Cell cycle regulation - Mitosis

Question 1

Cell division

Answer 1

The continuity of life is based on the reproduction of cells, or cell division. This takes place in a process called mitosis. It happens in the steps, G2 of interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. The functions of cell division is reproduction, growth and development and tissue renewal.

Question 2

Cell cycle

Answer 2

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells. The cell cycle consists of the mitotic phase (M), interphase which consists of G1 and G2 (G for gap), after M comes G1, after G1 is the S phase (synthesis), after the S phase comes G2 and after G2 is the M phase again.

Question 3

Genome

Answer 3

A cells endowment of DNA, its genetic information, is called its genome. Although a prokaryotic genome is often a single DNA molecule, eukaryotic genomes usually consist of a number of DNA molecules. A typical human cell, has about 2 meters of DNA.

Question 4

Ekaryotic chromosomes

Answer 4

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into structures called chromosomes, so named because they take up certain dyes used in microscopy. Each eukaryotic chromosome consists of one very long, linear DNA molecule associated with many proteins. The DNA molecule carries several 100 to a few thousand genes, the units of information that specify an organisms inherited traits.

Question 5

Chromatin

Answer 5

Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin.

Question 6

Somatic cells

Answer 6

These are all body cells except the reproductive cells. They have 46 chromosomes in humans.

Question 7

Gametes

Answer 7

Reproductive cells, or gametes, sperm and eggs, have half as many chromosomes as somatic cells, a set of 23 chromosomes per gamete in humans.

Question 8

Sister chromatids

Answer 8

Each duplicated chromosome has 2 sister chromatids, which are joined copies of the original chromosome. The 2 chromatids, each containing an identical DNA molecule, are initially attached all along their lengths, by protein complexes called cohesions. This attachment is known as sister chromatid cohesion. Each sister chromatid has a centromere.

Question 9

Centromere

Answer 9

Each sister chromatid has a centromere, a region of the chromosomal DNA where the chromatid is attached most closely to its sister chromatid. This attachment, mediated by proteins bound to the centromeric DNA sequences and gives the condensed, duplicated chromosome a narrow “waist”. The part of a chromatid to each side of the centromere is referred to as an arm of the chromatid.

Question 10

Mitosis

Answer 10

It is the division of the genetic material in the nucleus, and it is usually followed immediately by cytokinesis. One cell has become 2, each the genetic equivalent of the parent cell. A particular human cell might undergo one division in 24 hours. Of this, the M phase would be less than 1 hour, while the S phase would last 10-12 hours. G1 5-6 hours and G2 4-6 hours.

Question 11

Cytokinesis

Answer 11

Mitosis is the division of the genetic material in the nucleus, and it is usually followed immediately by cytokinesis, the division of the cytoplasm. The division of the cytoplasm is usually well underway by late telophase, so the 2 daughter cells appear shortly after the end of mitosis. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.

Question 12

Mitotic (M) phase

Answer 12

Mitosis is just one part of the cell cycle, and it is usually the shortest. In the cell cycle it is called the mitotic phase or the M phase. After the M phase comes the first part of interphase which is called G1.

Question 13

Interphase

Answer 13

Interphase is divided into 3 parts. G1, S and G2. G1 follows the M phase and G2 follows the S phase. The G phases were misnamed gaps, when they were first observed, because the cells appeared inactive, but actually there is an intense metabolic activity and growth throughout interphase. It grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmatic reticulum.

Question 14

G1 phase

Answer 14

The G1 phase follows the M phase.

Question 15

S phase

Answer 15

The S phase follows the G1 phase and is where duplication of the chromosomes occur, which is crucial for the eventual division of the cell.

Question 16

Prophase

Answer 16

The chromatin fibers become more tightly coiled, condensing into discrete chromosomes, visible with a light microscope. The nucleoli disappear. Each duplicated chromosome appears as 2 joined sister chromatids. The mitotic spindle begins to form. It is composed of centrosomes and microtubules that extend from them. The array of shorter microtubules that extend from the centrosomes are called asters (stars). The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.

Question 17

Prometaphase

Answer 17

The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes are even more condensed. Each of the two chromatids now has a kinetochore, a specialized protein structure at the centromere. Some of the microtubules attach to the kinetochores becoming kinetochore microtubules, which start to pull the chromosomes back and forth. Nonkinetochore microtubules interact with those from the opposite spindle.

Question 18

Metaphase

Answer 18

The centrosomes are now at opposite poles of the cell. The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindles 2 poles. The chromosomes’ centromeres lie at the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

Question 19

Anaphase

Answer 19

Anaphase is the shortest stage of mitosis, often only lasting a few minutes. Anaphase begins when the cohesin proteins are cleaved. This allows the sister chromatid pair to separate suddenly. Each chromatid thereby becomes a full chromosome. The 2 liberated daughter chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 micrometer/minute). The cell elongates as the nonkinetochore microtubules lengthen. By the end of anaphase, the 2 ends of the cell have equivalent and complete collections of chromosomes.

Question 20

Telophase

Answer 20

Two daughter nuclei form in the cell. Nuclear envelopes arise from the fragments of the parent cells nuclear envelope and other portions of the endomembrane system. Nucleoli reappear. The chromosomes become less condensed. Any remaining spindle microtubules are depolymerized. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

Question 21

Mitotic spindle

Answer 21

Many of the events of mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This structure consists of fibers made of microtubules and associated proteins. While the mitotic spindle assembles, the other microtubules of the cytoskeleton disassemble, providing the materials used to construct the spindle. The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten by losing subunits.

