This article is for anyone who is confused by mitosis, or who hasn’t yet started the topic.
Mitosis is part of cell division. It is key to understanding how multicellular organisms develop from a single cell. Mitosis is also similar to the second half of meiosis; learning mitosis first will make meiosis much easier to understand.
Remember that for A level biology, you always need to understand why, as well as how. So, get a strong grip of the fundamental concept of mitosis before you try to memorise the details. Understanding the purpose of mitosis will also make it a lot easier to remember the individual stages of the process.
Being a Cell
Most of the time, cells hang out just being cells. Doing cell stuff. In the cell cycle, this is called G1 phase. It’s just the normal state of a cell.
The other phases (S, G2, M) only happen when a cell is preparing to divide, and actually dividing. Some cell types will do this more often than others.
Dividing cells, Kuan-Chung Su, London Research Institute, Cancer Research UK. Source: Wellcome Collection. This Image is licensed under CC BY 4.0. DNA is shown in red. Read it like a cartoon strip, starting in the middle and spiraling out. The central image is of the mother cell, and the final image shows the separated daughter cells.
Mitosis is a part of Cell Division
Cells come from cells. New cells are created when existing cells divide into two by cell division.
The cell that is going to divide is called the “mother cell”, and the two resulting cells are called “daughter cells”.
But - this is not like a mother birthing two daughters to send out into the world. As these daughters arrive, the mother cell ceases to exist. Because it has become the daughter cells.
Mitosis is just one part of the cell division process (alternatively meiosis can be used instead, but most cells only do mitosis).
In cell division with mitosis, the daughter cells are both genetically identical to the mother cell.
How to Divide a Cell: why DNA matters
So, what does a cell need to be able to divide in two? If you divide a human in two you get … a mess. Cells are much easier to split in half.
For a start, most intracellular molecules and organelles have multiple copies. For example, human cells contain thousands of mitochondria. These can be shared out between the two daughter cells. And large organelles like the endoplasmic reticulum can be divided easily enough because their membranes are fluid. But there’s a big problem …
The mother cell has only one copy of the organism’s DNA.
Before it can divide to form two identical daughter cells, the mother cell will need to synthesise the extra DNA required to provide two identical sets (one for each daughter cell), and then separate the two sets of DNA ready for cell division. This DNA synthesis step is S phase in the cell cycle. It is followed by G2 phase where the cell prepares itself for mitosis.
Mitosis is the process of separating the two sets of DNA. Mitosis and the process of actually splitting the cell into two both happen during M phase.
Vocabulary check
Cell division is the process by which a cell divides in two, creating two separate cells.
The Cell Cycle describes different phases of a cell’s activity. All except G1 are related to cell division.
G1 phase = normal cell stuff
S phase = replication of DNA ready for cell division
G2 phase = preparation for M phase
M phase = organisation of the DNA into two different sets, and cytokinesis (the splitting of the cell into two)
Mitosis is the process by which a cell separates its DNA into two identical sets before and during cell division.
Meiosis is an alternative process to mitosis, used to make gametes (sperm, eggs, pollen). Meiosis is not discussed in this article.
What is a Chromosome
The very long DNA molecules inside cells are not just big messy tangles - they are organised with the help of proteins. These include proteins called histones. Sections of a DNA molecule that are not in use are coiled around histones to organise it into neat packages. Together, one molecule of DNA plus the proteins that help organise its structure is called a chromosome.
When a cell is doing its normal cell behaviours, just doing its job, it will be actively using its DNA as a template to make RNA. For this reason, large parts of its DNA will be uncoiled and exposed within the nucleus. A light microscope can’t see the DNA when it’s like this because the DNA molecule is extremely thin. And if you could see it, it’d look pretty messy.
But during cell division, the DNA is packaged up very neatly and densely. During this time, the chromosome can be seen as a dark body. In fact, this is where the word comes from (“chromo” - dark, “some”= body).
Just like cotton thread, DNA is much easier to see when it’s neatly organised for transport/storage, than when it is uncoiled and exposed for use. (Note: DNA does not coil around a single spool like thread does, it has a more complex but similarly dense structure.)
Normally, a human cell contains two sets of chromosomes. The DNA code in one set comes from the biological mother of the person, and that in the other set comes from the biological father. The two sets are very similar, but have small differences due to them having different versions of genes.
What do Chromosomes look like (top tip: don’t think of an X)
One normal chromosome, with its DNA neatly packaged up so we can see it. Notice that it’s not an X shape!
A chromosome is one molecule of DNA with the proteins that help manage its structure. Its structure varies a lot depending on how tightly it is package. When a chromosome is packaged up very densely, it looks like a long sausage shape - see the picture to the right. You can imagine this structure as the equivalent of a spool of thread as shown above (although the DNA is rolled up in a different structure from the thread).
But wait - if you google an image of a chromosome, you see images of a blobby X shape!
OK so yes, that’s a chromosome too, but they only look like that at one specific time during mitosis, after DNA replication! That image of an X-shape is NOT a good place to start your understanding! It’s a bit like looking at a woman half-way through giving birth and saying “that’s what a human looks like”.
Sister Chromatids
Chromosomes only look like X-shapes after the chromosome’s DNA has been replicated but before it separates.
The Chromosome on the left is a double stranded molecule of DNA. The chromosome on the right has been replicated and now has twice as much DNA as before. Each sister chromatid is a double stranded molecule of DNA.
Confusingly it’s still called “a” chromosome at this point, but it now has twice as much DNA and comprises two identical “sister chromatids”. The chromatids are joined together at their centromeres, creating the classic X shape (see diagram to right).
During cell division, the sister chromatids will separate and move to the two separate cells. After cell division has finished, they will just be called chromosomes.
To be clear, these are not the pairs of slightly-different chromosomes where one carries code from each parent. These sister chromatids are identical copies of the code from just one parent. There will be another pair of sister chromatids carrying the code from the other parent.
Semi-conservative Replication
The original chromosome, on the left, has two strands. These are separated and used to template new strands. This means that both of the sister chromatids have one of the original strands of DNA, now pair-bonded to a new strand that has been freshly synthesised using it as a template.
An important note:
Neither of the sister chromatids is “the original one”. The original DNA has separated into two strands, and each of the sister chromatids contains one of these original strands, now pair-bonded to a freshly synthesised complementary strand.
This is called semi-convervative replication .
Semi = half
Conserve = preserve / save
Mitosis: ensuring each cell gets a full set of chromosomes
After DNA replication, the cell contains duplicated (X-shape) versions of each version of the cell’s 46 chromosomes. This doubling gives it all the DNA it needs to split itself into two cells, each with a full set of DNA.
Mitosis is the process by which the cell organises the DNA to ensure each daughter cell gets a complete set, including both versions of each of the different chromosomes.
Ok, now you know what it’s all about, what it’s trying to achieve, and what is going on with the chromosomes, you’re ready to go on to look at the diagrams showing the individual steps of mitotis.
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Next: See this fantastic blog post which includes loads of mitosis resources including video explanations of the full process and A level biology exam questions.
This article was written by Jenny Shipway, with guidance from Tom.