Ramil Ephrem L.

Sanchez XII-Newton
1. Phases of Cell Cycle
Introduction

Have you ever watched a caterpillar turn into a butterfly? If so,

you’re probably familiar with the idea of a life cycle. Butterflies go

through some spectacular life cycle transitions—turning from something

that looks like a lowly worm into a glorious creature that floats on

the breeze. Other organisms, from humans to plants to bacteria, also

have a life cycle: a series of developmental steps that an individual

goes through from the time it is born until the time it reproduces.

The cell cycle can be thought of as the life cycle of a cell. In other

words, it is the series of growth and development steps a cell

undergoes between its “birth”—formation by the division of a mother

cell—and reproduction—division to make two new daughter cells.

Stages of the cell cycle

To divide, a cell must complete several important tasks: it must grow,

copy its genetic material (DNA), and physically split into two

daughter cells. Cells perform these tasks in an organized, predictable

series of steps that make up the cell cycle. The cell cycle is a

cycle, rather than a linear pathway, because at the end of each go-

round, the two daughter cells can start the exact same process over

again from the beginning.

In eukaryotic cells, or cells with a nucleus, the stages of the cell

cycle are divided into two major phases: interphase and the mitotic
 (M) phase.

 During interphase, the cell grows and makes a copy of its DNA.

 During the mitotic (M) phase, the cell separates its DNA into two sets

and divides its cytoplasm, forming two new cells.

Interphase

Let’s enter the cell cycle just as a cell forms, by division of its

mother cell. What must this newborn cell do next if it wants to go on

and divide itself? Preparation for division happens in three steps:

 G1 phase. During G1 phase, also called the first gap phase, the cell

grows physically larger, copies organelles, and makes the molecular

building blocks it will need in later steps.

In the great majority of cases, cells do indeed grow before

division. However, in certain situations during development, cells may

intentionally split themselves up into smaller and smaller pieces over

successive rounds of cell division. For instance, this happens in very

early development of an African clawed frog (Xenopus laevis) embryo.

See the end of the article for a video of cell divisions in early frog

embryos.

 S phase. In S phase, the cell synthesizes a complete copy of the DNA

in its nucleus. It also duplicates a microtubule-organizing structure

called the centrosome. The centrosomes help separate DNA during M

phase.
 G2 phase. During the second gap phase, or G2 phase, the cell grows

more, makes proteins and organelles, and begins to reorganize its

contents in preparation for mitosis. G2 phase ends when mitosis begins.

The G1, S, and G2 together are known as interphase. The

prefix inter- means between, reflecting that interphase takes place

between one mitotic (M) phase and the next.

Image of the cell cycle. Interphase is composed of G1 phase (cell

growth), followed by S phase (DNA synthesis), followed by G2 phase

(cell growth). At the end of interphase comes the mitotic phase, which

is made up of mitosis and cytokinesis and leads to the formation of two

daughter cells. Mitosis precedes cytokinesis, though the two processes

typically overlap somewhat.

M phase

During the mitotic (M) phase, the cell divides its copied DNA and

cytoplasm to make two new cells. M phase involves two distinct

division-related processes: mitosis and cytokinesis.

In mitosis, the nuclear DNA of the cell condenses into visible

chromosomes and is pulled apart by the mitotic spindle, a specialized

structure made from microtubules. Mitosis takes place in four stages:

prophase (sometimes divided into early prophase and prometaphase),

metaphase, anaphase, and telophase. You can learn more about these

stages in the video on mitosis.

In cytokinesis, the cytoplasm of the cell is split in two, making two

new cells. Cytokinesis usually begins just as mitosis is ending, with

a little overlap. Importantly, cytokinesis takes place differently in

animal and plant cells.

 Cytokinesis in animal and plant cells.

In an animal cell, a contractile ring of cytoskeletal fibers forms at

the middle of the cell and contracts inward, producing an indentation

called the cleavage furrow. Eventually, the contractile ring pinches

the mother cell in two, producing two daughter cells.

