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Preprophase

Preprophase is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Preprophase rather than just read about it. In short: Preprophase is an additional phase during mitosis in plant cells that does not occur in other eukaryotes such as animals or fungi. It precedes prophase and is characterized by two distinct events: The formation of the preprophase band, a dense microtubule ring underneath the plasma membrane.

Preprophase — main illustration
Preprophase — illustration

Key takeaways

  • Preprophase belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Preprophase to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Preprophase from memory before moving on to harder problems.

Reference excerpt

Preprophase is an additional phase during mitosis in plant cells that does not occur in other eukaryotes such as animals or fungi. It precedes prophase and is characterized by two distinct events:

The formation of the preprophase band, a dense microtubule ring underneath the plasma membrane. The initiation of microtubule nucleation at the nuclear envelope.

Function of preprophase in the cell cycle Plant cells are fixed with regards to their neighbor cells within the tissues they are growing in. In contrast to animals where certain cells can migrate within the embryo to form new tissues, the seedlings of higher plants grow entirely based on the orientation of cell division and subsequent elongation and differentiation of cells within their cell walls. Therefore, the accurate control of cell division planes and placement of the future cell wall in plant cells is crucial for the correct architecture of plant tissues and organs. The preprophase stage of somatic plant cell mitosis serves to establish the precise location of the division plane and future cell wall before the cell enters prophase. This is achieved through the formation of a transient microtubule structure, the preprophase band, and a so far unknown mechanism by which the cell is able to "memorize" the position of the preprophase band to guide the new cell wall growing during cytokinesis to the correct location. In gametophyte tissues during the reproductive phase of the plant life cycle, cell division planes may be established without the use of a preprophase band. In highly vacuolated plant cells, preprophase may be preceded by the formation of a phragmosome. The function of the phragmosome is to suspend the cell nucleus in the center of the cell in preparation for mitosis. If a phragmosome is visible, the preprophase band will appear at its outer edge.

Preprophase band formation

At the beginning of preprophase, the cortical microtubules of a plant cell disappear and aggregate into a dense ring underneath the plasma membrane. This preprophase band runs around the equatorial plane of the future mitotic spindle and marks the plane of cell division and future fusion site for the cell plate. It consists of microtubules and microfilaments (actin) and persists into prophase. Spindle formation occurs during prophase with the axis perpendicular to the plane surrounded by the preprophase band.

Microtubule nucleation In contrast to animal cells, plant cells do not possess centrosomes to organize their mitotic spindles. Instead, the nuclear envelope acts as a microtubule organizing center (MTOC) for spindle formation during preprophase. The first sign is a clear, actin-free zone appearing around the nuclear envelope. This zone fills with microtubules nucleating on the surface of the nucleus. The preprophase spindle forms by self-assembly of these microtubules in the cytoplasm surrounding the nuclear envelope. It is reinforced through chromosome (kinetochore)-mediated spindle assembly after the nuclear envelope breaks down at the beginning of prometaphase.

Transition into prophase During progression from preprophase into prophase, the randomly oriented microtubules align parallel along the nuclear surface according to the spindle axis. This structure is called the prophase spindle. Triggered by nuclear membrane breakdown at the beginning of prometaphase, the preprophase band disappears and the prophase spindle matures into the metaphase spindle occupying the space of the former nucleus. Experiments with drugs destroying microfilaments indicate that actin may play a role in keeping the cellular "memory" of the position of the division plane after the preprophase band breaks down to direct cytokinesis in telophase.

Notes and references

Bibliography P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2

Illustrations

Preprophase: Microtubule dynamics during preprophase and prophase in plant cell mitosis, modified from Donukshe et al.[1] The images follow a tobacco BY-2 cell through the first stages of mitosis (c. 12 minutes). The growing ends of microtubules are shown in green (labeled with green fluorescent protein fused to the microtubule plus end binding protein EB1 of Arabidopsis thaliana). N = Nucleus, V = Vacuole, PPB = Preprophase band, MTN = Microtubule nucleation starts at the nuclear envelope, NEB = Nuclear envelope breakdown at the onset of prometaphase. Also see the movie corresponding to this figure.
Microtubule dynamics during preprophase and prophase in plant cell mitosis, modified from Donukshe et al.[1] The images follow a tobacco BY-2 cell through the first stages of mitosis (c. 12 minutes). The growing ends of microtubules are shown in green (labeled with green fluorescent protein fused to the microtubule plus end binding protein EB1 of Arabidopsis thaliana). N = Nucleus, V = Vacuole, PPB = Preprophase band, MTN = Microtubule nucleation starts at the nuclear envelope, NEB = Nuclear envelope breakdown at the onset of prometaphase. Also see the movie corresponding to this figure.

Worked examples

Example 1 — a first encounter with Preprophase

Start with the simplest possible case. Write down what Preprophase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Preprophase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Preprophase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Preprophase

In research
Preprophase appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Preprophase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Preprophase is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cell cycle, Mitosis, Plant cells, so understanding it makes those chapters shorter.
In everyday life
Look for Preprophase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Preprophase in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Preprophase means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Preprophase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Preprophase in simple terms?

Preprophase is an additional phase during mitosis in plant cells that does not occur in other eukaryotes such as animals or fungi. It precedes prophase and is characterized by two distinct events: The formation of the preprophase band, a dense microtubule ring underneath the plasma membrane.

Why does Preprophase matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Preprophase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Preprophase.

Tags

  • Cell cycle
  • Mitosis
  • Plant cells

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