CGAR

Biology 11 – 05Prof. Wilma Giol

The cell cycle is the life of a somatic, eukaryotic cell.

A cell can go into cell division after receiving a an extra-cellular signal that tells it to.

It consists of cell growth (Interphase) and cell division (Mitosis and Cytokinesis).

There are four phases in the cell cycle: the G1 phase, S phase, G2 phase, and the M phase.

After all of these phases have passed, the cell has already become two identical daughter cells.

The “G” stands for gap or growth.During this phase, biosynthetic

activities that were suspended during mitosis are resumed and the synthesis of various enzymes needed during the S Phase takes place.

Or “synthesis” phase. DNA synthesis is the

starting point of this phase. After it is completed, each chromosome has two sister chromatids.

The processes of RNA transcription and protein synthesis are slowed down. An exception is the production of the histone protein.

Significant protein synthesis occurs during this phase. Mainly the production of microtubules.

This phase allows for DNA repair to take place if necessary. It is important in avoiding the proliferation of damaged cells which can lead to cancer.

During the M Phase there are two processes: Mitosis and Cytokinesis.

When they are completed, the result is two identical daughter cells.

Each of these cells are usually at the beginning of interphase (G1 phase) and continue to grow until they divide into

more daughter cells.

Sometimes cells can stop growing temporarily or permanently after they result from mitosis.

When this happens, they are at he G0 phase.

An example of a cell in permanent G0 phase is a Neuron.

There are two molecules that collaborate to regulate the cell cycle: Cyclins Cell-Dependent Kinases

These molecules are conserved in most eukaryotes. They were discovered by Leland H. Hartwell, R.

Timothy Hunt, and Paul M. Nurse who won the 2001 Nobel Prize in Physiology or Medicine for that discovery.

When these two molecules interact, they produce enzymes and proteins that control the cell’s processes.

The result of these two molecules joining is a cyclin-CDK complex.

It is regulated by kinases and phosphatases.

Mitosis is the process in which the cell creates a duplicate of its chromosomes in its nucleus to create a nucleus replica. The mitosis process is exclusive to eukaryotes, because prokaryotes lack a nucleus.

Mitosis is composed of 4 phases: Prophase Metaphase Anaphase Telophase

This process produces somatic cells, a cell that contains a complete set of chromosomes, unlike the gametes, and compose the body, its parts and organs except reproductive cells.

This phase takes 50 to 60 percent of the total time of the Mitosis phase

During this phase, the nucleolus disappears; chromatin, genetic material in the nucleus, condenses into chromosomes (two chromatins joined near the center by a centromere). After the chromosomes are formed, the nuclear envelope breaks. The two centrosomes near the nucleus move to opposite sites of the cell while microtubes called mitotic spindles start to grow from them.

This phase lasts only a few minutesDuring this phase the chromosomes

line up in the center of the cell and the mitotic spindles’s kinetchore connect to the chromosomes at the centromeres of each chromosome.

The chromosomes separate into what is referred as daughter-chromosomes. Each daughter-chromosome is pulled by the spindle microtube towards their respective centrosome. Some microtubes grow longer, further pushing the centrosomes apart.

During this phase, Chromosomes are close to the centrosomes, the nuclear envelope forms around the chromosomes and the chromosomes revert back to chromatin. Microtubes spindles disappear. The cell has finished the genetic duplication and assesment of the division process in the cell is completed.

Almost simultaneously, Cytokinesis occurs.

Cykokinesis is the physical division of the Mother cell into two individual cells.

This process occurs during the Telophase, after all the genetic material needed by the cell has been duplicated and organized.

During this process, a binuclear cell splits its cytoplasm, forming two new separate cells.

Binary Fission Binary fission is used by most prokaryotes

for asexual reproduction. This process replicates the original, or mother, cell, to produce two identical daughter cells.

In Binary Fission the DNA of the mother cell is replicated . Each set of chromosomes attaches to the plasma membrane, the outer wall of the cell. The cell then elongates and eventually splits, creating two daughter cells.

Budding Budding is used by yeast and some

bacteria as a method of reproduction. During budding, a bud grows from the

mother cell. The mother cell then duplicates its DNA and transports it to the buds and the bud eventually separates from the Mother cell and becomes and individual cell.

Process in which a parent (diploid, 2N) somatic cell divides into four daughter (haploid, N) cells. It’s a one-way process, unlike Mitosis, which is a cycle.

Meiosis enables organisms to reproduce sexually, because this process creates Gametes.

Meiosis occurs only in eukaryotes, because prokaryotes do not have a nucleus.

Gametes are haploid cells that fuse with another haploid cell to form a zygote, which is a diploid.

