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Lesson: Cell Cycle - Mitosis and Meiosis
Glossary
Allele: Alternate forms of a gene.
Allele: Alternate forms of a gene.
Anaphase: Third phase of mitosis in which sister chromatids separate and move to opposite sides of the cell.
Cell cycle: the stages between one cell division and the next.
Cell Plate: The wall material laid between the dividing plant cells during cytokinesis.
Centromere: Region of a chromosome where sister chromatids are joined together.
Chiasmata: the site of crossing over between the non sister chromatids of the homologous chromosomes. The chromosomes appear joined at the chiasmata when they begin separating at the beginning of the diplotene.
Chromatid: the duplicated chromosomes formed during S phase. Each dividing chromosome therefore has a pair of chromatids.
Chromatin: A complex of nucleic acids and proteins found in the nucleus.
Chromosome: compacted chromatin in M phase
Cohesins: a protein complex that holds the two chromatids together until they are separated in anaphase.
Crossing over (genetic recombination): the breakage, exchange and reunion of segments of homologous chromosomes.
Cytokinesis: Division of the cytoplasm.
Gametophyte: the haploid phase of life cycle of an organism.
Karyokinesis: Division of the nucleus.
Kinetochore: A complex of that assembles on the centromere and to which the microtubules of the spindle attach.
M Phase: the dividing phase of the cell cycle that includes
metaphase: Second phase of mitosis in which the chromosomes are aligned in the center of the cell.
Microtubules: Components of the cytoskeleton. Polymers of protein tubulin that associate together as heterodimers of Ī± and Ī² tubulins that form hollow cylinders. Supportive in function.
Nuclear lamina: the meshwork of intermediate filaments under the nuclear membrane.
Fuction: support and attachment.
Nucleus: the organelle containing the genetic material in eukaryotic cell.
Prometaphase: the phase of mitosis in which the spindle attaches to the kinetochores and the chromosomes move towards the centre of the spindle axis.
Prophase: Initial phase of mitosis in which chromosomes condense, the nuclear envelope dissolves and the spindle begins to form.
Sister chromatids: Two identical copies of a chromosome.
Spindle: Microtubules involved in the arrangement and sorting of chromosomes during cell division.
Spindle: Structure that helps separate the sister chromatids during mitosis; also separates homologous chromosomes during meiosis.
Telophase: Final phase of mitosis in which a nuclear envelop forms around each of the two sets of chromosomes
Table of Contents
Chapter: Cell Cycle
Glossary
Introduction
Phases of cell cycle
Interphase
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Meiosis
Stages of meiosis
Meiosis I
Meiosis I
Meiosis II
Cytokinesis
Significance of meiosis
Meiosis in animals: Oogenesis and Spermatogenesis Summary
Exercise/ Practice
Introduction
One of the characteristic features of living organisms is their capacity to grow and reproduce. As the cells grow in size, the limited capacity to expand as the cells increase in size to accommodate the increasing contents necessitates the cell to divide. Cells arise by division of existing cells (“Omnis cellula e cellula”, Rudlof Virchow, 1955). Cell division plays an important role in -
1. single celled prokaryotes, for e.g. amoeba that divided asexually by a simple process of binary fission.
2. growth and development of multicelled eukaryotic organisms. The multicelled organisms begin their life from a single celled zygote that divides and differentiates to assume the form and function of an adult
3. the continuous growth and renewal of cells in multicelled eukaryotes
There are two types of cell division:
1. Mitosis – is a type of cell division involved in the development of a single celled zygote into an adult organism, growth and repair of tissues and in asexual reproduction. In mitosis the parent cell divides into two daughter cells that are genetically identical to parent cell,i.e. the chromosome number is same to parent cell. The fidelity of the process ensures the heritable transmission of traits essential for maintaining the continuity of life.
2. Meiosis - by which the germ cells divide to form gametes. Each parent cell produces four daughter cells in which the chromosome number is reduced to half. Meiosis involves genetic recombination by the process of crossing over.
1. Mitosis – is a type of cell division involved in the development of a single celled zygote into an adult organism, growth and repair of tissues and in asexual reproduction. In mitosis the parent cell divides into two daughter cells that are genetically identical to parent cell,i.e. the chromosome number is same to parent cell. The fidelity of the process ensures the heritable transmission of traits essential for maintaining the continuity of life.
