allot mindorff mitosis and meiosis
TRANSCRIPT
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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.
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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
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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
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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]
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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
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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.
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