Allot Mindorff Mitosis and Meiosis

8/9/2019 Allot Mindorff Mitosis and Meiosis

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Course Compamon


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1 • Cells

Prokaryotic cells

Prokaryotes were the f i rs t organisms to evolve on Ea rth an d they

still have the simplest cell structure. Bacteria are prokaryotes.

They are mostly small in size, unicellular and are found almosteveryw here - in soil , in water , on o ur skin , in our in tes t ines and

even in poo ls of hot w ater in volcanic areas.

The electrón micrograph below shows a cell of Escherichia coli

( E . coli), a bacter ium found in the hu m an in tes tines . Most

strains of E. coli are harmless, but some cause food poisoning.

Cytoplasm

fluid filling the space inside the plasmamembrane

water with many dissolved substances

contains m anyenzymes

contains ribosomes

does not contain any membrane-bound

organelles

carries out the chemical reactions of

metabolism

Ribosomessmall granular structures (70 S)

smaller than eukaryotic ribosomes w hich

are 80S

synthesizes proteins

Nucleoid región of cytoplasm containing the

genetic material (usually one molecule of

DNA)

DNA molecule is circular and naked (not

associated with protein)

total amount of DNA is much smaller than

in eukaryotes

the nucleoid is stained less densely than

the rest of the cytoplasm bec ause there

are fewer ribosomes in it and less protein

Flagellastructures protruding from the cell wall

with a corkscrew shape

base is embedded in the cell wall

using energy they can be rotated, topropel the cell from one area to another

unlike eukaryotic flagella they are solid

and inflexible

Cell wall

always presentcomposed of peptidoglycan

protects the cell

maintains its shape

prevenís cell from bursting

Plasma mem brane

thin layer mainly composed of

phospholipids, pushed up against the

inside of the cell wall in healthy cells

partially permeable

Controls entry and exit of substances

can also pump substances in or out by

active transport

produces ATP by aerobic cell respiration

Pili-----------------------------------------------protein filaments protruding from the cell

wall

can be pulled in or pushed out by a

ratchet mechanism

used for cell to cell adhesión

used whe n bacteria stick together to form

aggregations of cells

used when two cells are exchanging DNA

during a process called conjugation

Figure 28 Electron micrographs of E. coli


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u aryo c censEukaryot ic cel ls have a m uch more cojnpl icated interna hst ruc turethan prokaryot ic cells . They have a ^u de us and prg a i i e lW in thecytoplasm with single or double membranes. Each organelle has adist inctive structure and function. Six types are described here.

How many of each type of organelle

are visible in the electrón micrograph

of liver tissue?

Nucleus

The nuclear memb rane is double and has pores

through it. Uncoiled chro mosom es are spread

through the nucleus and are called chromatin.

There are often densely staining areas of

chromatin around the edge of the nucleus. The

nucleus stores almost all the genetic material

of the cell. It is where DNA is replicated and

transcribed, and whe re mR NA is modified

before export to the cytoplasm.

Rough endoplasm ic reticulum (rER) Golgi apparatus

nucleus

free

ribosomes

The rER consists of flattened memb rane sacs

called cisternae. Attached to the outside of

these cisternae are ribosomes. The main

function of the rER is to synthesise protein for

secretion from the cell. Protein synthesised

by the ribosomes of the rER passes into the

cisternae and is then carried by vesid es (small

membrane sacs), which bud off and are moved

to the Golgi apparatus.

rough endoplasmic

reticulum (rER)

This organelle consists of flattened m embrane

sacs called cisternae, like rER. Howeverthecisternae are not as long, are often curved, do

not have ribosomes attached and have many

vesicles nearby. The Golgi apparatus processes

proteins brought in vesicles from the rER. Most

of these proteins are then carried in vesicles to

the plasma me mbrane for secretion.

