Botany Paper : Cell Biology Module : Cell cycle

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Botany Paper : Cell Biology Module : Cell cycle

Module Detail

Subject Name <Botany>

Paper Name <Cell Biology>

Module Name/Title <Cell cycle>

Module Id <Module Id>

Pre-requisites <Cell, cell organelles, general mitosis and meiosis, DNA replication>

Objectives <Objectives of the Module Ø Introduction

Ø Cell cycle

Ø Phases of cell cycle Ø Duration of cell cycle Ø Cell cycle control

Ø Cell cycle check points>

Keywords < Cell cycle>,< Phases of cell cycle>,< Cell cycle control>,

< Cell cycle check points >

Structure of Module / Syllabus of a module (Define Topic / Sub-topic of module )

< INTRODUCTION> < a single cell to a multicellular organism >, <Cell cycle>

< THE BASIC CELL CYCLE>

<The Cell Cycle Is an Ordered Series of Events Leading to Replication of Cells>,

Botany Paper : Cell Biology Module : Cell cycle

<PHASES OF CELL CYCLE>

< Gap 1 (G1) phase>, <Gap 0 (G0)>, <S phase>, <G2 phase >, <M Phase>

<DURATION OF THE CELL CYCLE>

<RESTRICTRICTION OF DNA

REPLICATION TO ONLY ONCE PER CELL CYCLE>

<Pre-replication complex>, < Initiation Complex>, < replisome>

Botany Paper : Cell Biology Module : Cell cycle

Role Name Affiliation

National Coordinator < >

Subject Coordinator < Prof Sujata Bhargav >

Paper Coordinator < Prof Nutan Malpathak >

Content Writer/Author (CW) < Renuka Diwan >

Content Reviewer (CR) <CR Name>

Language Editor (LE) <LE Name>

Botany Paper : Cell Biology Module : Cell cycle

1. Introduction

2. INTRODUCTION

a) a single cell to a multicellular organism, b) Cell cycle

3. THE BASIC CELL CYCLE

The Cell Cycle Is an Ordered Series of Events Leading to Replication of Cells 4. PHASES OF CELL CYCLE

a. Gap 1 (G1) phase, b. Gap 0 (G0), c. S phase, d. G2 phase, e. M Phase

5. DURATION OF THE CELL CYCLE

6. RESTRICTRICTION OF DNA REPLICATION TO ONLY ONCE PER CELL CYCLE

a. Pre-replication complex, b. Initiation Complex, c. replisome

Botany Paper : Cell Biology Module : Cell cycle

e-Text Example

Cell Cycle

Introduction

The journey of life that starts from a single cell to form a complex multicellular organism goes through numerous cell divisions. All organisms are made up of cells which constantly divide and multiply (Fig 1). Cells divide when they need to replace injured or dead cells, or as part of the development of a fertilized egg. 300 million cells are replaced in our bodies every minute — without our ever noticing!

Fig 1: From a single cell to a multicellular organism

Studies on yeast, worms, flies, frogs, mammals, and plants have contributed to a universal picture on how the basic cell cycle machinery is regulated, and research on these many divergent organisms is also elucidating how evolution modified the basic cell cycle machinery to cope with the specific developmental and environmental challenges of each organism.

Do these divisions occur randomly? Or are they regulated?

Cell division follows a specific pattern called the Cell Cycle (Fig 2). This cycle illustrates the different phases of division. These divisions are regulated and follow a strict chain of events. Most eukaryotic cells follow a process of growth and division called the cell cycle. These events include

Botany Paper : Cell Biology Module : Cell cycle

a growth stage (interphase), mitosis or nuclear division and cytokinesis or division of the cytoplasm (M phase) (Fig 2).

Fig 2: Cell division follows a specific pattern called the Cell Cycle

THE BASIC CELL CYCLE

The Cell Cycle Is an Ordered Series of Events Leading to Replication of Cells

The two main events of cellular reproduction that represent the two larger phases of the cell cycle are

1. the copying of cellular components - Interphase 2. the cleavage of the cell- Mitosis

During interphase, appropriate cellular components are copied. Before cell can enter mitosis, cell has to pass through rigorous controls known as checkpoints. Interphase is further didvided into sub-phases as will be discussed in the next section.

Mitosis, M phase, is the part of the cell cycle when the cell prepares for and completes cell division (Fig 3). Chromosomes condense during the prophase period of mitosis, by tightly folding loops of the 30-nm chromatin fiber attached to the chromosome scaffold. Sister chromatids,

Botany Paper : Cell Biology Module : Cell cycle

produced by DNA replication during the S phase, remain attached at the centromere and multiple points along their length and become aligned in the center of the cell during metaphase. During the anaphase portion of mitosis, sister chromatids separate and move to opposite poles of the mitotic apparatus, or spindle, segregating one of the two sister chromatids to each daughter cell.