Gametes can only be found in organisms that reproduce sexually.

A cell undergoing Meiosis divides two times.

The first division is Meiosis I: Prophase I, Metaphase I,

Anaphase I, Telophase IThe second, Meiosis II:

Prophase II, Metaphase II, Anaphse II, Telophase II

During Meiosis I, the number of cells doubles but each has half the chromosomes of the original cells.

Meiosis II is very similar to mitosis: the number of cells doubles but the chromosomes per cell stay the same.

Interphase:-This process precedes Meiosis.-In this process, DNA replicates.-Each individual chromatid replicates (similar to what happens in mitosis) and is held together to its homologue by a centromere.

Prophase I-The homologous chromsomes pair up, forming a structure similar to the letter “x”, called a tetrad.-Two non-sister chromosomes pair up, and crossing-over occurs between two of the non-sister chromatids. -It’s not visible until the chromosomes separate.-The point at which the two non-sister chromatids join is called the chiasma.

Metaphase-Homologous chromosomes prepare for separation.-Spindle fibers form from the centrosome, from a pair of organelles called centrioles.-The centrioles form poles in the cell, as they set themselves on opposite sides of the cell.-The chromosomes are lined by the spindle fibers at the metaphase plate, a divisory line in the middle of the cell.

Anaphase I-The chiasmata separate.-The chromosomes move to different poles.

Telophase I-Varies between organisms

Nuclear envelopes may form, or Prophase II may start as soon as Telophase I ends-Interkinesis occurs, a resting period for the cell between this phase and Prophase II-DNA replication does not occur

Meiosis II -The two haploids from meiosis I

divide into another two haploids, each with one chromatid, for a total of four haploids, each with different genetic data. This is where the gametes will be formed.

Prophase II -The nuclear envelope disappears.

-The chromatids shorten and thicken once more. -The centrioles (core of the spindle network) move to the poles of each cell again and align for the second meiotic division.

Metaphase II-The centromeres contain two kinetochores, which attach to spindle fibers from each pole. -Instead of one, now two metaphase plates are going to be formed, one for each cell, the new one being perpendicular to the last one, to divide the cell in four segments.

Anaphase II -The chromatids are pulled from

each other, in preparation for the 4 cells. -Each cell will have 1 chromatid, now called sister chromosomes, and are moved to opposing poles.

Telophase II-The chromosomes lengthen and the microtubules from the spindle network disappear once again. -The nuclear envelopes form once more, and cleavage or cell wall formation occurs to divide the cells that are put together, into four heterologous, genetic data-wise cells. >>Meiosis is complete.

It is essential for reproduction in animals, because it is the way the sperm in males and the eggs in females (both also known as gametes) multiply inside the body.

It produces diversity [ploidy, when the male and female chromosomes are put together and/or shared (diploid cells)] which makes a healthy cell.

Without ploidy, the zygotes during fertilization would end up with more chromosomes than they’re supposed to, which brings complications.

Most important aspect of meiosis is the fact that it gives genetic variety to the offspring during reproduction, giving it the ability to adapt to a certain environment.

In mitosis, the two cells that come out of the division are exactly the same in genetic makeup as the original mother cell. They have the very same DNA, so they behave the same. In meiosis, the four cells that come out from the process are completely different, which makes this process important in reproduction because of genetic diversity.

Nondisjunction: when homologues don’t separate in Meiosis I

Can cause:1) Death to embryo2) Having Trisomy 21 (21 instead of

23 chromosomes) which leads to Down’s Syndrome

3) Turner Syndrome (X)4) Klintfelter Syndrome (XXY)

Translocation and deletion: transfer a piece of a chromosome to another or loss of a fragment of chromosome.

Cancer is a type of disease that consists of uncontrolled cell growth.

If a cell continues to divide uncontrollably, it will lead to the creation of a tumor.

There are two types of tumors: Malignant Benign (self-limited growth)

Most are completely harmless to health. Controlled by a capsule surrounding its

surface. Have the potential to become malignant. Examples of benign tumors:

Moles Uterine Fibroids

Three characteristics that differentiate them from benign: Neoplasm (uncontrolled cell

proliferation) Invasion (intrusion on and destruction of

adjacent tissue) Metastasis (spreading to other locations

in the body via blood vessels)

Chemical Carcinogen When two genes are mutated, their functions

are altered. Two genes (responsible for avoiding cancer)

lead to the creation of a tumor when mutated:▪ Proto-Oncogene (which regulate cell growth and

differentiation)▪ Become Oncogene, which don’t regulate cell growth.

