Module 2.6 Flashcards
Mitosis
AA type of cell division wher eone cell splits into two .
-Mitosis is part of a much longer porcess called the cell cycle .
cell cycle
-the whole cell cycle takes a long time
-the duration of the cell cycle depends ont he type of cell (mammalian cell 24 hours)
-Some cells take months
-Some cels even exist the cell cycle either temporarily or permanently .
Cell cycle conists of two main parts
-The Mphase (or mitotic phase ) is when mitosis takes place and cells divide into two .
-However , the Mphse is actuallhy a relatively short period within the whole cell cycle .
Most of the cell cycle is called interphase .
during interphase , the cell ma apper to be relatively inactive , but actually lots of different processes are happening .
-dURING INTERPHSE , THE CELL IS CARRYING OUT THE FUNCTIONS OF THAT PARTICULAR CELL YPE ,.(iNTESTICAL EPITHELIAL CELLS ) - ARE ABSORBING NUTRIENTS from the small intenstine .
the interphase can be divided into three main phases .
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G1 PHASE (1st stage )
-during g1 phase - the cell replicaites organelles such as mitochondria
-Replicating organelles require a great deal of proteins .
-So durinng G1 , a high level of transcrption and translation take place .
(Loooked a in protein synthesis )
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G1 phase prt two
-During g1 , the size of the cell also increases .
-This ensures that when the cell duvudes , the two daughter ells are the correct size.
second phase of interpahse (second stage )
-SPHASE
Next the cell passes into the s or synthesisi phase .
-During the s phse all of the chromosomes are replicated + to do this , cells need to synthesise a great deal of DNA .
last phase of interphase is the G2 PHASE
now cell enters GN phase .
-During GN , the cellreplaces the energy stores sued during the s phase
-cell contitnues to carry out transcription and transaltion .
-If any of the chromosmes have been damaged , then theya re REPAIRED during gn .
-jUST LIKE g1 PHAE , cells continue to GROW .
Once the cell has fnished G2 phase , it now enters the M or mitotic phase
Mitotic pahse can be divided into two prts .
MITOSIS - chromosomes separate into two sepearae nueclei .
-CYTOKENSISI - cell divideds into TWO .
sometimes , cellcs can exit the cell cycle , if this happeens , then scientists say tht the cell is in the G0 phase .
cell enters g0 phase for three reasons.
reason 1
fully differentiated cells can enter g0 and stay there for the rest of their lives (adult neurones )
reason 2
overtime , DNA can be damaged .
At a certain point , the cell will no longer be bale to divide .
-Scienitsts call this senescent cells , At this stage , the cell enters G0 permanantly.
reason 3
lsdtly , cell enters g0 temporary for example b memory cells
-thee cells an the be triggered to renter the cellc ycle during n inreaction .
-the cell cycle is very tightly regulated .
at certain key points in th cell cycle , the cell chekcs that the cell cycle should continue , Scienitsts call these CHECKPOINTS .
-At any point , if a cell fails a checkpoint , it iwll leave the cell cycle and enter g0.
there are three checkpoints
first checkpoint
first checkpoint
before the spahse ,t he cell must pass thorugh the g1 checkpoint .
-Here the cell checks it has gorwn into the correct size and also checks for DNA damage .
secodn checkpoint
if the cell satisidies the g1 checkpoint , then it passes into the s pashe and carries out DNA repliation .
-Thr next check point is AT THE END OF GN and before cell passes into the mitotic pahse .
-Again at the G2 checkpoint , cell checks for DNA damage and if the cell has grown to the correct size .
third checkpoint
if this checkpoint is passed , then cell enters the mitotic pahse .
-Final checkpoint is called he SPINDLE CHECKPOINT .
-Checks that the chromosmes are assembled correectly on the mtioti spindle .
-If this is the case , then he cellc ompeltes mtiosis and proceeds thorugh cytokenesis .
chromosomes
a cell that is aout to udnergo mitosis
cell only contains two pairs of chrosmomes (just to make diagram more simplfied)
Most organsisms contain lot mroe (23 pirs )
Long chrosmomes are shown in green
and short pair is hsown in ellow
cheomosmoes that are in pairs are xcalled diploid cells .
in cell divisionby mitosis , we staert with one diploid cell and end up with two .
