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ABOUT DISHA DISHA PUBLICATION PUBLICATION
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CELL CYCLE AND CELL DIVISIO The cell cycle is the life cycle of a cell. During its life cycle, cell grow, perform its assign functions, replicate, and divide. For eukaryotic cells, the cell cycle consists of two general phases: interphase and the mitotic phase. Cell division is part of the life cycle for almost every cell. It is a more complicated process in eukaryotic than prokaryotic cells. Eukaryotes have multiple chromosomes and many organelles, all of which must be duplicated and separated for the cell to divide. CELL CYCLE y
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The sequence of events by which a cell duplicates its genome, synthesises other cell constituents, and eventually divides into two daughter cells is known as cell cycle. The events of the cell cycle are under genetic control.
Phases Phase s of Ce Cell ll Cycl e
Duration ofsemester the cell cycle varies with from organism to organism, Master your Scribd and from cell to cell. Duration of the cell cycle in humans is 24 hrs, and in yeast & The New York Times is 90 min. y
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Special offer forCell students: Only $4.99/month. cycle consists of two phases: y
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Interphase (Non-dividing phase)
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Interphase involves a series of changes that for division. It is the period during which the cell experi and DNA replication in an orderly manner. Interphase is the longest period in the cell cycl more than 95% of the duration of the cell cy It is divided into 3 phases: G1 phase, S p phase ² G (Gap 1) phase: It is the rst phase of 1 is the stage during which the cell grows its DNA for replication. In this phas metabolically active. A lot of protein a synthesized during this phase. A larg nucleotides, amino acids for histone s energy rich compounds are formed. ² S phase (Synthetic phase): It is the stage DNA synthesis occurs. In this phase, t DNA (per cell) doubles, but the chromo remains the same. Each chromosome no Read Free Foron 30this Days Sign up to vote title two sister chromatids. The sisterchrom Useful by Not useful together a structure called a centrome Cancel anytime. now committed to division. ² G phase: In this phase, the cell conti 2
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G0 or quiescent phase: Is the stage wherein cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for replacing the cells lost during injury. Interphase is followed by a shorter phase of the cell cycle called the mitotic phase.
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M Phase y
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A cell reproduces during the mitotic phase. The mitotic phase has two stages: Karyokinesis and Cytokinesis. ² Karyokinesis: It involves nuclear division, producing two daughter nuclei. ² Cytokinesis: It involves cell division, producing two roughly equal cells called as daughter cells. The mitotic (M) phase of the cell cycle alternates with the much longer interphase.
Fig.
Diagrammatic representation of Interph phase
1. PROPHASE: It can be divided into three stag
(i) Early prophase ² Centrioles start moving towards opposit Length of a Cell Cycle nucleolus cell. Both the centriole pairs ra microtubular brils called astral rays. Eac The time it takes a cell to complete the cell cycle depends y rays along with centriole pair is called as on the type of cell that is dividing. ² The chromatin reticulum disappears. Most of the cells in the human body can complete the cell y ² Chromosomes appear as long threads. cycle in about 24 hours. ² Nucleus become less distinct Of this time: y (ii) Middle prophase ² The M phase would occupy < 1 hour You're Reading a Preview ² The chromatin condenses into chrom ² The S phase would last 10-12 hours (~ ½ the cell dehydrating and coiling. cycle) Unlock full access with a free trial. ² The chromosomes consist of two ² The G phase typically takes 4-6 hours 2 chromatids, joined together by a centrom ² The G phase usually occupies 5-6 hours. This phase 1 rst time, they can be seen with a light m is, however, the most variable in length in differentWith Free Trial Download (iii) Late prophase types of cells ² A structure known as the centrosome dupl MITOSIS (also known as equational di visi on) form two daughter centrosomes that migra ends of the cell. Mitosis was rst discovered in animal cells by Walter y ² The centrosomes organise the production o Fleming (1878). He also coined the term mitosis. Read Free Forspindle 30this Days Sign up to vote on title that form the bres that constitu Strasburger (1879) identied mitosis in plant cells y Useful Not useful spindle. Mitosis is called somatic cell division since it occurs most y Cancel anytime. ² The nuclear membrane breaks down to f in vegetative cells or somatic cells. Special offer forcommonly students: Only $4.99/month. of small vesicles and the nucleolus disint In plants, active centres of mitosis are root apex and stem y
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2. Metaphase
Biology the Cell Cycle Worksheet
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3. Anaphase
(i) Pro-metaphase ² The chromosomes, led by their centromeres, migrate to the equatorial plane in the midline of cell. This region of the mitotic spindle is known as the metaphase plate. ² Each chromatid now has a specialized protein structure located at the centromere called a kinetochore through which spindle bres attach. ² The chromosomes continue to condense. (ii) Metaphase ² It is the longest phase of the cell cycle. It last for about 20 minutes. ² The spindle fibres pull and push the duplicated chromosomes to the middle of the cell during metaphase. ² Notice in the gure that the chromosomes line up along the middle of the cell. This makes sure that each new cell will receive one copy of each chromosome.
