Cell Division: True things!

INTRODUCTION

Cell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division is known as mitosis, and leaves the daughter cell capable of dividing again. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization. Mitosis and meiosis (also known as reduction division) are different processes by which cells reproduce. Cells within a plant or animal are constantly undergoing these processes to replace worn-out cells, grow, and produce offspring. Humans have 46 chromosomes in each somatic (regular body) cell. Since that is double the number of chromosomes found in gametes (sex cells), we refer to it as the diploid number. The number of chromosomes found in gametes is 23. Since it is half the number in the somatic cell, it is called the haploid number of chromosomes. Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new “gamete”. Mitosis maintains the cell’s original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished. Meiosis was discovered and described for the first time in sea urchin
eggs in 1876, by noted German biologist Oscar Hertwig (1849-1922). It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846-1910), in Ascaris worms’ eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann (1834-1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911 the American geneticist Thomas Hunt Morgan (1866-1945) observed crossover in Drosophila melanogaster meiosis and provided the first true genetics. (http://en.wikipedia.org/wiki/Cell_division)

MITOSIS

Mitosis is the process of forming (generally) identical daughter cells by replicating and dividing the original chromosomes, in effect making a cellular xerox. Mitosis deals only with the segregation of the chromosomes and organelles into daughter cells. Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic chromosomes. The condensed replicated chromosomes have several points of interest. The kinetochore is the point where microtubules of the spindle apparatus attach. Replicated chromosomes consist of two molecules of DNA (along with their associated histone proteins) known as chromatids. The area where both chromatids are in contact with each other is known as the centromere the kinetochores are on the outer sides of the centromere. During mitosis replicated chromosomes are positioned near the middle of the cytoplasm and then segregated so that each daughter cell receives a copy of the original DNA (if we start with 46 in the parent cell, we should end up with 46 chromosomes in each daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus) to “pull” chromosomes into each “cell”. The microtubules have the 9+2 arrangement discussed earlier. Animal cells (except for a group of worms known as nematodes) have a centriole. Plants and most other eukaryotic organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell membrane assumes this function when it pulls the by-then replicated chromosomes apart during binary fission. Cells that contain centrioles also have a series of smaller microtubules, the aster, that extend from the centrioles to the cell membrane. The aster is thought to serve as a brace for the functioning of the spindle fibers. The phases of mitosis are sometimes difficult to separate. (http://en.wikipedia.org/wiki/Mitosis)

Phases of Mitosis:

Interphase:

The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division. Interphase is divided into three phases, G₁ (first gap), S (synthesis), and G₂ (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G₁), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G₂), and divides (M). (http://en.wikipedia.org/wiki/Mitosis)

Pre-prophase:

In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, pre-prophase is characterized by the formation of a ring of microtubules and actin filaments (called pre-prophase band) underneath the plasmamembrane around the equatorial plane of the future mitotic spindle and predicting the position of cell plate fusion during telophase. The cells of higher plants (such as the flowering plants) lack centrioles. Instead, spindle microtubules aggregate on the surface of the nuclear envelope during prophase. The pre-prophase band disappears during nuclear envelope disassembly and spindle formation in pro-metaphase. (http://en.wikipedia.org/wiki/Mitosis)

Prophase:

The nuclear envelope disassembles and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome. Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor. When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to “crawl” up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome’s two chromatids. When the spindle grows to sufficient length, kinetochore microtubules begin searching for kinetochores to attach to. A number of nonkinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle.Prometaphase is sometimes considered part of prophase. (http://en.wikipedia.org/wiki/Mitosis)

Prometaphase:

The nuclear envelope disassembles and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Fungi and some protists, such as algae or trichomonads, undergo a variation called closed mitosis where the spindle forms inside the nucleus or its microtubules are able to penetrate an intact nuclear envelope. Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor.When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to “crawl” up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome’s two chromatids. When the spindle grows to sufficient length, kinetochore microtubules begin searching for kinetochores to attach to. A number of nonkinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle.Prometaphase is sometimes considered part of prophase. (http://en.wikipedia.org/wiki/Mitosis)

Metaphase:

As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene along the metaphase plate or equatorial plane, an imaginary line that is equidistant from the two centrosome poles.This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Metaphase comes from the Greek
μετα meaning “after.” Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres) , it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint. (http://en.wikipedia.org/wiki/Mitosis)

Anaphase:

