1.1: Eukaryote cells - Biology

1.1: Eukaryote cells - Biology

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Plants have eukaryote cells. The cell wall is made of cellulose but may be thickened and strengthened in some cells.

A eukaryotic plant cell differs considerably from a prokaryotic cell of a bacteria or archaea. These are much simpler and smaller. Their DNA is found in a single chromosome and is not bound by a membrane. Similarly photosynthetic cyanobacteria do not have chloroplasts but rather photosynthesis occurs within the general cavity of the cell.

Elasticity and Structure of Eukaryote Chromosomes Studied by Micromanipulation and Micropipette Aspiration

1. Abbreviation used in this paper: NEB, nuclear envelope breakdown.

Address all correspondence to Bahram Houchmandzadeh, CNRS, Laboratorie de Spectrométrie Physique, BP 57, 38402 Saint-Martin-d'Hères Cedex, France. Tel.: (33) 476 51 44 27. Fax: (33) 476 51 45 44. E-mail: [email protected]

We warmly thank M. Elbaum who developed the micromanipulation set-up, and S. Childress, M. Goulian, T. Hirano, H. Macgregor, W. Marshall, P. Moens, Y. Rabin, E. Siggia, and J. Swedlow for many helpful discussions.

Bahram Houchmandzadeh, John F. Marko, Didier Chatenay, Albert Libchaber Elasticity and Structure of Eukaryote Chromosomes Studied by Micromanipulation and Micropipette Aspiration . J Cell Biol 6 October 1997 139 (1): 1–12. doi:

The structure of mitotic chromosomes in cultured newt lung cells was investigated by a quantitative study of their deformability, using micropipettes. Metaphase chromosomes are highly extensible objects that return to their native shape after being stretched up to 10 times their normal length. Larger deformations of 10 to 100 times irreversibly and progressively transform the chromosomes into a “thin filament,” parts of which display a helical organization. Chromosomes break for elongations of the order of 100 times, at which time the applied force is around 100 nanonewtons. We have also observed that as mitosis proceeds from nuclear envelope breakdown to metaphase, the native chromosomes progressively become more flexible. (The elastic Young modulus drops from 5,000 ± 1,000 to 1,000 ± 200 Pa.) These observations and measurements are in agreement with a helix-hierarchy model of chromosome structure. Knowing the Young modulus allows us to estimate that the force exerted by the spindle on a newt chromosome at anaphase is roughly one nanonewton.

M itosis involves gross physical reorganization of chromosomes the duplicated chromatids are condensed, resolved, and finally segregated. These processes can be expected to change the material properties of chromosomes, notably their elasticity. Elasticity indicates the nature and strength of the interactions holding materials together, and thus can be used to probe chromosome structure. Given the poor state of understanding of chromosome structure, it is therefore remarkable how little this subject has been studied. In a pioneering work, Nicklas (1983) measured that the force applied to grasshopper chromosomes during anaphase was 700 piconewtons, from which he inferred the chromosome stiffness. More recently, Claussen et al. (1994) stretched human metaphase chromosomes spread on a cover glass. They found that after stretching of up to 10 times, the chromosomes returned to their original shape. However, these studies did not address the question of chromosome architecture.

An often-discussed model is one in which the “thick” metaphase chromosome is composed of a “thin filament” of diameter 200–300 nm (Sedat and Manuelidis, 1978 Manuelidis, 1990). In fact, Bak et al. (1977) reported that as isolated human metaphase chromosomes disintegrate, they can change into a thin filament of diameter 400 nm, five times the original chromosome length. They suggested that metaphase chromosomes were formed by helical wrapping of this thin fiber. On the basis of electron microscopy, they further proposed that the thin fiber had a helical structure.

The proposal for a helical structure of metaphase chromosomes is old. Observations of “spiral chromatonema” during meiotic metaphase I date to at least 1926. Ohnuki (1968) established that hypotonic treatment stabilized spiral structure in human mitotic metaphase chromosomes. Boy de la Tour and Laemmli (1988) observed that fluorescent anti–topoisomerase II was helically organized when bound to histone H1–depleted chromosomes. Recent work by Hirano and Mitchison (1994) revealed that a protein heterodimer required for chromosome condensation in vitro (XCAP-C/E) was localized along a helical track along the metaphase-like chromatids. These and other studies (Belmont et al., 1987, 1989 Saitoh and Laemmli, 1993) suggest a chromosome with an internal structure made of a coiled or folded fiber. However, the spiral structures observed may be the result of chemical treatments of the chromosomes (Cook, 1995).

In this paper, we report a simple mechanical study of mitotic chromosomes in living cultured newt lung cells using micropipette aspiration and manipulation. First, we find that chromosomes display remarkable elasticity, returning to their initial shape after being extended by up to 10 times. For larger deformations, the chromosome no longer returns to its initial length. Instead, the thick native chromosome is progressively converted into a thin filament 15 times the length of the original chromosome. This thin filament is itself elastic it can be stretched six times before breaking. After the filament is released, it takes on an irregular but unmistakably helical form. Furthermore, by measuring force versus deformation, we have determined the Young elastic modulus of the metaphase chromosome, the force at which the metaphase chromosome begins to be converted to thin fiber, and the force required to break the thin fiber. These measurements reveal the strength of interactions that stabilize the different levels of structure. Finally, we have observed that the Young modulus drops by about fivefold during the interval from nuclear envelope breakdown to metaphase.

Our results lead to a simple unifying picture of chromosome elasticity and structure: Our conclusion is that metaphase chromosomes are composed of an underlying thin filament. The large range over which the metaphase chromosome is elastic, the scale of its Young modulus, and the helical structure of the filament all argue in favor of its helical folding. By the same line of reasoning, the fact that the thin filament is elastic over a large range of extensions suggests that it also has a folded or helical structure. We also show how the force exerted by the mitotic spindle on a chromosome and the resistance of the cytoplasm to chromosome movement can be deduced from the Young modulus measurement and chromosome shape at anaphase.

Section Summary

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleolus within the nucleus is the site for ribosome assembly. Ribosomes are found in the cytoplasm or are attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria perform cellular respiration and produce ATP. Peroxisomes break down fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies, the centrioles, with an unknown role in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport membrane lipids and proteins.

The cytoskeleton has three different types of protein elements. Microfilaments provide rigidity and shape to the cell, and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural elements of centrioles, flagella, and cilia.

Animal cells communicate through their extracellular matrices and are connected to each other by tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other by plasmodesmata.

Additional Self Check Questions

1. What structures does a plant cell have that an animal cell does not have? What structures does an animal cell have that a plant cell does not have?

3.2.2 All cells arise from other cells

Opportunities for skills development

Within multicellular organisms, not all cells retain the ability to divide.

Eukaryotic cells that do retain the ability to divide show a cell cycle.

  • DNA replication occurs during the interphase of the cell cycle.
  • Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication.

The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids.

Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.

Meiosis is covered in section 3.4.3

Students should be able to:

  • recognise the stages of the cell cycle: interphase, prophase, metaphase, anaphase and telophase (including cytokinesis)
  • explain the appearance of cells in each stage of mitosis.

Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division.

Binary fission in prokaryotic cells involves:

  • replication of the circular DNA and of plasmids
  • division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids.

Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles.

Required practical 2: Preparation of stained squashes of cells from plant root tips set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.

Students should measure the apparent size of cells in the root tip and calculate their actual size using the formula:

Calculation of a mitotic index.


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