2. Meiosis - by which the germ cells divide to form gametes. Each parent cell produces four daughter cells in which the chromosome number is reduced to half. Meiosis involves genetic recombination by the process of crossing over.
Phases of cell cycle
The life of a cell from the time it is formed to its division is called as cell cycle. The cell cycle consists of several well-coordinated phases – growth of the cell, replication of DNA, distribution of replicated chromosomes between the two daughter cells and cell division. The entire cell cycle in a eukaryotic cell as shown by a cultured human cell takes 24 hrs to complete. This is divided in a long interphase (23 hrs) which is further divisible into three phases- G1, S and G2, and short divisional stage- mitosis (1 hr).
The duration of the four phases of cell cycle differs in various cell types. In a human cell which is rapidly proliferating the total 24 hr cycle has a 11 hr G1 phase, 8 hrs S phase, G2 of about 4 hrs and M phase of 1 hr. The cell cycle is considerably shorter in rapidly dividing cells like Saccharomyces cerevisiae (90 min) and early embryo cells (30 min). The latter has no gap phase and has a very short S phase.
Interphase – Before the cell enters into cell division the chromosomes in the nucleus are replicated in the interphase. The interphase is the period in between the dividing stages of mitosis. The cell spends 95% of the time in this phase. Interphase is divided into G1, S and G2 phases. As viewed in the microscope the interphase nucleus appears to be uniform as the chromatin is highly dispersed (decondensed) throughout the entire nucleus.
The G1 (Gap1) phase that follows mitosis corresponds to the metabolically active stage with abundant protein synthesis required for the subsequent S phase. The cell continues to grow throughout the interphase with the DNA replication occurring in the S (synthetic phase).
The replication of DNA in the S phase is followed by the G2 (Gap 2) phase in which there is cell growth and proteins synthesized in preparation for the M phase.
The G1 (Gap1) phase that follows mitosis corresponds to the metabolically active stage with abundant protein synthesis required for the subsequent S phase. The cell continues to grow throughout the interphase with the DNA replication occurring in the S (synthetic phase).
The replication of DNA in the S phase is followed by the G2 (Gap 2) phase in which there is cell growth and proteins synthesized in preparation for the M phase.
Mitosis
Mitosis comes from a Greek word mitos meaning thread. The term was coined in 1882 by Walther Flemming. Mitosis is divided in two phases:
Karyokinesis- division of the nucleus involving the equal distribution of replicated into two nuclei
Karyokinesis- division of the nucleus involving the equal distribution of replicated into two nuclei
Cytokinesis- partitioning of the cytoplasm into two daughter cells. Mitosis results in formation of two daughter cells genetically identical to the mother cell. The number of chromosomes remains the same (unlike meiosis where the chromosome number is halved). Mitosis occurs in both the haploid cells (e.g. fungi, plant gametophytes and animals like bees) and diploid cells for the growth and maintenance of the organism.
Karyokinesis is divided into several stages
Prophase Metaphase Anaphase Telophase
Karyokinesis is divided into several stages
Prophase Metaphase Anaphase Telophase
Prophase
The chromatin in the interphase occurs in a highly dispersed state, which is important for the replication and transcription to occur. However as the cell prepares to divide the chromatin undergoes compaction or condensation to form chromosomes. During prophase the chromosomes become condensed and the preparation for spindle attachment begins with key proteins binding to kinetochores.
The condensation is mediated by two groups of proteins called topoisomerase II and condensins. The topoisomerase II is a part of the nuclear matrix scaffold and uses the energy from ATP to untangle and condense the two sister chromatids. The proteins condensins bind the DNA of a chromatid at several sites and the coils and loops are formed by the twisting of the DNA.
Role of the proteins cohesins and condensins in the organization of mitotic chromosomes
The organization of chromosomes in the nucleus is mediated by a group of proteins – structural maintenance of chromosomes (SMC) proteins. Cohesins and condensins are two SMCs identified initially by their roles in restructuring chromosomes during mitosis. The protein cohesion holds the two sister chromatids together by forming a ring around thethem. Cohesins thus form topological linkages between different sites on the same DNA molecule as well as between the sister chromatids.
‘Condensins facilitate sister chromatids separation by resolving the catenation (topological linkages) between the two sister chromatids and compacting the chromatin.
