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7.1: Mitosis and the Cell Cycle - Biology

7.1: Mitosis and the Cell Cycle - Biology


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Each cell has a limited number of options for its future:

  1. grow and divide (though this can be delayed in some cells, such as primary oocytes)
  2. differentiate into a specialized cell and cease growing and dividing
  3. die (programmed cell death called apoptosis)

Each cell in a multicellular organism receives information from myriad sources and processes this information to decide its fate. The process goes like this:

The cell cycle is controlled at three checkpoints:

  1. G1 Checkpoint
  2. G2 Checkpoint
  3. Mitosis Checkpoint

At each checkpoint the cell is assessed. If all is well the cell is allowed to proceed to the next phase.

Mitosis

DNA molecules in the cell nucleus are duplicated before mitosis, during the S (or synthesis) phase of interphase. Mitosis is the process of nuclear division. At the end of mitosis, a cell contains two identical nuclei. Mitosis is divided into four stages (PMAT) listed below.

Prophase → Metaphase → Anaphase → Telophase

Cytokinesis, the process of cell division, occurs during the last stage of mitosis (telophase).

Some cells do not go though mitosis. In this case, these cells move from G1 of the cell cycle into a resting phase known as G0. Sometimes a cell in G0 will move back into G1 and continue through the cell cycle. Other cells will simply stay in G0 for their entire lifetime.

Part 1: Labeling Diagrams

Examine the images below. As completely as possible, list the key events that occur in each stage of mitosis. Compare your list to your classmates.

Part 2: Mitosis Bead Simulations

In this exercise you will make models of chromosomes to study the meiosis chromosome replication and

Comparing mitosis and process of mitosis.

Materials

  • 8 magnets (= centromeres)
  • 30 beads of one color
  • 30 beads of another color

Procedure

  1. Set up half of the beads exactly as follows, representing genes on the chromosome of a hypothetical critter. We will assume that the critter is diploid (2N) and has two different chromosomes. Since it has two copies of each chromosome the diploid number is 4 (2 × 2 = 4).

    This is what your critter’s chromosomes look like in the unreplicated form. Note that there are four chromosomes here, or two homologous pairs. Each chromosome pair consists of a maternal and paternal version of the chromosome. The maternal and paternal versions are represented by respective bead color.

  2. Replicate your chromosomes! Make enough copies of each chromosome to represent both paternal and maternal chromosomes in a replicated form, as shown below. Note that the sister chromatids are identical in color. Be sure you can identify the sister chromatids, chromosomes, and the difference between a replicated and non-replicated form.
  3. Using your maternal and paternal sets of replicated chromosomes and your notes as a reference, practice the process of mitosis until you are very comfortable with it. Each person in the group should practice the entire process.

Think about It

Draw your bead chromosomes in each stage of mitosis. Label each stage. Note: You do not need to draw every single bead . but be sure to accurately indicate the relative sizes and colors of each different chromosome pair.

Do NOT proceed until you are comfortable with this! When your entire group is ready, let your instructor know. He or she will choose a group member to walk him or her through your simulation. If it is done correctly, you may move on to the next part.

Part 3: Microscopic Mitosis

In this part of the lab, you will examine 2 different slides:

  1. A cross section of an onion root tip, where cell growth (and consequently mitosis) happens at a rapid rate.
  2. Blastula of a whitefish. The blastula is a distinct stage during embryonic development when a fertilized egg forms a hollow ball of cells. During embryonic development, cells are dividing quickly and we are more likely to be able to see the varying stages of mitosis.

Materials

  • Alium slide
  • Whitefish blastula slide
  • Microscope

Procedure

You must have your own microscope for this lab!

  1. Using correct microscope procedure, observe an onion root tip under high power (400X).
  2. Locate the region of active cell division, known as the root apical meristem, which is about 1 mm behind the actual tip of the root.
  3. Identify and draw a cell in each of the four stages of mitosis in the onion slide. Then draw cells in cytokinesis and interphase as well.
  4. Observe the prepared slide of a whitefish blastula under high power (400X).
  5. Identify and draw a cell in each of the four stages of mitosis in the whitefish blastula slide. Then draw cells in cytokinesis and interphase as well.