-Two identical diploid cells like this .
-so cell division by mitosis takes place whn we ant idneical copies of cell 9e,g whena n organsism grows and repairs itself )
-Mitosis also used in a sexul reporduction
-mitosis par of the cell cyce.
-cell cycle has 2 pairs m phase nd interphase
-during interpHAW , RHW Cell increases its energy stores and grows in size .
-cell also replicates its organelles nd copies its DNa .
in the miotitic pahse , the cell crries out two processes
nueclues divides into two daughter nueclei (mitosis )
2. cell divides intwo d=two daughter cells each with a nucleus (cytokensis (
-JEY ; mitsosis is specifically refers to the division of thenueclues
division of CELL is cytokenesis .
both cytokensis and mitosis take plce in m phase
CCloser look at key event in itnerphase
KEY ; during interphase , we CANNOT SEE chromosmes in the neucles .
-The chormosmes are there but not visile as DISTINC TSTRUCUTRES .
-Instead we see dark mateiral called CHROMATIN .
-In croamtin , the chormomsoes jage a loose , open strucutre and hti smeans the DNA is ccesile for transcirption dd REPLICTION +TRANSCIRPTION
DNA in a chromosome
KEY ; Dna is a single long moelucl of DN .
KEY ; during interpahse , chormosmoes replictae like this
-now we hve two dienticl moelcule of dna .
-Scienitsts now cllt hese sister chormatids and the chromatids are joined t the centromere .
WHEN DOES THISH HAPPEN ?
KEY ; this happens for every chormsome + in humans that emans 23a[ors .
-This tkes palce during interpahse and the choromsomes sre not VIsible as DISTINCT STRUCUTRES at this stgae .
-After the inerpahse NUCLEUS DIIDES NTO WO DURINGMITOSIS .
cell thzt has just finished interpahse and going to enter mitsois
there are four stages of mitosis
PROPAHSE
METAPHASE
ANAPAHSE
TELEPHASE
Prophase
during propahse , the chromoseomes condesne and beome visible in then ucleus .
-We can see tht each chromosome consits of two sister chromatidss joined at the centrosome .
-The nuecleolus s disappears snd the nuclear Mmembrane starts to break down .
-A pair of centrioles move to either side ( or pole of the cell )
propahse (2)
proteins start to form spindle fbres which attah to the centromere of ach chrosmosme .
-These spindle fibres strt to move the hromosomes towrds the center of the cell .
second stage of mitosis is metaphase
-at metapahse , the spindle apparatus ahs completely formed .
-The chromsomes are lined at the center / equator of the cell .
third stge of mitsooosis - anaphase
-in anapahse , the centrosomes divide into two .
-spindle fibres begin to shoreten .
-This pulls the sister chromarids towards opposite poles of the cell like this .
telepase part two
the chromosmes uncoil back to their chratin -the neucleolus also reperrear in eah nucleus .
-at this point ,t he ell has now finsihednuclear division by mtisoiss nd starts CYTOKENSIS .
foruth pahse is the telephase
in telophase , the chromatidfd hsve reached the poles of the cell .
+now scienitsts refer to thems chromosmes again .
-now the psindle appartus breaks don and the nuclear memebrane reforms .
What happens is cytokenesis
the central memebrne is puled inwrds b t he cytoskeleton .
-this cretes a groove or furroe which gradually deepens .
eventuLLY Tte memebrnes fuse and form two independan ceells .
-cell division by mtisiosiss has finsihed .
THIS P ORCESS IS FOR N ANIMAL CELL .
Variation in plant cells 1
centrioels do not play a role in forming hte psindle apapratus in plant cells .
-as plnt ells do not contain centrioles .
variation in plant cells 2
when plnt cells undergo cytokensis ,t he cell memebrne cannot form a furrow as in animal cells .
-INSTEAD , formt he GOLGI PPARATUS from membrane strcutures down the center of the cell .
-These then fuse together to form the cenral cell memrbane .
-dividing the cytoplasm ino to .
-a new cellulsoe cellw all also forms doen the center .
key
unlike mitosis , cell division by meisosis only takes place in SEX ORGANS .
