genetica per scienze naturali a.a. 03-04 prof s. presciuttini cell division cell division is the...

Genetica per Scienze Naturalia.a. 03-04 prof S. Presciuttini

Cell Division Cell division is the basis for all forms of organismal Cell division is the basis for all forms of organismal

reproduction.reproduction. Single-celled organisms divide to reproduce. Cell division Single-celled organisms divide to reproduce. Cell division

in multicellular organisms produces specialized in multicellular organisms produces specialized reproductive cells, such as egg and sperm.reproductive cells, such as egg and sperm.

In order for a cell to divide, the genome must also divide, In order for a cell to divide, the genome must also divide, so, so, in all types of cell division in all organisms, DNA in all types of cell division in all organisms, DNA replication precedes cell divisionreplication precedes cell division..

CCell division can be grouped into asexual and sexual cell ell division can be grouped into asexual and sexual cell division.division.


Types of cell divisionIn prokaryotes there is only one simple In prokaryotes there is only one simple type of cell division, which produces type of cell division, which produces two identical daughter cells from one two identical daughter cells from one progenitor cellprogenitor cell ( (asexual cell divisionasexual cell division)). . Eukaryotes also show asexual cell Eukaryotes also show asexual cell divisiondivision; this; this converts a single fertilized converts a single fertilized egg cell, a egg cell, a zygotezygote, into a, into a multicellular multicellular organism, or a single unicellular organism, or a single unicellular organism intoorganism into a a population or a colonypopulation or a colony..The asexual cell division in The asexual cell division in eukaryoteukaryotes es is called is called mitosismitosis (M) (M). Both haploid (n) . Both haploid (n) and diploid (2n) cells can divide and diploid (2n) cells can divide asexually.asexually.The sexual cell division in The sexual cell division in eukaryoteukaryotes is es is called called meiosismeiosis (Mei) and occur in (Mei) and occur in specialized cells, specialized cells, the the meiocytemeiocytess, which, which divides twice, resulting in four haploid divides twice, resulting in four haploid cells called a tetrad.cells called a tetrad.


Life cycles of humans, plants, and fungi

IIn humans and many n humans and many plants, three cells of plants, three cells of the meiotic tetrad the meiotic tetrad abort. The abort. The abbreviation n abbreviation n indicates a haploid indicates a haploid cell, 2n a diploid cell, 2n a diploid cell; gp stands for cell; gp stands for gametophyte, the gametophyte, the small structure of small structure of haploid cells that haploid cells that will produce will produce gametes. gametes.

In flowering plants the gametophyte stage is radically reduced, but in others (such as In flowering plants the gametophyte stage is radically reduced, but in others (such as mosses) the gametophyte is the main vegetative stagemosses) the gametophyte is the main vegetative stage


G1, S, G2, M CCells spend most of their life in ells spend most of their life in interphaseinterphase, the period between nuclear divisions, the period between nuclear divisions,,

and comparatively little time in mitosis. and comparatively little time in mitosis. IInterphase is divided into three stages, nterphase is divided into three stages, G1G1, , SS, and , and G2G2..

In G1cells are growing and synthesizing the materials necessary for their proper In G1cells are growing and synthesizing the materials necessary for their proper functioning. The cells are "doing their thing", so if they are nose cells they are functioning. The cells are "doing their thing", so if they are nose cells they are producing mucus, if they are muscle cells they are contracting and relaxing, etc. producing mucus, if they are muscle cells they are contracting and relaxing, etc. Some cells, such as our nerve cells (neurons) and red blood cells, never leave this Some cells, such as our nerve cells (neurons) and red blood cells, never leave this stage and it is then called stage and it is then called G0G0. A cell which is in the G0 stage will not divide. It . A cell which is in the G0 stage will not divide. It will not grow, either, but will continue to function until it dies.will not grow, either, but will continue to function until it dies.