When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase (from the Greek
ανα meaning “up,” “against,” “back,” or “re-”). Two events then occur; First, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids turned sister chromosomes are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Next, the nonkinetochore microtubules elongate, pushing the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement. These two stages are sometimes called early and late anaphase. Early anaphase is usually defined as the separation of the sister chromatids, while late anaphase is the elongation of the microtubules and the microtubules being pulled farther apart. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations. (http://en.wikipedia.org/wiki/Mitosis)

Telophase:

Telophase (from the Greek
τελος meaning “end”) is a reversal of prophase and prometaphase events. It “cleans up” the after effects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell’s nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete. (http://en.wikipedia.org/wiki/Mitosis)

Cytokinesis:

Cytokinesis is often mistakenly thought to be the final part of telophase, however cytokinesis is a separate process that begins at the same time as telophase. Cytokinesis is technically not even a phase of mitosis, but rather a separate process, necessary for completing cell division. In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei.In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell.In plants this structure coalesces into a cell plate at the center of the phragmoplast and develops into a cell wall, separating the two nuclei. The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase. (http://en.wikipedia.org/wiki/Mitosis)

Significance and Importance of Mitosis:

• The importance of mitosis is the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.
• Transcription is generally believed to cease during mitosis, but epigenetic mechanisms such as bookmarking function during this stage of the cell cycle to ensure that the “memory” of which genes were active prior to entry into mitosis are transmitted to the daughter cells.
• The cells divide and replace old worn out cells and more importantly be able to replicate the duties of the cells they replace.
• Mitosis is important for growth to take place.
• Mitotic divisions enable a single cell to grow eg. from conception, repeated cell division has allowed us to developed into multicellular organisms.
• By duplicating the exact copy of our genetic material it ensures that our genetic material is stable and able to carry out its function correctly because the instructions from the previous cells would have been passed on to the new daughter cells.
• It is also important for cell replacement, regeneration.
• It is also important in asexual reproduction.

(http://en.wikipedia.org/wiki/Mitosis and )

Problems:

Although errors in mitosis are rare, the process may go wrong, especially during early cellular divisions in the zygote. Mitotic errors can be especially dangerous to the organism because future offspring from this parent cell will carry the same disorder. In non-disjunction, a chromosome may fail to separate during anaphase. One daughter cell will receive both sister chromosomes and the other will receive none. This results in the former cell having three chromosomes coding for the same thing (two sisters and a homologue), a condition known as trisomy, and the latter cell having only one chromosome (the homologous chromosome), a condition known as monosomy. These cells are considered aneuploidic cells and these abnormal cells can cause cancer. Mitosis is a traumatic process. The cell goes through dramatic changes in ultrastructure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Occasionally, chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing deletion. The fragment may incorrectly reattach to another, non-homologous chromosome, causing translocation. It may reattach to the original chromosome, but in reverse orientation, causing inversion. Or, it may be treated erroneously as a separate chromosome, causing chromosomal duplication. The effect of these genetic abnormalities depends on the specific nature of the error. It may range from no noticeable effect, cancer induction, or organism death. (http://en.wikipedia.org/wiki/Mitosis)

MEIOSIS

In biology or life science, meiosis (pronounced my-oh-sis or mee-oh-sis) is a process of reduction division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes. The word “meiosis” comes from the Greek verb meioun, meaning “to make small,” since it results in a reduction in chromosome number in the gamete cell. Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid
rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. Each cell has half the number of chromosomes as the parent cell. During meiosis, the genome of a diploid
germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.

In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. In these groups, gametes are produced by mitosis. Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes. (http://en.wikipedia.org/wiki/Meiosis)

Phases of Meiosis:

Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I. (http://en.wikipedia.org/wiki/Meiosis)

Meiosis I:

In meiosis I, the homologous pairs in a diploid cell separate, producing two haploid cells (46, N). The 46 chromosomes number is significant. A regular diploid cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromosomes it is only considered N because later in anaphase I the identical sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, a process similar to mitosis will occur whereby the sister chromatids are finally split, creating 2 haploid cells (23, N). (http://en.wikipedia.org/wiki/Meiosis)

Prophase I:

Homologous chromosomes pair and crossing over, or recombination, occurs–a step unique to meiosis. Chromosomes form structures called synapsis. The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata. (http://en.wikipedia.org/wiki/Meiosis)

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning “thin threads.” During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane. Hence, this stage is called “bouquet stage”. (http://en.wikipedia.org/wiki/Meiosis)

Zygotene:

