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Do prokaryotic chromosomes have centromeres?


I found this here.

Eukaryotic chromosomes are always linear… In contrast, prokaryotic chromosomes are either completely devoid of centromeres or carry the so-called “plasmid centromeres” which are not essential (with a few exceptions, such as Caulobacter) (57,-60).

What does it mean by prokaryotic chromosomes carry "plasmid centromers"? Their chromosomes carry the centromeres of plasmids? Do they have centromeres or not?


No, prokaryotes do not have centromeres. They do, however, generally have a somewhat analogous structure used during cell division to partition the replicated chromosomes and plasmids between the two daughter cells. The analogous place on the chromosome or plasmid is the parS element, a DNA sequence on the chromosome or plasmid which is part of the parABS system. This system, consisting of two proteins (parA and parB) along with parS, forms a mechanism to segregate the copies of a chromosome or plasmid for the daughter cells.

parS is usually called a "centromere-like" site, but is sometimes referred to as a "plasmid centromere”, which is why the quote uses the phrase so-called “plasmid centromeres”.


Centromere

The centromere is the point on a chromosome where mitotic spindle fibers attach to pull sister chromatids apart during cell division.

When a cell seeks to reproduce itself, it must first make a complete copy of each of its chromosomes, to ensure that their daughter cell receives a full complement of the parent cell’s DNA.

The two copies of each chromosome often remain stuck together until they are separated, with one copy going to each daughter cell. While stuck together, these two copies are called “sister chromatids.”

As a cell prepares to divide, the sister chromatids begin to become unstuck from each other until they are almost completely separated. They remain joined, however, at the centromere – a special region that plays a vital role in cell division.

At the centromere, elements of the cell’s cytoskeleton assemble and attach. First, a complex of proteins called the kinetochore assembles around the centromere region of DNA then, mitotic spindle fibers attach to the kinetochore. The other end of these fibers are anchored to opposite ends of the parent cell, which will shortly split to become new daughter cells.

When the spindle fibers begin to contract, the chromatids are pulled to opposite ends of the parent cell. In this way, when the parent cells splits in two during cytokinesis, each sister chromatid becomes a chromosome of the new daughter cell.

In stages 3 and 4, the DNA condenses into tightly-packed chromosomes, in which sister chromatids are paired up and joined at their centromere. In stage 5 pictured below, the sister chromatids are pulled apart to opposite sides of the cell.

In stage 6, at last the cell splits in two, separating the sisters into daughter cells.


Chromosomes, Prokaryotic

The genetic material of microorganisms , be they prokaryotic or eukaryotic, is arranged in an organized fashion. The arrangement in both cases is referred to as a chromosome.

The chromosomes of prokaryotic microorganisms are different from that of eukaryotic microorganisms, such as yeast , in terms of the organization and arrangement of the genetic material. Prokaryotic DNA tends to be more closely packed together, in terms of the stretches that actually code for something, than is the DNA of eukaryotic cells. Also, the shape of the chromosome differs between many prokaryotes and eukaryotes . For example, the deoxyribonucleic acid of yeast (a eukaryotic microorganism) is arranged in a number of linear arms, which are known as chromosomes. In contrast, bacteria (the prototypical prokaryotic microorganism) lack chromosomes. Rather, in many bacteria the DNA is arranged in a circle.

The chromosomal material of viruses is can adopt different structures. Viral nucleic acid, whether DNA or ribonucleic acid (RNA ) tends to adopt the circular arrangement when packaged inside the virus particle. Different types of virus can have different arrangements of the nucleic acid. However, viral DNA can behave differently inside the host, where it might remain autonomous or integrating into the host's nucleic acid. The changing behavior of the viral chromosome makes it more suitable to a separate discussion.

The circular arrangement of DNA was the first form discovered in bacteria. Indeed, for many years after this discovery the idea of any other arrangement of bacterial DNA was not seriously entertained. In bacteria, the circular bacterial chromosome consists of the double helix of DNA. Thus, the two strands of DNA are intertwined while at the same time being oriented in a circle. The circular arrangement of the DNA allows for the replication of the genetic material. Typically, the copying of both strands of DNA begins at a certain point, which is called the origin of replication. From this point, the replication of one strand of DNA proceeds in one direction, while the replication of the other strand proceeds in the opposite direction. Each newly made strand also helically coils around the template strand. The effect is to generate two new circles, each consisting of the intertwined double helix.

