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Genetics And Evolution

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Genetic and Evolution PDF Notes, Important Questions and Formulas

BIOLOGICAL CLASSIFICATION

1.  BIOLOGICAL CLASSIFICATION

The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification.
The purpose of biological classification is to organize the vast number of known organisms into categories that could be named, remembered and studied.
According to "A.P. de Candolle", Classification is of two types

2. TYPE OF CLASSIFICATION

  1. Empirical Classification
  2. Rational Classification
  1. Artificial classification: - In this type of classification plants are classified on the basis of one or two morphological characters. i. e. over all morphology is not considered.
    For e.g. - Classification proposed by Linnaeus is Artificial
  2. Natural classification: - In this type, plants are classified on the basis of their complete morphological characters of stem, root, leaves, flowers etc. Maximum characters are taken as base in this classification.

Importance – Natural classification is believed to be the best classification, because it represents the natural similarities and dissimilarities of plants i.e. it represents the interrelationship among plants.
In this classification, the plants belonging to the same group shows many similarities, while in artificial classification, the plants belonging to the same group shows only, 1 or 2 similar characters. They have many dissimilarities.

Natural classification is of two types

  1. Natural formal
  2. Natural phylogenetic
    1. Natural formal→ In this classification, the phylogeny of the plant is not considered i.e. only the morphology of the plant is considered.
    2. Natural phylogenetic - In this classification, both morphology and phylogeny are considered. In phylogenetic classification, the plants are arranged on the basis of their evolution.

Lamarck: - Proposed the term "Phylogeny"
Ernest Haeckel: - Gave the concept of phylogeny
Charles Darwin: - Gave broad explanation of phylogeny in his book "ORIGIN OF SPECIES"(1859)

(Genealogy - Sequence of evolution)
Genealogy of plant kingdom:-
Thallophyta - Bryophyta - Pteridophyta -
Gymnosperm - Angiosperm (most advanced plants)
Note: Phylogenetic classification also known as cladistic classification

iii.  Adansonian system or phenetic classification or Numerical classification:-
Proposed by "Sokel and Sneath". In it plants are classified on the basis of numbers of similarities and dissimilarities.
Note: In this, importance to any one character is not given, all characters have same importance. While in natural classification floral (reproductive) characters have more importance than vegetative (root, stem and leaves) characters.

 

Evolution

Evolutionary Biology:

  • It is the study of history of life forms i.e. the changes in flora and fauna that have occurred over millions of years or  earth

What is Evolution?

  • The word evolution means to unfold or unroll or to reveal hidden potentialities. Evolution simply means an orderly change from one condition to another.
  • Evolution term was introduced by Herbert Spencer. It can be best explained by Darwin's concept of 'Descent with modification'.
  • This is the story of origin of life and evolution of life forms or biodiversity on Planet earth in the context of evolution of earth and against the background of evolution of universe itself.

What is Universe?

The universe is a huge cluster of galaxies.

  • Galaxies contain stars and clouds of gas and dust. Considering the size of universe, earth is indeed a speck i.e. the universe is vast.
  • When we look at stars on a clear night sky we are, in a way, looking back in time. Stellar distances are measured in light years. What we see today is an object whose emitted light started its journey millions of year back and from trillions of kilometers away and reaching our eyes now. However, when we see objects in our immediate surroundings we see them instantly and hence in the present time. Therefore, when we see stars we apparently are peeping into the past.
  • The Big Bang theory attempts to explain to us the origin of universe.
  • Bing Bang theory - Proposed by Abbe Lemaitre.
  • According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion of a dense entity. This single huge explosion which is unimaginable in physical terms is called as big bang.
  • The universe expanded and hence, the temperature came down.
  • The gaseous clouds which were formed by big bang condensed under gravitation and converted into many flat discs like structures called nebula, made up of atoms and small particles. Solar nebula was one of them, which formed our solar system.
  • The very hot central part of solar nebula became still hotter and converted into the sun.
  • Later on, due to condensation of atoms and dust particles moving around the sun other planets were formed.
  • In the solar system of the Milky Way galaxy, earth was supposed to have been formed about 4.5 billion years back.
  • There was no atmosphere on early earth. It was formed later.

 2.  THEORIES FOR ORIGIN OF LIFE:

  1. Theory of special creation –
  • The greatest supporter of this theory was Father Suarez. This is a mythology based theory. This theory has three connotations
  1. All living organisms that we see today were created as such.
  2. The diversity was always the same since creation and will be the same in future.
  3. The earth is about 4000 years old.
  • All these ideas were strongly challenged during the nineteenth century based on observations of Charles Darwin, Wallace etc. They believed that life forms varied over the periods of time.
  • From fossils records and their dating, we can conclude that earth is very old, not thousands of years as was thought earlier but billions of year old.

