Cell Division
Cell division is fundamental to all living organisms and required for growth and development. As an essential means of reproduction for all living thi…
The cell cycle refers to the sequence of events throughout a typical cell's life - involving growth, DNA replication, and preparation for cell division. For sexually-reproducing organisms, life begins as a zygote, a fertilized egg. Over time, that original single cell grows and divides in a controlled manner to produce a multicellular, complex individual. Cells continue this process, especially to maintain and repair tissues during aging.
Now let's take a closer look. In eukaryotes, double-stranded DNA is specially organized within a membrane-bound nucleus to accommodate the cell's limited space. At the first level of compaction, DNA is wrapped tightly around specific proteins called histones. This combination of DNA protein particles is then repeated and packed into arrays known as nucleosomes, which, along with linker DNA, form coiled chromatin fibers. Finally, additional fibrous proteins compact the chromatin even further, packing long lengths of DNA into tightly-condensed units. Recognized as chromosomes, depending on the phase of cell division.
To prepare for division, cells must go through interphase, which is divided into three stages. G1, S, and G2. In G1, the first gap phase, a newly-generated daughter cell grows in size and prepares for DNA duplication in the next phase. Now in S, the synthesis phase, cells duplicate their nuclear DNA, which remains packaged as chromatin. Cells also duplicate the centrosomes, the microtubule-organizing structures which form the mitotic spindle apparatus. Finally, in G2, the second gap phase, cells continue to grow, multiply organelles and proteins that are required for mitosis, and replenish their energy stores. The cell is now ready to enter the first stage of mitosis.
Comprised of five unique stages, mitosis is a form of division where a cell's genetic material is partitioned between two daughter cells. First during prophase in humans, nucleic chromatin condenses into X-shaped chromosomes, composed of sister chromatid pairs attached at centromere junctions. Concurrently outside the nucleus, centrosomes migrate to opposite sides of the cell. As they do so, microtubule rods begin to grow from each. Either towards the cell's interior or exterior, forming a web-like spindle apparatus. Next, the nuclear envelope dissolves during prometaphase, exposing the chromosomes to the cell's other contents. Protein structures also appear on both sides of the centromeres, one for every chromatid. Once these kinetochores form, extending interior microtubules fasten to them, with each sister chromatid being tethered to a different pole.
Mitosis then progresses to metaphase, where the spindle apparatus rearranges the chromosomes so that they are similarly oriented in a fixed row along the cell's equator. During anaphase, kinetochore-affixed microtubules shorten. And sister chromatids, now individually referred to as chromosomes, are dragged apart. These and other microtubule dynamics also elongate the cell. Finally, the chromosomes land at opposite cell sides during telophase. And the spindle apparatus disbands. The genetic material loosens, and two nuclear envelopes, one around each chromosome set, arise. During telophase, a distinct process, not technically a stage of mitosis, called cytokinesis, also divides the cell. Thus, the end result of mitosis is a cell pair genetically identical to their precursor.
Unfortunately, mutations can cause damage to genes controlling cell cycle regulation, which leads to cell division proceeding unchecked. In this case, each successive cell division produces daughter cells with even more damage…and the faulty growth regulation ultimately leads to the formation of cell masses called tumors.
In this lab, you will examine the different stages of mitosis using onion root tip cells. And then examine what happens when cell cycle control is lost.
The cell cycle refers to the sequence of events throughout a typical cell's life - involving growth, DNA replication, and preparation for cell division. For sexually-reproducing organisms, life begins as a zygote, a fertilized egg. Over time, that original single cell grows and divides in a controlled manner to produce a multicellular, complex individual. Cells continue this process, especially to maintain and repair tissues during aging.
Now let's take a closer look. In eukaryotes, double-stranded DNA is specially organized within a membrane-bound nucleus to accommodate the cell's limited space. At the first level of compaction, DNA is wrapped tightly around specific proteins called histones. This combination of DNA protein particles is then repeated and packed into arrays known as nucleosomes, which, along with linker DNA, form coiled chromatin fibers. Finally, additional fibrous proteins compact the chromatin even further, packing long lengths of DNA into tightly-condensed units. Recognized as chromosomes, depending on the phase of cell division.
To prepare for division, cells must go through interphase, which is divided into three stages. G1, S, and G2. In G1, the first gap phase, a newly-generated daughter cell grows in size and prepares for DNA duplication in the next phase. Now in S, the synthesis phase, cells duplicate their nuclear DNA, which remains packaged as chromatin. Cells also duplicate the centrosomes, the microtubule-organizing structures which form the mitotic spindle apparatus. Finally, in G2, the second gap phase, cells continue to grow, multiply organelles and proteins that are required for mitosis, and replenish their energy stores. The cell is now ready to enter the first stage of mitosis.
