Meiosis

Meiosis is a more complex process as compared to mitosis. It occurs in the reproductive cells of male and female eukaryotic organisms.
It can be divided into two distinct parts namely 'Meiosis I' and 'Meiosis II'.
     

The steps involved in meiosis are described below-
1. DNA replicates and the chromatin organizes to form chromosomes.
 Each chromosome consists of two identical sister chromatids joined at the centromeres.
2. .The homologous chromosomes line up next to each other and a unique event, called recombination or crossing over, occurs. In this step the homologous chromosomes exchange DNA between adjacent homologous chromatids. (The sister chromatids of any chromosome are no longer identical to each other.)
3. The centrioles move to the poles of the cell and spindle fibers start emerging from them. Nuclear membrane and nucleolus disintegrate.
4. Homologous chromosomes line up along the equatorial plate of the cell.
The orientation of the chromosomes is random resulting in different characteristics among the offsprings. This verifies the law of independent assortment.
5. The ends of the spindle fibers attach to the centromere of each chromosome. The homologous chromosomes move apart from one another, along the spindle fibers, to the opposite ends of the cell.
Each chromosome is still double stranded and has two sister chromatids.
6. The spindle fibers disintegrate and cytokinesis takes place forming two daughter cells.
Each new cell will have only one member of the homologous pair and will thus be haploid.

The second step called Meiosis II is identical to mitosis and the resulting cells retain the same number of chromosomes. (though without their sister chromatids)

7. The spindle fibers redevelop from the poles of the cell.
8. The chromosomes line up along the equator. The ends of the spindle fibers attach to the centromere of each chromosome.
9. The sister chromatids move to the poles of the cell along the spindle fibers.
10. Cytokinesis takes place and four genetically different haploid cells are formed.

The haploid cells thus formed (from both parents), after fertilization, result in the development of a diploid cell called zygote.

Inside the cell: Meiosis and Mitosis

Before I begin with the description of the process, I would like to introduce a few general terms.

Homologous Chromosomes: This refers to the same type of chromosomes, i.e. the chromosomes responsible for the same characteristics, that pair up (one maternal and one paternal) inside the cell during meiosis. E.g.- X and Y chromosomes
Cytokinesis: It is the process by which the cytoplasm of a single eukaryotic cell divides to form two daughter cells during cell division.
Centromere: It is the structure in a chromosome which holds the two sister chromatids (replicated copies of chromosomes) together.
Kinetochore: It is a special structure formed only during cell division. It is a protein structure which assembles at the centromere and link the chromosomes to the microtubule polymers of the mitotic spindle. 

I shall not mention the names of all the phases involved as I consider understanding of the process more important.

Prior to mitosis, the DNA in the cell replicates and proteins forming the mitotic spindle are synthesized.

A simplified diagram representing mitosis

Mitosis:
This is the process by which normal body cells divide. It is basically a nuclear process(takes place inside the nucleus). The following steps are involved:
1. The DNA condenses and the chromatin material organizes to form chromosomes. Nucleolus disappears.
2. The centriole divides and moves to the poles. Nuclear membrane disintegrates.
3. Fibers, called mitotic spindles emerge from the centrioles and some of these connect to the centromere of each chromosome forming kinetochores.
4. Chromosomes move to the equatorial plate of the cell.
5. The spindle fibers attached to the kinetochores begin to contract pulling the chromatids apart while the unattached fibers elongate.
6. The chromatids, now called chromosomes reach the poles of the cell as the unattached fibers continue to elongate.
7. Nuclear membrane develops at both ends, nucleoli reappear and the DNA unwinds.
8. A cleavage furrows through the top and bottom which divides the cell into two daughter cells.(cytokinesis)
With this we come to the end of mitosis. I shall discuss meiosis in my next post.

Electronic configuration of atoms?

The principle according to which electrons are filled within the orbits of an atom  is known as the Aufbau Principle. According to this principle, each shell consists of n number of sub-shells, where n is the sub-shell number. The electrons are filled within these sub-shells according to their energy levels. The order of filling is shown in the diagram. 

