Mitosis (OCR A-Level Biology A): Revision Notes

Mitosis

Introduction to mitosis

Mitosis is the type of nuclear division that produces two genetically identical daughter cells. This process occurs when new cells are needed for:

• Growth of tissues and organisms
• Repair of damaged tissues
• Asexual reproduction in eukaryotic organisms

The mother cell (original cell) must first duplicate its chromosomes so that each daughter cell receives a complete set of genetic information.

Interphase and the cell cycle

Interphase is not a stage of mitosis itself, but rather the period of the cell cycle when the cell is not dividing. During interphase, several preparations occur:

• DNA replication takes place during the S (synthesis) phase
• The DNA exists as uncoiled chromatin, making it invisible under a light microscope
• Organelles duplicate in preparation for cell division
• Centrioles replicate in animal cells

By the time mitosis begins, each chromosome already consists of two identical DNA molecules. This duplication is essential for producing genetically identical daughter cells.

Stages of mitosis

The process of mitosis is divided into four main stages: prophase, metaphase, anaphase, and telophase.

Prophase

Prophase marks the beginning of visible chromosome formation. During this stage:

• Chromosomes become visible because DNA undergoes supercoiling (the twisting of DNA around its own axis, winding the helix more tightly)
• Each chromosome appears as two sister chromatids joined at a single point called the centromere
• The nucleolus disappears
• The nuclear membrane breaks down by the end of prophase

At this point, each chromosome contains two DNA molecules as a result of earlier replication during interphase. The chromatids remain joined at the centromere throughout prophase and metaphase.

Metaphase

During metaphase, the cell prepares to separate the sister chromatids:

• In animal cells, centrosomes (each containing a pair of duplicated centrioles) move to opposite ends of the cell, called poles
• The centrosomes extend microtubules that form the spindle apparatus, creating a framework for chromatid movement
• Chromosomes attach to spindle fibres at the cell's centre (the equator)
• Attachment occurs via protein structures called kinetochores located on each chromatid at the centromere

Anaphase

Anaphase is the stage where sister chromatids separate:

• Centromeres divide, releasing the sister chromatids
• The newly-separated chromatids (now called chromosomes) are pulled toward opposite poles
• Spindle fibres shorten at both ends, pulling the chromatids apart

Telophase

Telophase represents the final stage of nuclear division:

• New nuclear membranes form around each group of chromosomes at opposite poles
• Chromosomes begin to uncoil, returning to their extended chromatin state
• New nucleoli form within each nucleus

At the end of telophase, the cell contains two nuclei, each with a complete set of chromosomes identical to the original cell.

Cytokinesis

After telophase, the cytoplasm divides in a process called cytokinesis. Although not technically part of mitosis (which refers only to nuclear division), cytokinesis completes the formation of two separate cells.

In animal cells:

• The cytoplasm near the equator pinches inward
• The plasma membrane tucks in, eventually splitting the cytoplasm
• Organelles, which increased in number during interphase, are distributed between the two cells
• The Golgi apparatus produces vesicles that fuse to form new sections of plasma membrane

Mitosis in plants

Mitosis in plant cells follows the same basic stages as in animal cells, with two key differences:

Mitosis and asexual reproduction

Asexual reproduction is reproduction involving the production of offspring from a single individual, without the fusion of gametes. In eukaryotic organisms, asexual reproduction always involves mitosis to generate the new cells.

Key characteristics of asexual reproduction:

• Produces organisms that are genetically identical to the parent
• All offspring are clones of each other and the parent
• The only genetic variation arises from spontaneous mutations

Asexual reproduction is far more common in plants than animals, as plants cannot move to find mates.

Budding in yeast

Yeast, a unicellular fungus, reproduces asexually through a process called budding. This process uses mitosis for nuclear division but differs from typical cell division:

Stages of budding:

1. DNA replicates inside the nucleus during interphase
2. An area of the cell wall weakens; turgor pressure causes the cytoplasm and cell wall to bulge outward, forming a bud
3. A ring of chitin forms at the junction between the parent cell and bud (this remains as a 'scar' after separation)
4. The nucleus migrates toward the bud region and undergoes mitotic division (the nuclear membrane remains intact throughout, unlike in animal and plant cells)
5. One daughter nucleus migrates into the bud along with organelles
6. Cytokinesis completes with formation of a new cell wall between mother and daughter cells
7. The daughter cell grows to full size before beginning its own budding cycle

Observing mitosis and the mitotic index

Mitosis can be observed in actively dividing tissues such as root tips. The mitotic index provides a quantitative measure of cell division activity:

$\text{Mitotic index} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells observed}} \times 100\%$

Practical observation method

Root tips from garlic or onion can be prepared for microscopic observation:

Key preparation steps:

1. Grow roots from a garlic bulb with its base just touching water
2. Cut $1-2$ cm root tips
3. Fix cells using ethanoic acid to stop all cellular processes
4. Treat with $1$ M hydrochloric acid at $60°\text{C}$ to break down cell wall bonds (allowing cells to separate)
5. Stain chromosomes using ethano-orcein stain
6. Squash the tissue to create a thin, single-cell layer
7. Observe under a microscope and count cells at each stage

Sample data analysis

Stage	Number of cells seen
Prophase	$54$
Metaphase	$6$
Anaphase	$2$
Telophase	$4$
Interphase	$934$
Total	$\mathbf{1000}$

Temporal variation in mitotic activity

Cell division rates can vary throughout the day due to biological rhythms:

Time	Mitotic index (%)
12 midnight	$5.1$
4 am	$5.4$
8 am	$6.1$
12 noon	$7.5$
4 pm	$6.6$
8 pm	$6.7$

This data shows peak mitotic activity at midday ($7.5\%$) and lowest activity at midnight ($5.1\%$), suggesting circadian regulation of cell division.

Cell cycle timing example

Different cell types have different cycle durations. For bone marrow cells with an $18$-hour cycle:

• Interphase: $280°$ of the cycle ($280°/360° \times 18$ hours $= 14$ hours)
• Mitosis: $70°$ of the cycle ($70°/360° \times 18$ hours $= 3.5$ hours)
• Cytokinesis: $10°$ of the cycle ($10°/360° \times 18$ hours $= 0.5$ hours)

Remember!