Chapter 7 The Cell Cycle and Cell Division

kinetochores

A specialized structure on a centromere to which microtubules attach. Kinetochores are attachment sites for microtubules and are critical for chromosome movement.

In addition to the condensing of chromosomes, two other events during prophase contribute to the orderly segregation of the replicated DNA:

(1) centrosome orientation and (2) spindle formation.

Binary Fission and Sexual Reproduction

-Binary fission and mitosis result in daughter cells with the same number of chromosomes as their parent cells. -Sexual reproduction, however, requires a process of cell division in which the number of chromosomes is halved.

DNA Segregation

-Replication begins near the center of the cell, and as it proceeds, the ori regions move toward opposite ends of the cell; segregation and replication co-occur. -DNA sequences adjacent to the ori region bind proteins that are essential for this segregation. This is an active process involving proteins that bind specifically to DNA, and ATP is hydrolyzed, indicating this is an energy-dependent process. -Components of the prokaryotic cytoskeleton (see Figure 4.12) are involved in the segregation process. In particular, a bacterial protein that is structurally related to actin but functionally related to tubulin provides a filament along which the ori regions and their associated proteins move.

More about asymmetrical and symmetrical cells:

-Single-celled microbes are usually characterized by symmetrical or asymmetrical cell division. -Multicellular organisms, however, usually exhibit both types of cell division (FIGURE 7.1C). -While some cell divisions may appear symmetrical in terms of the size and shape of the daughter cells, there can be functional differences between the daughters, indicating that the division was actually asymmetrical.

centrosomes

-The major microtubule organizing center of an animal cell. In many eukaryotic organisms, structures called centrosomes are required for spindle formation. Centrosomes orient at both ends of the cell during prophase. A centrosome consists of a pair of centrioles, each one a hollow tube formed by nine triplets of microtubules.

Two regions of the prokaryotic chromosome play functional roles in DNA replication:

-ori: the site where replication of the circular chromosome starts (the origin of replication) -ter: the site where replication ends (the terminus of replication) Chromosome replication begins at the ori site and moves toward the ter site. When replication is complete, the two daughter DNA molecules separate and segregate from one another to opposite ends of the cell.

multicellular eukaryotes

-plants, animals, fungi Many multicellular eukaryotes, including fungi and plants and some animals, can also reproduce asexually. -Perhaps the most dramatic example of this is a forest containing thousands of aspen trees (Populus tremuloides) in the Wasatch Mountains of Utah -All the aspen trees in this picture are essentially identical genetically, indicating that they all arose from a single tree by asexual reproduction.

Two mechanisms may operate to move the chromosomes along.

1) First, the kinetochores contain molecular motor proteins, including kinesin and dynein, which may use energy from ATP hydrolysis to move the chromosomes along the microtubules. 2) Second, and probably more important, the microtubules attached to the kinetochore and pole shorten from both ends, drawing the chromosomes toward the poles.

clones

1) Genetically identical cells or organisms produced from a common ancestor by asexual means. (2) To produce many identical copies of a DNA sequence by its introduction into, and subsequent asexual reproduction of, a cell or organism.

As noted above, meiosis consists of two nuclear divisions, meiosis I and meiosis II. Two features characterize meiosis I:

1) Homologous chromosomes pair. During prophase I, homologous, replicated chromosomes pair with each other along their entire lengths. Pairing is facilitated by special proteins that glue the homologs together. No such pairing of homologs occurs in mitosis (although sister chromatids do remain together until anaphase). This homolog pairing remains through metaphase I. 2) There is no centromere division. Instead of centromeres dividing as in mitosis, in anaphase I the homologous chromosome pairs separate. Anaphase I begins when the glue holding homologs together loosens. Homologs separate and then move to opposite poles.

mutations

A change in the genetic material that is not caused by recombination. RE: Any genetic variations among the parent and offspring of asexual reproduction are due to rare genetic changes called mutations, which are alterations in DNA sequence

life cycle

A life cycle is the series of stages through which an organism passes from one generation to the next. All sexual life cycles involve meiosis (the process by which diploid cells divide to produce haploid cells) and fertilization (fusion of haploid cells to produce diploid cells).

centrioles

A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.

somatic cell

A somatic cell is any cell in the body of a multicellular organism that is not specialized for reproduction. Somatic cells are derived by mitosis.

sister chromatids

An unreplicated chromosome has two arms, whereas a replicated chromosome has four. The two essentially identical daughter DNA molecules are at this point called sister chromatids. -Each of a pair of newly replicated chromatids.