X 14 400

mitochondriongolgi

apparatus

Figure 29 Electron micrograph of part of a liver cell

lysosome

Lysosomes

These are approximately spherical with a

single membrane. They are formed from

Golgi vesicles. Lysosomes contain high

concentrations of protein, which makes them

densely staining in electrón micrographs. They

contain digestive enzymes, which can be used

to break dow n ingested food in vesicles or

break down organelles in the cell or even the

whole cell.

Mitochondria

A double membrane surrounds mitochondria,

with the inner of these membranes invaginated

to form structures called cristae. The fluid

inside is called the matrix. The shape of

mitochondria is variable but is usually sphericalor ovoid. They produce ATP for the cell by

aerobic cell respiration. Fat is digested here if it

is being used as an energy source in the cell.

Free ribosomes

These appear as dark granules in the cytoplasm

and are not surrounded by a membrane. They

are the same size as ribosomes attached to the

rER - about 20 nm in diameter. Free ribosomes

synthesize protein, releasing it to work inthe cytoplasm, as enzymes, or in other ways.

Ribosomes are constructed in a región of the

nucleus called the nucleolus.


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1 • Cells

€ ) Chapte r 1 qu est io ns

1 Figure 30 represents a cell from a multicellular organism.

Figure 30

(a) Identify, with a reason, whether the cell is

(i) prokaryotic or eukaryotic; [1]

(ii) part of a root tip or a finger tip; [1]

(iii) in a phase of mitosis or in interphase. [1]

(b) The magnification of the drawing is 2500 x.

(i) Calcúlate the actual size of the cell. [2]

(ii) Calcúlate how long a 5 |jm scale bar should be

if it was added to the drawing. [1](c) Predict what would happen to the cell if it was placed

in a concentrated salt solution for one hour. Include

reasons for your answer. [3]

2 The electrón micrograph in Figure 31 shows part of an

animal cell.

(a) Identify the labelled structures. [3]

(b) The structure indicated by the first label is

1.5 |im long. Calcúlate the magnification of the

micrograph. [2]

(c) Determine how long a 10 nm scale bar would be on

the micrograph. [2]

(d) Calcúlate the length of the structure indicated by

label III. [3]

II

Figure 31

3 Siphonous green algae are marine organisms, found on

many coral reefs. They are ecologically very successfuland some species have even caused problems when

accidentally introduced to new areas. Codium fragüe for

example has damaged shellfish industries after spreading

off the north-west coast of the United States. Bryopsis

pennata has become a pest species in aquaria, after

accidentally being introduced on coral rock.

Figure 32 is a photograph of part of an individual of

Bryopsis pennata. It can be 100 mm tall overall and

consists of branched structures called siphons (scalebar = 0.6 mm).

(a) Calcúlate the length of the smallest branch of the

siphon, visible in the photograph. Give your answer in

micrometres. [2]

Figure 33 is a diagram of part of one siphon.

(magnification = 180 x)

Figure 33

(b) Calcúlate the actual diameter of the siphon. [3]

(c) The structure of coráis shows that they are animals.

Deduce whether Bryopsis pennata is an animal, from

the structure of its siphon. [2]

(d) According to the cell theory, living organisms are

composed of cells. Discuss whether Bryopsis pennata

should be described as multicellular, unicellular or

acellular. [4]

(e) The vacuoles in the branched siphons are all

interconnected and the fluid inside them is

under pressure Suggest one advantage and one

disadvantage of having interconnected, pressurised

vacuoles. [2]

(f) The aquaria in which this species has become a pest

contain water with salt dissolved, like the sea. Predict

the effect of transferring Bryopsis pennata from sea

water to fresh water. [2]


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• • • • •

Cell división

Cell cycle

Growth, asexual reproduct ion, t issue repai r and ma intenanc e areexamp les of processes tha t requ ire the creation of new cells.

In euka ryotic cells, división of the nu cleus to form tw o genetically

identical nuclei is termed mitosis. División of the cytoplasm to

form two cells is called cytokinesis.

Prokaryotic cells reproduce by a process called binary fission. Thisinvolves replication of the single circu lar chrom osom e. The twocopies of the ch rom osom e mov e to opposite ends of the cell, and

cytokinesis quickly follows.