Part 4: Estimating Relative Time Spent in Each Stage of Mitosis

If you froze time and took a snapshot of a group of cells in a living organism, you could estimate the relative amount of time a cell spends in each stage of the cell cycle simply by counting the number of cells in each stage. For example, if there are 100 cells in your view and 90 of them are in prophase, you can assume that the cells spend most of the time in prophase.

In this part, you will practice identifying cells in the various stages of mitosis, and then you will estimate the relative amount of time a cell spends in each stage.

Materials

  • Alium slide
  • Microscope

Procedure

  1. Return to the slide of the onion root tip. Using correct microscope procedure, observe an onion root tip under high power (400X).
  2. Choose ONE view and then carefully COUNT the number of cells in each stage of the cell cycle. Feel free to estimate the total number of cells in each stage.

Data

InterphaseProphaseMetaphaseAnaphaseTelophase
Number of cells in each phase:
Percent of cells in each stage (see Equation 1)
Estimated time a cell spends in each stage (see Equation 2)

Equation 1:

Equation 2:

Think about It

Are there any problems with this estimate? How could you make this exercise more effective?

Part 5: Mitosis Bingo

Fill in the boxes with the names of the different stages of Mitosis. You will use the same stage multiple times. Then watch as different stages of mitosis are shown on the screen. Every time you see a stage on your card, cover that spot with a marker. You must cover all the spots on a card to win! (Printable version here.)

Lab Questions

  1. Describe how cytokinesis is different in plant cells and animal cells.
  2. Make an estimate of how long (relatively speaking) a cell stays in each stage of mitosis (not including interphase). How did you determine your estimates?
  3. Can you think of any reasons why cells contain a genetic program that tells the cell to commit suicide (apoptosis)? Give reasons why this would be.
  4. Is the G0 phase a truly resting phase? If the cells in G0 are not truly resting, why do you think we use the term resting to describe the state of these cells? [Hint: think of your nerve cells and muscle cells.]

The Cell Cycle

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells. The cell cycle has two major phases: interphase and the mitotic phase ([link]). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides. Watch this video about the cell cycle: https://www.youtube.com/watch?v=Wy3N5NCZBHQ

Interphase

During interphase, the cell undergoes normal processes while also preparing for cell division. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G1, S, and G2.

G1 Phase

The first stage of interphase is called the G1 phase, or first gap, because little change is visible. However, during the G1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins, as well as accumulating enough energy reserves to complete the task of replicating each chromosome in the nucleus.

S Phase

Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome—sister chromatids—that are firmly attached at the centromere region. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. The centrosome consists of a pair of rod-like centrioles at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of many eukaryotic species, such as plants and most fungi.

G2 Phase

In the G2 phase, or second gap, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic spindle. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.

The Mitotic Phase

To make two daughter cells, the contents of the nucleus and the cytoplasm must be divided. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and moved to opposite poles of the cell, and then the cell is divided into two new identical daughter cells. The first portion of the mitotic phase, mitosis, is composed of five stages, which accomplish nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into two daughter cells.

Mitosis

Mitosis is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus ([link]).

Which of the following is the correct order of events in mitosis?

  1. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus re-forms and the cell divides. The sister chromatids separate.
  2. The kinetochore becomes attached to the mitotic spindle. The sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus re-forms and the cell divides.
  3. The kinetochore becomes attached to metaphase plate. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus re-forms and the cell divides.
  4. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus re-forms and the cell divides.

During prophase, the “first phase,” several events must occur to provide access to the chromosomes in the nucleus. The nuclear envelope starts to break into small vesicles, and the Golgi apparatus and endoplasmic reticulum fragment and disperse to the periphery of the cell. The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell. The microtubules that form the basis of the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly and become visible under a light microscope.

During prometaphase, many processes that were begun in prophase continue to advance and culminate in the formation of a connection between the chromosomes and cytoskeleton. The remnants of the nuclear envelope disappear. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and visually discrete. Each sister chromatid attaches to spindle microtubules at the centromere via a protein complex called the kinetochore.

