ELECTRIC SIGNALS IN NEURONS

        The transfer of information from receptors to brain or spinal cord and the appropriate response or command, to effectors is mediated by nerve impluses. Therefore, the generation and conduction of nerve impulses is essential aspect of nerve physiology. the mechanism involves the following terms:

(i) Resting membrane potential: It is the membrane potential of a nerve in the resting state, i.e., when a nerve is not stimulated or excited. It results from

*Unequal distribution of ions: The sodium and potassium ions are distributed unequally across the plasma membrane of a nerve cell.

Extracellular Na+ Na+ Na+

Na+ (High)  plasma membrane

K+ K+ +

K+ (high)  Instracelluar fluid

* Relative permeability of the plasma membrane: The plasma membrane shows differential permeability to sodium and potassium ions. It is more permeable to K+.

* Electrogenic pumps: They expel three sodium ions from the cytosol for every two potassium ions imported, and plus help in maintaining the resting membrane potential

. 

          These three factors create a difference in electrical charges on either side of the membrane. It is positive outside and negative inside creating a potential difference across it. this is known as resting membrane potential and it is about -70mv (millivolts) for nervous tissue. Such a membrane is said to be polarised.

(i) Na+ Ecf

+ + + + + + 

(ii) K+ ICF

(ii) Action Potential: when the nerve fibre is simulated. the membrane potential. It is polarity . It is known as action as potential. It consists of two phases: Depolarization and Repolarisation.

(a) Depolarisation: Depolarisation is brought about by the opening of sodium ion channels.

+ + + Na+

- - -

This causes reversal of the charge distribution or potential difference, which charges from negative potential (-70m V) to positive potential (+30m V)inside the nerve cell. the potential reaches a peak called "spike potential" or action potential.

- - - -         depolarisation

+ + + + 

(b) Reproduction: When the stimulus is over, the membrane of the  nerve fibre returns back to its original state and this phase is called as repolarisation. It's brought about by closure of sodium channels and opening of potassium channels.

- - - K+               + + + 

                                 repolarisation

+ + +               >  - - -

MITOSIS

(In Greek, “mitos” means thread). It’s a mechanism in which a cell distributes its different components in equivalent amounts that have already been duplicated during interphase. Therefore, it’s also called as educational division.

Prophase

The chromatin threads (each chromosome is made up of two chromatin threads) become more tightly coiled or condense into discrete chromosomes, which can be observed with a light microscope.

Meaphase  (It’s the longest phase of mitosis)

The chromosomes are thickest and shortest because of further condensation.

The chromosomes become attached to the microtubules of the mitotic spindle at kinetochore, aligned at the equatorial plate. Exactly in the middle of the spindle.

Anaphase (Shortest stage)

The centromere* splits causing the sister chromatids to separate. The two free chromatids now begin to move towards the opposite poles.

Telophase

The  polar migration of daughter chromosomes is completed i.e., daughter chromosomes reach the end of the poles.

Daughter chromosomes also start to uncoil and become less condensed. As a result they again form chromatin threads.

Nuclear envelope, nucleoli, golgi complex and ER reappear. In this way two daughter nuclei form within the cell.

The nuclear division is followed by division of cytoplasm, which will result in the formation of two daughter cells.

Cytokinesis: In animal cells, cytoplasm divided by forming a cleavage furrow (because of absence of cell wall). Whereas in a plant cell it starts with the formation of a cell plate at the centre, which later extends into both the sides towards the periphery.

MEIOSIS

       Meiosis is derived from the Greek work “ meioum” which means “to diminish”. Since in this division the original chromosome number (e.g., 2n = 46) present in the parents is reduced to half (e.g., n = 23) in the daughter cells, it’s also known as reductional division. Another special feature of this type of division is the separation of homologous chromosomes or sister chromosomes and not their halves (i.e., chromatids). Thirdly, meiosis is observed in sexually reproducing organisms. Since, sexual reproduction involves union of male gametes (spores in plants or sperms in animals) and female gametes (ovules in plants and ova in animals), the chromosome number is doubled and will continue to duplicate every time the cell undergoes meiosis. But, this does not happen in nature! Because, the chromosome number is halved during meiosis to keep it constant in a particular species or race.

   It involves two divisions – (a) Meiosis I, and (b) Meiosis II

   Meiosis shows many such unique features which are summarized in Fig. B.2.1.

Meiosis I

Prophase I

          It is far more complex than mitosis and is a long drawn process. In some, it may last for days (e.g., human male -24 days) or even months or several years (e.g., human female till puberty 15-16 years). It is further subdivided into five stages characterized by the following features:

1.Leptotene (Gr. “leptos” meaning thin, “nema” – meaning thread)

(a)The chromosomes become more apparent under microscope as thin, elongated threads (hence the name) aligned very close to each other. They show bead-like thickenings at regular intervals known as chromomeres.

