Mitosis is the process of cell division. Two identical daughter cells are created from a single parent cell. The division of cells allows organisms to grow, repair any damage and replace worn-out cells. It is a crucial process that explains how the body maintains itself and how our genetic material is passed on.
The article will explain the cell cycle and the stages of mitosis. These are interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. We will give a clear overview of each step and its relevance to living organisms. This guide is aimed at students undertaking GCSE revision. It is suitable for all major exam boards, including AQA, Edexcel and OCR. Quiz questions are included to test your understanding.
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Cell division
Cells are the building blocks of all life. They are abundant inside every living organism. For example, the human body is composed of trillions of cells. They give our bodies structure, convert nutrients from food into energy and carry out specific functions.
A multicellular organism - such as humans, dogs, cows and even trees - can't continue to function without renewal. Dividing cells ensures that the organism can continue to operate smoothly across its lifespan.
There are three reasons why mitosis is so important, which we will discuss below: growth, repair and replacement.
Growth
As an organism gradually grows from a fertilised egg into an adult, this creates billions of new cells. Mitosis is key to this, with the formation of two genetically identical daughter cells from the parent cell. This means the cells in the body all have the same instructions and can function as intended. If we continue this example, a child growing taller and turning into an adult requires continual rounds of mitosis to increase their muscle, bone and skin cells.
Repair
We damage our tissue all the time. This covers minor injuries, such as bruising, all the way to severe injuries, such as breaking your leg. For example, when you cut your skin, the body creates new cells rapidly to seal the wound. Each cell is identical to the original skin cells, so the tissue continues to function when it is healed.
If the genetics don't match, the tissue repair would not work correctly, resulting in mutations or reduced function.
Replacement
The lifespan of a cell varies depending on the type of cell. For instance, a red blood cell survives for roughly 120 days before it is broken down and replaced. Conversely, the cells in heart muscles can last for 40 years or more. The type of cell division in mitosis ensures that the cells lost by natural decay are constantly replenished. This is crucial to continue essential processes without delay, such as transporting oxygen around the body.
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What is the role of mitosis in the human body?
Cell cycle and mitosis
There is a controlled sequence of stages before a cell divides. These stages are known as the cell cycle, which ensures the new cells will function correctly. Mitosis is only one part of this cycle, and most of the cell's life is spent preparing for its division.
There are two parts to the cell cycle: interphase and mitotic phase.
Stage 1 - Interphase - Longest stage of the cell cycle
The cell spends most of its life in interphase. While this is a preparation phase, it is marked by intense activity. The phase is split into three stages:
- G1 (Gap 1) | The cell grows in size and builds new organelles, such as mitochondria and ribosomes. These ensure that both daughter cells can function independently after division. It also builds up energy in the form of adenosine triphosphate (ATP), which is needed later to fuel mitosis.
- S (Synthesis) | The DNA in the nucleus is copied, which allows each chromosome to be copied. The duplicates are joined at the centromere, and they are called sister chromatids. This duplication is crucial to give each daughter cell a full set of instructions.
- G2 (Gap 2) | The cell continues to grow. It checks that the DNA has been replicated correctly before the cell divides in the next phase. Mistakes that are found can sometimes be repaired before mitosis begins.
When interphase is complete, the cell now has double the amount of DNA material. It is ready to be split between two daughter cells.
Mitotic phase - Process of mitosis and cytokinesis
The mitotic phase consists of mitosis - the division of the nucleus - and cytokinesis, which is the division of the cytoplasm. Both processes ensure each daughter cell has an identical set of chromosomes and contains what the cell needs to survive.
The cell cycle guarantees that every cell is healthy and identical to the original. Mistakes in the process, such as DNA replication, can cause mutations and lead to diseases, including cancer.
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What happens during the synthesis (S) phase of interphase?
Stage 2 - Prophase
Mitosis begins with prophase. The cell begins to organise all the genetic material so the daughter cells receive an even amount. There are several key changes during prophase:
- Condense | The long strands of DNA coil up tightly, becoming so short and thick that they can be seen under a light microscope. This lets the DNA move more freely and stops it from tangling.
- Chromatids | Every chromosome has already been copied during the synthesis stage of Interphase. The sister chromatids (the two identical copies created) are joined at a central point, known as the centromere.
- Nucleus access | The nuclear membrane breaks down, which is a protective barrier that surrounds the nucleus. This means spindle fibres can now access the chromosomes.
- Spindle fibres | Structures called spindle fibres are developed from the centrioles (in animal cells). They stretch across the cell and prepare to attach to the chromosomes.
Prophase is needed to ensure accurate movement of the chromosomes. If the chromosomes don't condense and the spindle fibres aren't formed, the cell wouldn't be able to correctly divide its DNA.
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What happens during prophase?