Since the cell cycle is a "cycle" it has no distinct beginning or ending. Cells are continually entering and exiting the various phases of the cycle (Fig 3).

Fig 3: The fate of a single parental chromosome throughout the eukaryotic cell cycle

Although chromosomes condense only during mitosis, they are shown in condensed form to emphasize the number of chromosomes at different cell-cycle stages. The nuclear envelope is not depicted. Following mitosis (M), daughter cells contain 2n chromosomes in diploid organisms and 1n chromosomes in haploid organisms including yeasts maintained in the haploid state. In proliferating cells, G1 is the period between “birth” of a cell following mitosis and the initiation of DNA synthesis, which marks the beginning of the S phase. At the end of the S phase, cells enter G2 containing twice the number of chromosomes as G1 cells (4n in diploid organisms). The end of G2 is marked by the onset of mitosis, during which numerous events leading to cell division occur. The G1, S, and G2 phases are collectively referred to as interphase, the period between one mitosis and the next. Most nonproliferating cells in vertebrates leave the cell cycle in G1, entering the G0 state.

Botany Paper : Cell Biology Module : Cell cycle

Interphase generally lasts at least 12 to 24 hours in mammalian tissue. During this period, the cell is constantly synthesizing RNA, producing protein and growing in size. By studying molecular events in cells, scientists have determined that interphase can be divided into 4 steps: Gap 0 (G0), Gap 1 (G1), S (synthesis) phase, Gap 2 (G2) (Fig 3).

We will begin our discussion of the events that take place during interphase with those that occur immediately after a cell has successfully divided during mitosis. This phase is called G1.

Fig 3: The phases of cell cycle and their relative duration

G1 is typically the longest phase of the cell cycle. This can be explained by the fact that G1 follows cell division in mitosis; G1 represents the first chance for new cells have to grow. Cells usually remain in G1 for about 10 hours of the 24 total hours of the cell cycle. The length of S phase varies according to the total DNA that the particular cell contains; the rate of synthesis of DNA is fairly constant between cells and species. Usually, cells will take between 6 hours to complete S phase. G2 is shorter, lasting only 3 to 4 hours in most cells. In sum, then, interphase generally takes between 18 and 20 hours. Mitosis, during which the cell makes preparations for and completes cell division only takes about 1-2 hours.

Botany Paper : Cell Biology Module : Cell cycle

G1 is an intermediate phase occupying the time between the end of cell division in mitosis and the beginning of DNA replication during S phase. During this time, the cell grows in preparation for DNA replication, and certain intracellular components, such as the centrosomes undergo replication. Before a cell begins DNA replication, it must ensure that it is biologically ready to take on such a process. Cells increase in size in G1, produce RNA and synthesize protein. G1 is the phase when this cellular monitoring takes place. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis.

During G1, the cell reviews the cellular environment and the cell size to ensure that the conditions are appropriate to support DNA replication. Not until the cell is ready does it leave G1. If all is not ready to undergo DNA replication, cells can pause during G1 and enter a phase called G0.

Depending on a cell's preparedness to continue in the cell cycle, G0 can last days, weeks, or even years. When the cell has reached an appropriate size and is in a supportive environment for DNA replication, it will exit either G1 or G0 and enter the next phase of interphase called S phase (Fig 3).

Gap 0 (G0)

There are times when a cell will leave the cycle and quit dividing. This may be a temporary resting period or more permanent. Example - a cell that has reached an end stage of development and will no longer divide (e.g. neuron). A G0 cell is often called "quiescent". The term “quiescent” is used in respect to cell cycle activity and does in no way symbolize lack of metabolic activity. In fact many G0 cells are terminally differentiated cells carrying out their diverse functions in the organism. e.g., secretion, attacking pathogens.

Botany Paper : Cell Biology Module : Cell cycle

Many times a cell will leave the cell cycle, temporarily or permanently. It exits the cycle at G1 and enters a stage designated G0 (G zero) (Fig 3).

Example:

1. G0 cells are terminally differentiated will never reenter the cell cycle but instead will carry out their function in the organism until they die.

2. G0 can be followed by reentry into the cell cycle. Most of the lymphocytes in human blood are in G0. However, with proper stimulation, such as encountering the appropriate antigen, they can be stimulated to reenter the cell cycle (at G1) and proceed on to new rounds of alternating S phases and mitosis.

3. G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely.

S phase

S phase, or synthesis, is the phase of the cell cycle when DNA packaged into chromosomes is replicated. This event is an essential aspect of the cell cycle because replication allows for each cell created by cell division to have the same genetic make-up. During S phase a number of events additional to chromosome replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called G2(Fig 3).