The organization of chromosomes in the nucleus is mediated by a group of proteins – structural maintenance of chromosomes (SMC) proteins. Cohesins and condensins are two SMCs identified initially by their roles in restructuring chromosomes during mitosis. The protein cohesion holds the two sister chromatids together by forming a ring around thethem. Cohesins thus form topological linkages between different sites on the same DNA molecule as well as between the sister chromatids.
‘Condensins facilitate sister chromatids separation by resolving the catenation (topological linkages) between the two sister chromatids and compacting the chromatin.
A significant process that occurs in prophase is the breakdown of the nuclear envelope. This results from the depolymerization of nuclear lamins that underlie the inner nuclear membrane following their phosphorylation by cyclin B-Cdk. Interestingly most of the organelles remain intact and are partitioned between the two daughter cells.
Characteristics of Prophase:
1. Chromosomes condense
2. Centrioles if present divide and migrate to the poles
3. Kinetochore assembles
4. Cytoskeleton disassembled and spindle forms
5. ER and Golgi complex fragment and the nuclear envelope begins to disperse
Prometaphase
During prometaphase the nuclear envelope completely breaks down and the mitotic spindle is formed. The chromosomes attach to the microtubules in the spindle with their kinetochores.
The segregation of chromosomes is carried out by mitotic spindle – made up of microtubules that push the poles apart and pulls the chromosomes towards the poles. There are three types of microtubules:
1. Astral microtubules- these are arranged around the centrosome and help in positioning of the spindle apparatus in the cell and also in cytokinesis. 2. Chromosomal microtubules-these are a group of 20-30 microtubules that connect each of the centrosomes with the kinetochore. During metaphase these help positioning the chromosomes in between at the equatorial plane. In anaphase these shorten and are responsible in pulling the two chromatids apart towards the poles. 3. Polar microtubules- these extend from the centrosomes in between the chromosomes and overlap with their counterparts from the opposite poles.
Prometaphase is marked by the completion of spindle assembly and the movement of the chromosomes towards the center of the cell. As mentioned earlier each of the mitotic chromosomes are composed of two sister chromatids formed as a result of replication in the S phase of the interphase. The two sister chromatids are joined together at the centromere (primary constriction). The centromere is rich in highly repetitive sequences and serves as the site of attachment of spindle fibers.
During prophase a group of proteins assembles at the centromere forming a structure called kinetochore. Kinetochore serves as the site of attachment of the chromosomes to the microtubules of the spindle fibers. It also importantly controls the progression of cell cycle beyond the spindle assembly checkpoint.
In animal cells spindle assembly begins with the microtubules arranging around the centrosomes in a sunray like arrangements called as aster. This is followed by the separation of the two duplicated centrosomes towards the poles and forming the bipolar spindle. Microtubule associated motor proteins drive the movement of the chromosomes.
In case of plant cells that lack centrosomes the initial polymerization of spindle microtubules occurs close to the chromosomes. The motor proteins attached to the ends of these microtubules converge together forming a spindle pole. The process of spindle formation is therefore different in animals (centrosome-dependent) and plants (centrosome independent). Initially the microtubules of the spindle grow towards the center of the cell and contact the chromosomes via the kinetochore proteins. These chromosomes associated with the microtubules rather than move directly to the center of the cell oscillates back and forth in between the two poles and finally moves midway between the poles. The entire energy for the process is provided by the associated motor proteins.
Characteristics of Prometaphase:
1. Nuclear envelope dissolves
2. Chromosomes attach to the microtubules via their kinetochores.
3. Chromosomes move to the equator
Metaphase
The chromosomes finally align themselves at the central metaphase plate with the two chromatids attached via their kinetochores to the microtubules of the opposite pole.
Characteristic of metaphase:
1. The chromosomes align on the metaphase plate attached by the chromosomal microtubules.
1. The chromosomes align on the metaphase plate attached by the chromosomal microtubules.
Anaphase
Involves the separation of the two sister chromatids (now called as chromosomes) and their movement towards the poles. Early anaphase requires the dissolving of the connections between the two sister chromatids.
The microtubules gradually shorten and therefore pull the two chromatids apart. The chromosomes move with their centromere leading towards the poles and the arms of the chromosome trailing.