The circular arrangement of the so-called chromosomal DNA is mimicked by plasmids . Plasmids exist in the cytoplasm and are not part of the chromosome. The DNA of plasmids tends to be coiled extremely tightly, much more so than the chromosomal DNA. This feature of plasmid DNA is often described as supercoiling. Depending of the type of plasmid, replication may involve integration into the bacterial chromosome or can be independent. Those that replicate independently are considered to be minichromosomes.

Plasmids allow the genes they harbor to be transferred from bacterium to bacterium quickly. Often, such genes encode proteins that are involved in resistance to antibacterial agents or other compounds that are a threat to bacterial survival, or proteins that aid the bacteria in establishing an infection (such as a toxin).

The circular arrangement of bacterial DNA was first demonstrated by electron microscopy of Escherichia coli and Bacillus subtilus bacteria in which the DNA had been delicately released from the bacteria. The microscopic images clearly established the circular nature of the released DNA. In the aftermath of these experiments, the assumption was that the bacterial chromosome consisted of one large circle of DNA. However, since these experiments, some bacteria have been found to have a number of circular pieces of DNA, and even to have linear chromosomes and sometimes even linear plasmids. Examples of bacteria with more than one circular piece of DNA include Brucella species, Deinococcus radiodurans, Leptospira interrogans, Paracoccus denitrificans, Rhodobacter sphaerodes, and Vibrio species. Examples of bacteria with linear forms of chromosomal DNA are Agrobacterium tumefaciens, Streptomyces species, and Borrelia species.

The linear arrangement of the bacterial chromosome was not discovered until the late 1970s, and was not definitively proven until the advent of the technique of pulsed field gel electrophoresis a decade later. The first bacterium shown to possess a linear chromosome was Borrelia burgdorferi.

The linear chromosomes of bacteria are similar to those of eukaryotes such as yeast in that they have specialized regions of DNA at the end of each double strand of DNA. These regions are known as telomeres, and serve as boundaries to bracket the coding stretches of DNA. Telomeres also retard the double strands of DNA from uncoiling by essentially pinning the ends of each strand together with the complimentary strand.

There are two types of telomeres in bacteria. One type is called a hairpin telomere. As its name implies, the telomers bends around from the end of one DNA strand to the end of the complimentary strand. The other type of telomere is known as an invertron telomere. This type acts to allow an overlap between the ends of the complimentary DNA strands.

Replication of a linear bacterial chromosome proceeds from one end, much like the operation of a zipper. As replication moves down the double helix, two tails of the daughter double helices form behind the point of replication.

Research on bacterial chromosome structure and function has tended to focus on Escherichia coli as the model microorganism. This bacterium is an excellent system for such studies. However, as the diversity of bacterial life has become more apparent in beginning in the 1970s, the limitations of extrapolating the findings from the Escherichia coli chromosome to bacteria in general has also more apparent. Very little is known, for example, of the chromosome structure of the Archae, the primitive life forms that share features with prokaryotes and eukaryotes, and of those bacteria that can live in environments previously thought to be completely inhospitable for bacterial growth .

See also Genetic identification of microorganisms Genetic regulation of prokaryotic cells Microbial genetics Viral genetics Yeast genetics


Similarities between Prokaryotic and Eukaryotic Chromosomes

Ø The chromosome of both prokaryotes and eukaryotes contains the genetic material DNA.

Ø The chemical composition and structural organization of DNA is similar in both prokaryotes and eukaryotes.

Ø In both prokaryotes and eukaryotes, the expression of genetic material is facilitated by transcription and translation.

Ø In both groups, the negatively charged DNA interacts with some positively charged proteins to nullify their charges.

Ø The genetic material contains both coding and noncoding sequences.

Ø In both groups, the methylation of DNA in the chromosome causes its inactivation.