The zygotene stage, also known as zygonema, from Greek words meaning “paired threads,” occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. The two homologous chromosomes become “zipped” together, forming the synaptonemal complex, in a process known as synapsis. (http://en.wikipedia.org/wiki/Meiosis)

The pachytene stage, also known as pachynema, from Greek words meaning “thick threads,” contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. (Sex chromosomes, however, are not identical, and only exchange information over a small region of homology.) Exchange takes place at sites where recombination nodules or chiasmata (singular: chiasma) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope. (http://en.wikipedia.org/wiki/Meiosis)

Diplotene:

During the diplotene stage, also known as diplonema, from Greek words meaning “two threads,”the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in Anaphase I.

In fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia exist until meiosis begins at puberty. (http://en.wikipedia.org/wiki/Meiosis)

Diakinesis:

Chromosomes condense further during the diakinesis stage, from Greek words meaning “moving through.”This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. (http://en.wikipedia.org/wiki/Meiosis)

Synchronous processes:

During these stages, centrosomes, each containing a pair of centrioles are migrating to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.

Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are also nonkinetochore microtubules. (http://en.wikipedia.org/wiki/Meiosis)

Metaphase I:

Homologous pairs move together along the phase plate: as kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate. (http://en.wikipedia.org/wiki/Meiosis)

Anaphase I:

Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one functional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further a part. The cell elongates in preparation for division down the middle. (http://en.wikipedia.org/wiki/Meiosis)

Telophase I:

The last meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. This effect produces a variety of responses from the neuro-synchromatic enzyme, also known as NSE. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Telophase I contains no nucleus, two daughter cells, and chromatids remain attached. (http://en.wikipedia.org/wiki/Meiosis)

Meiosis II:

Meiosis II
is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I. End result is production of four haploid cells (23,1N) from the two haploid cells (46,1N) produced in meiosis I. (http://en.wikipedia.org/wiki/Meiosis)

Prophase II:

It takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division. (http://en.wikipedia.org/wiki/Meiosis)

Metaphase II:

The centromeres contain two kinetochores, that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. (http://en.wikipedia.org/wiki/Meiosis)

Anaphase II:

Here the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. (http://en.wikipedia.org/wiki/Meiosis)

Telophase II:

The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the microtubules. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete. (http://en.wikipedia.org/wiki/Meiosis)

The core knowledge of Genetics:

Genetics is a biological subject deals with heredity and variations. Heredity means the study of inheritance. Again, inheritance means the transfer of biological properties from parents to offspring. Both inheritance and crossing-over (for which variation occurred) occurred during meiosis process (easily understand after reading above writings). So, it can be said the core knowledge of genetics.

Significance and Importance of Meiosis:

• Meiosis facilitates stable sexual reproduction.
• Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation.
• Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species.
• Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality. Polyploidy is poorly tolerated in animal species.
• Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring.
• Recombination and independent assortment allow for a greater diversity of genotypes in the population.
• As a system of creating diversity, meiosis allows a species to maintain stability under environmental changes.

(http://en.wikipedia.org/wiki/Meiosis)

Non-disjunction:

The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called non-disjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis. (http://en.wikipedia.org/wiki/Meiosis)

Genetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically connected and tend to segregate together during meiosis, and are thus genetically linked. Alleles for genes on different chromosomes are usually not linked, due to independent assortment of chromosomes during meiosis. Because there is some crossing over of DNA when the chromosomes segregate, alleles on the same chromosome can be separated and go to different daughter cells. There is a greater probability of this happening if the alleles are far apart on the chromosome, as it is more likely that a cross-over will occur between them. and Reginald Punnett shortly after Mendel’s laws were rediscovered. (http://en.wikipedia.org/wiki/Genetic_linkage)

Kinds of Linkage:

The phenomenon of linkage is of following two kinds-

1. Complete Linkage:

When the linked genes are so closely located in chromosomes that they inherit in same linkage groups for two or more generations in a continuous and regular fashion, then, they are called completely linked genes and the phenomenon of inheritance of completely linked genes is called complete linkage.

2. Incomplete Linkage:

The linked genes do not always stay together because homologue non-sister chromatids may exchange segments of varying length (which bearing many linked genes) with one another during meiotic prophase, by the process of crossing over. The linked genes which are widely located in chromosomes and have chances of separation by crossing over are called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.

Significance of linkage:

• Prevent from unexpected variation.
  
• Maintains its variability.
  