 B.   Cosmic panspermia theory -

Some scientists believe that life came from outer space.

  • Early Greek thinkers thought units of life called spores were transferred to different planets including earth
  • 'Panspennia' is still a favorite idea for some astronomers.

C.   Theory of spontaneous generation

(Abiogenesis/Autogenesis) -

  • This hypothesis was supported by ancient Greek philosophers.
  • According to this theory life came out of decaying and rotting matter like straw, mud, etc. spontaneously.
  • They believed that the mud of Nile river could give rise to fishes, frogs, crocodiles etc. when warmed by light rays.

D.   Theory of biogenesis
Proposed by Harvey & Huxley

  • They stated "Omnis vivum ex ovo or vivo", which means "New life can be originated on earth only by pre existing life."
  • Experiments of Francesco Redi, Lazzaro Spallanzani, and Louis Pasteur etc. supported the theory of biogenesis and disproved the abiogenesis.
  • Experiment of Louis Pasteur is most renowned among all of these.
  • Hence spontaneous generation theory was dismissed once and for all. However, this did not answer how the first life form came on earth.

Experiment of Louis Pasteur:

  • His experiment is also known as 'Swan neck flask experiment'.
  • He prepared sterilized syrup of sugar and killed yeast by boiling them in flasks.
  • He took two flasks one of broken neck and another of curved neck (swan neck flask/"S" shaped neck flask).
  • He showed that in pre-sterilized swan neck flasks, life did not come froin killed yeast because germ laden dust particles in the air were trapped by the curved neck which serves as filter while in another flask open to air (broken neck), new living organisms arose.

E.   Oparin -Haldane theory (Modem theory)

  • Oparin of Russia and Haldane of England proposed that the first form of life could have come from preexisting non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents.
  • Oparin's theory was published in his book 'ORIGIN OF LIFE'.
  • First life originated in sea water, so water is essential for origin of life.

 

MOLECULAR BASIS OF INHERITANCE

  1. NUCLEIC ACIDS
  • F. Meischer discovered nucleus acid in nucleus of pus cell and called it “nuclein”. The term nucleic acid was coined by “Altman”.
  • Nucleic acids are polymer of nucleotides.
    =-Nitrogen base + pentose sugar + phosphate on the basis of structure nitrogen bases are broadly of two types: -
  1. Pyrimidines- Consist of one pyrimidine ring. Skeleton of ring composed of two nitrogen and four carbon atoms. E.g. Cytosine, Thymine and Uracil.

 

ii. Purines – Consist of two rings i.e. one pyrimidine ring (2N +4C) and one imidazole ring (2N+3C)
E.g. Adenine and Guanine

 

Pentose sugar:-

  

 

Nitrogen base forms bond with first carbon of pentose sugar to form a nucleoside. Nitrogen of third place (N3) forms bond with sugar in case of pyrimidines while in purines nitrogen of ninth place (N9) forms bond with sugar.

 

Phosphate forms ester bond (covalent bond) with fifth Carbon of sugar to form a complete nucleotide.

Types of Nucleosides and Nucleotides

  1. Adenine + Ribose = Adenosine Adenosine + Phosphate = Adenylic acid AMP)
  2. Adenine + Deoxyribose = Deoxy adenosine Deoxy adenosine + P = Deoxy adenylic acid (dAMP)
  3. Guanine + Ribose = Guanosine
    Guanosine + P = Guanylic acid (GMP)
  4. Guanine + Deoxyribose = Deoxy guanosine
     

    Deoxy guanosine + P = Deoxy guanylic acid (dGMP)
  5. Cytosine + Ribose = Cytidine
    Cytidine + P = Cytidylic acid (CMP)
  6. Cytosine + Deoxyribose = Deoxycytidine
    Deoxycytidine + P = Deoxycytidylic acid (dCMP)
  7. Uracil + Ribose = Uridine
    Uridine + P = Uridylic acid (UMP)
  8. Thymine + Deoxyribose = Deoxy thymidine
    Deoxythymidine + P = Deoxythymidylic acid (dTMP)

 2.  DNA

  • Discovered by - Meischer
  • DNA term was given by - Zacharis
  • In DNA pentose sugar is deoxyribose sugar and four types of nitrogen bases A,T,G,C
  • Wilkins and Franklin studied DNA molecule with the help of X-Ray crystallography.
  • With the help of this study, Watson and Crick (1953) proposed a double helix model for DNA. For this model Watson, Crick and Wilkins were awarded by Noble Prize in 1962.
  • According to this model, DNA is composed of two polynucleotide chains.
  • Both polynucleotide chains are complementary and antiparallel to each other.
  • In both strand of DNA direction of phosphodiester bond is opposite. i.e. If direction of phosphodiester
  • bond in one strand is 3'-5' then it is 5'-3' in another strand.
  • Both strand of DNA held together by hydrogen bonds. These hydrogen bonds are present between nitrogen bases of both strand.
  • Adenine binds to thymine by two hydrogen bonds and cytosine binds to guanine by three hydrogen bonds