Anaphase I

Anaphase I is when the halving of chromosome number occurs; meiosis I is thus sometimes called the reduction division. A cell that begins with a diploid number of replicated chromosomes produces two cells after meiosis I, each of which has a haploid number of replicated chromosomes.

DNA SEGREGATION

Because eukaryotes have multiple chromosomes, segregation of the two sets of DNA molecules produced by replication is somewhat more complicated than it is in prokaryotes. -When a cell divides, one copy of each chromosome in the parental cell must end up in each of the two new cells—for example, each new somatic cell in a human will have all 46 chromosomes. -Chromosomes become highly condensed before they segregate, which makes them easier to move into the two new nuclei. -The spindle, a special cytoskeletal structure composed of microtubules, is involved in moving chromosomes during segregation.

spindle

Array of microtubules emanating from both poles of a dividing cell during mitosis and playing a role in the movement of chromosomes at nuclear division. Named for its shape.

Since the origin of sexual reproduction, evolution has generated many different versions of the sexual life cycle.

At the two extremes, either the diploid zygote immediately undergoes meiosis and essentially all of the life cycle is haploid (the haplontic life cycle), or the haploid gametes immediately undergo fertilization and essentially all the life cycle is diploid (the diplontic life cycle). -Between these extremes are haplo-diplontic life cycles in which the organism spends significant time in both haploid and diploid stages.

homologous pairs

Because the two chromosomes in each pair are nearly identical in length and DNA sequence, they are called homologous pairs. -A pair of matching chromosomes made up of a chromosome from each of the two sets of chromosomes in a diploid organism. -One of the two sets of chromosomes comes from the organism's maternal parent (mother) and the other comes from its paternal parent (father). Thus, for each chromosome pair, one homolog is paternal and the other is maternal. -The two chromosomes in a homologous pair (called homologs of one another) bear corresponding, though not identical, genetic information.

7.2 In order for any cell to divide, the following four events must occur:

Cell division signals, DNA replication, DNA segregation, Cytokinesis

S phase

Cells in S phase (synthesis phase) of the cell cycle are actively replicating their DNA.

G1 (or G0) phase

Cells that are preparing to undergo cell division are in G1 phase of the cell cycle. G1 is the phase in which most eukaryotic cells are found, and this is where they carry out their normal functions. Cells that are not yet dividing are in an arrested state termed G0. Cells remain in G0 or G1unless they receive appropriate signals to enter the next phase of the cell cycle.

first stage of mitosis

Chromosomes begin to condense, and then, in fully condensed prophase chromosomes, specialized protein structures called kinetochores appear on each side of the centromere, one per chromatid.

Chromatin

DNA and its associated proteins together are termed chromatin. -The nucleic acid-protein complex that makes up eukaryotic chromosomes.

M phase

Finally cells enter M phase, which is when DNA segregation in mitosis and cytokinesis occur. Together, G1, S, and G2 are termed interphase; the cell cycle can be partitioned into M phase and interphase.

Each of the two centrosomes

Each of the two centrosomes serves as a pole from which tubulin dimers aggregate into microtubules that extend from the poles into the middle region of the cell, forming the spindle. -The spindle forms during prophase and prometaphase, when the nuclear envelope breaks down. -The microtubules are initially unstable, constantly forming and falling apart, until they contact kinetochores or microtubules from the other half spindle and become more stable.

Cell Division Signals

External factors such as environmental conditions and nutrient concentrations are common signals for prokaryotes. -For example, the bacterium Bacillus subtilis can divide every 30 minutes under ideal conditions. But when nutrients in its environment are low, it stops dividing. It then resumes dividing when conditions improve.

asymmetrical

In asymmetrical cell division, mother and daughter cells can usually be distinguished from one another by size: the mother cell produces a daughter cell and remains essentially unchanged. -Asymmetrical cell division is found in organisms such as yeast, which is the organism used to brew beer and leaven bread (FIGURE 7.1A). -It is also common in more complex eukaryotes, such as plants and animals.

As noted above, in eukaryotes cell division occurs within the context of the cell cycle. The eukaryotic cell cycle can be divided into four phases:

G1 (or G0), S, G2, and M

Gametes

Gametes contain only a single set of chromosomes—that is, the chromosomes of gametes do not have homologs. -Accordingly, gametes are said to be haploid.