• Linkage has an important significance for analysing gene flow and selection in a two-locus system.
  
• The
  possible significance of linkage as a mechanism for permitting
  a population of “track” spatial changes in the environment is
  considered. The results are that when the recombination fraction
  between the loci is of the same order of magnitude as the selection
  coefficients or smaller, then linkage is important in determining
  the gene frequencies and a substantial amount of linkage disequilibrium
  is present in the cline. Depending on the spatial pattern of
  selection on the two loci, linkage can either decrease or increase
  a population’s response to local selection.
  

CROSSING OVER

Significance of crossing over:

1. Linear arrangement of genes: Crossing over clearly illustrates the linear arrangement of genes in the chromosomes.

2. Chromosome Maps: The frequency of crossing over is very useful to construct the chromosome maps.

3. Recombination: Crossing over produces new combination of genes.
4. Variations: Crossing over leads to genetic variation which is the raw material for evolution. (http://www.studentsguide.in/genetics/chromosome-crossing-over/significance-of-crossing-over.html)

Problems:

Although crossing over typically occurs between homologous regions of matching chromosomes, similarities in sequence can result in mismatched alignments. These processes are called unbalanced recombination. Unbalanced recombination is fairly rare compared to normal recombination, but severe problems can arise if a gamete containing unbalanced recombinants becomes part of a zygote. The result can be a local duplication of genes on one chromosome and a deletion of these on the other, a translocation of part of one chromosome onto a different one, or an inversion.

MUTATION

In biology, mutations are changes to the nucleotide sequence of the genetic material of an organism. Mutations can be caused by copying errors in the genetic material during cell division, by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses, or can occur deliberately under cellular control during processes such as hypermutation. A new mutation that was not inherited from either parent is called a de novo mutation. The source of the mutation is unrelated to the consequence, although the consequences are related to which cells are affected. Mutations create variations in the gene pool. Less favorable (or deleterious) mutations can be reduced in frequency in the gene pool by natural selection, while more favorable (beneficial or advantageous) mutations may accumulate and result in adaptive evolutionary changes. For example, a butterfly may produce offspring with new mutations. The majority of these mutations will have no effect; but one might change the color of one of the butterfly’s offspring, making it harder (or easier) for predators to see. If this color change is advantageous, the chance of this butterfly surviving and producing its own offspring are a little better, and over time the number of butterflies with this mutation may form a larger percentage of the population. Neutral mutations are defined as mutations whose effects do not influence the fitness of an individual. These can accumulate over time due to genetic drift. It is believed that the overwhelming majority of mutations have no significant effect on an organism’s fitness. Also, DNA repair mechanisms are able to mend most changes before they become permanent mutations, and many organisms have mechanisms for eliminating otherwise permanently mutated somatic cells. Mutation is generally accepted by the scientific community as the mechanism upon which natural selection acts, providing the advantageous new traits that survive and multiply in offspring or disadvantageous traits that die out with weaker organisms. (http://en.wikipedia.org/wiki/Mutation)

Significance of Mutation:

• Mutation is the ultimate source of variation. Without variation there could be no evolution, so mutations are of great importance to evolution.
• The process of speciation depends variously on it.

CONCLUSION

From this topic, we can understand the process of cell division (Mitosis and Meiosis) which is very important for a life. We can also know their (Mitosis and Meiosis) significance, importance and also problems or errors occurring in a life. We can know the related (with cell division) three terms linkage, crossing over and mutation with their importance and problems. As a student of life science, we must know about cell division which is the very important portion of Genetics (Important branch of biology). So, this writings help to know the process and significance of cell division in a life.

http://en.wikipedia.org/wiki/Cell_division

http://en.wikipedia.org/wiki/Mitosis

http://en.wikipedia.org/wiki/Meiosis

http://en.wikipedia.org/wiki/Genetic_linkage

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html

http://en.wikipedia.org/wiki/Chromosomal_crossover

http://en.wikipedia.org/wiki/Mutation

http://www.studentsguide.in/genetics/chromosome-crossing-over/crossing-over.html

http://www.studentsguide.in/genetics/chromosome-crossing-over/significance-of-crossing-over.html

Hasan, M. A., 2000. Ucchomaddhomic Jibobigwan, 1^st part: Udvid Bigwan. Hasan

Book House, 65 Paridash road, Banglabazr, Dhaka-1100. 152-163 pp.

Photo Album:

Fig.: Phases of Mitosis.

Fig.: Different phases of Mitosis and Meiosis.

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