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Title: Cell cycle complete notes
Description: Complete description of cell cycle along with stages of mitosis and meiosis.
Description: Complete description of cell cycle along with stages of mitosis and meiosis.
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The cell cycle, mitosis and meiosis
Learning objective
This learning material is about the life cycle of a cell and the series of stages by which
genetic materials are duplicated and partitioned to produce two daughter cells with the same
genetic component as the parent cell
...
G1 phase
...
At a certain point - the restriction
point - the cell is committed to division and moves into the S phase
...
DNA synthesis replicates the genetic material
...
G2 phase
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M phase
...
The period between mitotic divisions - that is, G1, S and G2 - is known as interphase
...
Chromosomes replicated during the S phase are divided
in such a way as to ensure that each daughter cell receives a copy of every chromosome
...
The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then
separates the sister chromatids to produce an even partitioning of the genetic material
...
In some single-celled organisms mitosis forms the basis of asexual reproduction
...
Mitotic divisions of the zygote and daughter cells are then
responsible for the subsequent growth and development of the organism
...
Mitosis, although a continuous process, is conventionally divided into five stages: prophase,
prometaphase, metaphase, anaphase and telophase
...
The nuclear membrane breaks down to form a
number of small vesicles and the nucleolus disintegrates
...
The centrosomes organise the production of microtubules that form the spindle
fibres that constitute the mitotic spindle
...
Each replicated chromosome can now be seen to consist of two identical
chromatids (or sister chromatids) held together by a structure known as the centromere
...
This region of the mitotic spindle
is known as the metaphase plate
...
Individual spindle fibres bind to a
kinetochore structure on each side of the centromere
...
Metaphase
The chromosomes align themselves along the metaphase plate of the spindle apparatus
...
The centromeres divide, and the sister chromatids of each
chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled
by spindle fibres attached to the kinetochore regions
...
(It is the alignment and separation in metaphase and
anaphase that is important in ensuring that each daughter cell receives a copy of every
chromosome
...
The nuclear membrane reforms around the chromosomes grouped at either pole of the cell,
the chromosomes uncoil and become diffuse, and the spindle fibres disappear
...
In plants a cell plate forms along the line of
the metaphase plate; in animals there is a constriction of the cytoplasm
...
Meiosis
Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes
(which contain a single copy of each chromosome) from diploid cells (which contain two
copies of each chromosome)
...
As in mitosis,
meiosis is preceded by a process of DNA replication that converts each chromosome into two
sister chromatids
...
Prophase I
The homologous chromosomes pair and exchange DNA to form recombinant chromosomes
...
Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of
chromosomes (bivalents) consisting of four chromatids (tetrads)
...
chiasma)
...
Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends
of the chromosomes
...
Metaphase I
Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase
plate
...
(This is a source of genetic variation through
random assortment, as the paternal and maternal chromosomes in a homologous pair are
similar but not identical
...
Human beings have 23 different chromosomes, so the
number of possible combinations is 223, which is over 8 million
...
Telophase I
The chromosomes become diffuse and the nuclear membrane reforms
...
Meiosis I is a
reduction division: the original diploid cell had two copies of each chromosome; the newly
formed haploid cells have one copy of each chromosome
...
Meiosis generates genetic diversity through:
the exchange of genetic material between homologous chromosomes during Meiosis I
the random alignment of maternal and paternal chromosomes in Meiosis I
the random alignment of the sister chromatids at Meiosis II
Meiosis in females
This work is licensed under a Creative Commons Licence
Title: Cell cycle complete notes
Description: Complete description of cell cycle along with stages of mitosis and meiosis.
Description: Complete description of cell cycle along with stages of mitosis and meiosis.