Ø Both groups contain extra-chromosomal genetic materials. (plasmids in prokaryotes and DNA of mitochondria and chloroplasts in eukaryotes)


Centromere: Structure and Types | Chromosomes

The site of constriction in a chromosome under light microscope is generally taken as the po­sition of centromere. It is generally believed that constitutive heterochromatin is present in the centromeric region. The component of centromere is mainly the kinetochore, and DNA associated proteins.

Spindle fibres or microtubules are attached at this point which helps in moving the chromosomes or chromatids to the poles during cell division. When mi­crotubules of the spindle are attached at the centromere of metaphase chromosomes consist­ing of two chromatids, then sister chromatids separate and move to opposite poles of the spindle—and next step of division proceeds.

Thus centromere has two functions, one is the attachment of sister chromatids, and second is the site for attachment of spindle fibre.

It has been observed under the electron microscope that a single spindle fibre is attached to the centromere of yeast, Saccharomyces cerevisiae, while multiple spindle fibres are attached to the centromere of other organisms.

The chromatin segment of the centromere in yeast has been analysed and found to contain a Protein-DNA complex of 220 to 250 base pairs. Four regions have been identified in the centromere of yeast as CDE 1, CDE 2, CDE 3 and CDE 4.

The base sequences of first three regions are similar in all yeasts but the variation in base sequence is found in CDE 4. CDE 2 region is peculiar in having 90% of base pairs as AT rich. Inverted repeat segments are found in CDE 3 region.

The centromeric DNA is protected from the digestion of nuclease by forming a structure called centromeric core particle. This particle contains more DNA than normal nucleosome core particle and associated proteins. The spindle fibre is attached to this particle that helps to separate chromosomes during cell division.

When the centromeric DNA sequences an protein sequences of centromeric protein are compared, it has been found that protein se­quences are more conserved than DNA se­quences indicating thereby that DNA sequences may not be the important determinant factor in the function of the centromeric regions.

The centromeric regions of higher organisms contain large amounts of heterochromatin consisting of repetitive DNA.

The centromere of human chromosome contains tandemly repetitive DNA of 170 bp. These repeats are called a-satellite DNA. The number of copies may vary from 5,000 to 15,000. This a DNA is responsible, in most cases as a binding site, for centromeric protein. However the role of a satellite DNA on mammalian centromeres is yet to be established fully.

Mammalian centromeres bind about 30 to 40 spindle fibres or microtubules whereas only one microtubule is attached to the centromere of yeast. The two species of yeast, S. cerevisiae and Schizocharomyces pombe, show wide di­vergence in the size of centromeric DNA.

The centromenic DNA of S. pombe is 1,000 times larger than those of S. cerevisiae. The cen­tromere of S. Pombe is more complex in having the central core of unique sequence DNA and the flanking sequence of 3 tandem repeats. The function of centromeric DNA has been assayed clearly in yeast showing segregation of plasmids in daughter cells in mitosis when centromere is present.

Types of Centromere:

(a) Structure of Telomeres:

All chromosomes have a special DNA-protein structure at the end called Telomeres. The telomeres have some important role in chro­mosome replication and stability. Microscopic observations show that chromosomes with bro­ken ends become degraded leading sometimes to cell death.

In an experiment, telomeres from Tetrahymena were transferred to the ends of linear plasmid DNA of yeast and these were then allowed to replicate in yeast. It has been noted that the addition of telomeric DNA helps the plasmid DNA to replicate as linear molecules showing thereby that telomeres are needed for replication.

Telemore consists of repetitive DNA of large Kilo bases and are highly conserved containing clusters of G residues. The telomere sequences of mammals, including human, axe AGGGTT. In Tetrahymena, the sequences is GGGGTT.

Molecular studies show that the telomere sequences of a large number of eukaryotes are similar consisting of repeats of DNA sequences preferably clusters of G residues. The sequence of telomere repeats in human is AGGGTT, in Terahymean it is GGGGTT (Table 13.1). These telomeric sequences are repeated hun­dreds or telomeric replication thousands of times up to several kilo-bases.

(b) Telomeric Replication:

DNA polymerase has the capacity to synthesise a growing DNA chain in 5′ → 3′ direction but cannot synthesise up to telomeric ends. The replication process in telomere is unique and different.