The movement of chromosomes in the direction of the poles is referred to as Anaphase A and the movement of spindle poles apart is called Anaphase B. Unlike the metaphase there is removal of tubulin dimers from the plus ends as well in anaphase resulting in the shortening of the spindle.
Characteristic of anaphase:
1. The centromeres split and the chromatids separate
2. Chromosomes move towards the poles
3. Spindle poles also move further apart
2. Chromosomes move towards the poles
3. Spindle poles also move further apart
Telophase
The chromosomes reach their respective poles. The mitotic spindle disassembles with the microtubules depolymerizing. The condensed chromosomes begin to disperse and assume their extended interphase state. The nuclear envelope reforms and the nucleolus reappear.
Characteristic of telophase:
1. Chromosomes reach the poles
2. Nuclear envelope reappears
3. Nucleolus reappears
4. Chromosomes uncoil
5. Spindle disappears
Cytokinesis
After the nuclear material has been equally distributed between the two daughter cells the cytoplasm divides in two by a process called cytokinesis. The process is markedly different in animals and plants. In animals the cell gradually develops an indentation that deepens and completely pinches the cell in two. The plasma membrane is formed by the cytoplasmic vesicles that fuse with the cleavage furrow. The central portion of the mitotic spindle forms a cytoplasmic bridge called the mid body between the two daughter cells. Finally the surfaces of the cleavage furrow fuses dividing the cell in two (abscission).
Doughland Marsland in 1950s proposed a mechanism – the contractile ring theory, responsible for cytokinesis. The force to cleave the cell is generated in a actin –myosin contractile machinery located in the plane where the cleavage furrow is formed. These filaments form a contractile ring and pulls the cortex and the attached plasma membrane towards the center of the cell. Thus constricting the equatorial region of the cell. This process resembles the drawing of a purse string narrowing the opening of the purse. The assembly of the contractile ring is mediated by a protein called RhoA that facilitates the assembly of the actin filaments as well as activation of the myosin II’s motor activity.
In contrast the plant cells divides in two by laying down wall materials in the center forming a phragmoplast. The phragmoplast consists of bundles of microtubules (remnants of mitotic spindle) oriented at right angles to the direction of the cell plate. It also has actin filaments, membranous vesicles and wall material. The vesicles form the Golgi apparatus carrying the wall matrix material is directed towards the site by the microtubules and where they coalesce and add to the growing cell plate. Cellulose and other crosslinking polysaccharides are added to these to make the framework of the mature cell wall.
Difference between mitosis in plant and animal cells
Plant Cells Animal Cells
1. Centrioles absent Centrioles present
2. Asters formed No asters formed
2. Asters formed No asters formed
3. Cytokinesis occurs by cell plate formation Cytokinesis occurs by furrowing or cleavage
4. Cell plate grows centrifugally Cleavage proceeds centripetally
4. Cell plate grows centrifugally Cleavage proceeds centripetally
Meiosis
The sexual life cycle of an organism includes the two important events of fertilization and meiosis that alternate with each other. Sexual reproduction involves the fusion of two gametes (n) – fertilization to form a zygote (2n). The production of gametes involves the process of meiosis (Gk. Word meion meaning reduction). The number of chromosome sets is reduced from diploid (2n) to haploid (n) in meiosis. Meiosis and fertilization therefore ensure the haploid and diploid phases of the life cycle of an organism.
The process of meiosis involves two sequential phases- Meiosis I (reductional division) and Meiosis II( equational division). The latter is considered equivalent to mitosis. There are different types of organisms that differ in phase of life cycle that meiosis occurs and also the duration of the different phases.
1. Gametic meiosis – gametes in multicelled organisms and protists takes place by meiosis. In animals the cells in the male gametangia called as the primary spermatocytes undergo meiosis to form four spermatids that mature to form specialized spermatozoa. In female vertebrates the primary oocyte enters meiosis and gets arrested in prophase. It grows in size and gets filled with yolk and reserve materials. Meiosis II is completed only after fertilization has taken place. Until then the sperm stays in the egg cytoplasm.
2. Zygotic meiosis- this is commonly seen in organisms where the dominant generation is represented by the gametophyte for e.g. in fungi, algae and protists. Following fertilization the zygote formed undergoes meiosis to form haploid spores. The spores divide by meiosis to form the dominant gametophytic generation.