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It is the shortest stage of mitosis. It has two sub stages, A and B. Both an anaphase B contribute to the movement of toward the spindle poles
Anaphase A: ² In anaphase A, chromosomes are pulled p ² In anaphase A, the centromere divide, s two sister chromatids from each other. ² Each chromatid is pulled toward the pole of their chromosome bres. ² During anaphasic movement of ch centromeres lead the arms. ² As a result of movement, chromosomes a I shapes based on the position of centrom ² Once they are separated, the sister chrom referred to as daughter chromosomes.
Anaphase B: ² In anaphase B, poles move apart. ² As the single-stranded chromosomes mo sides of the cell, the cell begins to get lon ² Anaphase ends when the two sets chromosomes reach opposite ends of the You're Reading a Preview ² Most spindle bres disappear from near Unlock full access with a free trial. remain intact near the middle. ² The number and type of chromosomes a same as present in the parent nucleus. Download With Free Trial ² It is the alignment and separation in m anaphase that is important in ensuring that cell receives a copy of every chromosom
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4
4. Telophase y
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During telophase, the spindle bres that helped divide chromosomes begin to disappear. The chromosomes begin to uncoil and form chromatin which together appears as reticulum. Nucleolus becomes visible again. Nuclear envelope is re-organised from RER by a process called nucleogenesis around each set of chromosomes at either pole of the cell. Two new identical nuclei forms.
5. Cytokinesis y y
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Cytok inesis in plants, which have cell wal completely different mechanism. ² During telophase, vesicles from the Go move to the middle of the cell, where they other to form a cell plate or middle lame ² Cell wall materials carried in the vesicles cell plate as it grows. ² The plate enlarges until its membranes plasma membrane at the perimeter. ² The contents of the vesicles form new cell between the daughter cells.
Cytokinesis completes the process of cell division. Cytokinesis is the process in which the cell actually divides into two. With the two nuclei already at opposite poles of the cell, the cell cytoplasm separates, and the cell pinches in the middle, ultimately leading to cleavage. The two cells formed are roughly equal in size. The process of cytokinesis is different in animal and plant cells.
Fig.