DNA Replication

In most prokaryotic cells, almost all of the genetic information is carried on one single, circular chromosome.

haploid

Having a chromosome complement consisting of just one copy of each chromosome; designated 1n1n or n.n.(Contrast with diploid.) -During sexual reproduction, two haploid gametes fuse to form a diploid zygote in a process called fertilization.

histones

Histones have amino acids with positively charged side chains on the surfaces that interact with negatively charged DNA (recall the phosphate groups in the backbone of each strand in the helix.

during M phase

However, during M phase, chromosomes are fully condensed and visible after staining with special dyes. Early in M phase, replicated chromosomes consist of two DNA molecules (and their associated packaging proteins), which are held together at the centromere, a structure used to orient and move chromosomes during cell division.

The Karyotype of a Human Female

Humans have 23 pairs of homologous chromosomes, giving 46 total.

G1 of the cell cycle

In G1 of the cell cycle, each chromosome in the nucleus consists of a long linear DNA sequence. Each DNA molecule is many times longer than the diameter of the nucleus. -To fit, DNA is heavily compacted by being wrapped around special proteins. This packaging is highly organized and involves many different proteins, the most common of which are the histones.

G1, S, and G2,

In G1, S, and G2, chromosomes are relatively uncondensed and thus difficult to distinguish with a microscope. (From a distance, it is easier to see a wound-up spool of thread than a single strand.)

Animals cytokinesis

In animals, cytokinesis usually begins with a furrowing of the cell membrane, as if an invisible thread were cinching the cytoplasm between the two nuclei (FIGURE 7.10A), similar to binary fission in bacteria. -This contractile ring is composed of microfilaments and other proteins, including a myosin motor protein, which form a ring on the cytoplasmic surface of the cell membrane. -Cytokinesis in animals (A) differs from cytokinesis in plants (B), because of the plant cell wall.

symmetrical

Symmetrical cell division is seen in bacteria and several single-celled eukaryotes such as Paramecium (FIGURE 7.1B).

prokaryotes

In prokaryotes, cell division results in reproduction: one individual becomes two. -A dividing cell first grows in size and replicates its DNA, and then it separates the cytoplasm and segregates its DNA into two new cells in the process called binary fission

interphase

In the cell cycle, the period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. During interphase the cell is most active in transcribing and translating genetic information.

Later in M phase

Later in M phase, toward the end of mitosis, the two sister chromatids are separated into two chromosomes consisting of a single DNA molecule and its associated packaging proteins. For a given organism, the number and sizes of the condensed chromosomes constitute the karyotype.

CELL DIVISION SIGNALS

Like prokaryotes, single-celled eukaryotes often undergo cell division when environmental conditions are appropriate. -For multicellular eukaryotes, the internal environmental conditions experienced by cells are often suitable for cell division. -However, the fact that environmental conditions are suitable does not necessarily mean that it occurs. The reason is that cell division signals in multicellular eukaryotes are related to the function of the entire organism. -For example, there is no need to produce additional liver cells when the liver has reached its optimal size. Once a cell is stimulated to begin dividing, internal signals come into play to ensure that cell division occurs correctly.

Meiosis

Meiosis is the type of cell division that results in the production of daughter cells (gametes) that contain only half the DNA of the original cell. Meiosis thus increases genetic diversity, which is the raw material for natural selection and evolution. Sexual reproduction involving meiosis occurs only in eukaryotes. The majority of eukaryotes have a sexual (or meiotic) life cycle.

eukaryotes

Mitotic cell division in eukaryotes, as in prokaryotes, results in identical daughter cells. However, the details are quite different.

G2 phase

Once DNA synthesis is complete, cells enter G2 phase, where they prepare for mitosis.

multicellular organisms

Organisms composed of many cells (Ex. tissue, humans, etc.) -However, in multicellular organisms, many rounds of cell division can occur before reproduction, and only a subset of cells (in humans, sperm and egg cells) are involved in reproduction. -The multicellular adult derives from many cell divisions starting from a single cell.

Z-ring

Protein fibers on the inside of a prokaryotic cell membrane in the middle of the cell that tightens, causing cytokinesis.

haplontic

Referring to a life cycle in which the mature organism is haploid and the zygote is the only diploid stage.

diplontic

Referring to a life cycle in which the organism is diploid and the gametes are the only haploid stage.

haplo-diplontic

Referring to a life cycle in which the organism spends significant time in both the haploid and diploid stages.

