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cell cycle

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alaa mahmoud

on 8 January 2013

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Transcript of cell cycle

cell cycle and its regulation presented by : Introduction Human cells divide approximately every 24 hours. Why ? 1- Development of a single fertilized egg into
more than (1013) cells . 2- Replacing damaged cells by healthy cells The cell cycle is divided into two basic parts: 1-Interphase 2-Cell division :
Mitosis or Meiosis Interphase First gap phase
(G1 phase) DNA synthesis phase
(S phase) Second gap phase
(G2 phase) -First growth stage after cell division.

-This is the longest period of the cell cycle between
the end of the mitosis and the beginning of DNA replication.

-Cells mature by making more cytoplasm & organelles .

-Cells carry on their normal metabolic activity G0 phase : -Not all cells are continually replicated.

-Non-replicating cells are found in this stage. Mitosis -Mitosis means threads -Mitotic division is called :
(homotypic cell division)
because no crossing over occurs in it. -When the cell enters Mitosis there
are two molecules of DNA
in each chromosome. Chromosomes The word chromosome is used for
both for single threads present in
G1 and also for double threads
structure present in G2 . Stages of mitosis Prophase -The three major events of prophase are:
1-Chromosome coiling
2-Nuclealar disintegration
3-Nuclear envelop break down -At the beginning of prophase the two pairs Of centeriols start
to migrate towards the opposite ends of the cell . -In the same time microtubules grow out Surrounding each centeriole pair . Metaphase 1-The most striking feature Of metaphase that :
all chromosomes become aligned in the
equatorial plane .
This highly characteristic arrangement is
described as : Metaphase plate 2- Each chromosome is attached to the mitotic
spindles at the kinetochore
(kinote means movable-chora means space) -At first microtubules termed asters then spindle
cell mass is constituted termed mitotic spindles . Anaphase -It begines when the centromeres separate simultaneously. Separation allows one chromatid of each pair that makes up a metaphase chromosome to migrate to each pole of the spindle (Anaphase A). - Each chromatid contains a complete copy of the chromosomal DNA replicated during S phase . Telophase ( Growth of the body) -During telophase ,two new nuclei are formed in what is essentially the reverse of prophase.
-At telophase , the nucleolus re-forms at the nucleolar organizer again largely from preexisting structures .
-When the daughter cells separate at the end of cleavage ,
a remnant of the midbody may remain attached to one of the daughter cells and presist well into interphase . Meiosis Definition:
Meiotic cell division reduces the chromosome
from diploid number to haploid number
to form gametes .
it occurs in germ cells. Stages : 1)First Meiotic Division 2) Second Meiotic Division Prophase I : Leptotene: In which 46 d-chromosomes appear because the
DNA has been replicated during S-phase . Zygotene: The pairing of the homolog chromosomes
Each two homologous d-chromosomes and pair
up to form 23 bivalent. Pachytene: In which d-chromosomes condense and become
shorter and thicker . Diplotene In which two cromatides of each d-chromosome become apart from each other . Also contributing to separation of the daughter
chromosomes at anaphase is further elongation
of the spindle, which can double in length before
the cell reaches (Telophase). Crossing over happen between two homologous
chromosomes. Meiosis Definition:
Meiotic cell division reduces the chromosome
from diploid number to haploid number
to form gametes .
it occurs in germ cells. Stages : 2) Second Meiotic Division Prophase I : Leptotene In which 46 d-chromosomes appear because the
DNA has been replicated during S-phase . Zygotene The pairing of the homolog chromosomes
Each two homologous d-chromosomes and pair
up to form 23 bivalent. Pachytene In which d-chromosomes condense and become
shorter and thicker . Diplotene In which two cromatides of each d-chromosome become apart from each other . Crossing over happen between two homologous
chromosomes. Diakinasis: -There is little information on how the chromosomes condense to form the chromatin of the nucleus .
-During mitosis , both RNA and Protein synthesis are severely limited , but some protein synthesis does occur during telophase .
-These proteins might be involved in the transformation from chromosomes into chromatin . -In which each homologoue in the bivalent is mixture of both(maternal $ paternal).
-At the end of this stage the nuclear membrane and nucleolus disappear and the mitotic spindle is formed. Metaphase I -The centrioles are at opposite poles of the cell.
-The bivalents are now condensed and arranged on the
so called metaphase plate.
-Spindle fibers from each pole of the cell attach to one chromosome of each pair. -The chromosome alignment illustrated below is from a cell with a diploid chromosome number of 8. -Independent assortment refers to the random arrangement of pairs of chromosomes. The diagram below shows four possible arrangements of chromosomes during metaphase 1 from an individual that has 6 total chromosomes Anaphase I: -Anaphase I begins when the two chromosomes of each bivalent separate and
move to opposite poles of the cell.