Cell plate formation in a plant ce
Signicance You're Reading a Previewof Mitosis
It is an equational division, and the two dau y Unlock full access with a free trial. in all respects. identical
They receive the same number and kind of c were in the mother cells. Download With FreeasTrial ² It is the only mode of multiplication i organisms. ² It is the process by which growth takes pla and plants by constantly adding more an Somatic cells are formed by mitosis. ²Read It maintains the genetic Free For 30 Days Sign up to vote on this titlecontinuity and e Fig. Cytokinesis helps in proper coordination among diffe Useful Not useful Cancel anytime. ² It also plays a role in repair by growth In animal cells, cytokinesis occurs by a process called y Special offer for students: Only $4.99/month. wound healing, regeneration of damage cleavage. the tail of lizard), and replacement of ce The rst sign of cleavage is the appearance of a cleavage y
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(ii) Zygotene (Gk. ‘Zygos’-pairing) Homologous chromosomes become closely a y to the development of nucleoprotein between The process of attachment of the homologous y to form a complex structure called synaptone y is known as synapsis. y Synapsis may be y ² Proterminal, starting from ends and prce centromeres. y ² Procentric, starting from centromeres an y towards ends. ² Random, at various places between centr Two synaptonemal complexes further form a c y bivalent or tetrad. y Bivalent or tetrad is a pair of synapsed y chromosomes (iii) Pachytene (Gk. ‘pachus’ - thick) It is the longest phase of prophase I. y The chromosome becomes shorter and th y Stages of Meiosis contraction. Meiosis consists of two divisions: During pachytene, each chromosome shows tw y (i) Meiosis I (Reduction Division): Meiosis I start after DNA so that bivalent shows 4 chromatids. Hence t has been replicated in S phase. During meiosis I, th e number pachytene tetrads. of chromosome is reduced to half. The homologous pairs The two chromatids belonging to the same y You're Reading a Preview of chromosomes get separated and cells become haploid. are called sister chromatids while chromatid The two chromatids of the chromosome become genetically the two different chromosomes of a homolo Unlock full access with a free trial. as non-sister chromatid. different due to crossing over. termed Meiosis I is followed by meiosis II at the end of which four Crossing-over occurs at the end of pachyten y haploid cells are formed. break Download With Free Trialand exchange of parts (genes) betwe (ii) Meiosis II (Equational division): During meiosis II the chromatid i.e. chromatids of a homologous p sister chromatids of each chromosome are separated (as Crossing is mainly responsible for the genet y in mitosis). It is called equational division because the Crossing over is the exchange of genetic ma y chromosome number remains the same as produced after two homologous chromosomes with the he the end of the rst division. recombinase. Read Free Foron 30this Days Sign up to title The point of vote interchange and rejoining appe y Meiosis I and is known as chiasma or points of crossin Useful Not useful Like mitosis, meiosis also consists of four stages; prophase, CrossingCancel overanytime. results in recombinations of y Special offer for students: Only $4.99/month. metaphase, anaphase and telophase. which ultimately lead to evolution. (iv) Diplotene (Gk. ‘Diplous’-double) y
Meiosis occurs in reproductive cells, e.g. in the testes of male and in the ovaries of female animals; in the pollen mother cell of the anthers (male organs) and in the megaspore mother cells of the ovary (female organ) of the owers. Cells which undergo meiosis are called meiocytes. It involves sequential cycle of two nuclear division called Meiosis-I and Meiosis-II but only a single cycle of DNA replication takes place. It results in four haploid daughter cells. The cells produced by meiosis are not all alike. Each cell is unique and this variation is produced by two processes – independent segregation of the chromosomes and by crossing-over. Both take place during Meiosis I. Importance of meiosis: Every living organism has a denite number of chromosomes in its body cells. Therefore to keep the chromosome number constant, the reproductive cells of the parents (ovaries and testis in animals, and pollen mother cells and cells of ovary in plants) divide through meiosis.
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6 y
y
The homologous chromosomes of a bivalent move apart from each other by a phenomenon called terminalisation. Movement of intermediary chiasmata to ends of chromosomes due to repulsion between homologous chromosomes is called terminalisation.
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Nu clea r me mbra ne an d nu cleo lus di diakinesis. Formation of spindle also gets completed b diakinesis.
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Segregation of homologous chromosomes during anaphase I is called disjunction. Random segregation of chromosomes during anaphase I is y a cause for recombinations of non-linked genes. The number of chromosomes is reduced to half by the end y of anaphase - I. Each set of chromosomes that moves to one pole consists y of a mixture of paternal and maternal chromosome parts (new gene combination). Telophase-I The separated chromosomes form two haploid nuclei by y the end of telophase - I. The daughter cells are now called haploid (n) as they have y one set of chromosomes. The nucleolus reappears and nuclear membrane reforms. y The daughter nuclei begin the second meiotic division. y y
Interkinesis y
Biology the Cell Cycle Worksheet
It is the time gap between telophase of meiosis I and prophase of meiosis II. Chromosomes elongate but there is no formation of chromatin reticulum. Synthesis of RNA and protein may take place. Centrosome or centriole pairs undergo replication.