In a plant cell, vesicles derived from the Golgi apparatus move to the

middle of the cell, where they fuse to form a structure called the cell

plate. The cell plate expands outwards and connects with the side walls

of the cell, creating a new cell wall that partitions the mother cell

to make two daughter cells.

 In animals, cell division occurs when a band of cytoskeletal fibers

called the contractile ring contracts inward and pinches the cell in

two, a process called contractile cytokinesis. The indentation

produced as the ring contracts inward is called the cleavage furrow.

Animal cells can be pinched in two because they’re relatively soft and

squishy.

 Plant cells are much stiffer than animal cells; they’re surrounded by

a rigid cell wall and have high internal pressure. Because of this,

plant cells divide in two by building a new structure down the middle

of the cell. This structure, known as the cell plate, is made up of

plasma membrane and cell wall components delivered in vesicles, and it

partitions the cell in two.

Cell cycle exit and G0

What happens to the two daughter cells produced in one round of the

cell cycle? This depends on what type of cells they are. Some types of

cells divide rapidly, and in these cases, the daughter cells may
immediately undergo another round of cell division. For instance, many

cell types in an early embryo divide rapidly, and so do cells in a

tumor.

Other types of cells divide slowly or not at all. These cells may exit
the G1 phase and enter a resting state called G0 phase. In G0, a cell
is not actively preparing to divide, it’s just doing its job. For
instance, it might conduct signals as a neuron (like the one in the
drawing below) or store carbohydrates as a liver cell. G0 is a
permanent state for some cells, while others may re-start division if
they get the right signals.
 2. Process of Cell Division
Where Do Cells Come From?

Sometimes you accidentally bite your lip or skin your knee, but in a
matter of days the wound heals. Is it magic? Or, is there another
explanation?

Every day, every hour, every second one of the most important events
in life is going on in your body—cells are dividing. When cells
divide, they make new cells. A single cell divides to make two cells
and these two cells then divide to make four cells, and so on. We call
this process "cell division" and "cell reproduction," because new
cells are formed when old cells divide. The ability of cells to divide
is unique for living organisms.

Why Do Cells Divide?

Cells divide for many reasons. For example,

when you skin your knee, cells divide to
replace old, dead, or damaged cells. Cells
also divide so living things can grow. When
organisms grow, it isn't because cells are
getting larger. Organisms grow because cells
are dividing to produce more and more cells.
In human bodies, nearly two trillion cells
divide every day.

How Many Cells Are in Your Body?

You and I began as a single cell, or what you would call an egg. By
the time you are an adult, you will have trillions of cells. That
number depends on the size of the person, but biologists put that
number around 37 trillion cells. Yes, that is trillion with a "T."

How Do Cells Know When to Divide?

In cell division, the cell that is dividing is called the "parent"

cell. The parent cell divides into two "daughter" cells. The process
then repeats in what is called the cell cycle.

Cells regulate their division by communicating with each other using

chemical signals from special proteins called cyclins. These signals
act like switches to tell cells when to start dividing and later when
to stop dividing. It is important for cells to divide so you can grow
and so your cuts heal. It is also important for cells to stop dividing
at the right time. If a cell can not stop dividing when it is
supposed to stop, this can lead to a disease called cancer.
Some cells, like skin cells, are constantly dividing. We need to
continuously make new skin cells to replace the skin cells we lose.
Did you know we lose 30,000 to 40,000 dead skin cells every minute?
That means we lose around 50 million cells every day. This is a lot
of skin cells to replace, making cell division in skin cells is so
important. Other cells, like nerve and brain cells, divide much less
often.

How Cells Divide

Depending on the type of cell, there are two ways cells divide—mitosis
and meiosis. Each of these methods of cell division has special
characteristics. One of the key differences in mitosis is a single
cell divides into two cells that are replicas of each other and have
the same number of chromosomes. This type of cell division is good for
basic growth, repair, and maintenance. In meiosis a cell divides into
four cells that have half the number of chromosomes. Reducing the
number of chromosomes by half is important for sexual reproduction and
provides for genetic diversity.