▪ Tumor Suppressor Genes (Continue the cell cycle)▪ Become accelerated and lead to uncontrolled growth.

Ionizing Radiation Being exposed to sources of ionizing radiation.

Infectious Diseases Viral infection can affect the genetic code of cells and

lead to cancer. Hormonal Imbalances

Hormones can stimulate excessive cell growth. Immune System Dysfunction

Immunodeficiency can also lead to cancer.▪ Example: HIV

Heredity Damaged Tumor Suppressor alleles can be

transmitted from parent to offspring.

Surgery Removing the tumor

surgically. Radiation Therapy

Using ionization energy to kill cancer cells and shrink tumors.

Chemotherapy Using anti-cancer drugs

to destroy cancer cells. Targeted Therapies

Using agents to fix mutated proteins in cancer cells.

Immunotherapy Inducing the patients immune system to fight

the tumor. Hormonal Therapy

Blocking or providing hormones necessary to stop uncontrolled growth.

Angiogenesis Inhibitor Prevent the growth of blood vessels that tumors

require to survive. Symptom Control

Improving the quality of life of the patient by curing symptoms.

Stem cells have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

Stem cells are unspecialized. One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions.

Stem cells can give rise to specialized cells. This process is called differentiation.

Origin of stem cells Embryonic ▪ the inner cell mass of the blastocyst

Adult▪ Unknown

Adult stem cells have been identified in many organs and tissues. One important point to understand about adult stem cells is that there are a very small number of stem cells in each tissue. Stem cells are thought to reside in a specific area of each tissue where they may remain quiescent (non-dividing) for many years until they are activated by disease or tissue injury. The adult tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.

In a living animal, adult stem cells can divide for a long period and can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells.

Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets.

Bone marrow stromal cells (mesenchymal stem cells) give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.

neural stem cells in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.

Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.

Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis

A number of experiments have suggested that certain adult stem cell types are pluripotent. This ability to differentiate into multiple cell types is called plasticity or transdifferentiation. The following list offers examples of adult stem cell plasticity that have been reported during the past few years.

Hematopoietic stem cells may differentiate into: three major types of brain cells (neurons, oligodendrocytes, and astrocytes); skeletal muscle cells; cardiac muscle cells; and liver cells.

Bone marrow stromal cells may differentiate into: cardiac muscle cells and skeletal muscle cells.

Brain stem cells may differentiate into: blood cells and skeletal muscle cells.

As long as the embryonic stem cells in culture are grown under certain conditions, they can remain undifferentiated (unspecialized). But if cells are allowed to clump together to form embryoid bodies, they begin to differentiate spontaneously. They can form muscle cells, nerve cells, and many other cell types. Although spontaneous differentiation is a good indication that a culture of embryonic stem cells is healthy, it is not an efficient way to produce cultures of specific cell types.

So, to generate cultures of specific types of differentiated cells—heart muscle cells, blood cells, or nerve cells, for example—scientists try to control the differentiation of embryonic stem cells. They change the chemical composition of the culture medium, alter the surface of the culture dish, or modify the cells by inserting specific genes. Through years of experimentation scientists have established some basic protocols or "recipes" for the directed differentiation of embryonic stem cells into some specific cell types (

Studying stem cells will help us to understand how they transform into the dazzling array of specialized cells that make us what we are.

Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process.

In order to maintain their population, stem cells must self-renew at each division, which can be accomplished through asymmetric division to generate two different daughter cells – one that resembles the mother (a stem cell), and one that is committed to another cause.

Alternatively the division can result in the formation of two identical daughter cells that are indistinguishable from the mother. This symmetric mode of division enables stem cells to increase in numbers during development, or following an injury.

When environmental conditions are unfavorable to growth however, the rate at which organisms develop is delayed, owing to a general slowing in cell growth and division.

http://www.celldiv.com/content/1/1/29

In the unmanipulated blastocyst-stage embryo, stem cells of the inner

cell mass (ICM) promptly differentiate to generate primitive

ectoderm, which ultimately differentiates during gastrulation into the three embryonic germ (EG) layers.

Gastrulation is a phase early in the development of animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration.

When removed from their normal embryonic environment and cultured under appropriate conditions, ICM cells give rise to cells that proliferate and

replace themselves indefinitely. Yet, while in this undifferentiated state in culture, they maintain the developmental potential to form advanced derivatives of all three EG layers

Another potential application of stem cells is making cells and tissues for medical therapies

Until recently, there was little evidence that stem cells from adults could change course and provide the flexibility that researchers need in order to address all the medical diseases and disorders they would like to. New findings in animals, however, suggest that even after a stem cell has begun to specialize, it may be more flexible than previously thought.