-A key difference between mitosis and meiosis is that sister chromatids
remain joined at centomeres and move together toward the poles after
metaphase in meiosis I, whereas in mitosis they separate. Telophase I: -The homologous chromosome pairs complete their migration to the two poles with each chromosome still having two chromatids.
-Nucleoli reappear, spindle disappears, and cytokinesis follows.
-In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes.
-This stage is absent in some species. Definition : - The second part of meiotic division . - Similar to mitosis but its genetic results are different . - The result is 4 haploid cells (23 chromosomes)from 2 haploid cells (23 chromosomes)produced in meiosis I The 4 main steps of meiosis II : Prophase II -Disappearance of the nucleoli and the nuclear envelope. -Shortening and thickening of the chromatids as the chromatin condenses . -Centrioles move to the polar regions and the spindle
fibers appear and arrange for the second
meiotic division. Metaphase II -The chromosomes are arranged at the mid equatorial line of the cell . -The S phase replication of DNA takes place.

-Each chromosome has two (sister) chromatids.
Thus, duringthis phase, the amount of DNA in the cell
has effectively doubled. On discussing chromosomes , we should be aware that there are two
forms of chromosomes:
1- s-chromosome , each is made of one DNA molecule
( interphase , chromatin threads or chromatids )
2- d- chromosomes ( mitotic chromosomes) which are formed
during S phase .
Each is formed of two sisters chromatids that become linked to each other at the centromere . Each chromatid is made of a DNA molecule. -G2 phase, or pre-mitotic phase, is the third and final of interphase . -Curiously, G2 phase is not a necessary part of the cell cycle, as some cell types
(particularly young Xenopus embryos and some cancers ) proceed directly from DNA replication to mitosis -By the end of this phase : the RNA and proteins essential to
the cell division are synthesized , the energy for mitosis is
stored and duplication of the centrioles is completed forming
two pairs of centrioles . Anaphase II TelophaseII -The chromosomes are attached to the spindle fibers by their centromeres as the centromeres contain two kinetochores . - The centromeres begin to divide once the chromosomes are lined up. -Where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart .

-The sister chromatids move towards to the opposite poles of the cell. -The chromosomes reach the poles and uncoiling and lengthening of the chromosomes to form chromatin and the spindle fibers disappear . -Nuclear envelopes and nucleolus reform surrounding each chromatin network. -Cleavage or cell wall formation eventually produces a total of four daughter cells. Meiosis in Plant vs Animals : -Meiosis occurs in animals and plants :
1- In animals, meiosis is used to produce gametes directly.
2- In most plants, meiosis is first used to produce haploid spores. These spores eventually develop into gametophytes, which eventually give rise to gametes.
-In the end, both cases produce cells with half the number of chromosomes as the parent cell. Importance of meiosis : 1-It keeps the number of chromosomes of the cells of the same species constant.
2-It is a division necessary for the formation of the gametes in animals and spores in plants.
3-It provides chance for the appearance of a new gene combination and variation in human characters due to:
a-Random distribution of the chromosomes in the gametes.
b-Crossing over which leads to change in the arrangement of the genetic orders in the chromosomes
c-Random meeting The regulation of the cell cycle Checkpoints -The cell cycle is highly regulated .

-And the checkpoints detect the problems and its delay the progression to the next phase of cell cycle . Types of checkpoints 1-The restriction point : 2- DNA damage checkpoints Site:
This is present in G1,S,and G2 phases

-It is block cell cycle progression until repair of the damaged DNA
-or cell death by apoptosis 3-The unreplicated DNA checkpoint : Site:
It present in G2 phase .