During metaphase, all of the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other. At this time, the chromosomes are maximally condensed.

During anaphase, the sister chromatids at the equatorial plane are split apart at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule was attached. The cell becomes visibly elongated as the non-kinetochore microtubules slide against each other at the metaphase plate where they overlap.

During telophase, all of the events that set up the duplicated chromosomes for mitosis during the first three phases are reversed. The chromosomes reach the opposite poles and begin to decondense (unravel). The mitotic spindles are broken down into monomers that will be used to assemble cytoskeleton components for each daughter cell. Nuclear envelopes form around chromosomes.

This page of movies illustrates different aspects of mitosis. Watch the movie entitled “DIC microscopy of cell division in a newt lung cell” and identify the phases of mitosis.

Cytokinesis

Cytokinesis is the second part of the mitotic phase during which cell division is completed by the physical separation of the cytoplasmic components into two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.

In cells such as animal cells that lack cell walls, cytokinesis begins following the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. This fissure, or “crack,” is called the cleavage furrow. The furrow deepens as the actin ring contracts, and eventually the membrane and cell are cleaved in two ([link]).

In plant cells, a cleavage furrow is not possible because of the rigid cell walls surrounding the plasma membrane. A new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking up into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles move on microtubules to collect at the metaphase plate. There, the vesicles fuse from the center toward the cell walls this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell wall at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall of cellulose. The Golgi membranes become the plasma membrane on either side of the new cell wall ([link]).

G0 Phase

Not all cells adhere to the classic cell-cycle pattern in which a newly formed daughter cell immediately enters interphase, closely followed by the mitotic phase. Cells in the G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage, having exited the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently ([link]).

Control of the Cell Cycle

The length of the cell cycle is highly variable even within the cells of an individual organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or to an entire human lifetime spent in G0 by specialized cells such as cortical neurons or cardiac muscle cells. There is also variation in the time that a cell spends in each phase of the cell cycle. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is approximately 24 hours. In rapidly dividing human cells with a 24-hour cell cycle, the G1 phase lasts approximately 11 hours. The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell.

Regulation at Internal Checkpoints

It is essential that daughter cells be exact duplicates of the parent cell. Mistakes in the duplication or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced from the abnormal cell. To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell cycle checkpoints at which the cell cycle can be stopped until conditions are favorable. These checkpoints occur near the end of G1, at the G2–M transition, and during metaphase ([link]).

The G1 Checkpoint

The G1 checkpoint determines whether all conditions are favorable for cell division to proceed. The G1 checkpoint, also called the restriction point, is the point at which the cell irreversibly commits to the cell-division process. In addition to adequate reserves and cell size, there is a check for damage to the genomic DNA at the G1 checkpoint. A cell that does not meet all the requirements will not be released into the S phase.

The G2 Checkpoint

The G2 checkpoint bars the entry to the mitotic phase if certain conditions are not met. As in the G1 checkpoint, cell size and protein reserves are assessed. However, the most important role of the G2 checkpoint is to ensure that all of the chromosomes have been replicated and that the replicated DNA is not damaged.

The M Checkpoint

The M checkpoint occurs near the end of the metaphase stage of mitosis. The M checkpoint is also known as the spindle checkpoint because it determines if all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to spindle fibers arising from opposite poles of the cell.

Watch what occurs at the G1, G2, and M checkpoints by visiting this animation of the cell cycle.

Section Summary

The cell cycle is an orderly sequence of events. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages. In eukaryotes, the cell cycle consists of a long preparatory period, called interphase. Interphase is divided into G1, S, and G2 phases. Mitosis consists of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis is usually accompanied by cytokinesis, during which the cytoplasmic components of the daughter cells are separated either by an actin ring (animal cells) or by cell plate formation (plant cells).

Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell cycle: one near the end of G1, a second at the G2–M transition, and the third during metaphase.

Art Connections

[link] Which of the following is the correct order of events in mitosis?

