Lecture 9: Cell Division I                                                                                           pdf _download pdf _download

Fundamental Differences between PROkaryotes and EUkaryotes.

Such intracellular compartmentalization dramatically enhances the organization of eukaryotic organisms, allowing them the cellular structure to grow much larger than most prokaryotic cells. Such organization, however, presents some major barriers to important cellular functions, like cell division, that need to be overcome, and necessitate a complete reorganization of the process of cellular reproduction.

Systems of Cell Reproduction:

Four events need to occur before any given cell can undertake cell division.

A signal to "reproduce" must be received.

Replication of DNA and duplication (multiplication) of vital cell components must occur.

DNA must be distributed in to the new cells.

The cell membrane begins to separate and ultimately divide (along with a new cell wall in some organisms) into two new daughter cells.

How this process is brought about is well understood by biologists, but the precise signal(s) and ordered activities within the overall process are still the subject of intense research.

Prokaryotic cells divide by binary fission.

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Eukaryotic cells divide by MITOSIS or MEOSIS

Unlike prokaryotes, eukaryotic cells do not constantly divide whenever environmental conditions are adequate, although unicellular eukaryotes do so more often than the cells of multicellular organisms.

In higher, multicellular eukaryotes there is cellular development, in which differentiate into different ytypes of cells, some of which, once formed rarely or never divide. For example: nerve cells and muscle cells in humans.

Signals to divide are related to the needs of the entire organism, not simply the opportunity created by resources.

Moreover, eukaryotes usually have multiple linear chromosomes within their nucleus, and thus, the replication of each chromosomal piece of DNA must be synchronized with each other, and must be appropriately apportioned into the newly divided, daughter cells.

Mitosis generates two cells with the same genetic information as the original cell.

Meiosis is a specialized cell division used for sexual reproduction. The genetic information of the chromosomes is shuffled, and the cells, called gametes, typically get one-half of the original DNA complement.

Interphase is the period between divisions of the cytoplasm.

Most cells have two major phases: interphase and cell division, through mitosis, often referred to as the cell cycle.

 

    

Interphase consists of three sub-phases< ...

    G1 is Gap 1, the period just after mitosis and before the beginning of DNA synthesis.

    Next is S (synthesis) phase, which is the time when the cell's DNA is replicated.

     G2 is the time after S and prior to mitosis.

A typical eukaryotic cell will spend most of its life in interphase. Nerve cells etc. lose the capacity to divide altogether and stay in interphase indefinitely. Other cells divide regularly, others occasionally.

Consequently, at any given time the majority of cells are in interphase, while only a few cells are undergoing mitosis.

 

Mitosis and cytokinesis are referred to as the "M phase".

The G1-to-S transition commits the cell to enter into another cell replication cycle.

The complexity of these different phases is further enhanced by the complexities of the eukaryotic genome itself.....(which, as I stated before, normally consists of more than one chromosome).

Apart from gametes, most eukaryotic cells (like our own) contain TWO full sets of genetic information, one from one parent, and one from the other. As such they (we) are said to be diploid.

 

  

The number of chromosomes varies from organism to organism; for example, the single celled "baker's yeast" have 32, the more complex "humans" have 46... and the even more complex "horses" have 64.

Each eukaryotic chromosome consists of a single, double-stranded molecule of DNA; the molecule is extremely long -relative to the size of the cell.

As with the prokaryotes, DNA of a human cell is much larger than its nucleus in which it is contained,and, if put end to end has a total length of 2 meters, while the nucleus itself is just 10 um in diameter.

Unlike prokaryotes, however, in eukaryotes there are many proteins which are associated with the DNA molecule, forming chromatin, which is a well organized complex of DNA wrapped around a series of proteins, called histones.

These wraps of DNA and histone proteins are called nucleosomes and resemble beads on a string.

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The core of a nucleosome contains eight histone molecules, two each fom four of the histone classes.

There are 146 base pairs of DNA wrapped around the core, in 1.65 turns of DNA.

One molecule from the remaining histone class, histone (H1), clamps the DNA to the core, and helps form the next level of packaging.

In addition to the histones around which the DNA is wrapped, there are many proteins, which are intimately associated with the strands of DNA (chromatin), and depending upon the phase of the cell cycle, the density of this packing can be highly variable.

While the DNA carries the genetic information, the proteins organize the DNA physically and regulate the activities of the DNA.  Indeed (by mass), chromatin consists of about equal parts DNA and protein.

This makes it possible to allow the DNA to be loosely bound durig most of the cell cycle (interphase) and yet condenses the DNA into tightly packed "bundles", safely packaged for it's transport into the two separate cells during cell division.

Without condensation of the chromatin, the various DNA strands would fail to properly partition (divide equally) into the two daughter cells.

After the DNA of a given chromosome has been replicated during S phase, that chromosome now consists of two joined chromatids, joined at a region called the centromere.

During cell division (mitosis and meiosis), the chromatin becomes even more coiled and condensed.

 

Mitosis: is a process that produces Exact Copies of Genetic Information whereby a single cell, gives rise to two genetically identical daughter cells, but more importantly, a single nucleus gives rise to two genetically identical nuclei, one for each of the two new daughter cells.

Mitosis is a continuous event, but it is convenient to look at it as a series of steps. Mitosis_step by step 2D

    

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In many organisms, each centrosome contains a pair of centrioles that have replicated during interphase.

Centrosomes are regions where microtubules form. These microtubules will orchestrate the subsequnet movement of chromosomes as the cell divides during mitosis (launchpad)

The spindles begin to form during prophase.  In prophase, polar microtubules (spindle fibres) form between the two centrosomes and make up the developing spindle. (See Mitosis animation).