The capacity of the various sub-shells is as follows:

s(sharp) sub-shell - 2 electrons

p(principal) sub-shell - 6 electrons

d(diffuse) sub-shell - 10 electrons

f(fundamental) sub-shell - 14 electrons

These names are derived from the characteristics of their spectroscopic lines.

Therefore, the first orbit can accommodate a maximum of 2 electrons, the second orbit can accommodate 2 + 6 = 8 electrons, the third 18 and so on.

(Electrons do not go to the sub-shells shown in green under normal conditions.)

From this principle, we can understand why electrons come to the N shell before 18 electrons are filled in the M shell,  2 electrons are filled in the O shell before the ninth electron comes in the N shell etc.

What is light?

                                   

Light is an electromagnetic wave.  It consists of particles of energy called photons. Each photon consists of a fixed amount of energy called quantum. Each quantum consists of a wave of wavelength, λ and frequency, v. The energy of a photon, E is equal to- 

                                                     E = hv = hc/λ                (c = vλ )

                                     where,     h = Planck's constant = 6.625 X 10^-34

                                                     ^{v = frequency of the wave}

                                                     ^{c = speed of light}

                                                     λ = wavelength

^{Thus, a light wave can have energies in the multiples of hv i.e. hv, 2 hv, 3 hv etc.}

Photoelectric effect

When light shines on a metal surface it emits electrons. 

 This is due to the fact that when a photon of light incidents on the surface of metal (and hits an electron in an atom), it contains enough energy to knock the electron out of the atom.

In other words photoelectric effect is a phenomenon in which, when the light(photon) of appropriate wavelength or frequency incidents on the metallic surface called photo cathode, then due to the production of photo electrons, photocurrent is generated.

 In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron.

Radioactive series

Radioactive series refers to the radioactive decay of different unstable elements (called radioactive elements) until a stable nuclei is formed. There are four radioactive series, three of which are natural (Uranium series, Thorium series and Actinium series) and the fourth (Neptunium series) is an artificial radioactive series.

Uranium series:

 In this series the parent element is uranium ₉₂U²³⁸ and the end product of this series after theemission of six α particles and five β particles is obtained as radium lead (₈₂Pb²⁰⁶) which is indistinguishable chemically from the ordinary lead and it is a stable isotope of lead.

 

Thorium series:

  The parent element of this series is Thorium- ₉₀Th²³². It goes through a series of transformations in many respects similar to the uranium series and ends with a stable isotope

(₈₂Pb²⁰⁸).

 

Actinium series:

 The parent element of this series is an isotope of Uranium called action Uranium- ₉₂U²³⁵  and its end product is again a stable isotope of lead (₈₂Pb²⁰⁷).

 

Neptunium series:

 After the discovery of the unstable transurenic elements (the elements of atomic number greater than 92 are called transurenic elements), another radioactive series was traced out. This is called neptunium series and is named after its longest living member Neptunium. It doesn’t end in a stable isotope of lead  but in a stable isotope of Bismuth (₈₃Bi²⁰⁹).

 

Radioactivity

There are certain elements which emit some invisible rays (α, β, γ) by the process of self-disintegration. These elements have excess neutrons in their nucleus. By the process of nuclear disintegration into smaller fragments they emit invisible radiations. These atoms are called radioactive elements and the invisible rays emancipating from these elements are called radioactive rays.

All elements which lie in the range of atomic numbers 83 to105 are radioactive elements. Out of this atomic numbers 83 to 92 are natural radioactive elements while 93 to 105 are artificial. The radiations emitted by these elements are of three types:
α rays- These are positively charged and are equivalent to He⁺⁺  ions                
β rays- These are negatively charged electrons
γ rays- These are electrically neutral

The nuclei of radioactive elements are unstable and disintegrate by ejecting an α particle or a β particle. By ejection of α or β particles, new atoms are formed and if these atoms also have unstable nuclei then further emission also takes place, until a stable nuclei is found and a series of radioactive elements are obtained called radioactive series. There are four radioactive series-
i)Uranium series
ii)Thorium series
iii)Actinium series
iv)Neptunium series


I will discuss more about these series in my next post.