  1. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus re-forms and the cell divides. The sister chromatids separate.
  2. The kinetochore becomes attached to the mitotic spindle. The sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus re-forms and the cell divides.
  3. The kinetochore becomes attached to metaphase plate. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus re-forms and the cell divides.
  4. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus re-forms and the cell divides.

[link] D. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus reforms and the cell divides.

Multiple Choice

Chromosomes are duplicated during what portion of the cell cycle?

Separation of the sister chromatids is a characteristic of which stage of mitosis?

The individual chromosomes become visible with a light microscope during which stage of mitosis?

What is necessary for a cell to pass the G2 checkpoint?

  1. cell has reached a sufficient size
  2. an adequate stockpile of nucleotides
  3. accurate and complete DNA replication
  4. proper attachment of mitotic spindle fibers to kinetochores

Free Response

Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.

There are very few similarities between animal cell and plant cell cytokinesis. In animal cells, a ring of actin fibers is formed around the periphery of the cell at the former metaphase plate. The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two. In plant cells, a new cell wall must be formed between the daughter cells. Because of the rigid cell walls of the parent cell, contraction of the middle of the cell is not possible. Instead, a cell plate is formed in the center of the cell at the former metaphase plate. The cell plate is formed from Golgi vesicles that contain enzymes, proteins, and glucose. The vesicles fuse and the enzymes build a new cell wall from the proteins and glucose. The cell plate grows toward, and eventually fuses with, the cell wall of the parent cell.

Glossary


Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

Prophase

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

Metaphase

The chromosomes align themselves along the metaphase plate of the spindle apparatus.

Anaphase

The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

Telophase

The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.

Cytokinesis

The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.


Phases of Cell Cycle

Mitosis

Let’s start this cell cycle with “birth.”

During mitosis, the “parent” cell goes through a complex series of steps to ensure that each “daughter” cell will get the materials it needs to survive, including a copy of each chromosome. Once the materials are properly sorted, the “parent” cell divides down the middle, pinching its membrane in two.

You can read more about the detailed steps of mitosis and how a parent cell makes sure its daughter cells will inherit what they need to survive in our article on Mitosis (https://biologydictionary.net/mitosis/).

Each of the new “daughters” are now independently living cells. But they’re small, and have only one copy of their genetic material.

This means they can’t divide to produce their own “daughters” right away. First, they must pass through “interphase” – the phase between divisions, which consists of three distinct phases.

G1 Phase

In G1 phase, the newly formed daughter cell grows. The “G” is most often said to stand for “gap,” since these phases appear to an outside observer with a light microscope to be relatively inactive “gaps” in the cell’s activity.

However given what we know today, it might be more accurate to say the “G” stands for “growth” – for the “G” phases are flurries of protein and organelle production as well as literal increase in the size of the cell.

During the first “growth” or “gap” phase, the cell produces many essential materials such as proteins and ribosomes. Cells that rely on specialized organelles such as chloroplasts and mitochondria make a lot more of those organelles during G1 as well. The cell’s size may increase as it assimilates more material from its environment into its machinery for life.

This allows the cell to increase its energy production and overall metabolism, preparing it for…

S Phase

During S phase, the cell replicates its DNA. The “S” stands for “synthesis” – referring to the synthesis of new chromosomes from raw materials.

This is a very energy-intensive operation, since many nucleotides need to by synthesized. Many eukaryotic cells have dozens of chromosomes – huge masses of DNA – that must be copied.

Production of other substances and organelles is slowed greatly during this time as the cell focuses on replicating its entire genome.

When the S phase is completed, the cell will have two complete sets of its genetic material. This is crucial for cell division, as it ensures that both daughter cells can receive a copy of the “blueprint” they need to survive and reproduce.

However, replicating its DNA can leave the cell a little bit depleted. That’s why it has to go through…

G2 Phase

Just like the first “gap” phase of the cell cycle, the G2 phase is characterized by lots of protein production.

