Download The Summary of the Cell Division

The Summary of the Cell Division
1. Significance of the Cell proliferation and its regulation
a. For the growth and development of a multicellular organism, and for the generation of
offspring;
b. Produce new organisms in unicellular species;
c. Renew the aging, apoptotic cells, and damaged tissue;
2. Definition of the cell cycle: The cell cycle is the process by which a cell grows, duplicates
its DNA, and divides into identical daughter cells.
3. The cell cycle is divided into two main parts: interphase and mitosis . Interphase is
subdivided into three phases: gap phase 1 (G1), synthesis (S), and gap phase 2 (G2).
4. Cells divides into 3 categories of : Cycling cells(Dividing continuously, constantly—Stem
cells, skin cells, embryonic cells ;, G0 cells (Do not divide normally, cell never divides in
adult; out of cell cycle in Go but divide when given an appropriate stimulus:liver cells,
lymphocytes) and Terminal cells(Highly specialized, have lost the ability to divide until they
die;muscle cells, red blood cells, nerve cells )
5. Cell Cycle
Interphase - the phase of a cell's life cycle in which DNA and histon are replicated.
includes:
G1 phase:Gap phase 1 begins at the completion of mitosis and cytokinesis and lasts until the
beginning of S phase. This phase is generally the longest of the four cell cycle phases and is
quite variable in length. During this phase, the cell chooses either to replicate its
deoxyribonucleic acid (DNA) or to exit the cell cycle and enter a quiescent state (the G0 phase).
S phase:Replication of the chromosomes is restricted to one specific portion of interphase,
called S phase (DNA synthesis phase), which typically lasts about 6 h. In mammalian cells, the
start of S phase—the actual initiation of DNA synthesis—takes place several hours after the cell
has committed to carrying out DNA synthesis. During S phase, each chromosome replicates
exactly once to form a pair of physically linked sister chromatids. In animal cells, a pair of
centrioles is also duplicated during S phase. See also Chromosome; Genetics.
G2 phase:The portion of interphase that follows S phase is called gap phase 2. Some cells can
exit the cell cycle from G2 phase, just as they can from G1 phase.
M phase
M phase includes the overlapping processes of mitosis and cytokinesis. Mitosis is divided into
five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis usually
begins during anaphase and ends at a point after the completion of mitosis. At the end of
cytokinesis, the parent cell has formed its two G1 phase progeny and the cell is ready to repeat
the cycle. See also Cytokinesis; Mitosis
Prophase –
a. the nuclear envelope dissolves and nucleolus disappear,
b. the phase of mitosis in which the duplicated chromosomes condense,
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c. centrioles divide and move to opposite ends of the cell.
d. Spindle formed
Metaphase - the phase of mitosis in which the chromosomes line up at the equator (the central
plane) Golgi, ER reconstruct
of the cell.
Anaphase - the phase of mitosis in which the chromosomes begin to separate.
Telophase - the last phase of mitosis, when the chromosomes migrate to opposite ends of the cell,
two new nuclear envelopes and Nucleolus reassemble, and the chromosomes uncoil to
chromatin.
6. Meiosis: reduces the number of chromosomes by half.Daughter cells differ from parent,
and each other.
Meiosis involves two divisions, both divide into prophase, metaphase, anaphase and telophase.
Prophase futher divides into 5 subphases:
Leptotene: The chromosomes strands are very elongated,thin,visible single strands,we call it
“slender stage” too.The visible small dark granules are called CHROMOMERES.
Zygotene: Homologous chromosomes pairs off in zipper fashion,starting at various points
along this line,we call the phenomenon SYNAPSIS.The chromosome pairs is called
BIVALENTS
Pachytene: Chromosomes continue to shorten during this stage,each synapsis figure has four
strands called TETRADS.Two closely related non-sister chromatids changed one or several
parts by breaking and rejoining.We called it CROSSING OVER.
Diplotene: The stage also lenown as “doubled thread”.tetrads can be observed
microscopically,the doubled homologous repel each other.The points where crossing over had
occurred between non-sister chromatids are called CHIASMATA.
Diakinesis: Chromosomes shorten and thicken.The CHIASMATA moves toward the end of the
chromatids at the same time homologous pairs continue to repel further.So we can see the
shape like number “8”, “o” or “+”
7. There are 6 major take home lessons from the story of MEIOSIS.Let me summarize the
major features:
1.Chromosomes duplicate only at INTERPHASEⅠ, not at INTERPHASEⅡ.
2.SYNAPSIS is the process that brings the homologous strands into intimate contact.
3.These homologous strands then CROSS OVER and RECOMBINE.
4.The four stranded BIVALENTS are separated by the two ANAPHASES. During
ANAPHASESⅠ, the homologous chromosomes separate. During ANAPHASEⅡ , the sister
chromosomes separate.
5.The four cell that result from the two division process carry only half the number of
chromosomes from the original cell.
6.MEIOSIS is where increasing variation in the offspring take place such as:
← cross over
↑ random line up of chromosomes at METAPHASEⅠ so that paternal
and maternal chromosomes are mixed by the time a cell enters MEIOSISⅡ
→
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random selection of cells with polar bodies to be the egg cells take place.
8. Comparison of Meiosis with Mitosis
Before mitosis and meiosis, DNA replication occurs only once during the interphase prior to
cell division.
Mitosis requires one division, and Meiosis requires two divisions.
Two diploid daughter cells result from mitosis, and Four haploid daughter cells result from
meiosis.
Daughter cells from mitosis are genetically identical to parental cells
Daughter cells from meiosis are not genetically identical to parental cells.
Mitosis occurs in all somatic cells for growth and
repair.
Meiosis occurs only in the reproductive organs for the production of gametes.
Comparison of Meiosis I to Mitosis
Meiosis I:
Prophase I - pairing of homologous chromosomes
Metaphase I – homologous pairs line up at metaphase plate
Anaphase I – homologous chromosomes separate
Telophase I – daughter cells are haploid
Mitosis:
Prophase has no such pairing
Metaphase – chromosomes align at metaphase plate
Anaphase – sister chromatids separate
Telophase – diploid cells
Comparison of Meiosis II to Mitosis
The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the
haploid number of chromosomes.
At the end of telophase II of meiosis II, there are four haploid daughter cells that are not
genetically identical.
At the end of mitosis, there are two diploid daughter cells that are identical.
Genetic Recombination in Humans
There are three ways in which meiosis and fertilization ensure that a child has a different
combination of genes from that of either parent:
1. Independent assortment of chromosomes during metaphase I
2. Crossing-over during prophase I
3. Upon fertilization, recombination of chromosomes
from different individuals (via their
gametes)
occurs.
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