Cells which will divide pass through a specific phase in Cells which will divide pass through a specific phase in G1 which acts as a gateway into the S stage. Once cells G1 which acts as a gateway into the S stage. Once cells pass this "point of no return" they will proceed through pass this "point of no return" they will proceed through S, G2 and mitosis. S is the stage when S, G2 and mitosis. S is the stage when DNA synthesisDNA synthesis (chromosome replication) (chromosome replication) occurs. The chromosomes occurs. The chromosomes consist of two identical strands once replication is consist of two identical strands once replication is completed. Each of these strands is called a completed. Each of these strands is called a chromatidchromatid. . During mitosis the chromatids will separate and each During mitosis the chromatids will separate and each chromatid will become chromatid will become a separate chromosomea separate chromosome..


Mitosis Mitosis (M) is usually the shortest segment of the cell cycle, lasting for Mitosis (M) is usually the shortest segment of the cell cycle, lasting for

approximately 5 to 10 percent of the cycle. DNA synthesis takes place during the S approximately 5 to 10 percent of the cycle. DNA synthesis takes place during the S period. Gperiod. G11 and G and G22 are gaps between S and M. Together, G are gaps between S and M. Together, G11, S, and G, S, and G22 constitute constitute

interphase, the time between mitoses. (Interphase used to be called "resting period"; interphase, the time between mitoses. (Interphase used to be called "resting period"; however, cells are active in many ways during interphase, not the least of which, of however, cells are active in many ways during interphase, not the least of which, of course, is DNA replication.) The chromosomes cannot be seen during interphase, course, is DNA replication.) The chromosomes cannot be seen during interphase, mainly because they are in an extended state and are intertwined with one another mainly because they are in an extended state and are intertwined with one another (chromatine)(chromatine)..

For the sake of study, biologists divide mitosis into four stages called prophase, For the sake of study, biologists divide mitosis into four stages called prophase, metaphase, anaphase, and metaphase, anaphase, and ttelophase. It must be stressed, however, that any nuclear elophase. It must be stressed, however, that any nuclear division is a dynamic process on which we impose such arbitrary stages only for our division is a dynamic process on which we impose such arbitrary stages only for our own convenience.own convenience.

In each of the resultant daughter cells, the chromo-some complement is identical In each of the resultant daughter cells, the chromo-some complement is identical with that of the original cell. Of course, what were referred to as chromatids now with that of the original cell. Of course, what were referred to as chromatids now take on the role of full-fledged chromosomes in their own right.take on the role of full-fledged chromosomes in their own right.


The four stages of mitosis

ProphaseProphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase


Prophase The onset of mitosis is The onset of mitosis is characterizedcharacterized by the chromosomes becoming distinct. They by the chromosomes becoming distinct. They

get progressively shorter through a process of contraction, or condensation, into a get progressively shorter through a process of contraction, or condensation, into a series of spirals or coils; the coiling produces structures that are more easily moved series of spirals or coils; the coiling produces structures that are more easily moved around.around.

As the chromosomes become visible, they appear double-stranded, each As the chromosomes become visible, they appear double-stranded, each chromosome being composed of two longitudinal halves, the sister chromatids. The chromosome being composed of two longitudinal halves, the sister chromatids. The two chromatids formed by one chromosome each contain one of the replicated two chromatids formed by one chromosome each contain one of the replicated DNA molecules.DNA molecules.

Because of semiconservative replication these replicate DNA molecules are each Because of semiconservative replication these replicate DNA molecules are each "half old and half new"; that is, in each double helix one of the nucleotide strands is "half old and half new"; that is, in each double helix one of the nucleotide strands is newly polymerized. These sister chromatids are joined at a region called the newly polymerized. These sister chromatids are joined at a region called the centromere. At this stage the centromere has already divided into a pair of sister centromere. At this stage the centromere has already divided into a pair of sister centromeres.centromeres.

The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm become one.become one.