-that is bevause cell division by meisosis only ever used yto produce GAMETES .
(SPERM AND EGG in animals ) and (pollen and eggs in lanys )
-Iin cell diviosn by meisois , we start wih a diploid cell .
-at the end , we have four
four diploid hetes . (These cells ahve inidvidul chromosomes not pairs )
KEY why IS IT IMPORTAN that gametes are haploid cells ?
-diromg fertilisation , gametes fuse togetehr to porduce a fertilised egg or zygote .
-As gmetes are haploid , the zygote has the diploid number of chromosomes .
KEY ; - all the gametes made by meisois are geeticlly different
esssential ideas aour chromosomes .
-diploid cells chromosomes in humans are in pair s
scientits refrt to a chromosme pair like this is a homologous chromosome , (one rom hpomoglous pair from mternl one homoous pair from paternal ).
key ; both chromosomes in a ioloigicl pair have the exalt sane gene .
-e,g both copeis of chormsome have he same genes
however u can inherit different alleles .
-pallele a from mother
allel from dad
or allele a from andlele from mother and dad
allel b from mum and dad .
-gmetes porduced by meisois re genetically different why ?
homogous pair of chromosomes abt to undergo meisois (mternal and patenral )
-before the nueclues divides by meiosis , all the chromsoomes are copied in the interphase .
REMEBER - sister chromids are joined at the centromes .
-At the early stage of meisois ,t he two chromoesomes in a homoglous serieds com in a pair and
the chromaitds of the two chromsomes wrapp around echother .
-Scientists call this porcess CROSSIN OVER .
-The attached pair of chromsomes are called a BIVALENT .
-Ppoints were chromatids are joined is called chiasmata .
KEY ; parts of the chromtids can BREAKUp AND EXCHANGE BETWEEN THE HOMOLGOUS CHROMOSOMES LIKE THIS . SHOWWN ON PICTURE
-as you can see the maternal and paternal chromosomes hsve now exhaged dn .
-scientits sy this is yhe recombinant CHROMOSOMES .
-AS THESE CHROMOSOMES HAVE EXCHANGED dna . THIS MEANS THEY HAVE EXCHANGED ALLELES .
Looking at blood group chromosome , paternal choromsome on the left is materbal and pterbal choromsome has blue ,
adfter crossing over boh maternal and paternal chromsomes both contian a and b chromosome .
one crhomatid with b aellele other one has a allele . REDO THIS CARRD
bear in midn chiasmata can occcur at multiple points
so a larg number of alleels can e exhanged .
This exhange i alleres is a major source of GEENEIC variation in meisois .
Key about meiosis
Meiosis invovles two rounds of nuclear division .
-in meiosis 1 , homologous chromosomes are separated from eachother.
-in meiosis’s 2 , sister chromatids are separated from
Eachother .
What happens before meiosis ?
-the cell will have been through interphase .
-during interphase , the cell copied the chromosomes and the organelles .
(Remember : chromosomes are not visible as distinct structures during interphase ).
Stage one of meiosis 1
Cell enters neurosis. 1 and the first stage of this is PROPHASE ONE
-during prophase 1 , the chromosomes condense and become visible .
-Homolgpus chromosomes link together forming the chiasmata
REMEMBER
When the homolgpus chromosomes are paired , like this we call this a bivalent.
-at this point , crossing over can take place .Exchanging alleles between the homologous chromosomes .
During prophase (1) part two
The nuclear melegesnd also breaks down .
-the centrioles move to opposite poles of the cell .
-spindle fibres also start to assemble in the spindle apparatus .
Next step - meta phase 1
The pairs of homologous chromosomes are now lined up with the equator of the spindle apparatus
Next is anaphase 1
During anaphase 1 , the spindle fibres shorten .
The homogenised chromosomes , move towards opposite poles .
-for this to happen , the choasmata between the homologous chromosomes break .
Finally , in meiosis 1 , we have telephase 1
In telophase 1 the chromosomes have now reached the poles of the cell .
-At this point , the nuclear membranes reform and the chromosomes uncoil bsck
Into their chromatic state .
At this point cell undergoes cytokinesis dividing into two cells .
-These cells are haploid because they no longer contain pairs of homolgpus chromosomes .
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