 (b)Nucleus increases in size.

2.Zygotene (Gr. “zygon” – adjoining)

Homologous chromosomes, which are sister chromosomes, one coming from the male parent and other from the female parent, are aligned and undergo pairing. The pairing is known as synapsis. It can begin at scattered points but progresses in such a fashion that the homologous chromosomes are exactly paired chromomere –by-chromomere and gene-by-gene. The pairing of homologous chromosomes is known as synapsis forming synaptonemal complex.

3. Pachytene (Gr. “pachus”- thick)

(a) By the mid-stage, the nucleus contains half the number of chromosomes, because each unit is now a bivalent (“bi” meaning “two” referring to two homologous chromosomes) or a tetrad (“tetra “means “four” referring to four chromatids). Thus the two non-sister homologous chromosomes with their four arms or chromatids are intimately paired during this stage.

(b) Another important development which occurs during this phase and has far-reaching effects with respect to formation of recombinants is “crossing-over”. The two non-sister homologous chromosomes exchange segments containing genetic material, resulting in new combinations.

GENETICS

  Genetics is study of science of genes, heredity and variation in living organisms. Heredity is the cause of similarity between individuals and variation is the cause of differences between individuals. Genetics deals with transmission of characters from parents to offspring and the molecular structure and function of genes. The study of genes is done at three levels.

.Transmission Genetics

.Population Genetics    

.Molecular Genetics 

TRANSMISSION GENETICS          

The two most fundamental questions in biology are – what is the nature of genetic material and how is it transmitted from parents to offspring? The fact that living things inherit traits from their parents has been used in prehistoric times to improve crop plants and animals through selective breeding. However modern genetics only began with the work of Gregor Mendel. His breeding experiments in pea plants revealed that hereditary characteristics are determined by discrete factors (genes) that are transmitted to the next generation.

THE LAWS OF INHERTANCE

      The study of science of genetics begins with the work of Gregor Mendel. He was a clergyman and a teacher. He performed his own experiment on garden pea. He restricted his attention to the single character and kept pedigree records for each plant. Mendel studied the nature of inheritance in plants and presented his work in 1885 in the paper “Experiments on Plant Hybridisation “. The importance of his work did not get noticed until 1890s, after his death. The advantages of garden pea (Pisum sativum) as experimental system were:

The characters were readily observable.

Plants were easy to grow and self fertilise before the flowers opened

Mendel worked with 14 pure breeding varieties and selected seven pairs of characters and propounded the following laws:

The law of dominance: Mendel stated that the hereditary characters are determined by pair of factors (genes) in an individual. In a monohy--brid cross all the plants in F₁ generation showed only one character   but in F₂ the other character appears. The character in F₁ generation is dominant and the one which expresses in F₂ generation is recessive e.g., the cross between two homozygous parents (P), one with two dominant alleles for tall plant (TT) and the other with two recessive alleles for dwarf plant (tt). The phenotype of the offspring in F₂ generation is dominant and the one which expresses in F₂ generation is recessive e.g., the cross between two homozygous parents (P), one with two dominant alleles for tall plant (TT) and the other with two recessive alleles for dwarf plant (tt).  The phenotype of the offspring in F₁ generation is tall plant but the genotype is Tt.  The F₁ heterozygote produces four kinds of gametes. When this plant self-pollinates, the T and t egg and sperm cells combine randomly to form ¼TT (tall), 2/4 Tt (tall) and ¼ tt (dwarf) offspring. The F2 generation has 1:2:1 genotypic ratio and 3:1 phenotypic radio.

THE LAWS OF INHERITANCE

Parents	TT	X	tt
	Tall		(Dwarf)
F₂
Gametes	T		T
T	TT		Tt
	(Tall)		(Tall)
T	Tt		Tt
	(Tall)		(Dwarf)

The law of segregation or purity of gametes: These pair of characters in an individual segregate from each other during the formation of gametes. In one pair of contrasting character alleles segregate in 3:1 phenotypic ratio and 1:2:1 genotypic ratio.

The law of independent assortment: In the formation of gametes the segregation of one pair of allele is independent of the therefore gametes containing all possible combination of these alleles will be produced in equal numbers. In a dihybrid cross the F₂ phenotypic ratio is 9:3:3:1.

After 100 years of Mendel’s first publication, the three scientists Hugo de Vries, Carl Correns and Erich von Tschermak re-examined and verified Mendel’s laws and science of genetics was launched.