Metaphase At this stage, the nuclear spindle becomes prominent. The At this stage, the nuclear spindle becomes prominent. The

spindle is a birdcage-like structure that forms in the nuclear spindle is a birdcage-like structure that forms in the nuclear area; it consists of a series of parallel proteinaceous fibers area; it consists of a series of parallel proteinaceous fibers that point to each of two cell poles. These spindle fibers are that point to each of two cell poles. These spindle fibers are polymers of a protein called tubulin. The chromosomes polymers of a protein called tubulin. The chromosomes move to the equatorial plane of the cell, where one sister move to the equatorial plane of the cell, where one sister centromere becomes attached to a spindle fiber from one centromere becomes attached to a spindle fiber from one pole; the other sister centromere, to the other pole. pole; the other sister centromere, to the other pole.


Metaphasechromosomes

SSome of the orders of ome of the orders of chromatin packing chromatin packing thoughtthought to give rise to the to give rise to the highly condensed mitotic highly condensed mitotic chromosome.chromosome.

The folding of naked The folding of naked DNA into nucleosomes is DNA into nucleosomes is the best understood level the best understood level of packing.of packing.

The structures The structures corresponding to the corresponding to the additional layers of additional layers of chromosome packing are chromosome packing are more speculative.more speculative.


Anaphase and telophase AnaphaseAnaphase begins when the pairs of sister chromatids begins when the pairs of sister chromatids

separate, one of a pair moving to each pole. The separate, one of a pair moving to each pole. The centromeres, which now clearly appear to have divided, centromeres, which now clearly appear to have divided, separate first. As each chromatid moves, its two arms separate first. As each chromatid moves, its two arms appear to trail its centromere; a set of V-shaped structures appear to trail its centromere; a set of V-shaped structures results, with the points of the V's directed at the poles.results, with the points of the V's directed at the poles.

At telophase,At telophase, a nuclear membrane re-forms around each a nuclear membrane re-forms around each daughter nucleus, the chromosomes uncoil, and the nucleoli daughter nucleus, the chromosomes uncoil, and the nucleoli reappearreappear, , effectively re-forming the interphase nuclei. By effectively re-forming the interphase nuclei. By the end of telophase, the spindle has dispersed, and the the end of telophase, the spindle has dispersed, and the cytoplasm has been divided into two by a new cell cytoplasm has been divided into two by a new cell membrane.membrane.


Cells in active proliferation

The apical meristem x400. Most of the cells even in this The apical meristem x400. Most of the cells even in this area of active cell division are at interphase. Those with area of active cell division are at interphase. Those with visible chromosomes are at some stage of mitosis.visible chromosomes are at some stage of mitosis.

Longitudinal section of Longitudinal section of an onion root tip an onion root tip (apical (apical meristem) meristem) x40. x40.


Meiosis: Prophase 1 Meiosis consists of Meiosis consists of two nuclear divisionstwo nuclear divisions, , distinguished as distinguished as meiosis I meiosis I and and meiosis meiosis

IIII. The events of meiosis I are quite different from those of meiosis II, and the . The events of meiosis I are quite different from those of meiosis II, and the events of both differ from those of mitosis. Each meiotic division is formally events of both differ from those of mitosis. Each meiotic division is formally divided into prophase, metaphase, anaphase, and divided into prophase, metaphase, anaphase, and ttelophase.elophase.

PROPHASE I.PROPHASE I. The chromosomes become visible as long, thin single threadsThe chromosomes become visible as long, thin single threads (which(which are composed of pairs of replicated DNA molecules are composed of pairs of replicated DNA molecules), and continue ), and continue contractingcontracting during the entire during the entire stagestage. . Homologous Homologous chromosomes chromosomes form pairsform pairs (this (this does not happen in Mitosis); does not happen in Mitosis); each chromosome has a pairing partner, and the two each chromosome has a pairing partner, and the two become progressively paired, or become progressively paired, or synapsedsynapsed, along their lengths. Thus, the number of , along their lengths. Thus, the number of homologous pairs of chromosomes in the nucleus is equal to the homologous pairs of chromosomes in the nucleus is equal to the haploid haploid number number nn. The beadlike chromomeres align precisely in the paired homologs, producing a . The beadlike chromomeres align precisely in the paired homologs, producing a distinctive pattern for each pair. Since each member of a homologous pair produces distinctive pattern for each pair. Since each member of a homologous pair produces two sister chromatids, the synapsed structure now consists of a bundle of two sister chromatids, the synapsed structure now consists of a bundle of four four homologous chromatids, the tetradhomologous chromatids, the tetrad. At . At this stagethis stage, cross-shaped structures called , cross-shaped structures called chiasmatachiasmata (singular, chiasma) appear between nonsister chromatids. Each (singular, chiasma) appear between nonsister chromatids. Each homologous group of four generally has one or more chiasmata. Chiasmata are the homologous group of four generally has one or more chiasmata. Chiasmata are the visible manifestations of events called crossovers. A crossover is a precise visible manifestations of events called crossovers. A crossover is a precise breakage, swapping, and reunion between two nonsister chromatids.breakage, swapping, and reunion between two nonsister chromatids.