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The formation of spindle apparatus gets com Each chromosome gets connected with bot y poles by chromosome bres that develop surfaces of its centromere. Anaphase II The centromere in each chromosome divide y that there is one centromere for each chroma The chromatids get their centromere and bec y chromosomes and begin to move towards poles. At the end of anaphase II, four groups of chro y produced, each having haploid number. Telophase II On reaching the poles the chromosomes organi y into haploid daughter nuclei. The nucleolus and the nuclear membrane rea y Chromosomes elongate to form chromatin n y Spindle bres degenerate during telophase II y y
Cytokinesis
It can be of two types: Successive and simul In successive type, cytokinesis occurs tw meiosis -I and meiosis - II. y ² In simultaneous type, cytokinesis occu y meiosis II. You're Reading a Preview Meiosis II Meiosis results in four haploid cells. y It has the same four stages; Prophase II, Metaphase II, with a free trial. Unlock full access Signicance of Meiosis Anaphase II, Telophase II. Meiosis maintains the chromosome number fr y Though it is similar to mitosis, but meiosis II is not mitosis Download With FreetoTrial generation. It reduces the chromosome num because: that the process of fertilisation restores the It always occurs in haploid cells. y in the zygote. It is not preceded by DNA replication. y Meiosis occurs during gamete formation (ge y The daughter cells formed at the end of meiosis II is neither y and reduces the number of chromosomes from similar to each other nor to the parent cell. to haploid (n) in the gametes. These haploid g Prophase II Read Free Foron 30 Days Signdiploid up to vote this title form zygote during fertilization. The d The chromosomes shorten and reappear. The two chromatids y Not useful develops into a normal diploid individual. Useful are attached to the single centromere. Cancel anytime. Variations are caused by the cross-over an y formation spindle starts. Special offeryforThe students: Onlyof$4.99/month. distribution of homologous chromosomes betw Nucleolus and nuclear membrane begin to disappear. y y
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188
There is no pairing of chromosomes, crossing-over, or chiasmata- In the zygotene stage of prophase, the pairing of chromos formation during prophase. During pachytene, the crossing-over occurs. The chiasmata the diplotene stage. Synaptonemal complex is not formed.
Synaptonemal complex is formed during the zygotene stag
Anaphase involves the separation of the chromatids of each chromosome.
During anaphase I, the homologous chromosomes separa chromatids remain attached at their centromeres. During anaphase II, the chromatids separate as a result of th the centromere.
Mitosis plays a signicant role in the healing, repair, and growth of a cell.
Meiosis brings about variation and maintains the chromosom generation to generation.
In this way, the organism ends its diploid phase several haploid cells. These cells undergo m There are three types of meiosis, depending upon the stage when mitotically) to form either larger, multicellul meiosis occurs: Gametic meiosis, zygotic meiosis and sporic or more haploid cells. Two opposite types of meiosis. male and female) from these individuals or 1. Gametic meiosis become a diploid zygote. In gametic meiosis, the diploid zygote undergoes mitosis to y In the whole cycle, zygotes are the only diploi y produce multicellular diploid cells. Cells from the diploid occurs only in the haploid phase. individuals then undergo meiosis to produce haploid 3. Sporic meiosis gametes. In sporic meiosis mitosis occur in both the diplo y The haploid gametes fuse and produce the diploid zygote y phases. The organism exhibits alternation with gametes of the opposite type. which features spore-producing multicellula In the whole cycle, gametes are the only haploid cells; y (which are diploid) and gamete-producing You're Reading a Preview mitosis occurs only in the diploid phase. gametophytes (which are haploid). Diagrama 2. Zygotic meiosis meiosis Unlock full access with a free trial. looks like the complex halves of ga A zygotic meiosis is meiosis of a diploid zygote immediately y and zygotic meiosis are merged into one. after karyogamy, the fusion of two cell nuclei.
Types of Meiosi s
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