G2 phase

Similar to G1, G2 is an intermediate phase, a time for the cell to ensure that it is ready to proceed in the cell cycle. Occurring between the end of DNA replication in S phase and the beginning of cell

Botany Paper : Cell Biology Module : Cell cycle

division in mitosis, G2 can be thought of as a safety gap during which a cell can check to make sure that the entirety of its DNA and other intracellular components have been properly duplicated and to determine if the cell can now proceed to enter M (mitosis) and divide. In addition to acting as a checkpoint along the cell cycle, G2 also represents the cell's final chance to grow before it is split into two independent cells during mitosis (Fig 3).

Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells.

Mitosis is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division. Actual stages of mitosis can be viewed at Animal Cell Mitosis (Fig 3).

State Description Abbreviatio n

quiescent/

senescent Gap 0 G0 A resting phase where the cell has left the cycle and has stopped

dividing.

Interphase

Gap 1 G1 Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.

Synthesis S DNA replication occurs during this phase.

Gap 2 G2

During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divide.

Cell

division Mitosis M

Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells. A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.

DURATION OF THE CELL CYCLE

Botany Paper : Cell Biology Module : Cell cycle

The length of the cell cycle is important because it determines how quickly an organism can multiply. For single-celled organisms, this rate determines how quickly the organism can reproduce new, independent organisms. For higher-order species the length of the cell cycle determines how long it takes to replace damaged cells. The duration of the cell cycle varies from organism to organism and from cell to cell. Certain fly embryos sport cell cycles that last only 8 minutes per cycle! Some mammals take much longer than that--up to a year in certain liver cells. Generally, however, for fast-dividing mammalian cells, the length of the cycle is approximately 24 hours.

Most of the differences in cell cycle duration between species and cells are found in the duration of specific cell cycle phases. DNA replication, for example, generally proceeds faster in the simpler the organisms. One reason for this trend is simply that prokaryotes have smaller genomes and not as much DNA to be replicated. Across species and organismal complexity, embryonic cells have an increased need for rapidity in the cell cycle because they need to multiply for the development of the embryo. Early embryonic cell cycles often omit G1 and G2 and quickly proceed through successive rounds of S phase and mitosis. For these cells, the main concern is not the regulation of the cell cycle (which occurs largely in G1 and G2), but rather in the speed of cell proliferation (Fig 3).

As we discussed in the previous section, the lengths of G1 and G2 vary in cells based on the individual cell's level of preparedness for proceeding in the cell cycle. Remember, cells can enter G0 for extensive amounts of time during G1 before continuing on to S phase. If a cell has quickly undergone sufficient cell growth or DNA replication, the time spent in G1 and G2 will be decreased (Fig 3).

It is possible to determine the time a cell spends in different phases of the cell cycle and its specific location in the cycle by feeding cells with molecules that are only taken into the cell at a

Botany Paper : Cell Biology Module : Cell cycle

specific point in the cell cycle. For example, thymidine is only incorporated into a cell during S phase, and scientists will often use thymidine as a tool to mark the onset of S phase. The amount of DNA present in a cell is also a good indication of where a cell stands in the cell cycle. During S phase, DNA is replicated and, as a result, cells in G2 have higher levels of cellular DNA than cells in G1.

During interphase, there is visible change in appearance of the cell. Synthesis of RNA and proteins occurs continuously, but synthesis of DNA occurs only in the discrete periods of S phase (Fig 4).

Botany Paper : Cell Biology Module : Cell cycle

Fig 4: Cell growth is continuous but DNA synthesis is not

RESTRICTRICTION OF DNA REPLICATION TO ONLY ONCE PER CELL CYCLE

As discussed in above sections the two main events of cellular reproduction are the copying of cellular components followed by the cleavage of the cell. In each cell cycle the genetic material is replicated and then apportioned to the daughter cells. Eukaryotic genome has multiple replicons however origin of each replicon activated only once thus ensuring that DNA is copied only once in each cell cycle.

Progress through the cell cycle and in turn DNA replication is tightly regulated by the formation and activation of pre-replicative complexs (pre-RCs) which is achieved through the activation and inactivation of cyclin-dependent kinases (Cdks). Specifically it is the interactions of cyclins and cyclin dependent kinases that are responsible for the transition from G1 into S-phase (Fig 4).

