4.6: Cell Cycle

Can you imagine that the human body contains around 30 trillion cells, and yet starts as only one? So far in this course we've learned about how the body functions, but now it's time to learn how it's formed. Topic 4.6 goes over the processes that cells use to form and replicate themselves. Please keep note that this is not the same process that most organisms use to reproduce (more on that in unit 5).

Vocab List

• Cell cycle
  • Interphase
    • G1 phase
    • S phase
      • Genome
      • DNA replication
      • Sister chromatids
      • Centromere
    • G2 phase
    • G0
  • M phase
    • Mitosis
      • Prophase
        • Mitotic spindle
        • Kinetochores
      • Metaphase
      • Anaphase
      • Telophase
    • Cytokinesis
      • Contractile ring
      • Cell plate
    • Binary fission
• Somatic cells
  • Diploid
• Gametes
  • Haploid

Written Explanation

Overview:

Before we start with a deeper look into each part of a cell's life, we're going to get a general view of the situation. A cell's life cycle is known as the cell cycle, spanning from cell formation to division. This cycle is split into two main parts: the growth phase (interphase) and the replication phase (M-phase). In the growth phase, the cell carries out its bodily functions and builds new mitochondria, chloroplasts (in the case of plants), produces/enlarges other organelles, and replicates its current set of DNA. Most of those bodily functions are, of course, facilitated by enzymes. In the replication phase (which differs between eukaryotes and prokaryotes), the cell splits in half and divides the newly replicated DNA evenly between the two new "daughter" cells.

Interphase:

As mentioned before, interphase is the growth phase of a cell. It involves the cell growing and replicating its DNA, as it prepares for cell division and carries out bodily processes (protein synthesis). This phase takes up around 90% of a cell's total life cycle.

Interphase is split into three parts, all memorably named. The G1 phase is the first growth period of a cell's life cycle. The S phase involves the synthesis of a new set of DNA for the cell. Lastly, the G2 phase is the second growth period of a cell's life. Throughout interphase, the cell continues to carry out its cellular functions.

Overall, interphase results in a doubling of all a cell's organelles and DNA, so that when the cell splits and forms two daughter cells, they will each contain the necessary materials to function (and participate in the cell cycle themselves).

Additionally, some cells can live indefinitely in interphase by entering G0. In this state, cells continue to fulfill their cellular functions and produce/consume compounds, but they don't progress through the cell cycle. Some cells, like liver cells, can leave G0 and restart the replication process to heal from liver damage. Other cells, like muscle cells, can't re-enter the cycle, and instead grow by getting larger (creating larger cells as opposed to more cells). Neurons also can't leave G0 and just continue to fulfill their signaling functions. It's important to remember that there are always exceptions to these rules when in such complicated systems (for example, muscle stem cells can divide, as well as neurons in very specific conditions), but these rules of thumb are generally correct.

DNA Replication:

S phase is certainly the most complex stage of interphase. It involves DNA replication, in which a cell's normal full set of DNA turns into two identical (besides rare copying errors) full sets of DNA. This full set of DNA is known as an organism's genome, and is composed of multiple pieces of DNA called chromosomes. A human cell contains 46 chromosomes (23 pairs), with 23 of those coming from each parent. Dogs have 78 chromosomes (39 pairs). Chromosomes contain the genetic code for different traits and behaviors. Prokaryotes have a singular loop of DNA, which is their genome.

There is actually an exception to the rule that all cells contain a complete genome of their organism. Gametes (reproductive cells) have only half of a genome, because they merge with another reproductive cell to form a complete genome (remember that each parent contributes half the DNA). Cells like gametes that have half a genome, or more specifically have only one chromosome from each pair, are known as haploid cells, while cells with the full set of chromosomes are known as diploid cells.

The last few terms you need to know about DNA replication relate to the newly formed chromosomes, after replication has occurred. First, chromosomes that are "paired" together in a genome (one from each parent) are called homologous chromosomes. When a chromosome is replicated in S phase, we still call it a single chromosome, but also two sister chromatids, connected at the middle with a centromere. The wording with this can get confusing, but hopefully the diagram below clears everything up. Each pair of homologous chromosomes are also placed together in the diagram of the human genome above.

M Phase:

In M phase, the cell splits in half and divides the newly formed DNA and organelles between the two new cells. This phase occurs differently in prokaryotes compared to eukaryotes, with prokaryotes using binary fission, and eukaryotes using mitosis.

Mitosis:

Eukaryotic cells follow the four step process of mitosis to split their cells. Mitosis is the division of a cell's nucleus and the division of the chromosomes between the two new nuclei. It is followed by cytokinesis, which is the actual division of a cell.

Mitosis is divided into four main steps, defined by the acronym PMAT. The first phase, prophase, the condensing of genetic material, involves the fragmentation of the nucleus, and the forming of the mitotic spindle. First, the DNA is bundled together to form compact chromosomes (this uncondensed form of DNA is known as chromatin). The sister chromatids connect at the middle with a centromere. Then, the nuclear envelope fragments (including the nucleolus), letting out all of the genetic material. Lastly, the mitotic spindle, a microtubule structure that spans across the cell forms. The spindle is attached at opposite points of the cell, anchored to kinetochores (a protein).

In the second phase of mitosis, metaphase, all the chromosomes, which are currently paired sister chromatids, line up at the center of the cell, on the “metaphase plate” (which is not a physical structure). The chromosomes are attached to the microtubules that form the mitotic spindle.

In the third phase of mitosis, anaphase, the sister chromatids are separated and pulled apart by the microtubules so that each side of the cell contains a complete and equal genome. The centromere is broken, and the chromatids are pulled apart. The microtubules that are not attached to chromosomes elongate, pushing the two sides of the cell apart.

In the fourth and last phase of mitosis, telophase, two new nuclei form around the genetic material. The mitotic spindle breaks down, and the chromosomes decondense back into chromatin. The nuclear envelope reforms around the two new nuclei, and the nucleolus reappears. This is the end of mitosis, but not the end of the cell cycle.

After mitosis, the cell needs to actually be split. It does this through cytokinesis. Cytokinesis involves either the formation of a new wall separating the two new nuclei, or the pinching of the cell membrane in two. In plant cells, a new wall called the cell plate forms in the middle of the cell. In animal cells, the cell membrane is pinched closed by a contractile ring, creating a cleavage furrow.

Binary Fission:

To finish off the cell cycle, we need to understand the differences between prokaryotic and eukaryotic cell division. Firstly, prokaryotes go through the same interphase as eukaryotes (except that prokaryotes only replicate their single loop of DNA, as opposed to several strands). Moreover, they don't contain any organelles which are replicated throughout interphase. In M phase, prokaryotes undergo a process called binary fission. In binary fission, the cell stretches out, divides the chromosomes into opposite sides of itself, and finally splits in half (with a contractile ring). Binary fission skips most of the steps of mitosis, notably not condensing the chromosomes.