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Chapter 11

BIOL 243 Chapter Notes - Chapter 11: Gamete, Karyotype, Gang Of Youths


Department
Biology
Course Code
BIOL 243
Professor
Gordon Chua
Chapter
11

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Chap te r 11 Note s
- Section 11.1: Ce ll Division
- Succe s s ful cell divis ion must fulfill the following require me nts:
- First, the two daughter ce lls must ea ch re ceive the full comple me nt of gene tic
ma te rial (DNA) pre s e nt in the s ingle pa re nt ce ll
- Second, the pa re nt ce ll mus t be large e nough to divide in two a nd s till contribute
sufficient cytoplasmic compone nts s uch a s prote ins, lipids , a nd other
ma cromole cule s to e a ch da ughter ce ll
- Satisfying the s e requireme nts me a ns that key cellular compone nts must be
duplica ted be fore ce ll division take s place
- Proka ryotic ce lls (ba cte ria a nd archa e ons) divide by bina ry fiss ion
- When e ukaryotic cells divide , the y first divide the nucleus by mitosis , a nd the n divide
the cytopla s m into two da ughte r ce lls by cytokines is
- It is likely tha t mitos is e volve d from bina ry fission
- In bina ry fiss ion, a ce ll replica te s its DNA, incre ases in s ize, a nd divides into two
da ughte r cells . Ea ch da ughte r cell re ceive s one copy of the re plicated pa re nta l DNA
- Bina ry Fis s ion
- Mole cula r mecha nisms tha t drive bina ry fis sion have be e n studie d most exte ns ive ly
in ba cte ria
- Bina ry fiss ion is the s a me in a rcha e ons , chloropla sts, a nd mitochondria, a nd
orga ne lles within pla nt, funga l, a nd animal ce lls
- Basic ste ps of bina ry fission: re plication of DNA, s e gre gation of re plicated DNA to
da ughte r cells , a nd divis ion of one ce ll into two
- Bina ry fiss ion in E. coli
- The circular ge nome of E. coli is a tta ched by prote ins to the inside of the
plas ma me mbra ne
- DNA replica tion is initia ted a t a s pecific loca tion on the circular DNA molecule ,
ca lle d the origin of re plication, a nd proceeds in opposite directions around the
circle
- The re s ult is two DNA molecules , e a ch of which is a tta ched to the plasma
me mbra ne a t a diffe re nt s ite . The two a tta chme nt sites a re initia lly clos e
together. The ce ll the n elongate s a n, a s it does so, the two DNA a tta chment
sites move a pa rt
- When the ce ll is a bout twice its original s ize , a nd the DNA molecules are well
se pa ra ted, a contra ction forms a t the midpoint of the cell
- Eventua lly, ne w me mbrane a nd ce ll wall a re s ynthe s ized a t the s ite of
cons triction, dividing the s ingle ce ll into two
- The re s ult is two da ughte r ce lls , each having the same genetic ma terial as the
pa re nt cell
- Several ge ne s play a ke y role in ba cteria l ce ll divis ion, one of these ge ne s being
the Fts Z gene. This gene ha s be e n s tudie d a lot be caus e ma ny copies of the
prote in it e ncodes a s s e mble a nd form a ring a t the s ite of cons triction, where the
ne w ce ll wall forms be tween the two daughter ce ll
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- Fts Z is pre s ent in the genome s of dive rs e bacteria a nd archa e ons, s ugges ting
tha t it pla ys a fundame ntal role in proka ryotic ce ll division
- Fts Z is e volutiona rily re late d to tubulin, which ma ke s up the dynamic
microtubule s found in eukaryotic cells that are importa nt in intrace llula r tra ns port,
ce ll movement, a nd ce ll division
- Mitotic Ce ll Division
- The ge nome of e ukaryotic ce lls is typically much larger a nd is orga nize d into one or
more linear chromosomes , e a ch of which must be re plicate d a nd s e pa ra te d into
da ughte r cells . Whe rea s the DNA of prokaryotes is a tta che d to the inside of the
plas ma me mbra ne, a llowing re plica te d DNA to be s e para te d into da ughter ce lls by
ce ll growth, the DNA of euka ryote s is locate d in the nucle us
- Euka ryotic ce ll division require s first the bre akdown a nd then the re -forma tion of the
nuclear e nvelope , as well a s me cha nis ms othe r tha n cell growth to s eparate
re plica ted DNA
- Chromosomes of dividing e uka ryotic cells a tta ch to the mitotic s pindle , which
se pa ra tes them into da ughte r ce lls
- Some unicellular e uka ryote s exhibit forms of cell divis ion tha t ha ve chara cte ris tics
of bina ry fiss ion and mitosis
- For example, dinoflage llate s, like all e uka ryote s, ha ve a nucle us a nd line ar
chromosomes. However, unlike mos t euka ryote s, the nucle ar envelope doe s not
bre ak down, but sta ys inta ct during ce ll division. Furthermore, the replica te d DNA
