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G2 phase

G2 phase 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 G2 phase rather than just read about it. In short: The G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated.

G2 phase — main illustration
G2 phase — illustration

Key takeaways

  • G2 phase belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect G2 phase to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of G2 phase from memory before moving on to harder problems.

Reference excerpt

The G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with the onset of prophase, the first phase of mitosis in which the cell’s chromatin condenses into chromosomes. G2 phase is a period of rapid cell growth and protein synthesis during which the cell prepares itself for mitosis. G2 phase is not a necessary part of the cell cycle, as some cell types (particularly young Xenopus embryos and some cancers) proceed directly from DNA replication to mitosis. Though much is known about the genetic network which regulates G2 phase and subsequent entry into mitosis, there is still much to be discovered concerning its significance and regulation, particularly in regards to cancer. One hypothesis is that the growth in G2 phase is regulated as a method of cell size control. Fission yeast (Schizosaccharomyces pombe) has been previously shown to employ such a mechanism, via Cdr2-mediated spatial regulation of Wee1 activity. Though Wee1 is a fairly conserved negative regulator of mitotic entry, no general mechanism of cell size control in G2 has yet been elucidated. Biochemically, the end of G2 phase occurs when a threshold level of active cyclin B1/CDK1 complex, also known as Maturation promoting factor (MPF) has been reached. The activity of this complex is tightly regulated during G2. In particular, the G2 checkpoint arrests cells in G2 in response to DNA damage through inhibitory regulation of CDK1.

Homologous recombinational repair During mitotic S phase, DNA replication produces two nearly identical sister chromatids. DNA double-strand breaks that arise after replication has progressed or during the G2 phase can be repaired before cell division occurs (M-phase of the cell cycle). Thus, during the G2 phase, double-strand breaks in one sister chromatid may be repaired by homologous recombinational repair using the other intact sister chromatid as template.

End of G2/entry into mitosis

Mitotic entry is determined by a threshold level of active cyclin-B1/CDK1 complex, also known as cyclin-B1/Cdc2 or the maturation promoting factor (MPF). Active cyclin-B1/CDK1 triggers irreversible actions in early mitosis, including centrosome separation, nuclear envelope breakdown, and spindle assembly. In vertebrates, there are five cyclin B isoforms (B1, B2, B3, B4, and B5), but the specific role of each of these isoforms in regulating mitotic entry is still unclear. It is known that cyclin B1 can compensate for loss of both cyclin B2 (and vice versa in Drosophila). Saccharomyces cerevisiae contains six B-type cyclins (Clb1-6), with Clb2 being the most essential for function. In both vertebrates and S. cerevisiae, it is speculated that the presence of multiple B-type cyclins allows different cyclins to regulate different portions of the G2/M transition while also making the transition robust to perturbations. Subsequent discussions will focus on the spatial and temporal activation of cyclin B1/CDK in mammalian cells, but similar pathways are applicable in both other metazoans and in S. cerevisiae.

Cyclin B1 synthesis and degradation Cyclin B1 levels are suppressed throughout G1 and S phases by the anaphase-promoting complex (APC), an E3 ubiquitin ligase which targets cyclin B1 for proteolysis. Transcription begins at the end of S phase after DNA replication, in response to phosphorylation of transcription factors such as NF-Y, FoxM1 and B-Myb by upstream G1 and G1/S cyclin-CDK complexes.

Regulation of cyclin-B1/CDK1 activity Increased levels of cyclin B1 cause rising levels of cyclin B1-CDK1 complexes throughout G2, but the complex remains inactive prior to the G2/M transition due to inhibitory phosphorylation by the Wee1 and Myt1 kinases. Wee1 is localized primarily to the nucleus and acts on the Tyr15 site, while Myt1 is localized to the outer surface of the ER and acts predominantly on the Thr14 site. The effects of Wee1 and Myt1 are counteracted by phosphatases in the cdc25 family, which remove the inhibitory phosphates on CDK1 and thus convert the cyclin B1-CDK1 complex to its fully activated form, MPF.

Active cyclinB1-CDK1 phosphorylates and modulates the activity of Wee1 and the Cdc25 isoforms A and C. Specifically, CDK1 phosphorylation inhibits Wee1 kinase activity, activates Cdc25C phosphatase activity via activating the intermediate kinase PLK1, and stabilizes Cdc25A. Thus, CDK1 forms a positive feedback loop with Cdc25 and a double negative feedback loop with Wee1 (essentially a net positive feedback loop).

Positive feedback and switch-like activation

… excerpt ends here. Continue reading the full article.

Illustrations

G2 phase: Diagram of the G2 phase
Diagram of the G2 phase
G2 phase: This diagram illustrates the feedback loops underlying the G2/M transition. Cyclin-B1/CDK1 activates Plk and inactivates Wee1 and Myt1. Activated Plk activates cdc25. Activation of Cdc25 and inactivation of Wee1/Myt1 lead to further activation of Cyclin-B1/CDK1. Also shown is the putative role of cyclin-A/CDK2 and Cdc25A as initial activators of the feedback loop, discussed in a later section.
This diagram illustrates the feedback loops underlying the G2/M transition. Cyclin-B1/CDK1 activates Plk and inactivates Wee1 and Myt1. Activated Plk activates cdc25. Activation of Cdc25 and inactivation of Wee1/Myt1 lead to further activation of Cyclin-B1/CDK1. Also shown is the putative role of cyclin-A/CDK2 and Cdc25A as initial activators of the feedback loop, discussed in a later section.
G2 phase: This graph illustrates the stable equilibria for cyclin-B1/CDK1 activity at varying cyclin B1 concentrations, with the threshold of cyclin B concentration for entering mitosis higher than the threshold for exiting mitosis.
This graph illustrates the stable equilibria for cyclin-B1/CDK1 activity at varying cyclin B1 concentrations, with the threshold of cyclin B concentration for entering mitosis higher than the threshold for exiting mitosis.

Worked examples

Example 1 — a first encounter with G2 phase

Start with the simplest possible case. Write down what G2 phase 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 G2 phase 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 G2 phase 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 G2 phase

In research
G2 phase 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 G2 phase 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
G2 phase is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cell cycle, so understanding it makes those chapters shorter.
In everyday life
Look for G2 phase 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 G2 phase in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what G2 phase 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 G2 phase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is G2 phase in simple terms?

The G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated.

Why does G2 phase 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 G2 phase?

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 G2 phase.

Tags

  • Cell cycle

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