacteria starts with DNA replication at the replication origin attached to the cell wall, near the midpoint of the cell. New replication forks can form before the first cell division ends; this phenomenon allows an extremely rapid rate of reproduction.
The process of binary fission is the most commonly observed mechanism for cell division in bacteria and archaea (at least the culturable ones studied in the laboratory). The following is a description of a process that happens in some rod-shaped bacteria:
Since we must consider the replication of DNA one structural feature of relevance to DNA replication in the bacteria and archaea is that their genetic material is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell. Moreover, the DNA of the nucleoid is associated with numerous proteins that aid in compacting the DNA into a smaller, organized structure. Another organizational feature to note is that the bacterial chromosome is typically attached to the plasma membrane at about the midpoint of the cell. The starting point of replication, the origin, is close to this attachment site. Recall also that replication of the DNA is bidirectional, with replication forks moving away from the origin on both strands of the loop simultaneously. Due to the structural arrangement of the DNA at the midpoint this means that as the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell.
This process of DNA replication is typically occurring at the same time as a growth in the physical dimensions of the cell. Therefore, as the cell elongates, the growing membrane aids in the transport of the chromosomes towards the two opposite poles of the cells. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins.
The formation of a ring composed of repeating units of a protein called FtsZ (a cytoskeletal protein) directs the formation of a partition between the two new nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. Gradually, a septum is formed between the nucleoids, extending from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.
Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.
How does attaching the replicating chromosome to the cell membrane aid in dividing the two chromosomes after replication is complete?
These images show the steps of binary fission in prokaryotes. (credit: modification of work by “Mcstrother”/Wikimedia Commons)
Control of these processes
Not surprisingly, the process of binary fission is strictly controlled in most bacteria and archaea. Somewhat surprisingly, however, while some key molecular players are know, much remains to be discovered and understood about how decisions are made to coordinate the activities.