 

 

PRINCIPLES OF INHERITANCE AND VARIATION

1. INTRODUCTION

  • ü  Genetics term was given by W. Bateson. W. Bateson is Father of Modern Genetics.
  • ü  Genetics = Collective study of heredity & Variations.
    Heredity = Transmission of genetic characters from parent to off-springs.
    Variation = individuals of same species have some differences, these are called variation
  • ü  History of reserches in genetics.
    Muller – Proposed the term "Cytogenetics"
    (Cytology + Genetics)

 

Father of Actinobiology
Actinobiology - study the effect of radiation on living organisms.
Morgan – Father of Experimental genetics He experiment on Drosophila & proposed various concepts.
Gene theory - According to gene theory; genes are linearly located on chromosome.
Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance, Linkage map on Drosophila given by Morgan.

  • Garrod = Father of human genetics & Biochemical genetics. Garrod discovered first human metabolic genetic disorder which is called alkaptonuria (black urine disease). In this disease enzyme homogentisic acid oxidase is deficient. Gave the concept 'One mutant gene one metabolic block'

SOME GENETICAL TERMS

  1. Factors: - Unit of heredity which is responsible for inheritance and appearance of characters.
    These factors were referred as genes by Johannsen (1909). Mendel used term "element" for factor.
    Morgan first used symbol to represent the factor. Dominant factors are represented by capital letter while recessive factor by small letter.
  2. Allele: - Alternative forms of a gene which are located on same position [loci] on the homologous chromosome is called Allele. Term allele was coined by Bateson.
  3. Homozygous: - A zygote is formed by fusion of two gametes having identicle factors is called homozygote and organism developed from this zygote is called homozygous. Ex. TT, RR, tt
  4. Heterozygous :- A zygote is formed by fusion of two different types of gamete carrying different factors is called heterozygote (Tt, Rr) and individual developed from such zygote is called heterozygous. The term homozygous and heterozygous are coined by Bateson.
  5. Homozygous: - If individual contains only one gene of a pair then individual said to be Hemizygous. Male individual is always Hemizygous for sex linked gene.
  6. Phenotype: - It is the external and morphological appearance of an organism for a particular character.
  7. Genotype: - The genetic constitution or genetic make-up of an organism for a particular character. Genotype & phenotype terms were coined by Johannsen.
  8. Phenocopy: - If different genotypes are placed in different environmental conditions then they produce same phenotype. Then these genotypes are said to be Phenocopy of each other.


2.   MENDELISM
Experiments performed by Mendel on genetics and description of mechanisms of hereditory processes and formulation of principles are known as Mendelism. Mendel postulated various experimental laws in relation of genetics.

 

Gregor Johann Mendel (1822 - 1884):- Mendel was born on July 22, 1822 at Heinzendorf in Austria at Silesia village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.
In 1856-57, he started his historical experiments of heredity on pea (Pisum sativum) plant. His experimental work continued on pea plant till 1865 (19th century).
The results of his experiments were published in the science journal, "Nature For schender varein" in 1866.
This journal was in Germen language. Title was "verschue uber Pflangen Hybridan".
This journal was published by 'Natural History society of Bruno'.
A paper of Mendel by the name of "Experiment in plant Hybridization" published in this journal. Mendel was unable to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work (due to kidney disease (Bright disease)
After 16 years of Mendel's death in 1900, Mendel's postulates were rediscovered. Rediscovery by three scientists independently.

  1. Carl Correns - Germany - (Experiment on Maize)
  2. Hugo deVries (Holland) (Experiment on Evening Primerose)
    He republished the Mendel's results in 1901 in Flora magazine
  3. Erich von Tschermak Seysenegg - (Austria)
    (Experiment on different flowering plants)
    The credit of rediscovery of Mendelism goes to three scientists.
    Correns gave two laws of Mendelism.-
    Law of Heredity/Inheritance/Mendelism
    1st  Law - Law of segregation.
    IInd Law - Law of independent assortment.
    Mendel experiments remain hidden for 34 years.

Mendel results remain hidden due to:

  1. At that time Darwin's book "Origin of Species" published. Scientists were busy in discussion with this book.
  2. Mendel's ideas were ahead of that time.

 

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