Category: developmental biology

MITOSIS- its Occurrence, Stages and Significance.

by MicroScopia IWM, posted in developmental biology, Genetics/Molecular biology

                                       

INTRODUCTION:

  1. Mitosis is a type of cell division that takes place in living organisms and it is commonly defined as the process of duplication of chromosomes in eukaryotic cells and distributed during cell division.
  2.  The process where a single cell divides resulting in two identical cells, each resulted cell contains the same number of chromosomes and same genetic composition similar to the parent cell.
  3. Mitosis was first discovered in plant cells by Strasburger in 1875. In 1879, mitosis is also discovered in animal cells by W. Flemming. Flemming in 1882 gave the term Mitosis.
  4. The term mitosis is derived from the Greek word such as ‘Mitos’ means thread.

OCCURRENCE OF MITOSIS:

  1. The mitosis takes place in somatic cells. The cells which undergo mitosis are called  Mitocytes.
  2. In plants, the mitocytes are called meristematic cells. Some of the major sites of Mitosis in plants are root apex, shoot apex, intercalary meristem, lateral meristem, leaves, embryo, and seeds.
  3. In animals, the mitocytes are stem cells, germinal epithelium, and embryonic cells. In animals, it mainly takes place in Embryo, skin, and bone marrow.
  4. Mitosis also occurs during the regeneration of the cells. The mitosis takes place for three main reasons such as growth, repair, and asexual reproduction.

NUCLEUS:

It is a membrane-bound cell organelle present in both animal and plant cells. It is the center of the cell where genetic material is stored in the form of DNA. The DNA is arranged into a group of proteins into thin fibers. During the Interphase of the cell division, the fibers are uncoiled and dispersed into the chromatin. During mitosis, these chromatin condenses to become chromosome.

 CHROMOSOME:

The chromosomes carry genetic material and they are made of DNA. The mitotic chromosomes possess two sister chromatids, they are narrow at the centromere. They also contain identical copies of original DNA. These Mitotic chromosomes are homologous, they are similar in shape, size, and location of centromere.

STAGES OF MITOSIS :

The mitosis cell division is broadly explained in two stages such

  1. Karyokinesis: Division of Nucleus. Greek ‘karyon’ means the nucleus, whereas ‘kinesis’ means movement.
  2. Cytokinesis: Division of Cytoplasm.

KARYOKINESIS:

4 different stages that take place in Karyokinesis.

1.Prophase

2.Metaphase

3.Anaphase

4.Telophase

PROPHASE:

  1. The nucleus becomes spherical and the cytoplasm becomes more sticky.
  2. The chromatin slowly condenses into well-defined chromosomes.
  3. During Prophase, the chromosomes appear as a ball of wool. The chromosomes consists of two threads which are longitudinal known as chromatids.
  4. The chromosomes appear as two sister chromatids joined at the centromere.
  5. The microtubules are formed outside the nucleus.
  6. In plant cells, the spindle apparatus is formed without centriole. In animal cells, the centriole is divided into two moves towards opposite poles.

METAPHASE:

  1. Nuclear envelop breaks down into membrane vesicles and the chromosomes are set free into the cytoplasm.
  2. Chromosomes are attached to spindle microtubules through kinetochore.
  3. Nucleolus disappears.
  4. Kinetochore microtubules arrange the chromosomes in one plane to form a central equatorial plate.
  5. Centromeres lie on the equatorial plane while the chromosome arms are directed away from the equator called auto orientation.
  6. Smaller chromosomes remain towards the center while larger ones arrange at the periphery.
  7. Metaphase is the longest stage of Mitosis and takes place for about 20 minutes. It is the best stage to study the structure of chromosomes.

ANAPHASE:

  1. Chromosomes split simultaneously at the centromeres so that the sister chromatids separate.
  2. The separated sister chromatids move towards the opposite poles.
  3. The daughter chromosomes appear in different shapes such as V-shaped(metacentric), L-shaped(sub-metacentric), J-shaped(acrocentric), rod-shaped(telocentric).
  4. The spindle fibers are attached to the centromere and pull the chromosomes to the poles.
  5. Anaphase is the shortest stage of Mitosis.

TELOPHASE:

  1. Daughter chromosomes arrive at the poles. Kinetochore microtubules disappear.
  2. Chromosomes uncoil into chromatin.
  3. Nucleolus reappears. The formation of nuclear envelope occurs around each pair of chromosomes.
  4. The Viscous nature of the cytoplasm decreases.
  5. Telophase is called a reverse stage of the prophase.

