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Presentation Biology NCERT Solutions - Biology for Class 12th, ISBN: 9789351416203Presentation Biology NCERT Solutions - Biology for Class 12th, ISBN: 9789351416203R194,90
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Class notes 12th grade NCERT Solutions - Biology for Class 12th
Summary NCERT Solutions - Biology for Class 12th - Biology
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Chapter 5 The work of Mendel and others who followed him gave us an
Principles of Inheritance idea of inheritance patterns. However the nature of those ‘factors’
and Variation which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
Chapter 6 the structure of genetic material and the structural basis of
Molecular Basis of Inheritance genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
Chapter 7 molecular biology was a consequent development with major
Evolution contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
, James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received B.Sc. degree in Zoology. During these years his interest in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.
He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
JAMES WATSON England. He studied physics at University College, London and obtained
FRANCIS CRICK a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.
A critical influence in Crick’s career was his friendship with J. D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.
The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in 1960; the Research Corporation Prize, in 1962 and above all, the
Nobel Prize in 1962.
, CHAPTER 5
PRINCIPLES OF INHERITANCE
AND VARIATION
5.1 Mendel’s Laws of
Inheritance
5.2 Inheritance of One Gene Have you ever wondered why an elephant always gives
birth only to a baby elephant and not some other animal?
5.3 Inheritance of Two Genes Or why a mango seed forms only a mango plant and not
5.4 Sex Determination any other plant?
Given that they do, are the offspring identical to their
5.5 Mutation
parents? Or do they show differences in some of their
5.6 Genetic Disorders characteristics? Have you ever wondered why siblings
sometimes look so similar to each other? Or sometimes
even so different?
These and several related questions are dealt with,
scientifically, in a branch of biology known as Genetics.
This subject deals with the inheritance, as well as the
variation of characters from parents to offspring.
Inheritance is the process by which characters are passed
on from parent to progeny; it is the basis of heredity.
Variation is the degree by which progeny differ from their
parents.
Humans knew from as early as 8000-1000 B.C. that
one of the causes of variation was hidden in sexual
reproduction. They exploited the variations that were
naturally present in the wild populations of plants and
animals to selectively breed and select for organisms that
possessed desirable characters. For example, through
artificial selection and domestication from ancestral
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