3. Sporic meiosis- the plants with having both sporophytic and gametophytic body for (e.g. algae) in their life cycle. The meiotic divisions do not have any relation to gamete formation or fertilization.
Stages of meiosis
Meiosis I Prophase I is of a longer duration and further divided into five stages Leptotene- The first stage is leptotene in which the dispersed chromatin begins to condense forming thread like structures. The meiotic cells are generally larger than the surrounding tissues. The chromosomes are thin and long and difficult to distinguish individually. A series of dense granules called chromomeres occur at irregular intervals along their length. The chromomeres are charactereistic in number, size and position.
Zygotene - In zygotene the homologous chromosomes align together to form bivalents. This pairing is called synapsis and involves the formation of a complex called synaptonemal complex. This ladder like complex consists of transverse and lateral protein elements that form a mesh with which the loops of the chromatin of homologous chromosomes are attached. This paired arrangement of the chromosomes is called bivalents (two homologues) or tetrads (four chromatids).
Pachytene - The next stage pachytene is characterized by the formation of complete synaptenomal complex and crossing over involving genetic recombination. This exchange of genetic material between the homologous chromosomes generates genetic variability. The new combination of maternal and paternal alleles produces organisms with new phenotypes and genotypes.
Diplotene – Contraction of the bivalents and the repulsion of the homologous chromosomes are the characteristic features of diplotene. In diplotene the synaptonemal complex dissolves leaving the chromosomes attached at specific points called chiasmata (sing. chiasma). Chiasmata are the structures formed between the homologous chromosomes as a result of crossing over ( exchange of DNA segments) between the synapsed homologues. Each chiasma represents a single cross over event.
Diakinesis – this stage is characterized by the terminalization of chiasma (movement towards the ends of the chromosomes), assembly of the spindle, disappearance of the nucleolus and breakdown of the nuclear envelope. As the bivalents continue to contract and the repulsion between the homologues increases the chiasmata appears to move towards the ends of the chromosomes. The process is called terminalization of chiasmata.
Metaphase I – The bivalents are the maximum state of condensation as they arrange on the equatorial of the cell. Spindle fibers from the opposite poles connect the pair of homologous chromosomes. The bivalents then orient themselves on the spindle but instead of all the centromeres being on the equatorial plate as happens in mitosis each bivalent is located such that the two centromeres lie on either side of the plate. The maternal and paternal chromosomes of a bivalent are randomly oriented and have the equal likelihood of assorting to either pole. This results in a random collection of paternal and maternal chromosomes at each pole later.
Anaphase I- the chiasmata holding the homologues together dissolves and the homologues separate to move towards opposite poles. The sister chromatids arestill attached to each other and move together as a unit. The two centromeres of each bivalent remain undivided and their movement to opposite poles of the spindle causes the remaining chiasmata to slip off and free the homologues from each other. Each of the two poles therefore comes to have a reduced or haploid number of chromosomes. Unlike mitotic anaphase in which the chromosomes appear longitudinally single each chromosome has a pair of chromatids united at their centromeres. But it must be remembered that each of the chromatids is genetically different as a result of crossing over.
Telophase I – the intermediate stage between two meiotic divisions. The condensed chromosomes disperse but do not reach the highly extended state. The nuclear envelope may appear but not always. Each of the two cells formed at the end of meiosis I have one of the pair of the homologous chromosome and therefore half the number of chromosomes as the parent cell. After an interphase which depending upon the species may be long, short or completely absent the chromosomes in each of the two haploid cells enter into the next meiotic division. There is no further DNA replication.
Characteristics of Meiosis I
1. Produces to two haploid cells that has chromosomes with two sister chromatids
2. Prophase I- Pairing of homologous chromosomes and genetic recombination (crossing over)
3. Metaphase I – Bivalents align at the spindle equator
4. Anaphase I- Homologous chromosomes move to opposite poles.
5. Telophase I and cytokinesis- Two haploid cells are produced
1. Produces to two haploid cells that has chromosomes with two sister chromatids
2. Prophase I- Pairing of homologous chromosomes and genetic recombination (crossing over)
3. Metaphase I – Bivalents align at the spindle equator
4. Anaphase I- Homologous chromosomes move to opposite poles.
5. Telophase I and cytokinesis- Two haploid cells are produced
Meiosis II The second mitotic division is similar to a mitotic division and is often referred to as equational division. Each chromosome consists of two chromatids held together at the centromere. In meiosis II the centromere splits and separates into two chromatids (each now referred to as chromosome) one to each pole. The only difference with that of mitosis is the haploid number of cells.