Sexual reproduction

Reproduction in which the genes of two individuals are combined to produce offspring, typically involving the union of male and female gametes.

binary fission

Reproduction of a prokaryote by division of a cell into two comparable progeny cells. -Most single-celled prokaryotes reproduce by binary fission, a type of asexual cell division that produces two genetically identical daughter cells from a parent cell.

mitosis

Similarly, single-celled eukaryotes (such as yeast) can reproduce asexually through a type of cell division termed mitosis, which also produces two genetically identical daughter cells. -In addition to being used during asexual reproduction in single-celled eukaryotes, mitosis is the type of cell division used to add new cells in a multicellular eukaryote.

eukaryotic cells

The DNA in eukaryotic cells is organized into multiple structures called chromosomes. -Each chromosome consists of a double-stranded molecule of DNA and associated proteins. -Although all cells of multicellular organisms contain DNA, not all body cells are specialized for reproduction.

Independent Assortment Before INFO

The DNA of a eukaryote is packaged into chromosomes, and the number of chromosomes varies with species. -Clearly, the more the chromosome pairs there are, the lower the probability that a product of meiosis will have all of the chromosomes that were present in one of the parental gametes.

Cytokinesis

The actual division of a single cell and its contents into two cells begins soon after DNA segregation begins. -Initially there is a pinching in of the cell membrane caused by the contraction of a ring of fibers, the Z-ring, on the inside surface of the membrane (similar to a drawstring on shorts being slowly tightened). -The major component of these fibers is structurally similar to tubulin (the protein that makes up eukaryotic microtubules), but its function is analogous to that of actin. -As DNA segregation proceeds, the membrane pinches in more, and once the daughter DNA molecules are fully segregated, new cell wall materials are deposited, which finally separate the two cells.

Some regions of DNA on a chromosome are always highly condensed, while others are less condensed.

The advantage of a highly condensed chromosome is that it can move more easily, without getting entangled with other chromosomes or material in the cell. The disadvantage is that other proteins are unable to interact with it.

DNA replication

The creation of a new strand of DNA in which DNA polymerase catalyzes the exact reproduction of an existing (template) strand of DNA.

Cytokinesis

The division of the cytoplasm of a dividing cell. (Contrast with mitosis.)

zygote

The fertilized egg. The cell created by the union of two gametes, in which the gamete nuclei are fused. The earliest stage of the diploid generation. -In multicellular eukaryotes, the diploid zygote then undergoes many mitotic cell divisions, resulting eventually in the mature organism. The chromosome number in the diploid zygote is denoted by 2n, and in the haploid gamete by n, where n is the number of different (non-homologous) chromosomes.

Telophase

The final phase of mitosis or meiosis during which chromosomes become diffuse, nuclear envelopes re-form, and nucleoli begin to reappear in the daughter nuclei. -During telophase, a nuclear envelope forms around each set of new chromosomes, and the chromosomes become less compact. The spindle also disappears at this stage, as a result of depolymerization of microtubules. As a result, there are two genetically identical nuclei in a single cell.

first stage of mitosis, prophase

The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies.

Asexual reproduction

The formation of new individuals without the union of genetic material from two different parents. Often takes place by budding or fragmentation. The offspring are genetically identical to their parent

gametes

The mature sexual reproductive cell: the egg or the sperm.

karyotype

The number, forms, and types of chromosomes in a cell. -Each chromosome has a particular length, and the centromere is located at a particular position along its length. - For example, humans have 46 chromosomes (23 homologous pairs) that differ in their sizes and centromere positions

Cell division signals

The one or more signals required to initiate cell division. The signals may originate from either inside or outside the cell.

prometaphase

The phase of nuclear division that begins with the disintegration of the nuclear envelope. -As the cell enters prometaphase, the spindle becomes fully formed, the nuclear envelope breaks down, and the replicated chromosomes become attached at their kinetochores to microtubules coming from the centrosome at the poles of the cell.

chromosome arm.

The portion of the DNA molecule on each side of the centromere is called a chromosome arm -In linear, eukaryotic chromosomes, the part between a centromere and a telomere.

positions of the centrosomes

The positions of the centrosomes determine the plane at which the cell divides; therefore they determine the spatial relationship between the two new cells that result from cell division. This relationship may be of little consequence to single free-living cells such as yeasts, but it is important for development in a multicellular organism.

CYTOKINESIS

The process of cytokinesis in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls).

centromere

The region where sister chromatids join.

7.1 cell division

The reproduction of a cell to produce two new cells. In eukaryotes, involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).

