The cell cycle refers to the distinct stages through which a cell goes, from the moment it becomes a cell to the moment it divides to result in 2 separate cells. Bear in mind that some cells cease to divide any longer after a certain period of time, depending on cell type. If that’s the case, they are said to be in resting phase termed G0.
Within the dividing cell, it starts with gap 1, G1, continues into the S phase (S is for Synthesis) where DNA replicates, followed by gap 2, G2, and ending with mitosis.
G1 and G2 may sound like codes for some complex enzymes, but they are mere notations for gaps 1 and 2, which are just that: gaps between mitosis and DNA replication (in the S phase) respectively. G1 through to G2 – that’s G1, S phase and G2 – are all stages which collectively are known as interphase. Inter = between; phase = …phase, so interphase is just the stage between a cell’s creation and that cell’s division by mitosis.
Interphase is by far the stage in which most cells are in most of the time. The other stage, the small one, is called the mitotic phase and it encompasses mitosis (prophase, metaphase, anaphase and telophase) plus cytokinesis.
Overview of Mitosis
Mitosis is the process by which cells divide to achieve growth and repair by simply increasing cell number. For unicellular organisms, cell division is actually their reproduction itself (asexual reproduction). The dividing cell is called the parent cell, and the resulting two cells have inappropriately been called daughter cells by scientists so far. Now because cells don’t have a damn gender and we are better than accepting silly nonsensical received wisdom about what to call these cells, we will call them offspring cells instead. The offspring cells are genetically identical i.e. clones, as they contain copies of the parent cell’s DNA.
Stages of Mitosis
Prophase, metaphase, anaphase, telophase and cytokinesis.
There’s no easy way around these stages, so just bloody learn them. Actually there is an easy way. Awesome video time!
1. Chromosomes begin to appear visible under a microscope due to chromatin (the coiled and yet-again coiled DNA fibre) condensing. Before this the DNA is not specifically distinguishable in the shape of chromosomes. This is a terrible word tangle so this is how it is. From a bowl of spaghetti (the nucleus) put the spaghetti in the shape of several chromosomes. Chromatin is the spaghetti initially, and chromosomes are the spaghetti still, just turned and twisted and distinguishable as individual stick-shaped objects. That is all, that’s all it is. Before this happens though, the DNA must be replicated – that’s the reason behind the X shape of chromosomes; they are two “lines” a.k.a. chromatids joined together at their centres called centromeres.
2. The nuclear envelope breaks down.
3. Organelles known as centrioles migrate towards the poles of the cell. These organelles are involved in the act of pulling the chromosomes apart into the soon-to-be offspring cells. They achieve this by the microtubules that extend out of them and connect to the centromeres. Microtubules are like lassos. Sort of.
1. Chromosomes are aligned at the cell equator by spindle fibres (made of the aforementioned microtubules) which lengthen and shorten themselves on opposing sides (tug of war) until all chromosomes are lined up about halfway across the cell. This area is called the metaphase plate. It looks like a plate. Who said biology can’t be straightforward?
1. The chromatids split at their centromeres and are pulled towards opposite poles of the cell by the shortening spindle fibres.
1. Nuclear envelopes reform around the two new nuclei.
2. The chromosomes decondense and become indistinguishable under a microscope yet again, and the spindle fibres spread out.
This is the final step of mitosis when the cytoplasm of the parent cell divides to complete the cell division, resulting in two brand new and individual offspring cells.
Cell division in prokaryotic organisms such as bacteria is very simple. Termed binary fission (splitting in two), it involves duplication of the cell’s DNA and the even splitting of the copied genetic material into its two offspring cells, which in this unicellular organism effectively becomes two new individuals.
The cytoplasm is therefore also evenly divided alongside the respective genetic material. However, you might remember that these organisms have extra genetic information in plasmids alongside their main DNA. The number of copies of plasmid that each new cell receives from the parent cell during binary fission is variable.
In the diagram only the main DNA is pictured (plasmids are much smaller) as well as the cell wall which can be seen pinching from the sides of the new emerging offspring cells, and gradually tearing a path towards the middle to separate the two cells. Very much the same idea as cytokinesis in mitosis, although bear in mind these cells do also have a cell wall to worry about in addition to the membrane!