The life of a cell can be thought of as an ordered sequence of events,

called the cell cycle. The cell cycle refers to the even ts betw ee n onecell división and the n ex t in a e ukaryo tic cell. It can be rou ghly

divided into in terpha se a nd cell división. Interphas e is an active

perio d in th e life of a cell w h en m any m etab olic re act io ns occu r,including protein synthesis, DNA replication an d an increase in thenu m ber of mitocho ndria a nd /or chloroplasts. It is not necessarily a

perio d of p re para tion for m itosis, as a cell can rem ain in in te rp hase

indefinitely.

Interphase consists of three phases, the G1 phase, the S phase andthe G2 phase. D uring th e S phas e the cell copies all genetic material,so that after m itosis bo th n ew cells have a com plete set of genes.

mitosis and

cytokinesis

cell prepares

to dividecell grows

replication

of DNA

Figure 1 The cell cycle. Note that during

the S phase, the chromosome in the

model cell is duplicated through theprocess of replication.

Data-based question: cell size and the cell cycle

Figure 2 shows the daily life cycle pattern of Emiliania

huxleyi (a species of phytoplankton) as observed

under laboratory conditions. The hypothesis is that

the cell cycle appears to be timed so that the light

period can be used for photosynthesis linked to

growth whereas energy consuming processes can

occur in the dark, the daughter ceils being prepared for

photosynthesis by the onset of the next day.

1 State the time of day when:

(a) most DNA replication occurs

(b) when mitosis is most likely to occur. [2]

2 Identify the cell cycle stage when most of the

increase in cell size is occurring. [1]

3 Evalúate the claim that the timing of the cell cycle in

Emiliania huxleyi is an adaptation to take advantage

of light resources. [3] Time of day

Figure 2 The cell cycle in Emiliania huxleyi follows a

daily pattern.

The four phases of mitosis


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The four phases of mitosisMitosis is the división of a euk aryo tic nucleus intotwo genetically identical nuclei . Before mitosis can

occur, two copies of each ch romoso me are needed.Each c hrom osom e init ial ly consists of a single

DNA mo lecule. This has to be rep licated beforemitosis, a nd i t then consists of two iden tical DNA

molecules, called sister chrom atids. A lthoug h i tis a continu ou s process, cytologists hav e dividedthe events of mitosis into four phases: proph ase,metaphase, a naph ase and telophase. The events

that occur during these stages in an anim al cell

a re summ arized here .

Prophase

The chromosomes become shorter and fatter bycoiling (Figure 3a). To become sh ort eno ugh they

have to coil repeatedly. This is called supercoiling.At the en d of prophase the nu clear membrane

bre aks dow n.

M icrotub ules grow from the poles of the cell from

a structure called the microtubule organizingcentre (MTOC) to the chro moso mes (Figure 3b).

These microtubu les form a spindle shape and so theMTOCs togeth er with the microtubu les are referred

to as the m itotic spindle.

Metaphase

Spindle m icrotubules at tach to the cent romeres.Chromosom es are m oved to the eq uator of the cell(Figure 3c), with a spindle microtubule at tached

to one of the sister chromatids from one poleand another spindle microtubule at tached to the

opposite sister chromatid from the other pole.

Anaphase

At the start of anaphase, the pairs of sister

chromatids separate and the spindle microtubules pu lí th em to w ard s th e po les of th e ce ll (F igure 3d).

Unt il then the cent romeres had held them together.Mitosis produces two genetically identical nuclei

because sis te r chro m atids are p u ll ed to opposite poles. To en su re th is , th e cen trom ere s of si st erchrom at ids must be at tached in metapha se to

spindle microtubules from different poles.

Telophase

N ucle ar m em bran es re fo rm a ro u n d th echroma t ids, now called chromosomes, at each

pole (F ig ure 3e ). The chrom osom es uncoil , th ecell divides and the two daughter cells enter

interphase again.