Meiosis: Metaphase I, Anaphase I, and Telophase I METAPHASE IMETAPHASE I.. The nuclear membrane and nucleoli disappear, and each The nuclear membrane and nucleoli disappear, and each

pair of homologs takes up a position in the equatorial plane. pair of homologs takes up a position in the equatorial plane. TThe sister he sister centromeres do not appear to have divided, so centromeres do not appear to have divided, so they they act as one. act as one. This This apparent lack of division represents a major difference from mitosisapparent lack of division represents a major difference from mitosis . . The two nonsister centromeres attach to spindle fibers from opposite poles. The two nonsister centromeres attach to spindle fibers from opposite poles.

ANAPHASE IANAPHASE I.. Homologous c Homologous chromosomes move directionally to hromosomes move directionally to oppositeopposite poles. poles. ThThis is the stage at which haploid nuclei are formedis is the stage at which haploid nuclei are formed . .

TELOPHASE ITELOPHASE I.. In many organisms, thIn many organisms, thisis stage do not exist, no nuclear stage do not exist, no nuclear membrane re-forms, and the cells proceed directly to meiosis II. In other membrane re-forms, and the cells proceed directly to meiosis II. In other organisms, telophase I and the interkinesis are brief in duration; the organisms, telophase I and the interkinesis are brief in duration; the chromosomes elongate and become diffuse, and the nuclear membrane re-chromosomes elongate and become diffuse, and the nuclear membrane re-forms. In any case, there is never DNA synthesis at this time, and forms. In any case, there is never DNA synthesis at this time, and the the genetic state of the chromosomes does not changegenetic state of the chromosomes does not change..


Meiosis I Stages of Trillium erectum

Late prophaseLate prophase I I

MetaphaseMetaphase I I

Anaphase IAnaphase I


Meiosis 2 PROPHASE IIPROPHASE II.. The presence of the haploid number of The presence of the haploid number of

sister chromatid pairs in the contracted state characterizes sister chromatid pairs in the contracted state characterizes prophase II. prophase II.

METAPHASE IIMETAPHASE II.. The pairs of sister chromatids arrange The pairs of sister chromatids arrange themselves on the equatorial plane during metaphase II. themselves on the equatorial plane during metaphase II. Here the chromatids often partly dissociate from each other Here the chromatids often partly dissociate from each other instead of being closely pressed together as they are in instead of being closely pressed together as they are in mitosis. mitosis.

ANAPHASE IIANAPHASE II.. Centromeres split and sister chromatids Centromeres split and sister chromatids are pulled to opposite poles by the spindle fibers. are pulled to opposite poles by the spindle fibers.

TELOPHASE IITELOPHASE II.. The nuclei re-form around the The nuclei re-form around the chromosomes at the poles.chromosomes at the poles.


Metaphase IIMetaphase IIAnaphase IIAnaphase IIThe centromers have The centromers have separated, half chromosomes separated, half chromosomes are drawn to the polesare drawn to the poles

Meiosis II Stages of Trillium erectum

Early telophaseEarly telophase II. II. The nuclei are haploid, the The nuclei are haploid, the chromosomes single-strandedchromosomes single-stranded


Comparing mitosis with meiosis

genetica per scienze naturali a.a. 03-04 prof s. presciuttini cell division cell division is the...

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