Comprised of five unique stages, mitosis is a form of division where a cell's genetic material is partitioned between two daughter cells. First during prophase in humans, nucleic chromatin condenses into X-shaped chromosomes, composed of sister chromatid pairs attached at centromere junctions. Concurrently outside the nucleus, centrosomes migrate to opposite sides of the cell. As they do so, microtubule rods begin to grow from each. Either towards the cell's interior or exterior, forming a web-like spindle apparatus. Next, the nuclear envelope dissolves during prometaphase, exposing the chromosomes to the cell's other contents. Protein structures also appear on both sides of the centromeres, one for every chromatid. Once these kinetochores form, extending interior microtubules fasten to them, with each sister chromatid being tethered to a different pole.
Mitosis then progresses to metaphase, where the spindle apparatus rearranges the chromosomes so that they are similarly oriented in a fixed row along the cell's equator. During anaphase, kinetochore-affixed microtubules shorten. And sister chromatids, now individually referred to as chromosomes, are dragged apart. These and other microtubule dynamics also elongate the cell. Finally, the chromosomes land at opposite cell sides during telophase. And the spindle apparatus disbands. The genetic material loosens, and two nuclear envelopes, one around each chromosome set, arise. During telophase, a distinct process, not technically a stage of mitosis, called cytokinesis, also divides the cell. Thus, the end result of mitosis is a cell pair genetically identical to their precursor.
Unfortunately, mutations can cause damage to genes controlling cell cycle regulation, which leads to cell division proceeding unchecked. In this case, each successive cell division produces daughter cells with even more damage…and the faulty growth regulation ultimately leads to the formation of cell masses called tumors.
In this lab, you will examine the different stages of mitosis using onion root tip cells. And then examine what happens when cell cycle control is lost.
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Q1: What is the cell cycle and why is it important for organisms?
The cell cycle is the sequence of events throughout a cell's life involving growth, DNA replication, and preparation for division. For multicellular organisms, cell division produces new cells for development and replaces damaged cells from injury. For unicellular organisms, cell division generates a completely new organism, making the cell cycle essential for growth, reproduction, and tissue maintenance.
Q2: How is DNA organized and packaged inside eukaryotic cells?
In eukaryotes, double-stranded DNA is wrapped tightly around histone proteins, forming nucleosomes. These nucleosomes, along with linker DNA, create coiled chromatin fibers. Additional fibrous proteins then compact the chromatin further, packing long DNA lengths into tightly-condensed units called chromosomes, which are visible during cell division phases.
Q3: What happens during interphase and how long does it last?
Interphase comprises about 78% of a cell's life and contains three stages: G1, S, and G2. During G1, newly-formed cells grow and prepare for DNA replication. In S phase, cells duplicate their nuclear DNA and centrosomes. In G2, cells continue growing, multiply organelles and proteins needed for mitosis, and replenish energy stores before entering division.
Q4: What are the five stages of mitosis and what occurs in each?
Mitosis consists of prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, chromatin condenses into chromosomes and spindle fibers form. Prometaphase dissolves the nuclear envelope and attaches kinetochores to chromosomes. Metaphase aligns chromosomes at the cell's equator. Anaphase separates sister chromatids to opposite poles. Telophase reforms nuclear envelopes around each chromosome set.
Q5: How does cytokinesis differ from mitosis?
Cytokinesis is a distinct process occurring during telophase that physically divides the cell's cytoplasm, producing two separate daughter cells. While mitosis partitions genetic material into two nuclei, cytokinesis completes cell division by separating the cytoplasm around each nucleus, resulting in two genetically identical daughter cells.
Q6: What internal and external factors regulate cell division?
External regulation ensures division necessity, such as replacing damaged stomach lining cells. Internal regulation maintains daughter cell health through checkpoints that verify DNA integrity before S phase, complete replication before mitosis, and proper chromosome attachment before anaphase. Cells failing checkpoints may undergo apoptosis if unable to correct errors.
Q7: How do mutations in cell cycle control genes lead to cancer?
Mutations in tumor-suppressor genes prevent cells from uncontrolled division, relieving normal growth inhibition. When these mutations accumulate, cells divide unchecked without external signals, producing daughter cells with increasing damage. This aberrant growth regulation ultimately forms tumors and can lead to cancer, one of the leading causes of death in the United States.