 

    

Each "polar" microtubule runs from one mitotic center (centrosome) to just beyond the middle of the spindle, where it overlaps and interacts with a corresponding microtubule from the other side.

Initially, these microtubules are constantly polymerizing ("forming") and depolymerizing ("falling apart") during this period. Microtubules grow by addition of tubulin dimers to the "+" end of the microtubule.

When microtubules from one centrosome contact microtubules from the other, they become more stable.

The mitotic spindles serve as "railroad tracks", along which chromosomes will move later in mitosis.

As the movie indicates, in prophase each chromosomes begin to condense... and, having previously being replicated during the "S" phase of the cell cycle begin to show that they are actually in the form of two chromatids

The region of tight binding between the chromatids, the centromere, is where the microtubules will associate with the chromosomes,

Late in prophase, the kinetochores (which are located in the region around the centromere of each chromosome and define the site where microtubules attach to the chromatids) begin to develop.

  

As you might expect, chromosome movements are HIGHLY organized..

The movement phases of chromosomes are designated prometaphase, metaphase, and anaphase.

  

During prometaphase, the nuclear lamina disintegrates and the nuclear envelope breaks into small vesicles permitting the fibers of the spindle to "invade" the nuclear region.

The spindle microtubules then associate with the kinetochores, and are termed "kinetochore microtubules".

The microtubules from one pole associate with the kinetochore of one of the members of a pair of chromatids. Microtubules from the other pole associate with the kinetochore of the other member.

Repulsive forces from the poles push chromosomes toward the center, or "equatorial plate", in a rather aimless back and forth motion.

During metaphase, the chromosomes with kinetochores bound at their cenromeres arrive at the equatorial plate.

At this stage, all chromosomes are fully condensed and have distinguishable shapes.

    

Each "polar" microtubule runs from one mitotic center (centrosome) to just beyond the middle of the spindle, where it overlaps and interacts with a corresponding microtubule from the other side.

Anaphase begins when the centromeres separate.

The process takes any where for 10 to 60 minutes for the chromosomes to move to opposite poles.

Molecular "motors" at the kinetochores move the chromosomes toward the poles, accounting for about 75% of the motion.

    

 

About 25% of the motion comes from a shortening of the microtubules at the poles.

Additional distance is gained by the separating of the mitotic centers. This increase in distance between the poles is brought about by the polar microtubules, which have motor proteins associated in the overlapping regions. By this process the distance between the poles doubles.

Nuclei re-form during telophase.

When chromosomes finish moving, telophase begins; nuclear envelopes and nucleoli coalesce and re-form.

Cytokinesis: The Division of the Cytoplasm

Animal cells divide by a furrowing (or a "pinching in" or constriction) of the plasma membrane.

Microfilaments of actin and the motor protein filament myosin first form a ring beneath the plasma membrane. Thereafter the actin and myosin filaments contract to produce the constriction.

  

Plants have cell walls and their cytoplasm divides slightly differently.

After the spindle breaks down, vesicles from the Golgi apparatus appear in the equatorial region. The vesicles fuse to form a new plasma membrane, and the contents of the vesicles combine to form the "cell plate", which is the beginning of the new cell wall.

NB. While it is important that each daughter cell receives a single copy (and ONLY a single copy of each chromosome) Organelles and other cytoplasmic resources do not need to be distributed equally within the daughter cells (as long as some of each are present) their subsequent concentration can be controlled by other factors which ensure the presence of appropriate numbers of organelles -as needed.

 

Mitosis: The Distribution of EXACT COPIES of Genetic Information, whereby a single cell, gives rise to two genetically identical cells: but more specifically, a single nucleus gives rise to two genetically identical nuclei, one for each of the two new daughter cells.

 

Mitosis results in genetic constancy 

Asexual reproduction involves the generation of a new individual that is essentially genetically identical to the parent.  It involves a cell or cells that were generated by mitosis.

• Variation of cells is principally due, therefore to the "forces of evolution" that we have discussed previously... Natural selection, mutations, GF and GD -as well as other potential environmental effects.

Sexual reproduction, on the other hand, involves the process of Meiosis and provides for increase "inherent variation".

Two parents, each contribute to the formation of one cell that is genetically different from either of the two original parents.

This cellular division Meiosis is designed to create variety among the offspring beyond that which can be attributed to simple mutations or the environment.

Meiosis (launchpad): consists of two nuclear divisions that reduce the number of chromosomes in the diploid form to the haploid number.

Note: The nucleus divides twice, but the DNA is replicated only once.

Consequently, one of the major roles of meiosis is to reduce the chromosome number from diploid to haploid, and to ensure that each daughter cell (in diploids this would be a gamete) gets a complete set of each chromosome. 

     gametogenesis

Paradoxically, the process also promotes a quite incredible degree of inherent genetic diversity among the progeny(?).

This comes about, primarily because during prophase I of Meiosis I homologous chomosomes of the diploid cell undergo pairing or synapsis, which allows DNA/chromatin to be exchanged between the two sets of chromosomes.

gametogenesis

After metaphase I, (i.e. Anaphase I) ..homologous chromosomes separate in to the two different cells.

 

• Itis important to note that Individual chromosomes -each with two chromatids- remain intact until metaphase II of Meiosis II (second nuclear division) is completed and the chromatids separate to become chromosomes.

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The first meiotic division reduces the chromosomal number

Meiosis I is preceded by an interphase in which DNA is replicated.
Meiosis I begins with a long prophase, prophase I.

 

 

 

 


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