Mitosis Cell Division

Mitosis is how somatic—or non-reproductive cells—divide. Somatic cells

make up most of your body's tissues and organs, including skin,
muscles, lungs, gut, and hair cells. Reproductive cells (like eggs)
are not somatic cells.

In mitosis, the important thing to remember is that the daughter cells

each have the same chromosomes and DNA as the parent cell. The
daughter cells from mitosis are called diploid cells. Diploid cells
have two complete sets of chromosomes. Since the daughter cells have
exact copies of their parent cell's DNA, no genetic diversity is
created through mitosis in normal healthy cells.

Mitosis cell division creates two genetically identical daughter

diploid cells. The major steps of mitosis are shown here. (Image by
Mysid from Science Primer and National Center for Biotechnology
Information)
The Mitosis Cell Cycle

Before a cell starts dividing, it is in the "Interphase." It seems

that cells must be constantly dividing (remember there are 2 trillion
cell divisions in your body every day), but each cell actually spends
most of its time in the interphase. Interphase is the period when a
cell is getting ready to divide and start the cell cycle. During this
time, cells are gathering nutrients and energy. The parent cell is
also making a copy of its DNA to share equally between the two
daughter cells.

The mitosis division process has several steps or phases of the cell
cycle—interphase, prophase, prometaphase, metaphase, anaphase,
telophase, and cytokinesis—to successfully make the new diploid cells.

Meiosis Cell Division

Meiosis is the other main way cells divide. Meiosis is cell division
that creates sex cells, like female egg cells or male sperm
cells. What is important to remember about meiosis? In meiosis, each
new cell contains a unique set of genetic information. After meiosis,
the sperm and egg cells can join to create a new organism.
Meiosis is why we have genetic diversity in all sexually reproducing
organisms. During meiosis, a small portion of each chromosome breaks
off and reattaches to another chromosome. This process is called
"crossing over" or "genetic recombination." Genetic recombination is
the reason full siblings made from egg and sperm cells from the same
two parents can look very different from one another.

The meiosis cell cycle has two main stages of division -- Meiosis I
and Meiosis II. The end result of meiosis is four haploid daughter
cells that each contain different genetic information from each other
and the parent cell. Click for more detail. (Image from Science Primer
from the National Center for Biotechnology Information.)

The Meiosis Cell Cycle

Meiosis has two cycles of cell division, conveniently called Meiosis I

and Meiosis II. Meiosis I halves the number of chromosomes and is also
when crossing over happens. Meiosis II halves the amount of genetic
information in each chromosome of each cell. The end result is four
daughter cells called haploid cells. Haploid cells only have one set
of chromosomes - half the number of chromosomes as the parent cell.

Before meiosis I starts, the cell goes through interphase. Just like
in mitosis, the parent cell uses this time to prepare for cell
division by gathering nutrients and energy and making a copy of its
DNA. During the next stages of meiosis, this DNA will be switched
around during genetic recombination and then divided between four
haploid cells.
 3. Difference Between Mitosis, Amitosis and Meiosis
 Mitosis

-is a type of cell division in which one cell (the mother)

divides to produce two new cells (the daughters) that are genetically

identical to itself. In the context of the cell cycle, mitosis is the

part of the division process in which the DNA of the cell's nucleus is

split into two equal sets of chromosomes.

The great majority of the cell divisions that happen in your body

involve mitosis. During development and growth, mitosis populates an

organism’s body with cells, and throughout an organism’s life, it

replaces old, worn-out cells with new ones. For single-celled

eukaryotes like yeast, mitotic divisions are actually a form of

reproduction, adding new individuals to the population.

In all of these cases, the “goal” of mitosis is to make sure that each

daughter cell gets a perfect, full set of chromosomes. Cells with too

few or too many chromosomes usually don’t function well: they may not

survive, or they may even cause cancer. So, when cells undergo

mitosis, they don’t just divide their DNA at random and toss it into

piles for the two daughter cells. Instead, they split up their

duplicated chromosomes in a carefully organized series of steps.