The problem of replication of telomeres has been done by a special mecha­nism with the help of an enzyme telomerase having reverse transcriptase activity. This enzyme (telomerase) is able to add telomeric repeats at 3′ end of the DNA strand forming a single-stranded overhang at the 3′ end of both template and new strand.

Hence the 5′ end of each strand is shorter than 3′ end. Now the telomerase molecule incorporates an essential RNA molecule called Guide RNA at 5′ end which has specific sequences that are complementary to the telomere repeat.

It then serves as a primer for telomere at 5′ end of the strand. When the elongation of the strand at 5′ end is complete—i.e., two ends of the strand are equal—then the splicing of RNA primer takes place and the gap is filled up by the polymerase.

The control mechanism of the elongation of length of the telomere is not clearly known. But in case of yeast, it has been found that a special type of protein, called Rap 1p, has played an important role in regulating telomere length. It binds to the yeast telomere sequence and the elongation stops. The special mechanism of telomeric replication was first explained by C. Greider and E. Blackburn in 1986.


3. Prokaryotic cells are haploid, meaning they do not have chromosomes that occur in homologous pairs.

Most prokaryotic cells have just one chromosome, so they are classified as haploid cells (1n, without paired chromosomes). Even in Vibrio cholerae, which has two chromosomes, the chromosomes are unique from one another. That is, they are not a homologous pair, because they don’t contain the same genes in the same locations.

Many prokaryotes, such as bacteria, reproduce via binary fission. This is a method of asexual reproduction that is similar in its end result to mitosis—two daughter cells result, each with the same number of chromosomes as the parent cell. However, when bacteria undergo binary fission, no mitotic spindle forms. In addition, the replication of the prokaryotic cell’s chromosome can occur during the fission process.


2. Eukaryotic chromosomes are located within the nucleus, whereas prokaryotic chromosomes are located in the nucleoid.

The key difference between prokaryotic and eukaryotic cells is that eukaryotic cells have a membrane-bound nucleus (and membrane-bound organelles), whereas prokaryotic cells lack a nucleus. In eukaryotic cells, all the chromosomes are contained within the nucleus. In prokaryotic cells, the chromosome is located in a region of the cytoplasm called the nucleoid, which lacks a membrane.

One interesting implication of this difference in the location of eukaryotic and prokaryotic chromosomes is that transcription and translation—the processes of creating an RNA molecule and using that molecule to synthesize a protein—can occur simultaneously in prokaryotes. This is possible because prokaryotic cells lack a nuclear membrane, so transcription and translation occur in the same region. As the RNA is being transcribed, ribosomes can begin the translation process of stringing together amino acids. In contrast, in eukaryotic cells, transcription always occurs first, and it takes place within the nucleus. The RNA molecule needs to undergo editing before it leaves the nucleus. Then, translation is conducted by a ribosome in the cytoplasm.


Do prokaryotic chromosomes have centromeres? - Biology

When comparing prokaryotic cells to eukaryotic cells, prokaryotes are much simpler than eukaryotes in many of their features (Figure 1). Most prokaryotes contain a single, circular chromosome that is found in an area of the cytoplasm called the nucleoid.

Practice

Figure 1. A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid.

In eukaryotic cells, DNA and RNA synthesis occur in a separate compartment from protein synthesis. In prokaryotic cells, both processes occur together. What advantages might there be to separating the processes?

The size of the genome in one of the most well-studied prokaryotes, E.coli, is 4.6 million base pairs (approximately 1.1 mm, if cut and stretched out). So how does this fit inside a small bacterial cell? The DNA is twisted by what is known as supercoiling. Supercoiling means that DNA is either under-wound (less than one turn of the helix per 10 base pairs) or over-wound (more than 1 turn per 10 base pairs) from its normal relaxed state. Some proteins are known to be involved in the supercoiling other proteins and enzymes such as DNA gyrase help in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus (Figure 2). At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer (an eight protein complex). The DNA (which is negatively charged because of the phosphate groups) is wrapped tightly around the histone core. This nucleosome is linked to the next one with the help of a linker DNA. This is also known as the “beads on a string” structure. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage, the chromosomes are at their most compact, are approximately 700 nm in width, and are found in association with scaffold proteins.