Metaphase

sister chromatids

(b) nuclear

^/envelope

disintegrates

spindle

microtubules

píate

equator

(c)

Spindle apparatus

(e)

Cleavage

furrow

Nuclear

envelope

forming

Figure 3 The stages of mitosis:

(a) early and (b) late prophase.

(c) metaphase. (d) anaphase.

(e) telophase.

3 7


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h f Meiosis

homologous

chromosomes

Sexual reproduct ion i s a metho d oí producing offspring tha t also

generates genetic diversi ty in a species. In euk aryo tic organisms, it

involves the process oí fert il ization. F ert i l ization is the un ión of sex

cells, or gam etes, usually from two different parents. Fert i l izationwould double the he redi tary inform at ion each generat ion, i f the pr oce ss of creating gam ete s did n o t in volv e th e h a lv in g of h ered it a ry

information before fert i l ization.

As a consequence of fert i l ization, humans have pairs of

chromosomes, w i th one chromosom e in a pai r from each parent .

A nucleus l ike this with two chromosomes of each type is diploid.A nucleus wi th only one chrom osom e of each type i s haploid.

Meiosis is the process by which hereditary information is halvedduring the production of gametes. It is achieved by halving thenum ber of chromosomes. The process can be sum m arized as follows

(see Figure 1).

1 D u r in g i n te rp h ase , th e ch ro m o so m es rep lic at e. E ach

chromosom e consist of two ident ical chroma t ids.2 At the start of meiosis I, hom ologous chrom osom es pair up. The

homologous chromosom es exchange genet ic material wi th each

other in a process terme d Crossing over.3 D uring meiosis I, the homologou s pairs of chrom osom es

separate. On e of each pair goes to each of the tw o d aug hter cells.

The result is two haploid daughter cells.

4 In meiosis II, the two dau ghter nuclei divide again. This t ime thechrom atids of each chrom osom e separate. Meiosis II is similar tomitosis. The end result is four haploid cells.

Table 1 gives more de tailed inform ation ab out the sequenc e of events.

Table 1 Diagrams in the central column represent animal meiosis while most of the micrographs in the final column are cells from

the anther of a Lily.

daughter meiosis II

J \ nud* 1 f s

( n j i j f c x M daughta ( ' v i )

\ 3 / nucie¡" vS/n n n n

Figure 1 Outline of meiosis

Prophase I

• Cell has 2n c h r om osom es (dou b l echrom at id ) : n is hap loid nu mb er of

c h r om osom es .

• Hom ologous chromo som es pa ir (synapsis ) .

• Cro ssing over occurs.

N u cl ear en ve l o p edisintegrates

C h r o m o s o m e s each co n s i s t o f tw o s i s ter ch r o m at i d s

Spindle

m i cr o tu b u l es

P r o p h a s e I

C h i asm a (p o i n t o f cross over)

Metaphase I

• S p in d le m i c ro tu bu les m ov e h om olog ou s

pairs to equator of cel l .

• Or ien tat ion of paterna l and materna l

chromosomes on e i ther s ide of equator

is random and indep endent of other

homologous pa i r s .

E q u ato r

M e t a p h a s e I

1 2 7


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11 • Meiosis

Telophase I

• Chromosomes uncoi l . Dur ing in terphase

that fol lows, no replication occurs.

• R e d u ct io n o f c h ro m o s o m e n u m b e r f ro m

dip loid to hap loid completed.

• Cytokinesis occurs.

Prophase II

• Chrom osom es, wh ich st il l consist of two

chromat ids, condense and become v is ib le .

N u c l e a r e n v e l o p e s

f o r m i n g N u c l e o l u s C l e a v a g e

f o r m i n g f u r r o w

M e t a p h a s e II

P r o p h ase I I

C h r o m o so m es l i n e u p a l o n g eq u ato r

T e l o p h a s e I

S p i n d l e m i c r o t u b u l e s N u c l e a r e n v e l o p e

fo r m i n g a t r i g h t an g l es d i s i n teg r a tes

to p r ev i o u s sp i n d l e

Metaphase II

A n a p h a s e I I

Anaphase II

• C entrom eres separate and chromat ids are

moved to opposi te poles.

