Cell life cycle. Cell division. Mitosis. Meiosis. Amitosis. Endomitosis

1. Cell life cycle. Cell division. Mitosis. Meiosis. Amitosis. Endomitosis.

MD SHIHAB SHAHRIAR
LA1-201(2)

2. Cell Life Cycle

• The cell cycle, or cell-division cycle, is the series of events
that take place in a cell that cause it to divide into two daughter
cells. These events include the duplication of its DNA (DNA
replication) and some of its organelles, and subsequently the
partitioning of its cytoplasm and other components into two
daughter cells in a process called cell division.

3. Cell division

• Cell division is the process by which a parent cell divides into two or
more daughter cells.[1] Cell division usually occurs as part of a
larger cell cycle. In eukaryotes, there are two distinct types of cell
division: a vegetative division, whereby each daughter cell is
genetically identical to the parent cell (mitosis), and a reproductive
cell division, whereby the number of chromosomes in the daughter
cells is reduced by half to produce
haploid gametes (meiosis).[2] Meiosis results in four haploid daughter
cells by undergoing one round of DNA replication followed by two
divisions. Homologous chromosomes are separated in the first
division, and sister chromatids are separated in the second division.
Both of these cell division cycles are used in the process of sexual
reproduction at some point in their life cycle. Both are believed to be
present in the last eukaryotic common ancestor.

4.

• Prokaryotes (bacteria and archaea) usually undergo a vegetative cell division known
as binary fission, where their genetic material is segregated equally into two daughter
cells. While binary fission may be the means of division by most prokaryotes, there are
alternative manners of division, such as budding, that have been observed. All cell
divisions, regardless of organism, are preceded by a single round of DNA replication.
• For simple unicellular microorganisms such as the amoeba, one cell division is equivalent
to reproduction – an entire new organism is created. On a larger scale, mitotic cell division
can create progeny from multicellular organisms, such as plants that grow from cuttings.
Mitotic cell division enables sexually reproducing organisms to develop from the onecelled zygote, which itself was produced by meiotic cell division from gametes.[3][4] After
growth, cell division by mitosis allows for continual construction and repair of the
organism.[5] The human body experiences about 10 quadrillion cell divisions in a
lifetime.[6]
• The primary concern of cell division is the maintenance of the original cell's genome.
Before division can occur, the genomic information that is stored in chromosomes must be
replicated, and the duplicated genome must be separated cleanly between cells.[7] A great
deal of cellular infrastructure is involved in keeping genomic information consistent
between generations.

5. Types of cell division

• Cells are the most basic units of life, and every living organism is made up of one or more cells. These
cells reproduce by copying their genetic information and undergoing cell division, where the parent cell
gives rise to two daughter cells.
• There are three major types of cell division, which are:
• Binary fission
• Mitosis
• Meiosis
• Whereas binary fission takes place in prokaryotic cells of simple single-celled organisms such as
bacteria. Mitosis and meiosis take place in eukaryotic cells and are more advanced.
• Although there are differences between prokaryotes and eukaryotes, there are a number of features that
are common during their processes of cell division.

6. Mitosis cell division

• In cell biology, mitosis is a part of the cell cycle, in which,
replicated chromosomes are separated into two new nuclei. Cell
division gives rise to genetically identical cells in which the total
number of chromosomes is maintained.[1] In general, mitosis
(division of the nucleus) is preceded by the S stage
of interphase (during which the DNA is replicated) and is often
followed by telophase and cytokinesis; which divides
the cytoplasm, organelles and cell membrane of one cell into
two new cells containing roughly equal shares of these cellular
components.The different stages of Mitosis all together define
the mitotic (M) phase of an animal cell cycle—the division of
the mother cell into two daughter cells genetically identical to
each other.

7. Mitosis cell division in animal cell

8.

• The process of mitosis is divided into stages corresponding to the
completion of one set of activities and the start of the next. These
stages are prophase, prometaphase, metaphase, anaphase,
and telophase. During mitosis, the chromosomes, which have
already duplicated, condense and attach to spindle fibers that pull
one copy of each chromosome to opposite sides of the cell.[3] The
result is two genetically identical daughter nuclei. The rest of the cell
may then continue to divide by cytokinesis to produce two daughter
cells.[4] The different phases of mitosis can be visualized in real time,
using live cell imaging.[5] Producing three or more daughter cells
instead of the normal two is a mitotic error called tripolar mitosis or
multipolar mitosis (direct cell triplication / multiplication).[6] Other
errors during mitosis can induce apoptosis (programmed cell death)
or cause mutations. Certain types of cancer can arise from such
mutations.[7]

9.

• Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which
lack a nucleus, divide by a different process called binary
fission. Mitosis varies between organisms.[8] For
example, animal cells undergo an "open" mitosis, where
the nuclear envelope breaks down before the chromosomes
separate, whereas fungi undergo a "closed" mitosis, where
chromosomes divide within an intact cell nucleus.[9] Most animal
cells undergo a shape change, known as mitotic cell rounding,
to adopt a near spherical morphology at the start of mitosis.
Most human cells are produced by mitotic cell division.
Important exceptions include the gametes –
sperm and egg cells – which are produced by meiosis.