In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. The tightly packaged region is known as heterochromatin, and the less dense region is known as euchromatin. Heterochromatin usually contains genes that are not expressed, and is found in the regions of the centromere and telomeres. The euchromatin usually contains genes that are transcribed, with DNA packaged around nucleosomes but not further compacted.

Figure 2. These figures illustrate the compaction of the eukaryotic chromosome.


Dark centers of chromosomes reveal ancient DNA

Geneticists exploring the dark heart of the human genome have discovered big chunks of Neanderthal and other ancient DNA. The results open new ways to study both how chromosomes behave during cell division and how they have changed during human evolution.

Centromeres sit in the middle of chromosomes, the pinched-in "waist" in the image of a chromosome from a biology textbook. The centromere anchors the fibers that pull chromosomes apart when cells divide, which means they are really important for understanding what happens when cell division goes wrong, leading to cancer or genetic defects.

But the DNA of centromeres contains lots of repeating sequences, and scientists have been unable to properly map this region.

"It's the heart of darkness of the genome, we warn students not to go there," said Charles Langley, professor of evolution and ecology at UC Davis. Langley is senior author on a paper describing the work published in an upcoming issue of the journal eLife.

Langley and colleagues Sasha Langley and Gary Karpen at the Lawrence Berkeley Laboratory and Karen Miga at UC Santa Cruz reasoned that there could be haplotypes -- groups of genes that are inherited together in human evolution -- that stretch over vast portions of our genomes, and even across the centromere.

That's because the centromere does not participate in the "crossover" process that occurs when cells divide to form sperm or eggs. During crossover, paired chromosomes line up next to each other and their limbs cross, sometimes cutting and splicing DNA between them so that genes can be shuffled. But crossovers drop to zero near centromeres. Without that shuffling in every generation, centromeres might preserve very ancient stretches of DNA intact.

The researchers looked for inherited single nucleotide polymorphisms -- inherited changes in a single letter of DNA -- that would allow them to map haplotypes in the centromere.

They first showed that they could identify centromeric haplotypes, or "cenhaps," in Drosophila fruit flies.

That finding has two implications, Langley said. Firstly, if researchers can distinguish chromosomes from each other by their centromeres, they can start to carry out functional tests to see if these differences have an impact on which piece of DNA is inherited. For example, during egg formation, four chromatids are formed from two chromosomes, but only one makes it into the egg. So scientists want to know: Are certain centromere haplotypes transmitted more often? And are some haplotypes more likely to be involved in errors?

Secondly, researchers can use centromeres to look at ancestry and evolutionary descent.

Turning to human DNA, the researchers looked at centromere sequences from the 1000 Genomes Project, a public catalog of human variation. They discovered haplotypes spanning the centromeres in all the human chromosomes.

Haplotypes from half a million years ago

In the X chromosome in these genome sequences, they found several major centromeric haplotypes representing lineages stretching back a half a million years. In the genome as a whole, most of the diversity is seen among African genomes consistent with the more recent spread of humans out of the African continent. One of the oldest centromere haplotype lineages was not carried by those early emigrants.

In chromosome 11, they found highly diverged haplotypes of Neanderthal DNA in non-African genomes. These haplotypes diverged between 700,000 to a million years ago, around the time the ancestors of Neanderthals split from other human ancestors. The centromere of chromosome 12 also contains an even more ancient, archaic haplotype that appears to be derived from an unknown relative.

This Neanderthal DNA on chromosome 11 could be influencing differences in our sense of smell to this day. The cells that respond to taste and smell carry odorant receptors triggered by specific chemical signatures. Humans have about 400 different genes for odorant receptors. Thirty-four of these genes reside within the chromosome 11 centromere haplotype. The Neanderthal centromeric haplotypes and a second ancient haplotype account for about half of the variation in these odorant receptor proteins.

It's known from work by others that genetic variation in odorant receptors can influence sense of taste and smell, but the functional effects of the variation found in this study are yet to be discovered and their impact on taste and smell analyzed.


