How Does the Cell Divide

7/31/2019 How Does the Cell Divide

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How Does the Cell

Divide?


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Cell division is an important life

process. It results in the productionof new cells. As organisms live,they are in a constant state of

repair and disrepair. Their cellsgrow old or are damaged. And tosurvive, organisms must have a

means to replace dying or injuredcells.


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All cells come from preexistingcells. The process that results

in the formation of new cells iscalled cell division.

Chromosomes and CellDivision


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The nucleus dividesfirst, followed by thedivision of the

cytoplasm, thus,

resulting first in theformation of twodaughter nuclei, and

later in the formation of

two daughter cells.Since the nucleuscontains the

chromosomes of the

cell, division alsoinvolves the equal

How Does the Cell Divide?


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There are two kinds of cells in

multicellular organisms based onfunction, namely,

a. vegetative, somatic or body

cells; andb. reproductive cells.

Cell Types Based onFunction and Chromosome

Number


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Vegetative orSomatic Cells

Reproductive Cells

They form the bodyof the organism, like

the cells of thestomach and thelegs of an animal orthe cells of the

leaves, roots andstems of a floweringplant. They aremainly concerned

with themaintenance of thelife processes of thewhole organism.

Such processes may

Involved in theproduction of

offspring. Inanimals, thereproductive cellsare called

gametes, of whichthere two kinds,sperm and egg. Inplants, like in

mosses, ferns andflowering plants,there are two kindsof reproductive

cells, namely, the


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In terms of chromosomenumber

Vegetative CellsReproductive CellsDiploidHaploid


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Each kind of plant and animal has a definitenumber of chromosomes found in each of its

body cells. This number of chromosomes whichis established at the time of the formation of

the zygote or fertilized egg is often referred toas the chromosome number of the species.

Human Species 46Fruitfly 8Onion 16

Chromosome Number


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Since the zygote is the result of the

union of the egg cell and the spermcell, it may be said that the zygote

consists of two sets of

chromosomes. One set iscontributed by the sperm cell or is

of paternal origin; and the other set

is contributed by the egg cell or isof maternal origin.


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In man, the zygote has 46 chromosomes andis said to be diploid (2N), whereas the spermcell and the egg cell has only 23chromosomes and is said to be haploid (N).

The zygote of all the vegetative or somatic

cells that resulted from the cell division ofthe zygote are diploid or have 2Nchromosome number. Thus, the chromosomenumber of the species, in general, also refers

to the diploid number (2N).


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Through breeding techniques or throughfreaks of nature, the chromosome

number of the somatic or vegetativecells, originally 2N, may be 3N, or threehaploid (N) number of chromosomes. Or

it may be doubled (4N), quadrupled(8N), etc. this chromosomal condition is

generally calledpolyploidyand thechromosome number of the somatic or

vegetative cells is calledpolyploid.Hence the terms triploid (3N), tetraploid(4N), etc.


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Although polyploid organisms are

rare in the animal kingdom, theyare fairly common in the plantkingdom. At present, polyploidy

can be and has been induced inboth plants and animals foreconomic purposes.


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For example, many of thecommercial roses and strawberriesare polyploid plants. They producelarger flowers and/or larger fruits.

Triploid animals are sexually sterile,hence bigger since food is

allocated to growth of the bodyrather than to development of

reproductive cells.


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Prior to division, eachchromosome in a cell isdouble-stranded. Eachstrand is called

chromatid. Since the twochromatids of achromosome containidentical genes or genetic

information, they are alsosometimes referred to assister chromatids. Theyare joined together by a

centromere.

Chromosome Structure


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The location of the centromere is specific foreach kind of chromosome found in thenucleus of a cell. The genes that control theinheritance and expression of a particularcharacteristics have specific positions, or loci(singular, locus), along the length of thechromosomes.

Thus, with around 35,000 genes in a humancell, there may very well be severalhundreds to around one thousand genes that

are linearly arranged along the entire lengthof every chromosome.


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In a diploid cell, each kind of chromosome isrepresented by a pair. Each pair ofchromosomes is referred to as homologouschromosomes.

Homologous Chromosomes


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Homologous chromosomes are

structurally alike. They are ofthe same size or length. Their

centromeres are similarly

positioned so that the so-calledarms of the homologous

chromosomes are of the samelength.


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In onion, where 2N is equal to 16chromosomes, there are 8 pairs of

homologous chromosomes.In humans, females have diploid chrosome

number (2N) equal to 46. Thus, there are 23pairs ofhomologous chromosomes, of which

22 pairs are somatic or body chromosomesand the last pair are sex chromosomes. The23rd pair consists of two X chromosomes.

In males, where 2N is equal to 46, there are

22 pairs of homologous chromosomes andone pair of chromosomes that are not strictlyhomologous since they are of different sizes.

