The Reproduction of cells


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Life arises only from life. "Omnis cellula e cellula," Virchow's Biogenetic Law meaning "All cells from cells," is the organizing principle for this unit. The perpetuation of life is based on cell reproduction.


The Functions of Cell Division

 

  1. asexual reproduction of single celled organisms
  2. growth and development during early stages of an organism
  3. repair and renewal of aging tissues.

 


Bacterial Reproduction -
Prokaryotic Cell Division

 

Bacteria and cyanobacteria reproduce by a type of cell division called binary fission during which the prokaryotic cell elongates, duplicates its chromosome, and finally at about twice its original size the membrane pinches inward and a cell wall forms between the daughter chromosomes. For most bacteria this process may take 20 to 40 minutes. (see cells alive)

Most bacterial genes are found on a single circular helix of DNA and associated proteins. After a bacterial cell copies its chromosome in preparation for fission, allocation of the duplicate chromosomes to daughter cells depends on the attachment of both chromosomes to an extension of plasma membrane called the mesosome. It is the growth of the membrane between the two attachment sites that separates the chromosomes.

 


Eukaryotic Chromosomes, an Introduction

 

Eukaryotes have much larger genomes than prokaryotes. The average bacteria may have 1000 genes while the average eukaryote may have 50,000 genes. The problem of replicating and distributing so many genes is only manageable because the genes are grouped into multiple chromosomes.

Most eukaryotic species have two distinct generations; haploid and diploid. In the haploid generation the characteristic number of chromosomes in their gametes is "n" (23 for humans). After fertilization two nuclei unite and the organism becomes diploid, "2n", with double the haploid number of chromosomes in body cells (somatic cells). For humans the diploid number is 46.

Chromosomes are a DNA and protein complex which has coils on top of coils on top of coils.

 


The
Cell Cycle: An Overview

 

The cell cycle can be divided into two general periods - a long period of synthesis called interphase and then mitosis, the short period in which a cell unerringly distributes the replicated chromosomes to its new daughter cells.

Interphase, comprising 90% of the cell cycle, has three parts G1, S, and G2. During interphase's G1 and G2 the cell grows by synthesizing proteins and producing cytoplasmic organelles. During the S phase the cell replicates its genome.

Mitosis has 5 major subdivisions, the first 4--prophase, metaphase, anaphase, telophase--separate and distribute chromosomes while the 5th-- cytokinesis--splits the cell and its cytoplasmic constituents in two. (Illustrate)

Chemical regulation of the cell cycle involves a biological clock which involves the rise and fall of concentrations of several proteins including cyclin, MPF, cdc2. The exact mechanisms of gene regulation and internal and external signaling have not been fully worked out.

Find out about who won the recent Nobel Prize for Physiology for helping to unlock the secrets of the cell cycle -- and learn about cyclin and cyclin kinase.


The Mechanics of Cell Division

 

Mitosis in eukaryotic cells depends on the mitotic spindle. This spindle shaped structure made of microtubules begins to form during prophase. The assembly of spindle microtubules is initiated in the centrosome or animal cells. Plants are able to make a spindle apparatus without a centrosome however.

Visit the Mitchison Lab for images which show the localization of microtubules and DNA in interphase and mitotic frog egg cells.

Microtubules are polar, with distinct ends. The positive (+) end is where tubulin molecules are added or removed, and the negative (-) is anchored in the centrosome. Thus once initiated in the centrosome, spindle microtubules extend or retract at their tips, not at their bases. (Illustrate)

Although centrioles divide, migrate, and are located in the middle of each centrosome of animal cells they are not necessary to cell division since most plants lack them and if destroyed by a laser microbeam, spindles will still form and function during mitosis.

During interphase the single centrosome (in animal cells) replicates to form two centrosomes located just outside the nucleus. During prophase they migrate, producing spindle fibers elongating at their + ends, until the two centrosomes (presumably being moved apart by the growing microtubules) reach opposite ends of the cell.

Fifteen to thirty-five microtubules attach to the chromosomes at specialized regions of the centromeres called kinetochores.

Other microtubules overlap at the center of the cell.

These connections and the motor proteins which do the work of moving the microtubules will lead to the eventual separation of the chromatids.

In prophase microtubules from one pole of the cell may attach to a kinetochore first, and the chromosome begins to move toward that pole. However, this movement is checked as soon as microtubules from the opposite pole attach to the chromosome's other kinetochore and a tug-of-war ensues.

Metaphase

The back and forth motion equalizes the number of microtubules attached to the two kinetochores of a chromosome and ends momentarily when chromosomes line up at the metaphase plate. This period of alignment is called metaphase.

Anaphase commences when the chromosome's centromeres divide and the sister chromatids, now separate chromosomes, move apart toward the poles of the cell. It is the + end of a microtubule--the end attached to a kinetochore--where addition or removal of tubulin proteins must occur.

The exact mechanism of this interaction between kinetochores and microtubules is still unresolved, but researchers have recently found the motor protein dynein in kinetochores. Dynein may "walk" a chromosome along the shortening microtubules in a fashion similar to microtubule movement in cilia.

Nonkinetochore microtubules that overlap at the middle of the cell may be responsible for elongating the whole cell along the polar axis during anaphase. It is hypothesized that a sliding motion of neighboring microtubules could be explained by numerous dynein cross-bridges similar to those found in cilia or flagella.

Telophase, in many ways the opposite of prophase, is marked by the uncoiling of chromosomes, return of the nuclear envelope, reappearance of a nucleolus and other normalizing events.

For a summary of the steps in cell division of whitefish go to this site.


Mechanisms of Cytokinesis

 

Concurrent with telophase, the process of cytokinesis generally divides the cell's cytoplasm evenly. Animal cells are pinched in two, cleaved, when a contractile ring of microfilaments made of actin create a furrow by shrinking the diameter of the cell perpendicular to the polar axis.

In plant cells cytokinesis is very different since cell walls prevent simple cleavage. Instead a structure called the cell plate forms during telophase across the midline of the parent cell where the old metaphase plate was located. Vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell. These vesicles collide and fuse together to form the new plasma membranes. A region rich in pectins and other polysaccharides called a middle lamella forms between the two new membranes of the cell plate, and eventually primary cell walls develop on either side.

 


The Control of Cell Division

 

Timing and rate of cell division in different parts of a plant or animal are critical to normal growth, development, and maintenance. Some cells such as skin and intestinal epithelium must divide throughout a person's life to replace those lost or shed. Most cells in muscle and nervous tissue do not divide at all once they reach maturity.

Many factors both physical and chemical can either promote or inhibit cell division. Organisms produce chemicals (hormones) called growth factors which stimulate certain cells to divide. For instance platelets with encounter injured tissue release PDGF, platelet derived growth factor, which triggers cell division of fibroblasts, a response that helps heal the wound.

Crowding inhibits cell division. This phenomenon is called density-dependent inhibition of cell division. This type of inhibition may to due to a variety of factors including shortages of micronutrients, increase in concentrations of wastes, and inability to properly anchor themselves.

A critical checkpoint in the cell cycle occurs in G1 phase, which if the chemistry is right allows the cell to duplicate its DNA. Otherwise this restriction point directs the cell not to proceed and switch to a nondividing state called the G0 phase.


Meiosis


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Modified Nov. 6, 2005