Do prokaryotic chromosomes have centromeres? - Biology

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

  • Describe the process of binary fission in prokaryotes
  • Explain how FtsZ and tubulin proteins are examples of homology

Prokaryotes, such as bacteria, produce daughter cells by binary fission. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms, daughter cells are individuals.

To achieve the outcome of cloned offspring, certain steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells the cytoplasmic contents must also be divided to give both new cells the cellular machinery to sustain life. As we’ve seen with bacterial cells, the genome consists of a single, circular DNA chromosome therefore, the process of cell division is simplified. Karyokinesis is unnecessary because there is no true nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter cell. This type of cell division is called binary (prokaryotic) fission.

Binary Fission

Due to the relative simplicity of the prokaryotes, the cell division process is a less complicated and much more rapid process than cell division in eukaryotes. As a review of the general information on cell division we discussed at the beginning of this chapter, recall that the single, circular DNA chromosome of bacteria occupies a specific location, the nucleoid region, within the cell ((Figure)). Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus no nucleosomes in prokaryotes. The packing proteins of bacteria are, however, related to the cohesin and condensin proteins involved in the chromosome compaction of eukaryotes.

The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The starting point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane ((Figure)). Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ (short for “filamenting temperature-sensitive mutant Z”) directs the partition between the nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. A septum is formed between the daughter nucleoids, extending gradually from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.

Figure 1. These images show the steps of binary fission in prokaryotes. (credit: modification of work by “Mcstrother”/Wikimedia Commons)

Evolution Connection

Mitotic Spindle Apparatus

The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo karyokinesis and therefore have no need for a mitotic spindle. However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules which make up the mitotic spindle fibers that are necessary for eukaryotic nuclear division. FtsZ proteins can form filaments, rings, and other three-dimensional structures that resemble the way tubulin forms microtubules, centrioles, and various cytoskeletal components. In addition, both FtsZ and tubulin employ the same energy source, GTP (guanosine triphosphate), to rapidly assemble and disassemble complex structures.

FtsZ and tubulin are considered to be homologous structures derived from common evolutionary origins. In this example, FtsZ is the ancestor protein to tubulin (an evolutionarily derived protein). While both proteins are found in extant organisms, tubulin function has evolved and diversified tremendously since evolving from its FtsZ prokaryotic origin. A survey of mitotic assembly components found in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular eukaryotes ((Figure)).

Cell Division Apparatus among Various Organisms
Structure of genetic material Division of nuclear material Separation of daughter cells
Prokaryotes There is no nucleus. The single, circular chromosome exists in a region of cytoplasm called the nucleoid. Occurs through binary fission. As the chromosome is replicated, the two copies move to opposite ends of the cell by an unknown mechanism. FtsZ proteins assemble into a ring that pinches the cell in two.
Some protists Linear chromosomes exist in the nucleus. Chromosomes attach to the nuclear envelope, which remains intact. The mitotic spindle passes through the envelope and elongates the cell. No centrioles exist. Microfilaments form a cleavage furrow that pinches the cell in two.
Other protists Linear chromosomes wrapped around histones exist in the nucleus. A mitotic spindle forms from the centrioles and passes through the nuclear membrane, which remains intact. Chromosomes attach to the mitotic spindle, which separates the chromosomes and elongates the cell. Microfilaments form a cleavage furrow that pinches the cell in two.
Animal cells Linear chromosomes exist in the nucleus. A mitotic spindle forms from the centrosomes. The nuclear envelope dissolves. Chromosomes attach to the mitotic spindle, which separates the chromosomes and elongates the cell. Microfilaments form a cleavage furrow that pinches the cell in two.

Section Summary

In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome but no nucleus. Therefore, mitosis (karyokinesis) is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell wall material from the periphery of the cells results in the formation of a septum that eventually constructs the separate cell walls of the daughter cells.


Section Summary

In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and each copy is allocated into a daughter cell. The cytoplasmic contents are also divided evenly to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome and no nucleus. Therefore, mitosis is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell-wall material from the periphery of the cells results in a septum that eventually forms the separate cell walls of the daughter cells.