CYTOKINESIS:

Cytokinesis is defined as the division of cytoplasm.

  1. It starts during the anaphase and is completed by the end of the telophase.
  2. It takes place in 2 different methods.

a) Cell plate method: It takes place in plant cells. The vesicles of Golgi fuse at the center to form a barrel-shaped phragmoplast. The contents of the phragmoplast solidify to become a cell plate, this cell plate separates the two daughter cells.

b) Cleavage or cell furrowing method: It takes place in Animal cells. In this method, a Cleavage furrow appears in the middle, which gradually deepens and breaks the parent cell into two daughter cells.

SIGNIFICANCE OF MITOSIS:

  1. Mitosis is called as an equational division in which daughter cells produced are identical.
  2. It maintains the constant number of chromosomes and genetic stability in somatic and vegetative cells of the living organisms.
  3. It helps to increase the cell number so that zygote transforms into a multicellular adult.
  4. Healing wounds takes place by Mitosis.
  5. It helps in asexual reproduction.
  6. Mitosis is necessary for growth, maturity and to repair damaged cells.

BY: ABHISHEKA (MSIWM013)

Metamorphosis and its type

by MicroScopia IWM, posted in developmental biology

Metamorphosis is the phenomenon of dramatic developmental changes in which an immature organism turns into a sexually mature form. In this process, large proportions of structural changes occur in such a way that its younger form and older form are unrecognizable. These changes are initiated by endocrine factors and few hormones that will possibly affect every cell in the developing body.

Metamorphosis can be said as both developmental and an ecological conversion–

  • Developmental– Metamorphosis is initiated by specific hormones that reactivate the developmental process whereby it is adjusting itself morphologically, physiologically and behaviorally.
  • Ecological– This sort of evolution is associated with fluctuations in habitat, food and behavior.

Metamorphosis can be categorized in 2 types on the basis of formation: Complete and Incomplete metamorphosis-

Complete metamorphosis-It is a type of insect development in which egg, larva, pupal and adult stages differ among them greatly during the process of metamorphosis. The four stages that can be categorized are- Egg, larva, pupa and adult. Here, the metabolically active form is larvae and the inactive one is the pupa. The exoskeleton, particularly of the insect is completely molted. Sexually active is the final stage of the insect (adult). Examples of this type include Wasps, ants and fleas.

Incomplete metamorphosis-It is a type of insect development where gradual changes occur in the insect during the development from egg to adult. The three stages categorized are- Egg, nymph and adult. The nymph can be identified as a miniature adult. Certain portions of exoskeleton in the adults remain permanent throughout its life. Sexually active is the former stage of the insect. Examples of this type include termites, mantis and cockroach.

On the basis of the mode of developing, it is divided as: Direct and Indirect developers

Direct developers– Those organisms whose young structurally mimic the adult’s structure and are sexually inactive. Example- Humans.

Indirect developers– Those organisms which includes a larval stage with characteristics and features very different from those of adult organism. They can further be classified as primary and secondary larvae.

  • Primary larvae represent different body plans than adult one and are structurally very distinct. Example is sea urchin.
  • Secondary larvae are those larvae which are also the adults which possess basic body plan. Example is Butterfly.

Heterochrony– It is the phenomenon whereby there is a genetically controlled time difference in the rate of development process in an organism compared to its ancestors, therefore, showcasing a marvelous morphological innovation. The notion “heterochrony” was first introduced by the German zoologist Ernst Haeckel in 1875 for which he mainly used it explain the rare conditions of “recapitulation theory” (a hypothesis where a developing embryo, when going through all walks of life resembles the adult form of distinct ancestors.) For now, we will ponder over pedomorphosis’s subtypes.

PAEDOMORPHOSIS:

* Progenesis– It involves the retention of juvenile form, but in this case, the gonads and germ cells develop at a faster rate than normal. This becomes sexually mature while the rest of the body is still in a juvenile phase.

* Neotony– It refers to the retention of the juvenile form owing to the retardation of body development relative to that of the germ cells and gonads, which achieve maturity at the normal time.

* Direct development– Here, the embryo skips the larval step and proceeds with the creation of adult stage.

PERAMORPHOSIS:

* Hypermorphosis- During the course of evolution, the rate of development is unchanged but the relative time duration is increased, thereby allowing the addition of new stages to the end of the ancestral categorization process.

* Acceleration- The essential developmental changes, but in a shorter time (varying growth rates).

* Predisplacement- A modification in the ontogeny of a successor such that some developmental process begins earlier than in its ancestor.

BY-  N. Shreya Mohan (MSIWM042)