D a u g h t e r c h r o m o s o m e s

s e p a r a t e

Telophase II

• C hromat ids reach opposi te poles

• Nuc lear envelo pe formsCytokinesis occurs

T el o p h ase I I

- For each of the following


9/11

Meiosis is sometim es subject to errors. One exam ple of this is w hen

homologous chromosomes fail to separate at anaphase. This isterme d non-d isjunct ion. In other words, segregation does not occurfor a certain pair of homologous chromosomes. The result will bea gamete that ei ther has an extra chromosome or is deficient in a

chromosom e. If the g amete i s involved in hu m an ferti lization, the

resul t wi ll be an individual w i th ei ther 45 or 47 chromo somes.

An a bnorm al num ber of chromosomes will often lead to a person po sses sing a sy ndro m e, i.e. a co lle ct ion of ph ys ical signs or sy mptoms.

For example trisomy 21, also known as Down syndrome, is dueto a non-d isjunction event that leaves the individual with three of

chromosome number 21 instead of two. While individuáis vary, someof the com ponent features of the syndrom e include hearing loss, hea rt

and visión disorders. M ental and g row th retardation are also comm on.

syndromes, research the

chromosomes involved in the

non-disjunction event and some

of the component features of the

resulting syndrome.

a) Turner's syndrome

b) Klinefelter's syndrome

c) Patau syndrome.

diploid parentcell with two

chromosome 21

gamete with two

chromosome 21

trisomy: zygote

with three

chromosome 21

gamete with no

chromosome 21

cell dies

Figure 2 How non-disjunction can give rise to Down syndrome

Data-based question: risk of chromosomal abnormalities

with advancing age o f the parent

The data presented ¡n Figure 3 shows the relationship betweenmaternal age and the incidence of trisomy 21 and of other

chromosomal abnormalities.

trisomy 21

all

chromosomal

abnormalities

20 40 60

maternal age (years)

Figure 3 The incidence of trisomy 21 and other chromosomalabnormalities as a function of maternal age

1 Outline the relationship between

maternal age and the incidence of

chromosomal abnormalities in live

births. [2]

2 (a ) For mothers 40 years of age,determine the probability that

they will give birth to a child

with trisomy 21. [1]

(b ) Using the data in Figure 3,

calcúlate the probability thata mother of 40 years of agewill give birth to a child with

a chromosomal abnormality

other than trisomy 21. [2]

3 Only a small number of possible

chromosomal abnormalities are ever

found among live births, and trisomy

21is much the commonest. Suggest

reasons for these trends. [3]

4 Discuss the risks parents face

when choosing to postpone

having children. [2]

1 29


10/11

11 Meiosis

i^ n

Testing for Do wn syndrome

For older paren ts, a stand ard clinical practice is to ad m inister a

seru m b lood test such as the triple test or the qu ad test . These are

blo od te sts p erform ed on ex p ec ta n t m oth ers th a t lo ok fo r u n u su a l

levels of such chem icals as alpha -fetopro tein (AFP) and h um anchorionic go nad otrop in (HCG). AFP is produ ced in th e yolk sac andin the l iver of the developing fetus. HCG is produc ed by the placenta.The levels of each of these chemicals in th e m othe r's blood varies

with the gestational age of the pregnancy. With trisomy 21, the

m othe r's blood will show levels of AFP tha t are abo ut 25 per cent

lower tha n norm al levels an d HCG levels that are approximately two

t imes h igher tha n the n orm al HCG level .

