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My Pages On Different Subjects which Hyperlinked to all my Blog Posts

Saturday, 19 May 2012

Cell Division : Mitosis and Meiosis : Birth of new cells from old cells

3D pictures of All Stages of Mitosis cell division

For eukaryotic cells in general, the life cycle extends from the time of the cell`s formation until its own subsequent division is completed.Mitosis is a very small part of this cycle; it only lasts for few minutes or an a hour or more.Normally, the non dividing cell spends about ninty percent of its life in Interphase, when its increase it mass, approximately double the number of its cytoplasmic components, and finally duplicates its DNA. Interphase actually consists of three phases of activities: the gaps were so named before we knew much about what was going during the cell cycle.

Initial preparation stages of Mitosis
 Today we know that new cell components are synthesized during G1 and assembled fr distribution to daughter nuclei during G2. Among multi celled organisms, the duration of the cell cycle varies considerably. Most of the variation is associated with the G1 phase. For example, the mature cells of a complex plant may stay in G1 phase for days even years.In contrast , the cells of an early sea Urchin embryo may double in number every two hours.

We know that cells must have built-in instructions regarding the duration of a cell cycle, because the duration of cell cycle, because the duration is fairly consistent for all the members of a species. Sometimes adverse environmental conditions do arrest cells in G1 phase.(For example, this happens among ameobas and other Protistans when they are deprived of vital nutrient.) Even so, if a cell progresses beyond a certain point in G1 (The so called restriction point), the cycle normally will be completed regardless of outside conditions.
Chromosome Number in Mitosis


Microscopic pictures of stages of Mitosis cell division


3D Picture of Interphase

Two daughter nuclei are formed, each with a diploid number of chromosomes (The same as the parental nucleus).

3D Picture of Early Prophase

The DNA and associated proteins start condensing into the threadlike chromosome form. (Chromosomes are already duplicated.) Two chromosomes derived from themale parent are in blue ; their homologues from the female are red.

3D Picture of late prophase

Chromosomes continue to condense.Microtubule start to assemble outside the nucleus; they will form the spindle,which will establish poles for division.The nuclear envelope starts breaking up centrioles(if present) are moved by the microtubules toward opposite poles.

3D Picture of Metaphase

Nuclear envelope is gone; spindle penetrates region of nucleus . Sister chromatids of each chromosome are attached at their kintochores t the spindle. All the chromosomes are lined up at the sindle equator.

3D Picture of Anaphase

Sister chromatids of each chromosome will now be separated from each other and moved to opposite poles.

3D picture of Telophase

Chromosomes decondense.New  nuclear membranes start forming.Most often, cytokinesis occurs before the end of telophase.

                          3D Animation  of  Mitosis Cell Division  created by Me (Manash Kundu)

3D pictures of All Stages of Meiosis cell division

It took billions of cell division to produce all the tissues of your body, and every day millions of cells still divide every second just to replace worn-out or damaged predecessors and to keep that body running smoothly. Most are somatic cells; they are the ones making up all body tissues expect for the germ cells, a cell lineage set aside for sexual reproduction.

sexual reproduction begins with meiosis and proceeds through the formation of gametes ( sex cells, such as sperms and eggs). It ends at fertilization, when the sperm nucleus and egg nucleus fuse in the first cell of the new individual (the zygote). A zygote grows into a multi celled adult by way of  mitosis, which faithfully maintains the  chromosome number characteristic of the species, division after division For example, there are 46 chromosomes in your somatic cells, 48 in Gorillas, and 14 in garden peas with each mitotic cell division, that number does not change.

Number of Chromosomes during  Meiosis

Meiosis only occurs in germ cells that are destined a give rise to gametes. With this division mechanism, the chromosome number is not maintained; it is reduced in half. and not just any half - each gamates ends up with of each pair of homologous chromosomes. ( It does not matter which of the two it gets.) It takes Two nuclear divisions  to put one chromosome of each type into each gamete: Thus gamates end up with a haploid number of chromosomes, meaning "half" the parental number. Then when a sperm nucleus and an egg nucleus fuse at fertilization, the diploid number is restored. " Diploid" means having two chromosomes of each type (that is, pairs of homologous chromosomes) in the somatic cells of sexually reproducing species.


3D picture of Prophase-1

Each chromosome condenses, then pairs with its homologue crossing over and recombination occurs.

3D picture of Metaphase-1

Spindle apparatus has formed and nuclear envelops broken down during transition to metaphase 1. Homologues align randomly at spindle equator; some spindle microtubules attach each pair of homologues to opposite poles.

3D picture of Anaphase-1

Each hmlogue is separated from its partner, and the two are moved to opposite poles.

3D picture of Telophase-1

A haploid number of chromosomes (still duplicated) ends up at each pole.

3D picture of Prophase-2

There are no DNA replication interkinesis, which precedes prophase 2 in most species.Sister chromatids of each chromosome are still attached at the centromere during this often brief stage.

3D picture of Metaphase-2


Each chromosome is aligned at the spindle equator; microtubules attach its sister chromatids to opposite poles.

3D picture of Anaphase-2


Each chromosome splits what were once sister chromatids are now chromosomes in their own right and are moved to opposite poles.

3D picture of Telophase-2


Four daughter nuclei form; each has a haploid number of chromosomes, all of which are in the unduplicated state.

                               3D Animation  of  Meiosis Cell Division  created by Me (Manash Kundu)

These are larger contractile protein fibres that are involved in movement of
Organelles within the cell
Chromosomes during cell division
Cell extensions

This directs organisation of microtubules within the cell.It consists of a pair of centrioles (Small clusters of microtubules)
and plays an important role during cell division.

Microscopic pictures of stages of Mitosis cell division

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