10. Onion (Allium) cells in different phases of the cell cycle enlarged 800 diameters

11. Meiosis cell division

MEIOSIS from Greek μείωσις, meiosis, meaning "lessening") is a
special type of cell division of germ cells in sexuallyreproducing organisms used to produce the gametes, such
as sperm or egg cells. It involves two rounds of division that
ultimately result in four cells with only one copy of each paternal
and maternal chromosome (haploid). Additionally, prior to the
division, genetic material from the paternal and maternal copies
of each chromosome is crossed over, creating new combinations
of code on each chromosome.[1] Later on, during fertilisation, the
haploid cells produced by meiosis from a male and female will
fuse to create a cell with two copies of each chromosome again,
the zygote.

12. Meiosis

13.

• In meiosis, the chromosome or chromosomes duplicate (during interphase)
and homologous chromosomes exchange genetic information (chromosomal crossover)
during the first division, called meiosis I. The daughter cells divide again in meiosis II,
splitting up sister chromatids to form haploid gametes. Two gametes fuse
during fertilization, creating a diploid cell with a complete set of paired chromosomes.
• In meiosis, DNA replication is followed by two rounds of cell division to produce four
daughter cells, each with half the number of chromosomes as the original parent
cell.[1] The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis
begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so
that it consists of two identical sister chromatids, which remain held together through
sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or
"meiotic S-phase". Immediately following DNA replication, meiotic cells enter a
prolonged G2-like stage known as meiotic prophase. During this time, homologous
chromosomes pair with each other and undergo genetic recombination, a programmed
process in which DNA may be cut and then repaired, which allows them to exchange
some of their genetic information. A subset of recombination events results in crossovers,
which create physical links known as chiasmata (singular: chiasma, for the Greek
letter Chi (X)) between the homologous chromosomes. In most organisms, these links
can help direct each pair of homologous chromosomes to segregate away from each
other during Meiosis I, resulting in two haploid cells that have half the number of
chromosomes as the parent cell.

14.

• During meiosis II, the cohesion between sister chromatids is released and they segregate
from one another, as during mitosis. In some cases, all four of the meiotic products
form gametes such as sperm, spores or pollen. In female animals, three of the four
meiotic products are typically eliminated by extrusion into polar bodies, and only one cell
develops to produce an ovum. Because the number of chromosomes is halved during
meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two
copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis
and fertilization enable sexual reproduction, with successive generations maintaining the
same number of chromosomes. For example, diploid human cells contain 23 pairs of
chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and
half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one
set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting
zygote is once again diploid, with the mother and father each contributing 23
chromosomes. This same pattern, but not the same number of chromosomes, occurs in
all organisms that utilize meiosis.
• Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms
(which are all eukaryotes), including animals, plants and fungi.[3][4][5] It is an essential
process for oogenesis and spermatogenesis.

15. Amitosis

• Amitosis (a- + mitosis), also called 'karyostenosis' or direct cell
division or binary fission. It is cell proliferation that does not
occur by mitosis, the mechanism usually identified as essential
for cell division in eukaryotes. The polyploid macronucleus
found in ciliates divides amitotically. While normal mitosis
results in a precise division of parental alleles, amitosis results
in a random distribution of parental alleles. Ploidy levels of
>1000 in some species means both parental alleles can be
maintained over many generations, while species with fewer
numbers of each chromosome will tend to become homozygous
for one or the other parental allele through a process known as
phenotypic or allelic assortment.

16. Amitosis

17.

• The research that is accumulating about amitosis suggests that such
processes are, indeed, involved in the production of the breathtaking
37 trillion or so cells[22] in humans, perhaps particularly during the
fetal and embryonic phases of development when the majority of
these cells are produced, perhaps within the complexity of
implantation, perhaps when large numbers of cells are being
differentiated, and perhaps in cancerous cells.
• A word of caution: some examples of cell division formerly thought to
belong to this "non-mitotic" class, such as the division of some
unicellular eukaryotes, may actually occur by the process of "closed
mitosis" different from open or semi-closed mitotic processes, all
involving mitotic chromosomes and classified by the fate of
the nuclear envelope.

18. Endomitosis

• endomitosis The replication of chromosomes in the absence of
cell or nuclear division, resulting in numerous copies within
each cell. It occurs notably in the salivary
glands of Drosophila and other flies. Cells in these tissues
contain giant chromosomes (see polyteny), each consisting of
over a thousand intimately associated, or synapsed,
chromatids.
• endomitosis The doubling of the chromosomes within
a nucleus that does not divide. The doubling may be repeated a
number of times in a single nucleus. It occurs mainly in insects,
where different adult tissues have specific polyploid numbers; it
also occurs in some vertebrate tissues.