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Chromosomes and Mitosis

Chromosomes are coiled DNA carrying genes in pairs, and the cell cycle copies DNA before mitosis separates the copies into two genetically identical cells for growth and repair. AQA GCSE Biology Higher, spec references 4.1.2.1, 4.1.2.2 and 1.2.
  • GCSE
  • AQA
  • Biology
  • Higher
  • Spec 4.1.2.1 · 4.1.2.2 · 1.2
  • 5 min read
  • 7 revision boxes

Introduction

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Chromosomes, DNA and genes

Most cells in your body have a nucleus, and the nucleus holds the cell's genetic material. That genetic material is stored as chromosomes, which are coiled up lengths of DNA molecules.
Each chromosome carries a large number of genes. Different genes control the development of different characteristics, such as hair colour.
In body cells, chromosomes are normally found in pairs. You have two copies of each chromosome, one that came from your mother and one that came from your father, so there are two copies of chromosome 1, two copies of chromosome 2 and so on.

Learning

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The cell cycle: an overview

Body cells in multicellular organisms divide to make new cells, and they do it through a series of stages called the cell cycle. The cycle is a continuous loop: the cell grows and replicates its DNA, then goes through mitosis, then the new cells begin the loop again.
Mitosis is the stage of the cell cycle when the cell actually divides. It is easy to treat 'mitosis' and 'the cell cycle' as the same thing, but mitosis is only one part of the loop.
Across the cycle, the genetic material is first doubled and then divided between the two new cells. That is why the two cells at the end are identical to the original cell and contain the same number of chromosomes.
You only need the three overall stages of the cycle, not the individual phases that happen inside the mitosis stage.
A circular loop showing the cell cycle, with a large arc labelled growth and DNA replication leading into a smaller arc labelled mitosis, and arrows returning around the loop to show it repeats.
First stage of the cell cycle

Stage 1: growth and DNA replication

In a cell that isn't dividing, the DNA sits in the nucleus spread out as long, tangled strings.
Before the cell can divide, it grows and increases the number of subcellular structures it contains, such as mitochondria and ribosomes.
The DNA then replicates, making two copies of each chromosome, one for each new cell. Once copied, a chromosome takes on its familiar X shape: the left arm and the right arm are exact duplicates of each other, carrying the same DNA.
An X-shaped chromosome with one arm shaded red and the other shaded blue, showing that the two arms are exact copies carrying the same DNA.
With its contents built up and its DNA copied, the cell is now ready for mitosis.
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Stage 2 and 3: mitosis and cell division

  1. The chromosomes line up across the centre of the cell, ready to be separated.
    Flow chart showing chromosomes lining up at the centre of a cell, cell fibres pulling their arms to opposite ends, membranes forming two new nuclei, then the cytoplasm and cell membrane dividing to give two identical daughter cells.
  2. Cell fibres pull the chromosomes apart, so the two arms of each chromosome are dragged to opposite ends of the cell. Each end now has one full set of chromosomes.
  3. A membrane forms around each set of chromosomes. These become the nuclei of the two new cells, so the nucleus has divided.
  4. Finally the cytoplasm and cell membrane divide, splitting the cell into two new daughter cells.
  5. The two daughter cells contain exactly the same DNA as each other, and the same DNA as the parent cell they came from, so all three are genetically identical.
The chromosomes line up across the centre of the cell, ready to be separated.
Cell fibres pull the chromosomes apart, so the two arms of each chromosome are dragged to opposite ends of the cell. Each end now has one full set of chromosomes.
A membrane forms around each set of chromosomes. These become the nuclei of the two new cells, so the nucleus has divided.
Finally the cytoplasm and cell membrane divide, splitting the cell into two new daughter cells.
The two daughter cells contain exactly the same DNA as each other, and the same DNA as the parent cell they came from, so all three are genetically identical.
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Why mitosis matters, and how to spot it

Multicellular organisms rely on mitosis for two jobs: growing bigger and developing, and replacing cells that have been damaged. Both need lots of new body cells that carry exactly the same instructions as the ones already there.

  • Growth and development

    As an organism grows, its body cells divide by mitosis to produce more cells, which is how it gets bigger and develops.
    A plant root getting longer, or a young animal growing.

  • Repair

    Mitosis also replaces cells that have been damaged, making new cells to take their place.
    New skin cells forming where skin has been damaged.

  • Identical cells every time

    Mitosis produces genetically identical cells, so every body cell keeps the same genetic information as the original cell.

  • Spotting mitosis in a sample

    In a cell that is not dividing, the DNA is spread out as long strings, so you cannot pick out separate chromosomes. A cell undergoing mitosis has visible chromosomes that are being separated.
    Under a microscope, look for cells with visible chromosomes being separated rather than DNA spread out as long strings.

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Examples

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Calculating the percentage of root tip cells in mitosis

You look at cells from the tip of a plant root under a microscope. You count 11 cells undergoing mitosis and 62 cells that are not. Calculate the percentage of cells that are undergoing mitosis.
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How can you tell a cell is undergoing mitosis?

A student looks at cells in the tip of a plant root under a microscope. Suggest how she can tell whether a cell is undergoing mitosis or not. [1 mark]

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Common questions

Is mitosis the same as the whole cell cycle?
No. Mitosis is the part of the cell cycle when the cell divides, while the cycle also includes growth and DNA replication before mitosis.
Why are the two daughter cells genetically identical?
DNA is replicated before the cell divides, making two copies of each chromosome. The copies are separated so each new cell receives one complete, identical set of chromosomes.
How do you calculate the percentage of cells undergoing mitosis?
Add the cells in mitosis to the cells not in mitosis, divide the number in mitosis by this total and multiply by 100. For example, 11 cells in mitosis out of 73 gives 15.1%.