Pre-replication complex

Botany Paper : Cell Biology Module : Cell cycle

The first step in the assembly of the pre-replication complex (pre-RC) is the binding of the origin recognition complex (ORC) to the replication origin. The ORC is a six-subunit, Orc1p-6, protein complex that selects the replicative origin sites on DNA for initiation of replication and ORC binding to chromatin is regulated through the cell cycle. Generally, the function and size of the ORC subunits are conserved throughout many eukaryotic genomes with the difference being which ORC unit actually contacts the DNA. When the ORC binds to DNA at replication origins, it then serves as a scaffold for the assembly of other key initiation factors of the pre-replicative complex, which includes Cdc6, Cdt1, and minichromosome maintenance (Mcm 2-7) complex proteins. This pre-replicative complex assembly during the G1 stage of the cell cycle is required prior to the continuation of DNA replication during the S phase (Fig 4).

In late mitosis, Cdc6 protein joins the bound ORC followed by the binding of the Cdt1 protein.

ORC, Cdc6, and Cdt1 are all required to load the six protein minichromosome maintenance (Mcm 2-7) complex onto the DNA.

Botany Paper : Cell Biology Module : Cell cycle

Initiation Complex

During the G1 stage of the cell cycle, the replication initiation factors, origin recognition complex (ORC), Cdc6, Cdt1, and minichromosome maintenance (Mcm) protein complex, bind sequentially to DNA to form the pre-replication complex (pre-RC). At the transition of the G1 stage to the S phase of the cell cycle, S phase–specific cyclin-dependent protein kinase (CDK) and Cdc7/Dbf4 kinase (DDK) transform the pre-RC into an active replication fork. During this transformation, the pre-RC is disassembled with the loss of Cdc6, creating the initiation complex. In addition to the binding of the Mcm proteins, cell division cycle 45 (Cdc45) protein is also essential for initiating DNA replication. Studies have shown that Mcm is critical for the loading of Cdc45 onto chromatin and this complex containing both Mcm and Cdc45 is formed at the onset of the S phase of the cell

Botany Paper : Cell Biology Module : Cell cycle

cycle.Cdc45 targets the Mcm protein complex, which has been loaded onto the chromatin, as a component of the pre-RC at the origin of replication during the G1 stage of the cell cycle.

Once the initiation complex is formed and the cells pass into the S phase, the complex then becomes a replisome.

Fig 5: Transformation of the pre-replicative complex into an active replisome.

Mcm 2-7 complex loads onto DNA at replication origins during G1 and unwinds DNA ahead of replicative polymerases. Cdc6 and Cdt1 bring Mcm complexes to replication origins. CDK/DDK-dependent phosphorylation of pre-replicative proteins leads to replisome assembly and origin firing. Cdc6 and Cdt1 are no longer required and are removed from the nucleus or degraded. Mcms and associated proteins, GINS and Cdc45, unwind DNA to expose template DNA. At this point replisome assembly is completed and replication is initiated. "P" represents phosphorylation.

During the G1 phase of the cell cycle there are low levels of CDK activity. This low level of CDK activity allows for the formation of new pre-RC complexes but is not sufficient for DNA replication to be initiated by the newly formed pre-RCs. During the remaining phases of the cell cycle there are elevated levels of CDK activity. This high level of CDK activity is responsible for

Botany Paper : Cell Biology Module : Cell cycle

initiating DNA replication as well as inhibiting new pre-RC complex formation. Once DNA replication has been initiated the pre-RC complex is broken down. Due to the fact that CDK levels remain high during the S phase, G2, and M phases of the cell cycle no new pre-RC complexes can be formed. This all helps to ensure that no initiation can occur until the cell division is complete (Fig 6).

Fig 6: Levels of CDK activity regulate initiation of DNA replication

In addition to cyclin dependent kinases (Fig 7.c) a new round of replication is thought to be

prevented through the downregulation of Cdt1. This is achieved via degradation of Cdt1 (Fig 7.b) as well as through the inhibitory actions of a protein known as geminin (Fig 7.b). Geminin binds tightly to Cdt1 and is thought to be the major inhibitor of re-replication. Geminin first appears in S- phase and is degraded at the metaphase-anaphase transition, possibly through ubiquination by anaphase promoting complex (APC).

Botany Paper : Cell Biology Module : Cell cycle

Fig 7: Regulation of DNA replication by the downregulation of Cdt1

Botany Paper : Cell Biology Module : Cell cycle

1. Interphase is made up of three distinct phases: G1, S phase, and G2.

2. The G1 and G2 phases serve as checkpoints for the cell to make sure that it is ready to proceed in the cell cycle.

3. If it is not, the cell will use this time to make proper adjustments that can include cell growth, correction or completion of DNA synthesis, and duplication of intracellular components.

4. S phase involves the replication of chromosomes.

5. All three stages of interphase involve continued cell growth and an increase in the concentration of proteins found in the cell.

6. Synthesis of RNA and proteins occurs continuously, but DNA synthesis of DNA occurs only in the discrete periods of S phase.

7. Restrictriction of DNA replication to only once per cell cycle