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    which of the following statements about microtubules during anaphase is true?

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    Mitosis

    Cells duplicate and condense their DNA prior to entering mitosis. During mitosis, chromosomes attach to a spindle of microtubules that distribute them equally to two daughter cells.

    Mitosis

    Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. These tubules, collectively known as the spindle, extend from structures called centrosomes — with one centrosome located at each of the opposite ends, or poles, of a cell. As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell. The spindle tubules then shorten and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.

    What Are the Phases of Mitosis?

    Mitosis consists of five morphologically distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase. Each phase involves characteristic steps in the process of chromosome alignment and separation. Once mitosis is complete, the entire cell divides in two by way of the process called cytokinesis (Figure 1).

    Figure 1: Drawing of chromosomes during mitosis by Walther Flemming, circa 1880

    This illustration is one of more than one hundred drawings from Flemming's \"Cell Substance, Nucleus, and Cell Division.\" Flemming repeatedly observed the different forms of chromosomes leading up to and during cytokinesis, the ultimate division of one cell into two during the last stage of mitosis.

    © 2001 Nature Publishing Group Paweletz, N. Walther Flemming: pioneer of mitosis research. 2, 72 (2001). All rights reserved.

    What Happens during Prophase?

    Prophase is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase. During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms.

    The mitotic spindle also begins to develop during prophase. As the cell's two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.

    What Happens during Prometaphase?

    When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles. As a result, the spindle microtubules now have direct access to the genetic material of the cell.

    Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome. Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere. The actual number of microtubules that attach to a kinetochore varies between species, but at least one microtubule from each pole attaches to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth toward the two poles.

    What Happens during Metaphase and Anaphase?

    As prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole. At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. Kinetochore microtubules attach the chromosomes to the spindle pole; interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules extend from the spindle pole to the cell membrane.

    Metaphase leads to anaphase, during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur. Upon separation, every chromatid becomes an independent chromosome. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. Then, in the second part of anaphase — sometimes called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes (Figure 2).

    Source : www.nature.com

    Biology: Chapter 8 Assignment Flashcards & Practice Test

    Start studying Biology: Chapter 8 Assignment. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

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    What event does not occur in prophase of mitosis?

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    The chromosomes are replicated.

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    The mitotic spindle fibers attach to chromosomes via special structures termed

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    kinetochores

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    Essentials of Human Anatomy and Physiology

    12th Edition

    Elaine N. Marieb, Suzanne M. Keller

    642 explanations

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    Terms in this set (26)

    What event does not occur in prophase of mitosis?

    The chromosomes are replicated.

    The mitotic spindle fibers attach to chromosomes via special structures termed

    kinetochores

    What statement about microtubules during anaphase is true?

    - Those attached to chromosomes elongate, while those that are unattached shorten.

    - Those attached to chromosomes shorten, while those that are unattached elongate.

    - Both attached and unattached microtubules shorten.

    - Both attached and unattached microtubules elongate.

    - Both attached and unattached microtubules elongate at first and then shorten.

    Those attached to chromosomes shorten, while those that are unattached elongate.

    Centromeres split during what phase of mitosis?

    anaphase

    Cytokinesis in plant cells occurs by means of a cleavage furrow.

    False

    The ______ checkpoint makes sure that DNA replication has been completed successfully.

    G2

    In order for a cell to enter S phase, it must progress through the ______ checkpoint.

    G1

    The final preparations for cell division occur in the ______ phase of interphase.

    G2

    After cytokinesis, the cell enters the S phase.

    False

    When is the DNA in chromosomes replicated?

    interphase

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    6.2 The Cell Cycle – Concepts of Biology – 1st Canadian Edition

    6.2 THE CELL CYCLE

    Learning Objectives

    By the end of this section, you will be able to:

    Describe the three stages of interphase

    Discuss the behavior of chromosomes during mitosis and how the cytoplasmic content divides during cytokinesis

    Define the quiescent G0 phase

    Explain how the three internal control checkpoints occur at the end of G1, at the G2–M transition, and during metaphase

    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 (Figure 6.3). 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

    Figure 6.3 A cell moves through a series of phases in an orderly manner. During interphase, G1 involves cell growth and protein synthesis, the S phase involves DNA replication and the replication of the centrosome, and G2 involves further growth and protein synthesis. The mitotic phase follows interphase. Mitosis is nuclear division during which duplicated chromosomes are segregated and distributed into daughter nuclei. Usually the cell will divide after mitosis in a process called cytokinesis in which the cytoplasm is divided and two daughter cells are formed.

    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 (Figure 6.4).

    Figure 6.4 Animal cell mitosis is divided into five stages—prophase, prometaphase, metaphase, anaphase, and telophase—visualized here by light microscopy with fluorescence. Mitosis is usually accompanied by cytokinesis, shown here by a transmission electron microscope. (credit “diagrams”: modification of work by Mariana Ruiz Villareal; credit “mitosis micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: modification of work by the Wadsworth Center, NY State Department of Health; donated to the Wikimedia foundation; scale-bar data from Matt Russell)

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

    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.

    Source : opentextbc.ca

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