If the serum test raises concern, than parents will be advised ofthe opt ion to have a karyotype produced w hich can give a moredefinit ive diagnosis. A kary otype (see Figure 4) is an orga nized

image of m etaphase fetal chromosomes. T echnicians stain thechromosom es, wh ich resul ts in a band ing pat tern. The techn ician

can the n organize the chrom osomes by thei r length, the posi t ion of

the i r cen t romere and by the band ing pa t te rn .

ultrasound transducerbladder/

(a)S S a ■m yT s

? !■sy y

a = « i a üT7K « i

I ¡ f § g s g! S B ¡

8 *** » I Ií iSE it q: « -

s s

10

Í l S I

13 14 1 5

Nm■m..m

I I■ Mn ■

10 11

8 8 atón ¡¡16 17

mm

19 2 0 21 2 2

es

6

8 1

M12

gtt■ ■

18

• li

Y

(b)

chorionic vífli

amniotic fluid

fetus (8 -10 weeks) ^ cathete

Figure 5 Ultrasound sean ¡mage of

12-week-old fetus

uterine cavity

Figure 6 Chorionic villus sampling

vagina

There are two procedures for obtaining the fetal chromosomesto produce the karyotype. One procedure, called amniocentesis,involves passing a needle through the mother's abdominal wall, usingultrasound to guide the needle. Figure 5 shows an ultrasou nd sean

image of a fetus. The needle is used to w ithdra w a sample of am nioticfluid from the am niotic sac of a developing fetus. Cells from th e fetus in

the am niotic fluid are cultured and th en used to prepare a karyotype.

A second procedure for obtaining fetal chromosomes is chorionicvil lus sam pling, or cvs. This proce dure samples cells from the

pla centa , sp ecif ic ally th e chori on , ra th e r th a n th e am nio ti c fluid . Itcan be done earl ier tha n amn iocentesis and the sam pl ing tool can

enter throu gh th e vagina (see Figure 6) .

¡r i;| V1¡í í :

ff Ir ji {> 1/ Ü ¡iII k n !» ¡f ir ** I > ■# *» f *Figure 4 A diagram of the 24 types

of chromosomes in humans

(Figure 4a) and photograph of a

human karyotype (Figure 4b)

1 (a) For Figure 4a, distinguish between:

(i) chromosome 5 and chromosome 6

(¡i) chromosome 17 and chromosome 18

(iii) the X and Y chromosome. [3]

2 (a) State the gender of the subject of the human karyotype in Figure 4b. [1]Q+3+o x A/ h o t h o r t h o l e a n / n h / n o c h n\ A /c a n\ / p h n n r m ^ l l t i p rn

u m asm a a


11/11

D uring pro pha se I of meiosis, al l of the chrom atids of two

homologous chromosom es become t ight ly associated in a processcalled synapsis. The result ing combined pair of homologouschromosomes is called a bivalent (referring to the two homologous

chromosom es) or a tet rad (referring to the four chrom at ids wi thinthe st ructure) .

The materna l and p a te rna l chromosomes exchange correspondingsections of DNA and once Crossing over is complete, newcombinations of al íeles will have been created. The process by whichoffspring possess a com bination of al íeles different from th at ofeither parent is called recombination (see Figure 7).

A chiasma is an X-shaped structu re formed betw een n on-sister

chrom atids du ring p roph ase I of meiosis. The chiasm a is a physicalmanifestation of Crossing over. Usually between one and three

chiasm ata form per homologous pai r (Figure 8) . The chiasma ta persis t th ro u g h m eta ph ase I an d play a ro le in th e p rev enti o n of n on-disjunction.

Meiosis and genetic variety

The random orientat ion of chromosom es at metaphase I leads tovariat ion wi thin offspring. For every chromosom e pair , the num ber

of possible chromosome combinations doubles. For a haploid

number of n, the n um ber of possible com binat ions i s 2". For hum answ ith a haploid num be r of 23 this am ou nts to 223 or over 8 m illion

com binations. Crossing over increases this nu m ber st il l furth er - somuch so that meiosis can produce an effectively l imitless number of

genetically d ifferent haploid cell types fro m one diploid cell type.

Figure 7 The process of Crossing over

Figure 8 This image shows that múltiple chiasmata can formwithin one tetrad.

131

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Allot Mindorff Mitosis and Meiosis

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