Introduction
In order for organisms to grow, cells have two options: they must either replicate themselves to create more cells, or the cells themselves must expand in volume. In humans, tissues such as the skin and blood contain cells that are actively dividing, whilst other tissues such as fat contain cells that expand (good if you need energy for winter, bad if you are trying to fit into some expensive jeans). Other cells, such as neurons, will never divide again once they are terminally differentiated; they are post-mitotic.
In the process of replicating themselves, cells have another choice: do they want to make an identical copy and be left with two cells? Or do they want to make four “half-copies”, in preparation for sexual reproduction, where their genetic content will be made whole again by the process of fertilisation? This choice is the choice between mitosis and meiosis.
Difference Between Mitosis and Meiosis
This article will explore the characteristics of both kinds of cell division, shining a light on how they are similar and in which aspects they are crucially distinct. We will also explore the research into these processes and how cell division might go awry to cause disease states such as cancer and Down’s Syndrome.
Mitosis vs. Meiosis: Overview and commonly asked questions
Mitosis
Meiosis
What is the purpose of this process?
In a unicellular organism, the purpose of mitosis is to proliferate asa species. In a multicellular organism, the purpose can be to grow during development, or to repair or regenerate
adamaged tissue, for example.
To create gametes with only one copy of the organism’s genetic information, in preparation for sexual reproduction. Various steps in meiosis create opportunity for genetic diversity in the daughter cells. This is the raw substrate for evolution.
What is the outcome of this process?
Two diploid cells with identical genetic information.
Four haploid cells with different genetic information.
Which organisms perform this process?
Mitosis is performed by unicellular and multicellular eukaryotes.Bacteria have their own version of mitosis called “binary fission”.This is distinct from meiosis as bacteria typically have one circular chromosome,which is not contained within a nucleus, like eukaryotic chromosomes.
Only organisms which perform sexual reproduction. Archaeaand bacteria do not do this, so it might be tempting to think that unicellularorganisms do not sexually reproduce. However, there are exceptions; buddingyeast will form haploid spores under nutritional deprivation.
How long does this process take?
Mitosis is usually shorter than meiosis. The process can take over 10 hours for mammalian cells in culture [2], budding yeast can take ~80 minutes to complete a cell cycle [3], whilst bacteria can divide every 20 minutes.
Meiosis has various timescales in different organisms, which can be affected by several factors including temperature and environment of the organism, and the amount of nuclear DNA. The process lasts 6 hours in yeast but can last more than 40 years in human females, due to a developmental hold at prophase I, until ovulation. Other examples are 1-2 days in male fruit flies and ~ 24 days in human males. [1]
What is an example of a disease caused by an error in this process?
Uncontrolled mitosis occurs in cancer, where either genes that stop cell division (tumour suppressors) are switched off, or genes that encourage cell division (oncogenes) are overactive.
Errors in meiosis can lead to the wrong number of chromosomes ending up in germ cells, this is called aneuploidy. This can trigger miscarriage, but is occasionally tolerated. One example is Down’s syndrome, caused by trisomy 21. Another example is Klinefelter syndrome, where XY males have an additional X chromosome.
Summary
Meiosis and mitosis both have a prophase, metaphase, anaphase, telophase and cytokinesis.
In meiosis, prophase, metaphase, anaphase and telophase occur twice. The first round of division is special, but the second round is more like mitosis.
In mitosis, prophase, metaphase, anaphase and telophase occur once.
Prophase
Chromosomes condense and the centrosomes begin to form an early spindle.
Meiotic prophase I is much longer that mitotic prophase.
During prophase I homologous chromosomes make contacts with each other called chiasmata and “crossing over” occurs. This is where chromosomes exchange sections of DNA. This is important for generating genetic diversity but is also crucial mechanically to hold homologous chromosomes together.
Mitotic prophase is much shorter that meiotic prophase I.
There is no crossing over in mitosis.
(Im a really fast typer)