During G2, many cells also check to make sure that both copies of their DNA are correct and intact. If a cell’s DNA is found to be damaged, it may fail its “G2/M checkpoint” – so named because the this “checkpoint” happens at the end of the G2 phase, right between G2 and “M phase” or “Mitosis.”

This “G2/M checkpoint” is a very important safety measure for multicellular organisms like animals. Cancers, which can result in the death of the entire organism, can occur when cells with damaged DNA reproduce. By checking to see if a cells’ DNA has been damaged immediately before replication, animals and some other organisms reduce the risk of cancer.

Interestingly, some organisms can skip G2 altogether and go straight into mitosis after DNA is synthesized during S phase. Most organisms, however, find it safer to use G2 and its associated checkpoint!

If the G2/M checkpoint is passed, the cell cycle begins again. The cell divides through mitosis, and new daughter cells begin the cycle that will take them through G1, S, and G2 phases to produce new daughter cells of their own.

Unless of course they’re meant for…

An Alternative Path: G0 Phase

After being born through mitosis, some cells are not meant to divide themselves to produce daughter cells.

Neurons, for example – animal nerve cells – do not divide. Their “parent cells” are stem cells, and the “daughter” neuron cells are programmed not to go through the cell cycle themselves because uncontrolled neuron growth and cell division could be very dangerous for the organism.

So instead of entering G1 phase after being “born,” neurons enter a phase scientists call “G0 phase.” This is a metabolic state meant only to maintain the daughter cell, not prepare for cell division.

Neurons and other non-dividing cell types may spend their whole lives in G0 phase, performing their function for the overall organism without ever dividing or reproducing themselves.


Interphase

Interphase is made up of 3 separate phases: G1, S, and G2. During G1, the cell grows and acquires essentials for the upcoming DNA replication and mitosis. In the S phase, the DNA of the cell undergoes replication and the organelles and centrosomes start to duplicate. Organelles are membrane-bound structures in a cell. Centrosomes are organelles that produce spindle fibers during cell division. DNA replication is a complex process, but to put it in simple terms, the DNA replicates so that the two cells produced from mitosis have the same DNA. Finally, in the G2 phase, there is more growth, and the duplication of the organelles and centrosome complete. Upon completing these 3 sections of interphase, the cell may now undergo mitosis.


Chapter 12 Cell Cycle

1. Explain how cell division functions in reproduction, growth, and repair.

2. Describe the structural organization of a prokaryotic and a eukaryotic genome.

3. Describe the major events of cell division that enable the genome of one cell to be passed on to two daughter cells.

4. Describe how chromosome number changes throughout the human life cycle.

The Mitotic Cell Cycle

5. List the phases of the cell cycle and describe the sequence of events that occurs during each phase.

6. List the phases of mitosis and describe the events characteristic of each phase.

7. Recognize the phases of mitosis from diagrams and micrographs.

8. Draw or describe the spindle apparatus, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, asters, and centrioles (in animal cells).

9. Describe what characteristic changes occur in the spindle apparatus during each phase of mitosis.

10. Explain the current models for poleward chromosomal movement and elongation of the cell’s polar axis.

11. Compare cytokinesis in animals and in plants.

12. Describe the process of binary fission in bacteria and explain how eukaryotic mitosis may have evolved from binary fission.

Regulation of the Cell Cycle

13. Describe the roles of checkpoints, cyclin, Cdk, and MPF in the cell cycle control system.

14. Describe the internal and external factors that influence the cell cycle control system.

15. Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls.


Clinical Relevance - Neoplasia

Neoplasia is a disease of unchecked cell division and its progression is attributed to a change in activity of cell cycle regulators. If a mutation occurs in a protein that regulates the cell cycle, e.g. p53, it can lead to rapid, uncontrolled multiplication of these cells.

When there is a defect in p53 tumour suppressor gene, it cannot detect and bind to cells with damaged DNA to either repair the damage or cause apoptosis. This leads to unchecked replication of cells in the cell cycle and an increase in mutated p53. This increases the risk of neoplasms and also brings out the cancerous properties in the mutant p53.


Watch the video: The Cell Cycle and cancer Updated (January 2023).