Continuity of Life: Processes


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Ben Whitten

Mitosis (Updated)

Introduction to mitosis.

What is mitosis?

Image: Mitosis cells sequence image, Image by LadyofHats, Sourced Under a Creative Commons 4.0 License from Wiki Commons

For a number of students, mitosis is one of those topics that you've done to death. It is however essential to know for your exam, as it could come up in an extended response, or as a short answer question. 

Mitosis is what takes place in the 'M' phase of the cell cycle, where cell division of somatic cells actually occurs. Cells in the body undergo mitotic divisions for two main purposes; growth and repair. The result of mitosis is two genetically identical daughter cells, which will go on to perform mitosis over and over again. 

There are four main phases of mitosis. 

  1. Prophase 

  2. Metaphase 

  3. Anaphase 

  4. Telophase 

These can be remembered as 'PMAT', or 'IPMAT' if you choose to include interphase (which isn't really a part of mitosis!).

What is prophase?

In this first phase of mitosis (prophase), chromatin threads found in the nucleus condense, and coil up tightly to become chromosomes, which are two sister chromatids held together by a centromere. The nuclear membrane disintegrates, and the nucleolus disappears. Mitotic spindle forms from the centrioles, and spindle fibres attach to each chromosome at its centromere. Two centrosomes which contain two centrioles move to opposite poles of the cell.

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What is metaphase?

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In metaphase, chromosomes move to the centre of the cell and line up on the equator of the cell (note: the equator may also be called the metaphase plate). The centromeres of the chromosomes are aligned together on the equator, and the centrioles are located at opposite poles of the cell.

What is anaphase?

During anaphase, the spindle fibres shorten and pull on the centromeres. The chromosomes separate into sister chromatids, and are pulled to opposite poles. At the end of anaphase, each pole has an identical set of maternal and paternal chromosomes as DNA replication took place during interphase.

What is telophase?

In the last stage of mitosis (telophase), chromosomes decondense (uncoil) to form chromatin. Two new nuclear membranes form, one for each new daughter cell. The nucleoli reappear and the spindle apparatus disappears. The cell elongates, and a cleavage furrow (the dip between the (almost) 2 new cells) to become ready for cytokinesis.

Answering past exam questions!

In 2020, one of the extended response questions was as follows: 

"Compare mitosis and meiosis." (10 marks) (SCSA 2020) 

A very simple question from the surface, but easy to make mistakes on if not answered carefully! Step one of answering a question like this would be to look at the 'directional' word in the question, in this instance being 'compare'. This should instantly tell you that they are looking for the similarities and the differences between the processes. 

For 4 marks, you are required to state similarities between the two processes.

  • Both are types of cell division 

  • Both begin with a diploid cell 

  • Both contain similar stage/substages (PMAT) 

  • DNA replication occurs during interphase in both processes 

  • Sister chromatids separate in mitosis and meiosis II 

For the other 6 marks, you are required to state the differences between the processes. It may be worthwhile organising this into a table format for simplicity sake, making it clear to the marker that you understand what you're being asked.