These are the sex chromosomes, consisting

of the larger X chromosome and the smaller


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Cell Division


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There are two common types of celldivision that cells say undergo,

namely, mitosis and meiosis. Mitosisand meiosis are types of nuclear

division, with the division of cytoplasm

referred to as cytokinesis. However,the term mitosis and meiosis are often

used and understood in the broad

sense as including not only nucleardivision but also cytoplasmic division.


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Did you know that you began as a singlecell called zygote? The zygote is the

product of the union of your fathers spermcell and your mothers egg cell. It is but

one-tenth of a millimeter. So how did youreach the size you now have? Did you alsoknow that 28 days from now, every single

layer of skin that covers your body will allbe gone replaced by a set of completelynew ones?

Mitosis: Growth, Repairand Asexual Reproduction


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Multicellular organisms, cell division causesan increase in cell number, can lead togrowth and repair of damaged body parts.

Unicellular organisms, cell division is a formof asexual reproduction that produces newindividuals.

Cell Division for


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The type of cell divisioninvolved in growth, repairand asexual reproduction

is called mitosis.


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Cells go through acycle of alternatingstages of division andrest from division.

The Cell Cycle


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The nondividing stage, which is

devoted largely to cell growth, iscalled interphase. Most activelydividing cells spend some 90% oftheir time at this stage.When cells divide, two parts may beinvolved, the nucleus and thecytoplasm. The division of the

nucleus is called mitosis and thedivision of cytoplasm is calledcytokinesis.


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A type of nuclear division that produces twodaughter nuclei, each containing exactlythe same number of chromosomes as theparent nucleus.

Includes four stages, namely:

-prophase

-metaphase

-anaphase-telophase

Mitosis


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When cytokinesisoccurs, it happens

simultaneously with thelast stage of nucleardivision. Every time acell divides, it mustensure that a completeand faithful copy ofgenetic informationfound in the DNA(packaged into

chromosomes) ispassed on to thedaughter cells.


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Mitosis ensures that each

daughter cell receives acomplete set of chromosomes.Every part of the body, every

cell of the skin, bone or blood,originated from one cell and

hence contains identical sets ofchromosomes and copies of

genetic information.


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Some cells complete the cycle of interphaseand mitosis within 24 hours, while othersmay take years before they go through theprocess of cell division.

How Often Do Cells Divide?


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What Changes Occur inEach Stage of the Cell

Cycle?


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Interphase is thestage between two

successive celldivisions. Includesthree phases, asfollows:

-Gap 1 (G1) phase-Synthesis (S) phase

-Gap 2 (G2) phase


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The period when the cell increases in size inpreparation for cell division. RNA andproteins including enzymes needed formaking DNA are synthesized.

Gap 1 (G1) Phase


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The period during which DNA is synthesizedand chromosomes are replicated. Eachstrand of the double stranded produced iscalled a sister chromatid.

Synthesis (S) Phase


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The period when the cell continues tosynthesize RNA and proteins and increase insize.

Gap 2 (G2) Phase


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Although DNA synthesis is confined at the Sphase, the synthesis of organelles occurs

throughout interphase. Before division, thecells grow to their characteristic adult size.After division, the cells may go through G1 toprepare for the next division.

Or they may go through into an arrested,quiescent stage known as G0 statedifferentiate.

Some cells, such as nerve cells and bloodcells, remain in G0 all their lives.


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Prophase is when chromosomes coil up intorod-shaped structures, nucleoli and nuclearmembrane disappear and spindle fibers areformed.

Mitosis involves thefollowing stages:


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Metaphase involvesthe alignment

double-strandedchromosomes at theequatorial plate,with the

kinetochoresattaching thechromosomes to thespindle fibers.


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Anaphase beginswith the division of

the centromeresand ends with themigration of single-strandedchromosomes to thepoles.


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Telophase is alsoknown as reverse

prophase since itinvolves theuncoiling ofchromosomes,

reappearance of thenucleoli and nuclearmembrane, anddisappearance of

the spindle fibers.


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Involves the formation of a cell plate thateventually develops into the cell wall and themiddle lamella.

Cytokinesis in Plant Cells

C t l i Di i i i


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Occurs with the formation of a cleavagefurrow.

Cytoplasmic Division inAnimal Cells


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Sometimes, nucleardivision is not followedby cytoplasmic divisionand a cell with twonuclei, called abinucleated cell, isformed. Many cells of

the human liver arebinucleated. If there arerepeated nucleardivisions without

cytoplasmic divisiontaking place, then a cellwith many nuclei, calleda multinucleated cell, is

formed. The cells of

Wh C ll Di i i G


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Cell division may not always go on as itnormally should. Some cells may divide toofast, while others may divide too slowly ornot at all.