This detects the presence of delayed
DNA replication forks . 4- The spindle assembly checkpoint
(metaphase checkpoint) : Function:
It delays the chromosome separation
Until all chromosomes have attached
To the mitotic spindle . 5-The chromosome segregation checkpoint : Function:
-It prevents the process of cytokinesis until all
of the chromosomes .
-Is correctly separated . References 1- Alaa Mohamed Mohamed Shakhlab (291)
2- Alaa Mahmoud Abdoh EL-reffaiy (292)
3- Alaa Mahmoud Mohamed El-bestawy (293)
4- Alaa Mahmoud Mohamed Abd El-glel (294)
5- Ayat Saad Abdo Omer (296)
6- Ayten Ahmed Magdy El-Domiaty (297)
7- Ayten Sherif Mostafa Sadek (298)
8- Ithar Hany Mohamed Abd El-Lateef (299)
9- Aysel Mohamed Ahmed Ali Mostafa (300) Agenda : 1- Introduction
2- Interphase
3- Mitosis
4- Meiosis
5- Regulation of the cell cycle How is the cell cycle regulated ?? Maturation promoting factor (MPF): The cyclin&cdk complexes: Regulation of the cell cycle byCyclin_Cdk complexes Site :
In the G1 phase Function : It is sensitive to the physiological state of the cell.
The cells don't receive appropriate growth stimuli from
their environment don't progress past this point in the
G1 phase but die by apoptosis. 1) First Meiotic Division Text books 1)Paux E, Sourdille P, Salse J, et al. (2008).
"A Physical Map of the 1-Gigabase Bread Wheat
Chromosome 3B". Science 322 (5898): 101–104.doi:10.1126/science.1161847. PMID 18832645.
2)Pellicer J, Fay M F and Leitch I J (2010).
"The largest eukaryotic genome of them all?".
Botanical Journal of the Linnean Society 164: 10–15. doi:10.1111/j.1095-8339.2010.01072.x.
3)Wilson, E.B. (1925). The Cell in Development and
Heredity, Ed. 3. Macmillan, New York. p. 923
4)Krebs, JE; Goldstein, ES; Kilpatrick, ST (November 2009). Lewin's Genes X (10th ed.). Jones & Barlett Learning. ISBN 978-0-7637-6632-0. 5)Raven, Peter H.; Johnson, George B.; Mason,
Kenneth A.; Losos, Jonathan & Singer, Susan. Biology,
8th ed. McGraw-Hill 2007.
6)Petronczki M, Siomos MF, Nasmyth K
(February 2003). "Un ménage à quatre: the molecular
biology of chromosome segregation in meiosis".
Cell 112 (4): 423–40. doi:10.1016/S0092-8674(03)00083-7.
PMID 12600308.
7)Javaux EJ, Knoll AH, Walter MR (July 2001).
"Morphological and ecological complexity in early
eukaryotic ecosystems". Nature 412 (6842): 66–9. doi:10.1038/35083562. PMID 11452306. 8)Harris Bernstein, Carol Bernstein and
Richard E. Michod (2011). "Meiosis as an
Evolutionary Adaptation for DNA Repair".
In Inna Kruman. DNA repair. InTech.
9)Bernstein H, Bernstein C (2010). "Evolutionary
origin of recombination duringmeiosis".
BioScience 60 (7): 498–505. doi:10.1525/
10)Johnsborg O, Eldholm V, Håvarstein LS
(December 2007). "Natural genetic
transformation: prevalence, mechanisms and
function". Res. Microbiol. 158 (10): 767–78. doi:10.1016/j.resmic.2007.09.004. PMID 17997281. Websites 1)http://biosiva.50webs.org/cellcycle.htm
File:Cell_C 5)http://www.google.com.eg/imgres?q=
20lectures/meiosis/meiosis.htm 9)http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html
12)http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/ cells2.html THANK YOU Alexandria University,
Faculty of Medicine,
Histology Department. Under Supervision of:
Prof.Dr/ Nadia Amin Sharaf El-din -G2 phase is a period of rapid cell growth and protein synthesis during which the cell readies itself for mitosis.
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