Download Central Dogma,Genetic Code

Central Dogma,Genetic Code !!
CENTRAL DOGMA
The chromosomal DNA contains complete information about all
the specific proteins to be synthesized in the cells for their
needs.
This information is passed on by the DNA of the gene located
in the sense strand to the mRNA transcribed from it as a
single chain of complementary bases.
The mRNA, with the help of tRNA & ribosome then directs the
specific amino acids to come together and join in a sequence
communicated by DNA.
By this arrangement, the information initially coded as a
specific sequence of deoxyribonucleotides in the DNA (gene) is
transcribed as a specific sequence of complementary
ribonucleotides in the mRNA, & is finally translated into a
specific order of the amino acids in the polypeptide chain.
DEFINITION
Information flows from genes which are composed of DNA to
polypeptides which are composed of amino acids through a
intermediate, which is composed of mRNA. Thus the flow of
information is DNA —— mRNA ———- polypeptide, a progression
often called as central dogma of molecular biology.
In 1970, a little change occurs in central dogma, where RNA is
used as a template for the synthesis of DNA by the help of
reverse transcriptase, called reverse transcription.
Central dogma of molecular biology showing how genetic
information is transmitted through DNA replication and
expressed through transcription & translation. In reverse
transcription, RNA is used as a template for synthesis of DNA.
GENETIC CODE
The uniqueness of every cell, individual or species lies in
the uniqueness of its protein.
The cells are enabled to synthesize their specific proteins by
the information flowing from the DNA.
This information exists as the particular sequences of bases
in the DNA strands & is called genetic code.
NATURE OF GENETIC CODE
There are twenty kinds of amino acids & only four types of
nucleotides namely adenine, guanine, thymine and cytosine.
It is clear that one to one correlation between nucleotides
and amino acids is not possible.
If one base coded for one amino acid, only four amino acids
could be specified
If a sequence of two bases coded for one amino acid, the four
bases could specify 16(4×4) amino acids. This is also
inadequate.
If a sequence of three bases coded for one amino acid, the
four bases would specify 64(4×4×4) amino acids.
So a sequence of three bases, called the triplet code is the
simplest code that can account for 20 amino acids.
DEFINITION
Genetic code defined as the sequence of base triplets in DNA
molecule, each triplet representing one amino acid of a
polypeptide.
It is written in the conventional 5′-3′ direction.
CHARACTERISTICS OF GENETIC CODE
Triplet nature
The genetic code is a triplet code. Three adjacent beses
termed codon specify one amino acid.
No overlapping
The adjacent codons do not overlap; they do not share any
base.
The sequence CCUCAG is read only as CCU and CAG
No punctuation
The genetic code is commaless. There are no “punctuation
marks” / gaps between the coding triplets.
Not write as CCU, CUC and UCA it should write as CCUCUCUCA.
Universality
Genetic code is universal, means given codon in the DNA & mRNA
specifies the same amino acid in the protein synthesizing
system of all organisms, from bacteria to human beings also in
viruses.
Degeneracy
The genetic code is degenerate, means it lacks specificity &
one amino acid often has more than one code triplet. Only
methionine amino tryptophans have single triplet codon
. All
other amino acids are specified by 2–6 triplets.
E.g. phenylalanine has six codons namely UUU & UUC.
GENETIC CODE TABLE
Amino acid chart
Amino acids are put together into a polypeptide chain on
ribosome during protein synthesis.
20 different amino acids most commonly found in nature.
All amino acid contain both amino & carboxylic acids in their
side chain.
The properties of amino acids are determined by the functional
subunits linked on their side chains known as R—groups.
We listed amino acid which describes 20 amino acid residues
found in nature.
Aliphatic amino acids with hydrophobic side chain.
Alanine
Isoleucine
Leucine
Valine
Aromatic amino acid with hydrophobic side chain.
Phenylalanine
Tryptophan
Tyrosine
Amino acids with neutral side chain.
Asparagine
Cysteine
Glutamine
Methionine
Serine
Threonine
Amino acids with positive charged side chain.
Arginine
Histidine
Lysine
Amino acids with negative side chain.
Aspartic acid
Glutamic acid
Unique amino acids.
Glycine
Proline
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