When cells divide faster than they should,they may develop into cancerous growth.

When cells divide too slowly, wounds wouldtake a while to heal; damaged tissues arenot repaired soon enough.

When cells cease to divide as in the brain,

and damage nerve cells are not replaced,resulting in memory loss.

When Cell Division GoesWrong


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Old cells eventually sufferfrom complete failure of

division. As our cells growold and die, so do we.


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Scientists may not be very farfrom unlocking the mysteries ofyouth and aging, immortality and

death. The secret is believed tobe in that part of thechromosome called telomere and

in that enzyme known astelomerase.

When Cells Stop Dividing

Wh t t l d


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What are telomeres andtelomerase?


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The human body is made up of over 100trillion cells. Each cell contains a nucleus,

which contains chromosomes that are madeup of linearly arranged cells. In order togrow, the cells in our body must undergodivision (mitosis). Before diving, our cells

make duplicate copies of chromosomes.These chromosomes are distributed equally

to the daughter cells. Each chromosome hasa special protective cap called a telomere.

With each cell division, the telomere getsshorter. After about 50 divisions, thetelomere becomes so short that the cellstops dividing. This means the cell has

become old and may malfunction and die


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Is there a way to keepthe telomere from

shortening?


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We may become immortal if ourcells could go on dividing

forever.To prevent premature aging anddeath, cells naturally produce

telomerase, an enzyme thatelongates telomeres.

Telomerase delays cell agingand death by ensuring thattelomeres do not becomecritically short.


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Meiosis: Preparing for

Sexual Reproduction


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The sex cells are produced

through a type of nucleardivision that reduces

chromosome number to halfof that of the parent cell.This reduction-division is

called meiosis.

Meiosis involves two


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Meiosis IMeiosis IIA cell that divides by meiosis produces four

new daughter cells.

Meiosis involves twoconsecitive divisions


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Prophase 1 homologous chromosomes pairup. This is called synapsis. At this time,exchange of genetic material may occurthrough a process referred to as crossing-

over.Metaphase 1 each pair of homologous

chromosomes attach to a single spindle fiber.Anaphase 1 one double-stranded

chromosome in the pair moves to one pole,while the other double-strandedchromosome in the pair moves to theopposite pole

Meiosis 1


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Telophase 1 two nuclei are produced, eachcontaining only half the chromosomenumber of the original parent cell.


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Similar to mitosis.Each double stranded chromosome attaches

to a spindle fiber, its centromere splits intotwo, and the two strands of the chromosome

migrate to opposite poles.At the end of the cell division, four haploid

(N) daughter cells are produced.

Meiosis II

What is the significance of


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Allows the formation of haploid gametesEnsures that, even after fusion of gametes,

the chromosome number of the zygote oroffspring remains the same as the diploidchromosome number of the parents.

Makes possible the maintenance of constant

chromosome number in the speciesgeneration after generation.

What is the significance ofthe reduction of

chromosome numberduring meiosis?


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ChromosomalAbnormalities


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About 1 in 150 babies in the United States isborn with a chromosomal abnormality. Theseabnormalities are caused by errors in thenumber or structure of chromosomes. Manychildren with a chromosomal abnormalityhave mental and/or physical birth defects.Some chromosomal abnormalities result inmiscarriage or stillbirth. Understanding whatchromosomes are may make it easier tounderstand the wide range of problemschromosomal abnormalities can cause.

What are the causes of


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Chromosomal abnormalities usually resultfrom an error that occurs when an egg orsperm cell develops. It is not known whythese errors occur. As far as we know,

nothing that a parent does or doesnt dobefore or during pregnancy can cause achromosomal abnormality in his or herchild.

chromosomal

abnormalities?


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Egg and sperm cells each contain 23chromosomes. When they join together, they

form a fertilized egg with 46 chromosomes.But sometimes something goes wrong beforefertilization. An egg or sperm cell may divideincorrectly, resulting in an egg or sperm cell

with too many or too few chromosomes.


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When this cell with the wrong number of

chromosomes joins with a normal egg orsperm cell, the resulting embryo has achromosomal abnormality. A common type ofchromosomal abnormality is called a trisomy.

This means that an individual has threecopies of a specific chromosome, instead oftwo. For example, individuals with Downsyndrome generally have three copies ofchromosome 21 (though a small number ofcases are caused by chromosomalrearrangements).


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What are some commonchromosomal

abnormalities?


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This is one of the most commonchromosomal abnormalities, affecting about1 in 800 babies. Individuals with downsyndrome have varying degrees of

intellectual disability, characteristic facialfeatures and, often, heart defects and otherproblems.