Viruses cannot divide outside a host cell, or indeed even inside a host cell. They simply do not have any machinery to do so. Their replicative activity lies solely in their genetic material and protein units needed to house the genetic material and target new host cells. That’s it.
Meiosis is a type of cell division which results in 4 cells that are genetically non-identical from one parent cell. In order for once cell to divide to result in 4 cells, how many divisions must take place?
Two. 1 cell becomes 2, then 2 become 4:
The first division is called meiosis I, and the second is called meiosis II.
…so far so easy? (it should be!)
Cells resulting from meiosis are gametes such as egg cells and sperm cells, hence meiosis only occurs in sexually reproducing organisms. There are 2 key points about this:
1. Gametes are genetically unlike one another – while cells in other tissues such as muscle or blood must be genetically identical to one another (clones), the very basis of sexual reproduction is genetic diversity. So somewhere in the process of division, something takes place which creates genetic diversity (we’ll come to that shortly).
2. If gametes are to fuse and result in a new organism, it is essential that the number of chromosomes should stay constant. Humans have 46 chromosomes in each cell (of course, apart from cells without DNA in them, and “spoiler alert!”, gametes) – if each gamete had 46 chromosomes, then fusing 2 together would result in a zygote with 92 chromosomes, whose offspring would have 184 chromosomes, and before you know it something terrible would have happened.
The above picture illustrates how the number of chromosomes is halved in the final 4 cells. The terms diploid and haploid refer to the number of sets of chromosomes. In humans, somatic cells (i.e. cells other than gametes) are diploid because there are two sets of chromosomes. Gametes are haploid because they have only one set of chromosomes.
A “set” is made up of all chromosomes which are unique, i.e. are not paired with any homologous chromosomes.
X x X X x X x x <———- haploid = 1 set
XX xx XX XX xx XX xx xx <———- diploid = 2 sets
In the first XX, X and X are homologous chromosomes because they occupy the same space and contain DNA with similar purpose/function. Essentially, they are more or less copies of each other. So when 2 gametes fuse, they form a diploid cell with the complete number of chromosomes.
Wikipedia does us the honour with this epic picture:
On to the very important bit now…
How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?
10 words: Independent Assortment of Homologous Chromosomes, &
Genetic Recombination by Crossing Over
What an unnecessary mouthful. You still have to learn them though.
Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X’s and which small x’s end up with each other in each cell is random. Pretty simple concept.
If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.
Not assigning the expected chromosome or chromatid during meiosis is called chromosome non-disjunction and results in a cell with a different number of chromosomes.
Translocation is a type of chromosomal mutation where one or more nucleotide bases are moved between non-homologous chromosomes e.g. AAGCTT on human chromosome 1 is moved and becomes AAGCTT on human chromosome 3.
Genetic recombination by crossing over is a lot more interesting. It’s like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:
Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs.
As a finishing touch, I read of this mnemonic to remember the purpose of meiosis.
It is so cringe-worthy, I would rather memorise meiosis off by heart.
Down’s syndrome and Turner’s syndrome
During meiosis, different chromosome distribution in the gametes can occur which can leave them without the expected number of chromosomes. If there are more chromosomes than expected (2 of each in humans), this is termed polysomy and can result in conditions such as Down’s syndrome. If there are fewer chromosomes than expected, it is monosomy and can result in conditions such as Turner’s syndrome.
Down’s syndrome involves an extra chromosome 21, and expresses itself in terms of many different features, some of which are detrimental to health. Common outcomes include unique facial features, slower overall development, higher incidence of congenital heart abnormalities, decreased or absent fertility and overall lower life expectancy.
Turner’s syndrome is in a way the “reverse” of Down’s syndrome as it presents one fewer chromosome rather than one extra. Specifically, it is a diminished or absent X chromosome. Since XY embryos missing their only X chromosome would not be viable, this syndrome only presents itself in births of would-be XX babies who end up having just one X chromosome or one X and a partial X.
As many as 99% of Turner’s syndrome cases are thought to terminate via miscarriage or stillbirth. For those who survive and are born alive, common features include a webbed neck, low-set ears, short stature, lack of puberty without hormonal treatment and heart defects. Their overall life expectancy is shorter due to the development of heart disease, diabetes, thyroid problems and others, and constant health monitoring is required.