PRINCIPLES OF INHERITANCE AND VARIATIONS
GENE, ITS EXPRESSION AND REGULATION
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Genetic material is
the substance that not only control the formation and expression of traits in
organisms, but can also replicate and pass on information from a cell to its
daughter cells.
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It is now proved
that genetic material is the nucleic acids of the cell.
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Nucleic acids are
long polymers of nucleotides. Two types of nucleic acids are found in living
system, i.e. RNA and DNA.
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DNA acts as a
genetic material in most organisms. It is a long polymer of deoxyribonucleotides,
etc. which codes for all the metabolic processes of life.
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RNA is the first
genetic material to evolve and DNA was derived from RNA. But still RNA acts a
genetic material in some plant viruses.
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A nucleotide is
composed of a nitrogenous base, a pentose sugar and a phosphate group while, a
nucleoside is formed when a nitrogenous base is linked to a pentose sugar
through a N-glycocidic linkage.
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Nitrogenous bases
are two types, i.e. purines (adenine, guanine) and pyrimidines (cytosine,
uracil, thymine). Uracil is present in RNA only in place of thymine.
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In 1953, JD
Watson and FHC Crick proposed the 3-d model of physiological DNA on the basis
of X-ray diffraction data of DNA obtained by Franklin and Wilkins.
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In a DNA, adenine
pairs with thymine through two H-bonds while, guanine pairs with cytosine
through three H-bonds. This makes one strand complementary to the other.
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Replication of DNA
is the process by which DNA molecule makes its identical copies.
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Watson and Crick
in 1953 proposed a semiconservative mechanism of replication on the basis of
their model of DNA molecule. According to them, two strands of double helix
would separate and act as a template for the synthesis of new complementary
strands.
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Evidence in support
of semiconservative model of DNA replication was given by Meselson and Stahl
(1958), Cairn (1963) and Taylor (1969) independently in their experiments.
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DNA replication
begins at a specific and fixed position in a DNA molecule known as origin of
replication (ori).
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On 3’ → 5’ strand replication is continuous and on 5’ → 3’ strand it is discontinuous.
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The process of
replication is guided by complimentary H-bonding. The process is catalysed by
various sets of catalysis (enzyme).
‣
DNA dependent DNA polymerase which uses DNA template to catalyse the polymerisation of
deoxynucleotides.
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DNA helicase, which
unwinds DNA strand for the formation of a replication fork.
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DNA ligase, which
facilitates the joining of DNA strands together by catalysing the formation of
phosphodiester bond.
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These are three
types of RNAs found in all living cells, i.e. mRNA, tRNA and rRNA. Out of which
mRNA provides the template for transcription, tRNA brings amino acids and reads
the genetic code and rRNA plays structural and catalytic role during
translation.
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tRNA is an
adapter molecule that read the code of mRNA and on the other end would kind to
the specific amino acid. tRNA has five loops, i.e. D-loop, anticodon loop,
amino acid loop, T-loop, and variable loop. It is a clover leaf shape molecule.
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Central dogma is
the unidirectional flow of information from DNA to RNA and from RNA to
polypeptide.
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DNA contains
hereditary information in the form of sequence of nitrogenous bases. Three
bases together constitute a codon and the whole array of these codon is termed
genetic code.
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Genetic code has
following features
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It is unambiguous
and specific, i.e. one codon codes for only one amino acid.
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Codon is triplet
and degenerate.
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The genetic code
is nearly universal, i.e. one codon codes for same amino acid is all organisms
except in protozoa.
‣
AUG codon has
dual function, i.e. it codes for the amino acid methionine (met) and
also act as an initiator codon.
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Transcription is
the process through which genetic information of DNA is transferred to mRNA by
an enzyme called RNA polymerase in the presence of several other factors.
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Capping is
the addition of methyl guanosine triphosphate at 5’ and mRNA. Tailing is the addition of adenylate residues at 3’ end.
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In eukaryotes the
genes are split. The coding sequences, i.e. exons are incrupted by non-coding sequences, i.e. introns. These introns are removed from the mRNA by splicing.
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The process of
polymerisation of amino acids from a polypeptide is known as translation. The proteins are synthesized from mRNA with the help pf
ribosomes.
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Different phases
of translation are
‣
Activation of
amino acids
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Initiation of
polypeptide synthesis at the start codon (AUG)
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Elongation of
polypeptide chain
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Termination of
polypeptides synthesis on reaching stop codons (UAG, UAA or UGA).
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Regulation of
gene expression occurs at various levels. It results in the formation of a
polypeptide.
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In prokaryotes,
it is regulated by the rate of initiation of transcription and in eukaryotes,
regulation is achieved at the following four levels.
‣ Transcriptional
level (formation of primary transcript)
‣
Processing level
(regulation of splicing)
‣ Transport of mRNA
from nucleus to the cytoplasm.
‣ Translational
level
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Lac-operon was proposed by Jacob and Monad. It is a transcriptionally regulated
system, where a polycistronic structural gene is regulated by a common promotor
and regulatory gene.
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Inducible operon
system normally remains switched off, but becomes functional in the presence of
an inducer, e.g. lac operon in E. coli.
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In repressible operon system regulator produces aporepressor. These aporepressor binds with the
corepressor to form active repressor. It binds to the operator gene and blocks
the functions of RNA polymerase.
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