Down Syndrome


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The outlook for children with Down syndromeis far brighter than it once was. Most haveintellectual disabilities in the mild tomoderate range. With early intervention andspecial education, many learn to read andwrite and participate in diverse childhoodactivities.

The risk of Down syndrome and othertrisomies increases with the mothers age.

The risk of having a baby with Downsyndrome is about:

1 in 1,300 at age 25

1 in 1,000 at age 30

1 in 400 at age 35


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These trisomies usually are more severethan Down syndrome, but fortunately lesscommon. About 1 in 16,000 babies is bornwith trisomy 13 (also called Patau

syndrome), and about 1 in 5,000 withtrisomy 18 (also called Edwards syndrome).Babies with trisomies 13 or 18 generallyhave severe intellectual disabilities and

many physical birth defects. Most affectedbabies die before their first birthday.

Trisomies 13 and 18

Common Sex Chromosome


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The last pair of chromosomes are the sexchromosomes, called X and Y. Generally,females have two X chromosomes, andmales have one X chromosome and one Y

chromosome. Sex chromosomeabnormalities may cause infertility, growthabnormalities, and, in some cases,behavioral and learning problems. However,

most affected individuals live fairly normallives.

Common Sex ChromosomeAbnormalities


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This abnormality affects about 1in 2,500 girls. Girls with Turnersyndrome have one Xchromosome and are missing allor part of the other Xchromosome. They usually areinfertile and do not undergonormal puberty changes unless

they are treated with sexhormones. Affected girls areshort, though treatment withgrowth hormone can helpincrease height. Some haveother health problems, includingheart and kidney defects. Girls

with Turner syndrome generallyhave normal intelligence, thoughsome have learning difficulties,particularly with mathematicsand spatial concepts.

Turner Syndrome


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About 1 in 1,000 females has an extra Xchromosome. Affected girls tend to be tall.

They usually have no physical birth defects,experience normal puberty and are fertile.

Affected girls usually have normalintelligence, though many have learningproblems. Because these girls are healthyand have a normal appearance, their parents

often dont know they have a chromosomalabnormality. Some parents may learn thattheir daughter has this abnormality if theyhave prenatal testing (with amniocentesis or

CVS).

Triple X


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This abnormality affects about 1 in 500 to1,000 boys. Boys with Klinefelter syndromehave two, or occasionally more, Xchromosomes along with their Y

chromosome. Affected boys usually havenormal intelligence, though many havelearning problems. As adults, they producelower-than-normal amounts of the male

hormone testosterone (and often are treatedwith this hormone) and are infertile.

Klinefelter Syndrome


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About 1 in 1,000 males is born with one ormore extra Y chromosomes. Affected malesare sometimes taller than average, havenormal sexual development and are fertile.

Most have normal intelligence, though somehave learning, behavioral andspeech/language problems. As with triple Xfemales, many affected males and their

families dont know they have achromosomal abnormality unless it isdiagnosed with prenatal testing.

XYY


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Less CommonChromosomalAbnormalities

D l i


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A small section of a chromosome is missing.

Deletions

Mi d l ti


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An extremely small amount of achromosome is missing, possibly only asingle gene.

Microdeletions


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Translocations: A section of a chromosome isattached to another chromosome.

Inversions: A section of chromosome issnipped out and reinserted upside down.

Duplications: A section of a chromosome isduplicated, so there is extra genetic material.

Ring chromosome: Material is deleted atboth ends of a chromosome, and the new ends

join together to form a ring.


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Some uncommon disorders

can be caused by smallchromosomal deletions

Cri-du-chat (cat cry)d (d l ti


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Affected children have a cat-like, high-pitched cry during infancy, intellectualdisabilities and physical abnormalities.About 1 in 20,000 to 50,000 babies is born

with this disorder.

syndrome (deletion onchromosome 5)

Prader-Willi syndrome(d l ti h


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Affected children usually have intellectualdisabilities or learning disabilities,behavioral problems and short stature. Theyalso may develop extreme obesity. About 1

in 10,000 to 25,000 babies is affected.

(deletion on chromosome15)

22q11 deletion syndrome(d l ti h


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About 1 in 4,000 babies is born withdeletions in a specific region of chromosome22. These deletions cause a variety ofproblems that can include heart defects,

cleft lip/palate, immune systemabnormalities, characteristic facial featuresand learning disabilities. Certaincombinations of these features are

sometimes called DiGeorge orvelocardiofacial syndrome. Individuals withthis disorder have a 50-percent chance ofpassing the chromosomal abnormality on to

their offspring with each pregnancy.


Wolf-Hirschhorn syndrome(d l ti h


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This disorder is characterized by intellectualdisabilities, heart defects, poor muscle tone,seizures and other problems. It affects about1 in 50,000 babies.


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How Does the Cell Divide