is a ttache d to the nuclear e nve lope. The nucleus then grows a nd divide s in a
ma nne r reminis cent of bina ry fiss ion. Thes e a nd other obs e rva tions of
interme diate forms of ce ll divis ion in a dditiona l orga nis ms s trongly s ugge st tha t
mitosis e volve d from bina ry fission
- Cell divis ion in e uka ryotic ce lls proce e ds through a number of steps tha t ma ke up
the cell cycle
- The ce ll cycle cons ists of two dis tinct pha s e s:
- M pha se
- During this phas e , the pare nt ce ll divide s into two da ughte r ce lls
- M pha se consis ts of two diffe re nt e vents:
1. Mitos is
- The s e paration of the chromos omes into two nucle i
2. Cytokinesis
- The divis ion of the ce ll itse lf into two se pa ra te ce lls
- Usually the two e vents , go hand in hand, with cytokines is typica lly be ginning
even be fore mitosis is comple te
- In most mammalia n cells , M pha se lasts a bout an hour
- Interpha s e
- The pha se is the time be tween two s ucce s s ive M pha s e s
- During this s tage, the cell make s pre parations for divis ion. Thes e pre pa rations
include re plication of the DNA in the nucle us s o tha t e a ch da ughter ce ll
re ce ives a copy of the genome , a nd an increa s e in ce ll s ize , s o tha t e ach
da ughte r ce ll rece ives sufficie nt a mounts of cytoplas mic a nd membrane
components to a llow it to s urvive on its own
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- Interpha s e can be divide d into thre e pha s e s
- S pha s e (S for s ynthe s is )
- Replica tion of the e ntire DNA content of the nucleus (particula rly
importa nt ta sk)
- In most ce lls , S pha s e doe s not imme dia te ly pre ce de or follow mitos is but
is s epa rate d from it by two ga p pha s es :
1. G1 pha se: betwee n the e nd of M pha s e and the s tart of the S pha se
2. G2 pha se: betwee n the e nd of S pha se a nd the s ta rt of M pha se
- Ma ny e sse ntial proce s s e s occur during both “gap” pha ses . For example,
during the G1 phase, s pe cific regulatory prote ins a re ma de and a ctivate d.
Once active, the re gulatory prote ins, ma ny of which a re kina s e s, the n
promote the a ctivity of enzyme s that s ynthe size DNA. In the G2 phase,
bothe r the s ize a nd prote in content of the ce ll increase in prepara tion for
divis ion. Thus , G1 is a time of prepa ration for S-phase DNA s ynthe s is ,
and G2 is a time of pre pa ra tion for the M pha se mitosis a nd cytokines is
- The time it take s for a ce ll to pa s s through the ce ll cycle de pe nds on the type
of ce ll a nd the organism’s s tage of de ve lopme nt. Thus, ce lls tha t require rapid
re plenis hing pass the cycle in less time tha n thos e that don't ne e d cons tant
and quick re plenis hing
- Cells of tiss ues which don't re quire ra pid re plenis hing of ce lls pa us e in the ce ll
cycle some whe re be twee n M phas e a nd S phas e for periods ra nging from
da ys to more than a ye a r. This pe riod is ca lle d the G-knot (G0) pha s e
- The G0 phase is distinguishe d from G1 by the a bs e nce of pre pa ra tions for
DNA synthesis
- Liver ce lls re main in G0 for as much as a ye a r
- Othe r ce lls s uch a s ne rve ce lls a nd those that form the le ns of the e ye enter
G0 pe rma nently; these ce lls are non-dividing
- Although ce lls in G0 ha ve e xite d the ce ll cycle , the y a re active in othe r wa ys
in particula r, ce lls in G0 still pe rform the ir spe cialize d functions . For
example, live r cells in G0 s till carry out me ta bolism a nd de toxifica tion
- Section 11.2: Mitotic Ce ll Divis ion
- Mitotic ce ll division (mitos is followed by cytokinesis ) is the normal mode of as e xual
re production in unicellular e uka ryote s , and it is the mea ns by which a n organism’s ce lls ,
tiss ues , a nd organs de ve lop a nd a re ma inta ined in multicellular e uka ryotes
- During mitosis and cytokine s is , the pare nta l cell’s DNA is divide d and pass e s on to two
da ughte r cells
- One of the ke y challe nges fa ce d by a dividing e ukaryotic ce ll is e ns uring tha t both
da ughte r cells re ceive a n equa l a nd complete set of chromos ome s
- The length of DNA e xceeds the diameter of the cell its e lf, therefore ne e ds to be
conde nsed to fit into the nucleus . DNA ne e ds to be conde ns e d furthe r during ce ll
divis ion s o that it doe s not be come ta ngle d a s it segrega te s into da ughte r cells
- In e ukaryotic cells, DNA is orga nize d with his tone s , a nd other prote ins into chroma tin,
which ca n be loope d a nd pa cka ge d to form the structure s we know a s chromosome s
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