Question 22

Centrosome

Answer 22

In animal cells, the assembly of spindle microtubules starts at the centrosome, a subcellular region containing material that functions throughout the cell cycle to organize the cell’s microtubules. It is also a microtubule organizing center. A pair of centrioles is located at the center of the centrosome, but they are not essential for cell division. They are not even present in plant cells which also form spindle.

Question 23

G2 phase

Answer 23

G2 follows the S phase. The nuclear envelope encloses the nucleus. The nucleus contains one or more nucleoli. 2 centrosomes have formed by duplication of a single centrosome. Centrosomes are regions in animal cells that organize the microtubules of the spindle. Each centrosome contains 2 centrioles. Chromosomes duplicated during the S phase cannot be seen individually because they have not yet condensed.

Question 24

Aster

Answer 24

An aster, a radial array of short microtubules, extends from each centrosome. The spindle includes the centrosomes, the spindle microtubules and the asters.

Question 25

Kinetochore

Answer 25

Each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure made up of proteins that have assembled on specific sections of chromosomal DNA at each centromere. The chromosomes 2 kinetochores face in opposite directions. During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called kinetochore microtubules.

Question 26

Metaphase plate

Answer 26

At metaphase, the centromeres of all the duplicated chromosomes are on a plane midway between the spindles two poles. This plane is called the metaphase plate, and it is an imaginary plate rather than an actual cellular structure.

Question 27

Cleavage

Answer 27

In animals, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow.

Question 28

Cleavage furrow

Answer 28

In animals, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. The contraction of the dividing cell’s ring of microfilaments is like pulling a drawstring. The cleavage furrow deepens until the parent cell is pinched in two.

Question 29

Contractile ring of microfilaments

Answer 29

On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract.

Question 30

Cell plate

Answer 30

Cytokinesis in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect inside the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell.

Question 31

Binary fission

Answer 31

Prokaryotes (bacteria and archaea) undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form 2 cells. The term binary fission, meaning (dividing in half) refers to this process. And to the asexual reproduction of single-celled eukaryotes, such as the amoeba. However, this process in eukaryotes involves mitosis, while that in prokaryotes, it does not.

Question 32

Origin of replication

Answer 32

In E. coli, the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. While the chromosome is replicating, the cell elongates. When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent E. coli cell into 2 daughter cells.

Question 33

Dinoflagellates

Answer 33

In unicellular eukaryotes called dinoflagellates, the chromosomes attach to the nuclear envelope, which remains intact during cell division. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a process reminiscent of bacterial binary fission.

Question 34

Diatoms and some yeasts

Answer 34

In two other groups of unicellular eukaryotes, diatoms and some yeasts, the nuclear envelope also remains intact during cell division. In these organisms, the microtubules from a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into 2 daughter cells.

Question 35

Cell cycle control system

Answer 35

Experiments have demonstrated that the sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. It is regulated by certain checkpoints.

Question 36

Checkpoint

Answer 36

The cell cycle is regulated by certain checkpoints. A checkpoint in the cell cycle is a control point where stop and go ahead signals can regulate the cycle. The signals are transmitted within the cell by signal transduction pathways. Animal cells typically have built in stop signals that halt the cell cycle at checkpoints until overridden by go ahead signals.

Question 37

G0 phase

Answer 37

For many cells, the G1 checkpoint, dubbed the restriction point, in mammalian cells, seems to be the most important. If a cell receives a go ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2 and M phases and divide. If it does not receive a go ahead signal at that point, it may exit the cycle, switching into a nondividing state called the G0 phase. Most cells in the human body are in the G0 phase.

Question 38

Growth factor

Answer 38

If all other conditions are favorable, most types of mammalian cells divide in culture only if the growth medium includes specific growth factors. A growth factor is a protein released by certain cells that stimulate other cells to divide. Different cell types respond specifically to different growth factors or combinations of growth factors.

Question 39

Density-dependent inhibition

Answer 39

The effect of an external physical factor on cell division is clearly seen in density-dependent inhibition, a phenomenon in which crowded cells stop dividing.

Question 40

Anchorage dependence

Answer 40

Most animals also exhibit anchorage dependence. To divide, they must be attached to a substratum, such as the inside of a culture flask or the extracellular matrix of a tissue.. Experiments suggest that like cell density, anchorage is signaled to the cell cycle control system via pathways involving plasma membrane proteins and elements of the cytoskeleton linked to them.

Question 41

Transformation

Answer 41

Cells in a culture that develops the ability to divide indefinitely are said to have undergone a process called transformation, causing them to behave (in cell division at least) like cancer cells. By contrast, nearly all normal non-transformed mammalian cells growing in culture divide only about 20 to 50 times before they stop dividing, age and die.

Question 42

Benign tumor

Answer 42

Abnormal cells may remain at the original site if they have too few genetic and cellular changes to survive at another site. In that case the tumor is called a benign tumor. Most benign tumors do not cause serious problems and can be removed by surgery.

Question 43

Malignant tumor

Answer 43

A malignant tumor includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs; based on their ability to divide indefinitely in a culture. These cells are also considered transformed cells.

Question 44

The growth and metastasis of a malignant breast tumor

Answer 44

A series of genetic and cellular changes contribute to a tumor becoming malignant (cancerous). The cells of malignant tumors grow in an uncontrolled way and can spread to neighboring tissues and via lymph and blood vessels, to other parts of the body.

Question 45

Metastasis

Answer 45

The spread of cancer cells to locations distant from their original site is called metastasis.