Stage 3 - Prometaphase
Prometaphase is a transitional step that is sometimes overlooked. Its purpose is to ensure the chromosomes are ready to be moved and aligned correctly. These are the key events:
- Nuclear membrane breaks down | The nuclear membrane has already begun to break down during prophase. At this stage, it disappears completely. The spindle fibres now have complete access to the chromosomes.
- Spindle fibres attach to kinetochores | There are specialised protein structures in the centromere of each chromosome. These proteins are called kinetochores. The spindle fibres connect to these proteins, letting them control the movement of the chromosomes.
- Chromosomes move | The spindle fibres can now push and pull the chromosomes after attaching to the kinetochores. They begin to move towards the middle of the cell and prepare for alignment in the next phase.
In prometaphase, the spindle fibres attach to kinetochores to move the chromosomes to the middle of the cell. This phase is essentially a quality check to guarantee proper division.
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How do the chromosomes start to move?
Stage 4 - Metaphase
At the start of the metaphase, the spindle fibres are created and the chromosomes are in the middle of the cell, ready to be positioned for separation. These are the key events:
- Chromosomes at the equator | The chromosomes position themselves at the centre of the cell, which is called the metaphase plate. This means each new cell will get a copy of every chromosome.
- Spindle fibres attach | The spindle fibres connect to the centromeres of each chromosome. It is a vital connection, which lets the fibres pull the sister chromatids apart in the anaphase stage.
- Checkpoint control | The cell makes sure that every chromosome is attached to spindle fibres. The division will be paused if a chromosome isn't attached, and the problem will be fixed.
Metaphase makes sure the genetic material is equally distributed. The chromosomes have to be lined up properly at the metaphase plate, and the process is paused if any chromosome isn't correctly attached to the spindle fibres.
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What happens in the metaphase?
Stage 5 - Anaphase
The sister chromatids are separated and pulled apart during the anaphase stage. It is a dramatic moment in the process, as the identical copies of DNA are split between the two ends of the cell. These are the key events:
- Centromeres split | The centromere holding each pair of sister chromatids divides. This lets the chromatids separate.
- Sister chromatids pulled apart | The spindle fibres shorten and pull each chromatid to opposite ends of the cell. The chromatids are separated and can be considered individual chromosomes.
- Equal distribution | Each daughter cell receives the same set of chromosomes. The previous stages in this process have numerous checks to guarantee these identical chromosomes. A cell with too many or too few chromosomes can cause serious problems in the organism.
This stage is quicker than the other stages. It is also crucial to ensure genetic stability across the cells.
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What happens in anaphase?
Stage 6 - Telophase
In the telophase stage, the order in the cell is rebuilt and restored after the separation of the chromosomes. These are the key events:
- Chromosomes uncoil | The separated chromosomes relax back into their thread-like form, known as chromatin. They are harder to see under a microscope and are ready for normal cell activity.
- Nuclear membranes reform | A new membrane forms around each set of chromosomes at opposite parts of the cell. There are now two nuclei inside the cell, each with the same genetic material.
- Spindle fibres break down | The spindle fibres are no longer needed as the chromosomes are securely in place. The fibres are now dismantled.
This is the end of the nuclear division. The cell has two genetically identical nuclei. In the final cytokinesis stage, the cytoplasm is divided to complete the cell division.
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What happens during telophase?
Stage 7 - Cytokinesis
During the final cytokinesis stage, the cytoplasm is divided. This completes the cell division and creates two daughter cells. It is technically not part of the mitosis process, which focuses on the division of the nucleus. However, cytokinesis is always included as it completes the process. These are the key events:
- Cytoplasm splits | The cell splits into two. Each daughter cell has enough organelles, enzymes and energy stores to survive and function.
- Animal cells | The cell membrane is drawn inwards by a contractile ring. This ring is composed of protein filaments, and it pinches the cell into two. This action is called the cleavage furrow.
- Plant cells | Plant cells can't pinch upwards. Instead, a cell plate is created across the middle of the cell. New cell wall material is put along this plate, which gradually separates the daughter cells.
The cell cycle is now complete. There are two genetically identical daughter cells, each with the same number of chromosomes as the original parent cell.
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How does cytokinesis differ between animal and plant cells?
Conclusion | Stages of Mitosis, GCSE Biology revision
Mitosis is a vital process in every living organism. It allows cells to grow, repair tissues, and replace themselves while maintaining their genetic stability. There are seven stages to this process, with checks along the way to ensure stability is ensured to avoid mutations and diseases, such as cancer. These stages are interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. This is a secure, structured process that ensures a single parent cell will create two genetically identical daughter cells, each with the same number of chromosomes.
For further reading, you can learn more about animal and plant cell structure from SaveMyExams. You can also test your understanding with this past paper on mitosis by MME Revise.
If you need extra help learning this topic, TeachTutti has experienced GCSE Chemistry tutors who can teach you in person or online using the TeachTutti learning platform. Every tutor has an enhanced DBS check before they can begin tuition.
