The cell cycleThe sequence of events in a cell’s life, including growth, DNA replication, and division. is an ordered sequence of events that occur in the life of a cell. Just as humans progress from infant to toddler to child and so forth, a cell has an ordered sequence of life events. In order to progress, there are checkpoints to ensure readiness and DNA integrity. Important genes control these checkpoints, resulting in cancerous conditions when mutated enough to make a difference.
The cell cycle actually has only 2 phases to it: interphaseThe phase of the cell cycle in which the cell grows, performs its functions, and replicates DNA. and a cell division phase. Of course it can’t be that simple, and it’s not, there are subphases to these two main phases. When a cell is born, it enters into interphase. This is a time when the cell just lives. It makes proteinsLarge molecules made of amino acids with various functions in the body., maybe grows or becomes more complex. At some point, it duplicates its chromosomes. It grows a little more and then it enters into a cell division phase, either mitosis or meiosis. If this divisional phase is mitosis, the original cell dies and two, completely new cellsThe basic structural and functional units of life. are born.
In order to understand the cell cycle, it’s important to understand the states of being in which DNA is found. DNA is a double helixThe twisted-ladder structure of DNA molecules., found in pieces called chromosomes. Chromosomes can be found in a condensed state or a dispersed state. Chromosomes can only be seen under a compound microscope if they are condensed with DNA wrapped around histones and coiled. This type of packaging of DNA only occurs when the cell is dividing. Thus, we can really only see chromosomes under a compound microscope if the cell is in some phase of mitosis.
When the cell is in interphase the DNA is in a dispersed state, allowing enzymesProteins that speed up chemical reactions in the body. to access the DNA for protein synthesis and DNA replication. In this dispersed state, DNA is referred to as chromatinDNA and associated proteins in a loose, uncondensed form inside the nucleus.. Chromosomes can exist in a single state, where one arm of condensed DNA is connected to another arm via the centromereThe region of a chromosome where sister chromatids attach and spindle fibers bind.. They exist like this in the G1 or Gap 1 phase of interphase, which is the phase a cell enters upon being born. At some point, the cell will duplicate its chromosomes in the S phaseThe DNA synthesis phase of the cell cycle where chromosomes are replicated. (S for synthesis). Chromosomes can also exist in a duplicated state where an identical copy is made, but not separated from the original. When in the duplicated state, the identical copies are referred to as sister chromatids. They exist like this is the G2 or Gap 2 phase of interphase, which is after the S phase. These sister chromatids will be separated in mitosis.
The phases of mitosis can be split into 5 or 4 phases depending on the preference of your teacher and the person writing your book. I recognize 4 phases and let’s just list them right here: prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes., metaphaseA stage of mitosis where chromosomes align at the metaphase plate., anaphaseA stage in mitosis or meiosis where sister chromatids or homologous chromosomes separate., and telophaseThe final stage of mitosis where nuclear envelopes reform around separated chromosomes.. To tell these phases apart there are a few things that you can look at: the size of the cell the extent of the mitotic spindleThe structure made of microtubules that separates chromosomes during mitosis. and the placement of the chromosomes.
When a cell approaches mitosis with its chromosomes in a condensed state it enters prophase. In prophase the nuclear envelopeA double membrane that surrounds the nucleus and separates it from the cytoplasm. disappears which is why you see a dotted line in this first picture here the central songs move to opposite ends of the cell and the mitotic spindle starts growing some fibers.
In this picture we can see the most defining characteristic of metaphase which is that the chromosomes are lined up in this orientation along an imaginary line called the metaphase plateThe imaginary plane in the center of the cell where chromosomes line up during metaphase.. If alignment is not correct in metaphase it could result in two cells that are not identical copies of each other. The third phase here is called anaphase and this phase is actually quite energyThe capacity to do work or cause change. intensive even though sometimes it looks like nothing is happening when you view it under the microscope.
During metaphase, the chromosomes align along the imaginary equatorial plane (the metaphase plate) of the cell. The chromosomes must align themselves to split correctly, giving each new cell one sister chromatidOne of two identical copies of a duplicated chromosome.. Each chromosome is attached to spindle fibers originating from opposite poles of the cell at specialized structures called kinetochores.
Anaphase is basically a tug of war period what’s happening is that the spindle fibers that are attached to the centromeres start to break the centromere apart and pull the sister chromatids to opposite ends of the cell. This ensures that each daughter cell will receive a complete set of chromosomes. It is at this point that mistakes can happen and sister chromatids can fail to separate.
The last phase which is called telophase is basically just the opposite of prophase. The chromosomes become dispersed, the mitotic spindle starts to disappear, the nuclear envelope begins to form voted by the dotted lines and this structure here between the two potential cells becomes developed. In an animal cell this is called a cleavage furrowThe indentation in the cell membrane that forms during cytokinesis in animal cells..
The cleavage furrow will separate these two cells entirely in a process that is called cytokinesis. Many students wrongly think that telophase and cytokinesis are the same phase. Telophase results in the complete separation of the genetic material that is potentially within the nuclear envelope. Cytokinesis is the separation of the contents of each cell so all of the stuff that is not in the nucleusThe control center of the cell that contains DNA and directs cellular activities..
Notice how, now, we have 8 separate chromosomes with 4 going to each of the new cells. These chromosomes are no longer connected to their sister chromatids.
List of terms
- cell cycle
- interphase
- proteins
- cells
- double helix
- enzymes
- chromatin
- centromere
- S phase
- prophase
- metaphase
- anaphase
- telophase
- mitotic spindle
- nuclear envelope
- metaphase plate
- energy
- sister chromatid
- cleavage furrow
- nucleus