 Amitosis

- Amitosis (a- + mitosis), also called 'karyostenosis', 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.

It does not involve maximal condensation of chromatin into

chromosomes, observable by light microscopy as they line up in pairs

along the metaphase plate. It does not involve these paired structures

being pulled in opposite directions by a mitotic spindle to form

daughter cells. Rather, it effects nuclear proliferation without the

involvement of chromosomes, unsettling for cell biologists who have

come to rely on the mitotic figure as reassurance that chromatin is

being equally distributed into daughter cells. The phenomenon of

amitosis, even though it is an accepted as occurring in ciliates,

continues to meet with skepticism about its role in mammalian cell

proliferation, perhaps because it lacks the reassuring iconography of

mitosis. Of course the relatively recent discovery of copy number

variations (CNV's) in mammalian cells within an organ, significantly

challenges the age-old assumption that every cell in an organism must

inherit an exact copy of the parental genome to be functional. Rather

than CNV's resulting from mitosis gone awry, some of this variation

may arise from amitosis, and may be both desirable and necessary.

Furthermore, it is well to remember that ciliates possess a mechanism

for adjusting copy numbers of individual genes during amitosis of the

macronucleus.(Prescott, 1994)
  Meiosis

- Meiosis is a type of cell division that, in humans, occurs only

in male testes and female ovary tissue, and, together with
fertilization, it is the process that is characteristic of sexual
reproduction. Meiosis serves two important purposes: it keeps the
number of chromosomes from doubling each generation, and it provides
genetic diversity in offspring. In this it differs from mitosis, which
is the process of cell divisionthat occurs in all somatic cells.

-All of our somatic cells except the egg and sperm cells contain
twenty-three pairs of chromosomes, for a total of forty-six individual
chromosomes. This number, twenty-three, is known as the diploid
number. If our egg and sperm cells were just like our somatic cells
and contained twenty-three pairs of chromosomes, their fusion during
fertilization would create a cell with forty-six chromosome pairs, or
ninety-two chromosomes total. To prevent that from happening and to
ensure a stable number of chromosomes throughout the generations, a
special type of cell division is needed to halve the number of
chromosomes in egg and sperm cells. This special process is meiosis.

Meiosis creates haploid cells, in which there are twenty-three

individual chromosomes, without any pairing. When gametes fuse at
conception to produce a zygote, which will turn into a fetus and
eventually into an adult human being, the chromosomes containing the
mother's and father's genetic material combine to form a single
diploid cell. The specialized diploid cells that will eventually
undergo meiosis to produce the gametes are called primary oocytes in
the female ovary and primary spermatocytes in the male testis. They
are set aside from somatic cells early in the course of fetal
development.

Even though meiosis is a continuous process in reality, it is

convenient to describe it as occurring in two separate rounds of
nuclear division. In the first round (meiosis I), the two versions of
each chromosome, called homologues or homologous chromosomes, pair up
along their entire lengths and thus enable genetic material to be
exchanged between them. This exchange process is called crossing
over and contributes greatly to the amount of genetic variation that
we see between parents and their children. Subsequently, the two
homologues are pulled toward opposite ends of their surrounding cell,
thus creating a haploid cluster of chromosomes at each pole, at which
point division occurs, separating the two clusters. Meiosis I is
therefore the actual reduction division. At the end of meiosis I, each
chromosome is still composed of two sister strands (chromatids) held
together by a particular DNA sequence of about 220 nucleotides, called
the centromere. The centromere has a disk-shaped protein molecule
(kinetochore) attached to it that is important for the separation of
the sister chromatids in the second round of meiosis (meiosis II).
Meiosis II is essentially the same division process as mitosis.
Through the separation of the two sister chromatids, a total of four
daughter cells, each with a haploid set of chromosomes, are created.