DNA segregation

The separation of two DNAs formed by replication into two new cells during cell division.

diploid

The somatic cells of many familiar organisms, including most vascular plants and animals, are diploid, meaning they each contain two paired sets of chromosomes. -RE: Having a chromosome complement consisting of two copies (homologs) of each chromosome. Designated 2n. (Contrast with haploid.) -The chromosomes in each set are numbered 1, 2, 3, and so on and each chromosome differs from another in the same set in both length and the DNA sequences they contain.

anaphase

The stage in cell nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologs) occurs. -At the end of anaphase, one of the sister chromatids from each of the replicated chromosomes that entered mitosis is present at each pole, indicating that the same genetic information is present at each pole.

metaphase

The stage in nuclear division at which the centromeres of the highly supercoiled chromosomes are all lying on a plane (the metaphase plate) perpendicular to a line connecting the division poles. -Once all of the chromosomes are aligned, the cell is ready for the next stage of mitosis. -Note that each replicated chromosome aligns on the metaphase plate independently of its homolog. This is different than what we will see in meiosis

fertilization

The union of gametes.

After meiosis I ends, meiosis II can begin.

There may be considerable time—even months to years!—between the two meiotic divisions. During the time between meiosis I and II, chromosomes may become less condensed than they were during meiosis I. But there is no replication of chromosomal DNA. -However, centrosome replication does occur, allowing new spindles to be formed in each cell.

DNA REPLICATION

Unlike prokaryotes, eukaryotes have more than one chromosome. While the mechanics of DNA replication are essentially the same in eukaryotes and prokaryotes, there are two major differences. 1) First, eukaryotic DNA replication starts at numerous origins of replication, not at just one as in prokaryotes. 2) Second, DNA replication occurs during a specified part of the cell cycle and does not overlap with the segregation of DNA into daughter cells.

When the time for reproduction of an organism comes, it may be accomplished through two basic strategies:

asexual reproduction or sexual reproduction. Some organisms are capable of both.

Cell division may be:

asymmetrical or symmetrical

Production of a haploid cell is required for sexual reproduction

in which two haploid cells (gametes) fuse during fertilization.

As mentioned above, however, the four daughter cells produced by meiosis differ from one another genetically. The shuffling of genetic material occurs by two processes:

independent assortment and crossing over.

Meiosis consists of two nuclear divisions:

meiosis I and meiosis II -that together produce four nuclei, each with the haploid number of unreplicated chromosomes -Although the nucleus divides twice during meiosis, the DNA is replicated only once, during S phase of the cell cycle. Unlike the products of mitosis, the haploid cells produced by meiosis are genetically different from one another and from the parent cell. -Meiosis replaces mitosis in the eukaryotic cell cycle in diploid cells that are destined to produce haploid daughter cells.

Although mitosis is a continuous process in which each event flows smoothly into the next, it is convenient to subdivide it into a series of stages:

prophase, prometaphase, metaphase, anaphase, and telophase

In a diploid sexual organism, every somatic cell has two sets of chromosomes (2n):

one set derived from its male parent (paternal chromosomes), the other from its female parent (maternal chromosomes). -If the organism is multicellular, all of its cells are derived from many mitotic divisions of the initial zygote.

single-celled organisms

prokaryotes and eukaryotes In single-celled organisms, cell division and reproduction (the production of offspring) are equivalent.

Meiosis II is indistinguishable from mitosis:

replicated chromosomes condense and attach to the spindle by their two kinetochores (prophase II), unpaired replicated chromosomes align on the metaphase II plate, centromeres divide and sister chromatids move to opposite poles (anaphase II), and the nuclear envelope re-forms and chromosomes uncondense (telophase II). -Following cytokinesis, the products of meiosis are four cells, each containing the haploid number of unreplicated chromosomes. Importantly, these four cells are not genetically identical.

The fact that the genetic material is packaged into multiple pairs of chromosomes has an important consequence:

the cells formed after meiosis (which you know from earlier in this chapter are or will be gametes) differ from one another because of random alignment of homologous chromosomes on the metaphase I plate.

During S phase

the centrosome becomes duplicated, and at the G2-M transition, the two centrosomes separate from one another, and eventually function as the "poles" toward which chromosomes move during segregation.

Cytokinesis

the division of the cell's cytoplasm, is the final stage of cell reproduction. This process occurs differently in plants and animals.

Mitosis refers only to

the division of the nucleus.

Each replicated chromosome is attached on both sides of its centromere:

the kinetochore on one side attaches to one pole, and the kinetochore on the other side attaches to the other pole.

In eukaryotes, cell division occurs within the context of the cell cycle:

the phases through which a single eukaryotic cell passes to produce daughter cells by cell division.

In addition to providing more cells for the growing organism, cell division is important for maintaining functional integrity:

when cells are damaged or die, cell division of neighboring cells often provides replacements.