Prophase II – nuclear envelope breaks down and the chromosomes again become compacted.
Metaphase II - the chromosomes align on the metaphase plate. However the positioning of the sister chromatids is opposite facing the poles and therefore become connected with the spindle fibers from the opposite poles.
Anaphase II - the sister chromatids separate with the splitting of the centromeres that holds them together allowing them to move towards the opposite poles of the cell.
Telophase II- the nuclear envelope and the nucleolus reappears and the chromosomes disperse to enter into interphase. Finally the division of the cytoplasm results in formation of four haploid cells with 1C DNA content and genetically distinct from each other and also from the parent cell.
Characteristic Meiosis II
1. Resembles mitotic division – equational division
2. Separates each chromosome into two chromatids
1. Resembles mitotic division – equational division
2. Separates each chromosome into two chromatids
Significance of meiosis
Meiosis as the basis of the independent assortment of chromosomes/genes.
In a sexually reproducing organism meiosis is a necessary part of the life cycle. It is the opposite of fertilization as regards to the number of chromosomes. The chromosome number is reduced to half in meiosis during gamete formation and is restored to the diploid number by fertilization. The other significant feature of meiosis is the reassortment of maternal and paternal genetic material so that the resulting gametes carry new combinations of genes. The recombination results from
o The independent segregation of chromosomes at the first meiotic division and
o Crossing over- the exchange of genetic material between paired homologous chromosomes during the first meiotic prophase.
Meiosis in animals: Oogenesis and Spermatogenesis
In sexually reproducing forms the doubling of chromosomes alternates with the halving of chromosome numbers by meiosis. Sexual reproduction is based on the union of two monoploid cells – gametes- sperms and eggs. The formation of these gametes – gametogenesis (spermatogenesis- development of sperm; oogenesis- development of egg) involves meiosis. Spermatogenesis occurs in the male reproductive organ or testis where the spermatogonia or the sperm mother cells undergo meiosis. chromosome duplication and cell enlargement transform a spermatogonium into a primary spermatocyte. Meiosis I transforms one primary spermatocyte into two secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II so a total of four spermatids are formed. Each spermatid without further division develops a tail a neckpiece and other specialized features so that a total of four sperm are formed from each spermatogonium completing the process of spermatogenesis.
Oogenesis or the formation of sperm cells is similar in many ways to the formation of sperm cells. However there is a difference- cytoplasmic division is unequal in oogenesis resulting eventually in one large, functional egg cell (containing food reserves) and three small polar bodies.
Another important difference relates to the stage where the sperm and the egg cells acquire their specialized functions. The sperm cells generated after meiosis undergo development to loose their cytoplasm and develop flagella. Whereas the egg cells acquire their characteristic features during the process of meiosis itself. Many of the specialized features of the egg cell are acquired during the prophase I when the growth of the cell is temporarily halted.
Difference between mitosis and meiosis
Mitosis involves production of two daughter cells genetically identical to the parent cell and having the same number of chromosomes. Meiosis which occurs in sex cells includes two nuclear divisions and results in formation of four daughter cells with half the number of chromosomes. Additionally in meiosis the homologous chromosomes synapse and crossing over occurs between non sister chromatids resulting in genetic recombination that ensures the genetic variability.
Significance of errors in meiosis
Aneuploidy arises due to aberrant meiotic behavior – the failure of the homologous chromosomes to segregate properly- non-disjunction. It can arise in either of the two meiotic divisions.
Non-disjunction in the first division occurs when the two homologous chromosomes move to the same pole. This results in four cells arising from meiosis- two will be disomic (contain two copies of the same chromosome) and two will be nullisomic ( contain no copy of the chromosome) . Where two chromatids remain together at the anaphase II the tetrads contain two normal monosomic cells (containing one copy of the chromosome), one
nullisomic cell and one disomic cell. The resulting gametes thus have an incorrect number of chromosomes and produce defective embryos that do not survive. Down’s syndrome is an example where the disomic gamete (two copies of chromosome 21) participates in formation of embryo that survives but the resulting child exhibits developmental abnormalities like- short stature, low intelligence, fold over the eyes and broad hands amongst others.
