Biology - An Illustrated Guide to Science (794127), страница 9
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Eachchromosome has duplicated and nowconsists of two sister chrom atidsvisibly connected at their cen trom eres.The nuclear envelope disintegrates,and the nucleolus disappears.● In m etaphase the chromosomesarrange themselves around theequator of the cell. Microtubules formthe mitotic spindle.● An aphase begins when thecentromere in the chromosomedivides and starts to move to oppositeends of the cell.● By telophase the chromosomes are atopposite ends of the cell, and a newnuclear membrane begins to form.
Itends when the cell pinches in toproduce daughter cells.© Diagram Visual Information Ltd.●Uses of mitosis●Mitosis produces daughter cells thatare genetically identical to the parentcell. The growth and repair ofmulticellular organisms require mitosisto produce new cells.chromatidsseparatespindle fiberLate telophaseDaughter cellscentriolesreplicatenuclearmembranereformscytoplasmdivides51Asexual reproduction:fissionFission in amebasCONTINUITYKey wordsclonecystfissionmitosispseudopodiumFully grown amebapseudopodiaTwo types of fissionFission is the splitting of a parent cellinto a number of daughter cells. Inbinary fission two cells are produced.In multiple fission many moredaughters are produced.● The daughters produced by fission aregenetically identical to the parent, i.e.they are clon es.●nucleusMultiple fissionBinary fissionAmebawithdraws itspseudopodia,and its nucleusdivides bymitosis.Amebawithdraws itspseudopodia.Amebasecretesa cyst wall.Binary fissionIn Am oeba proteu s (ameba), binaryfission begins when the pseu dopodia(false feet) are withdrawn to make aslightly more spherical shape.● The nucleus divides by m itosis toproduce two identical nuclei.
Thesemove to opposite ends of the cell.● The ameba constricts around themiddle and forms two daughter cells.●Multiple fissionDaughternuclei separate,and cytoplasmconstricts.In multiple fission the amebawithdraws its pseudopodia to form amore spherical shape as in binaryfission but then secretes a wall aroundthe cell to form a cyst.● The cyst can survive harsherconditions than the normal amebacell.● Inside the cyst the ameba dividesmultiple times by mitosis to producemany small daughter cells. Thesedaughters are released when the cystwall breaks.●Amebadivides manytimes bymitosis.Under favorable conditions the cystruptures, releasing daughter amebas.© Diagram Visual Information Ltd.Daughteramebas52CONTINUITYKey wordsclonestarchtuberVegetative propagation●In vegetative propagation a plant willproduce daughters that are geneticallyidentical to the parent.
These clon esare produced without any sexualprocess and develop from roots,stems, or leaves.Asexual reproduction:vegetative propagationPotato reproduction1 A shootgrows froma lateralbud.2 The shootforms leaves,and rootsgrow.3 Side stemsgrow out andswell up intotubers.4 Food made inthe leaves is storedin the tubers.5 The leaves, stem,and old tuber die, butnew tubers remaindormant in the soil.Potato tubersPotato plants form fleshy stems in theroots that act as a store for starch.These organs, called tu bers, can growinto new plants if separated from theparent plant.● In the second half of the growingseason, parts of the root will swell asstarch is deposited in them.
Thisstarch acts as an energy store for theplant.● At the end of the growing season, theaerial parts of the potato wilt and die,leaving the tubers undergroundprotected from frost. In the nextseason the tubers will use their starchto provide energy to develop into newpotato plants.●Strawberry reproductionrunnerStrawberry runnersStrawberry plants produce shoots thatgrow out from the side of the planthorizontally.
These above groundstems are called runners.● Where runners touch the ground, theydevelop their own roots and aerialshoots. These develop into a newstrawberry plant.● Over time the original runner candecay to produce a completelyindependent plant.© Diagram Visual Information Ltd.●parent plantnew plantdevelopingfrom runnerroot53Meiosis: first divisionCONTINUITYKey wordsMeiosis IInterphaseEarly prophase IplasmamembranecytoplasmhomologouschromosomesanaphasechiasmachromatidgametegenehaploidMid prophase IhomologouschromosomeshomologouschromosomeinterphasemeiosisprophasetelophaseMeiosis●●Meiosis is a two-stage form of celldivision used only in the sex organs toproduce gametes.Meiosis differs from mitosis in thatduring prophase genetic material maybe exchanged between thechromatids.●chromatinthreadnucleolusbivalentnuclearmembranecentrioles●centromeres●Late prophase IMetaphase IAnaphase Ichromatid●●●●chiasmametaphaseplatespindlefiberscentriole●Telophase Icleveage furrowInterphase in meiosis is identical tointerphase in mitosis (see page 50).In early prophase I, homologouschromosomes,which have the samegenes but may have different alleles,are attached along their lengths in aprocess called synapsis.Synapsis is completed in midprophase I, and the chromosomes aresaid to be bivalent, a reference to thefact that two chromosomes are united.In late prophase I, chromosomes mayexchange segments of geneticinformation at locations calledchiasmata (see page 55).During prophase, the centrioles, whenpresent, begin migrating to the twopoles of the cell.In metaphase I, the chiasmata slipapart, and the chromosome pairs alignon either side of the metaphase plate.During anaphase I, spindle fibers pullthe chromosomes toward each pole ofthe cell, and the cell elongates inpreparation for division.During telophase I, spindle fibersdisappear, a cleavage furrow forms,and the cell splits.
