8.4 - 8.6 Flashcards
Note 1 —-»
Cell division is the basis of reproduction for every organism; it enables a multicellular organism to grow to adult size; and it replaces worn-out or damaged cells. In your body, for example, millions of cells must divide every second to maintain the total number of about 200 trillion cells. Some cells divide once a day, others less often; and highly specialized cells, such as mature muscle and nerve cells, do not divide at all. The fact that some mature cells never divide explains why certain kinds of damage—such as the death of cardiac muscle during a heart attack or the death of brain cells during a stroke—can never be reversed. The process of cell division is a key component of the cell cycle, an ordered sequence of events that run from the instant a cell is first formed from a dividing parent cell until its own division into two cells. The cell cycle consists of two main stages: a growing stage (called interphase), during which the cell approximately doubles everything in its cytoplasm and replicates its DNA, and the actual cell division (called the mitotic phase).
Cell Cycle
An ordered sequence of events that extends from the time a eukaryotic cell is first formed from a dividing parent cell until its own division into two cells.
Interphase
The period in the eukaryotic cell cycle when the cell is not actually dividing.
Note 2 —-»
During the process of interphase, the cell’s metabolic activity is very high as it performs its normal functions. For example, a cell in your small intestine might release digestive enzymes and absorb nutrients. Your intestinal cell also grows in size during interphase, making more cytoplasm, increasing its supply of digestive proteins, and creating more cytoplasmic organelles such as mitochondria and ribosomes. In addition, the cell duplicates its chromosomes during this period. Typically, interphase lasts for at least 90% of the total time required for the cell cycle.
Note 3 —-»
Interphase (illustrated in the beige portion of the figure) can be divided into three subphases: the G1 phase (“first gap”), the S phase (“synthesis” of DNA—also known as DNA replication), and the G2 phase (“second gap”). Calling the G phases “gaps” is a misnomer; cells are actually quite active and grow throughout all three subphases of interphase. The chromosomes are duplicated during the S phase, which typically lasts about half of the interphase. At the beginning of the S phase, each chromosome is single. At the end of this subphase, after DNA replication, the chromosomes are doubled, each consisting of two sister chromatids joined along their lengths. During the G2 phase, the cell completes preparations for cell division.
Mitotic Phase
The part of the cell cycle when the nucleus divides, its chromosomes are distributed to the daughter nuclei, and the cytoplasm divides, producing two daughter cells.
Note 4 —-»
The mitotic phase accounts for only about 10% of the total time required for the cell cycle. The mitotic phase is divided into two overlapping stages, called mitosis and cytokinesis. In mitosis, the nucleus and its contents—most important, the duplicated chromosomes—divide and are distributed into two daughter nuclei. During cytokinesis, which usually begins before mitosis ends, the cytoplasm is divided in two. The combination of mitosis and cytokinesis produces two genetically identical daughter cells, each with a single nucleus, surrounding cytoplasm stocked with organelles, and a plasma membrane. Each newly produced daughter cell may then proceed through G1 and repeat the cycle.
Mitosis
The division of a single-cell nucleus into two genetically identical nuclei.
Cytokinesis
The division of the cytoplasm to form two separate daughter cells.
Note 5 —-»
Mitosis is unique to eukaryotes and is the evolutionary solution to the problem of allocating an identical copy of the whole set of chromosomes to two daughter cells. Mitosis is a remarkably accurate mechanism. Experiments with yeast, for example, indicate that an error in chromosome distribution occurs only once in about 100,000 cell divisions. The extreme accuracy of mitosis is essential to the development of your own body. You began as a single cell. Mitotic cell division ensures that all your body cells receive copies of the 46 chromosomes that were found in this original cell. Thus, every one of the trillions of cells in your body today can trace its ancestry back through mitotic divisions to that first cell produced when your father’s sperm and mother’s egg fused about nine months before your birth. During the mitotic phase, a living cell viewed through a light microscope undergoes dramatic changes in the appearance of the chromosomes and other structures.
A researcher treats cells with a chemical that prevents DNA synthesis from starting. This treatment would trap the cells in which part of the cell cycle?
G1
Prophase
The first stage of mitosis, during which the chromatin condenses to form structures visible with a light microscope and the mitotic spindle begins to form, but the nucleus is still intact.
Prometaphase
The second stage of mitosis, during which the nuclear envelope fragments and the spindle microtubules attach to the kinetochores of the sister chromatids.
Metaphase
The third stage of mitosis, during which all the cell’s duplicated chromosomes are lined up at an imaginary plane equidistant between the poles of the mitotic spindle.
Anaphase
The fourth stage of mitosis, beginning when sister chromatids separate from each other and ending when a complete set of daughter cells arrives at each of the two poles of the cell.
Telophase
The fifth and final stage of mitosis, during which the daughter nuclei form at the two poles of a cell.
Note 6 —-»
The chromosomes are the stars of the mitotic dance. Their movements depend on the mitotic spindle, a football-shaped structure of microtubule fibers and associated proteins that guides the separation of the two sets of daughter chromosomes. The spindle microtubules emerge from two centrosomes, microtubule-organizing regions in the cytoplasm of eukaryotic cells.
Mitotic Spindle
A football-shaped structure formed of microtubules and associated with proteins that are involved in the movement of chromosomes during mitosis and meiosis.
Centrosome
A structure found in animal cells from which microtubules originate and that is important during cell division.
You view an animal cell through a microscope and observe dense, duplicated chromosomes scattered throughout the cell. Which state of mitosis are you witnessing?
Prophase (because the chromosomes are condensed but not yet aligned)
Note 7 —-»
Cytokinesis typically overlaps with telophase. Given the differences between plant and animal cells—particularly the stiff cell wall found in plant but not animal cells—it isn’t surprising that cytokinesis proceeds differently for these two types of eukaryotic cells. In animal cells, cytokinesis occurs by cleavage. The first sign of cleavage in animal cells is the appearance of a cleavage furrow, a shallow groove in the cell surface. At the site of the furrow, the cytoplasm has a ring of microfilaments made of actin, associated with molecules of myosin. When the actin microfilaments interact with the myosin, the ring contracts. Contraction of the myosin ring is much like pulling a drawstring on a hoodie: As the drawstring is pulled, the ring of the hood contracts inward, eventually pinching shut. Similarly, the cleavage furrow deepens and eventually pinches the parent cell in two, resulting in two completely separate daughter cells, each with its own nucleus and share of cytoplasm.
Cleavage
Cytokinesis in animal cells and in some protists, characterized by pinching in of plasma membrane.
Cleavage Furrow
The first sign of cytokinesis during cell division in an animal cell.
Note 8 —-»
Cytokinesis is markedly different in plant cells, which possess stiff cell walls that prevent contraction. During telophase, membranous vesicles containing cell wall material collected at the middle of the parent cell. The vesicles fuse, forming a membranous disk called the cell plate. The cell plate grows outward, accumulating more cell wall materials as more vesicles fuse with it. Eventually, the membrane of the cell plate fuses with the plasma membrane, and the cell plate’s contents join the parental cell wall. The result is two daughter cells, each bounded by its own plasma membrane and cell wall.