Aneuploidy can also arise due to nondisjunction at an early mitosis in the embryo. This results in two different populations of cells – mosaic. This is common in Turner’s syndrome.
Meiosis in higher plants
In angiosperms or flowering plants meiosis occurs in two locations –
1. Pollen mother cells (PMC) located in the anther undergo meiosis to form microspores. The haploid nucleus of each microspore subsequently divides mitotically to give rise to a three nucleate male gametophyte. Two of the three nuclei are sperm nuclei that function in the double fertilization process.
2. Meiosis in the ovule located inside the ovary leads to the formation of megaspores. In 70% of the angiosperms meiosis in ovules leads to the formation of four megaspores, three of which disintegrate. The one functional megaspore divides mitotically forming two haploid nuclei which further divide resulting in four monoploid nuclei. Finally a third mitotic division produces eight monoploid nuclei which eventually become partitioned as cells within the female gametophyte. The female gametophyte is located inside the ovule and consists of one egg cell, two polar nuclei, three antipodal cells and two synergids.
Summary
The cell cycle refers to the events that a cell undergoes from one cell division to another. The cell cycle is divided into two main stages - Interphase and the M phase. Interphase is further divided into G1, S and G2. G1 is the phase that follows mitosis and is followed by S phase during which the DNA replication takes place. G2 is the phase that follows DNA
The cell cycle refers to the events that a cell undergoes from one cell division to another. The cell cycle is divided into two main stages - Interphase and the M phase. Interphase is further divided into G1, S and G2. G1 is the phase that follows mitosis and is followed by S phase during which the DNA replication takes place. G2 is the phase that follows DNA
synthesis and is followed by the divisional M phase. The duration of the various stages of the cell cycle varies according the type and developmental state of the cell.
Mitosis is a process that ensures that genetic material is completely equally distributed between the two daughter cells. Mitosis is divided into prophase, metaphase, anaphase and telophase. In prophase the interphase chromatin gets compacted into chromosome along with the assembly of the spindle. The kinetochore proteins assemble on the primary constriction –the centromere. The microtubules of the spindle attach at the kinetochore. The nuclear membrane disorganizes.
During prometaphase and metaphase the chromosomes attached to the spindle microtubules arrange themselves in the equatorial plane. The anaphase is characterized by the separation of the sister chromatids from each other and their movement towards the poles. This is followed by the chromosome assuming their dispersed interphase state in telophase. Division of the cytoplasm – cytokinesis occurs by formation of median furrow in animals and by cell plate formation on plant cells.
Meiosis occurs in germinal cells during gamete formation and also in organisms with gametophytic plant body (n) for e.g. some algae, fungi and protists. In meiosis the nuclear division includes two sequential stages that results in formation of haploid nuclei with half the number of chromosomes as the parent cell. Crossing over involving exchange of genetic material between the two non sister chromatids results in recombination of the genetic material.
Mitosis is a process that ensures that genetic material is completely equally distributed between the two daughter cells. Mitosis is divided into prophase, metaphase, anaphase and telophase. In prophase the interphase chromatin gets compacted into chromosome along with the assembly of the spindle. The kinetochore proteins assemble on the primary constriction –the centromere. The microtubules of the spindle attach at the kinetochore. The nuclear membrane disorganizes.
During prometaphase and metaphase the chromosomes attached to the spindle microtubules arrange themselves in the equatorial plane. The anaphase is characterized by the separation of the sister chromatids from each other and their movement towards the poles. This is followed by the chromosome assuming their dispersed interphase state in telophase. Division of the cytoplasm – cytokinesis occurs by formation of median furrow in animals and by cell plate formation on plant cells.
Meiosis occurs in germinal cells during gamete formation and also in organisms with gametophytic plant body (n) for e.g. some algae, fungi and protists. In meiosis the nuclear division includes two sequential stages that results in formation of haploid nuclei with half the number of chromosomes as the parent cell. Crossing over involving exchange of genetic material between the two non sister chromatids results in recombination of the genetic material.
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CHEMISTRY ASSIGNMENT ON THE TOPIC ELECTRONEGATIVITY, DIBORANE &ALLOTROPHY
CHEMISTRY ASSIGNMENT ON THE TOPIC ELECTRONEGATIVITY, DIBORANE &ALLOTROPHY
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