Each daughter cellnow haploid. It has half the number ofchromosomes as the parent.© Diagram Visual Information Ltd.Meiosis I54CONTINUITYKey wordsanaphase IIcentriolecentromerechromatidchromosomegametehaploidmeiosismetaphase IIprophase IItelophase IIzygoteMeiosis: second divisionMeiosis IIProphase IIspindlefiberscentriolesmetaphase platenuclearmembraneGamete formation●Metaphase IIspindle fibersMeiosis II, is the mitotic division of thehaploid cells produced in meiosis I.Second stage: cell divisionIn prophase II, chrom atids shortenand thicken into visible chrom osom es.Cen trioles (when present) move tothe poles of the cell, spindle fibersform, and the chromosomes movetoward the equator of the cell.● In m etaphase II, the chromosomesline up along the metaphase plate.● In an aphase II, the cen trom eresdivide to produce four separatechromatids from the original twochromosomes.
These sisterchromatids move toward oppositeends of the cells.● Telophase II doubles the number ofcells without a corresponding increasein the number of chromosomes soeach new cell has only one chromatid.These haploid daughter cells have halfthe number of chromosomes found inthe parent cell.● These cells develop further intofunctional gam etes.© Diagram Visual Information Ltd.●chromosomechromatidscentromereAnaphase IIcentromereTelophase IIsisterchromatidhaploiddaughter cells55Crossing over andgenetic variationCONTINUITYKey wordsallelechiasmachromatidgametehomologouschromosomeCrossing overNo crossing overSingle crossing overDouble crossing overhomologous chromosomesmeiosiscentromeresHomologous chromosomesHom ologou s chrom osom es have thesame genes in the same positions butmay have different variants, or alleles,of the same gene.● The gene for eye color in humansalways resides at a particular place,called a locus, on a chromosome.However, there are a number ofdifferent forms (alleles) for this gene,for example blue, brown, or green.●A AB BCCa abA AallelesB BbcCcCA Aa abB Bbca aCcbbcCcchromatidsCrossing overCrossing over occurs when one lengthof chrom atid is exchanged for theequivalent length on a homologouschromosome.● Crossing over occurs during the firstdivision in m eiosis.
This ensures thatnew arrangements of alleles areproduced during the process thatleads to the formation of gam etes.● The further apart two genes are on achromosome, the more likely they areto be separated and remixed during acrossover.●a aB BCbBbcCA ACca abBcCa aA Achiasmab BBbCcCbccchiasmataA AA Aa aB BbbBCa abB bcCBbBCcCAAaaAAaaAAaaBBbbBbBbBbBbCCccCcCcCCccccCchromosome variations in the four haploid gametesCrossing over occurs when chiasm ataform, temporarily joining thechromatids of homologouschromosomes together at a particularpoint.● When the chiasmata break apart, thechromatids can be re-attached to thebroken end of a different chromatid.In this way lengths of chromatid canbe exchanged.●bCcCrossed shapesa aA Ac© Diagram Visual Information Ltd.A A56CONTINUITYKey wordsanthercarpelfruitgameteovarypistilovuleseedstamenstigmastyleFlower structureVertical section through flowerstamenantherfilamentpetalFlowers and fruits●Flowers are the sexual organsof a group of plants knownas angiosperms.
They areresponsible for producingseeds enclosed in structuresthat aid in their dispersal. Thecombinations of seeds andthese structures are calledfru its.pistil(carpel)Sequencesof ringsAll flowers havethe same basicstructure—a series ofrings or whorls arrangedon each other ona highlycondensedstem calledthe receptacle.● The lowest ring lookslike simple leaves or bractsand is the sepal.● The next ring up includes the petals.In insect-pollinated flowers, the petalsare often brightly colored and mayproduce scent, which attracts insects.● Inside the ring of petals are the maleparts of the flower. These are thestam en s.
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