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Genetic information, stored in the chromosomes and
transmitted to the daughter cells through DNA
replication is expressed through transcription to
RNA and, in the case of mRNA, subsequent
translation into polypeptide chains.
This flow of information from DNA to RNA to
protein is termed the "central dogma“
The process of translation requires a genetic code ,
through which the information contained in the
nucleic acid sequence is expressed to produce a
specific sequence of amino acids.
Any alternation in the nucleic acid sequence may
result in an improper amino acid being inserted into
the polypeptide chain, potentially causing diseases
or even death.
central
dogma
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The genetic code is a dictionary that identifies
the correspondence between a sequence of
nucleotide bases and a sequence of amino
acids.
Each individual word in the code is composed
of three nucleotide bases. These genetic words
are called codons
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The codons are usually presented in the messenger
RNA language of A, G, C, and U.
Their nucleotide sequence are always written from
the 5'-end to the 3'end.
The four nucleotide bases are used to produce the
three-base codons. There are therefore 64 different
combinations of bases, taken three at a time (43).
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Three of the codons, UAG, UGA, and UAA
do not code for the amino acids, but rather are
termination codons .
when one of these codons appears in an mRNA
sequence, it signals that synthesis of the peptide
chain coded for by that mRNA is completed.
AUG is a start codon.
1. Specificity: a specific codon always codes for
the same amino acid.
2. Redundancy or Degeneracy: Although each codon
corresponds to a single amino acid, a given amino
acid may have more than one triplet coding for it. (61
sense codes for 20 a. a… i.e. more than 3 sense codes
for 1 amino acid).
3. Triplicity: Triplet of nucleotides for each a.a., 4=64,
61 sense code, 3 non-sense.
4. Non overlapping and comma less: the code is read
from a fixed starting point as a continuous sequence
of bases, taken three at a time.
For example, ABCDEFGHIJKL…is read as
ABC/DEF/GHI/JKL…without any "punctuation"
between the codons.
5. Co-linearity: correspondence between the linear seq.
of the codon in mRNA+ a.a in the protein.
6. Universality: The genetic code is universal. The
specificity of the genetic codes has been conserved
from very early stages of evolution.
Amino Acid
Isoleucine
Leucine
Valine
Phenylalanine
Methionine
Cysteine
Alanine
Glycine
Proline
Threonine
Serine
Tyrosine
Tryptophan
Glutamine
Asparagine
Histidine
Glutamic acid
Aspartic acid
Lysine
Arginine
SLC
I
L
V
F
M
C
A
G
P
T
S
Y
W
Q
N
H
E
D
K
R
DNA codons
ATT, ATC, ATA
CTT, CTC, CTA, CTG, TTA, TTG
GTT, GTC, GTA, GTG
TTT, TTC
ATG
TGT, TGC
GCT, GCC, GCA, GCG
GGT, GGC, GGA, GGG
CCT, CCC, CCA, CCG
ACT, ACC, ACA, ACG
TCT, TCC, TCA, TCG, AGT, AGC
TAT, TAC
TGG
CAA, CAG
AAT, AAC
CAT, CAC
GAA, GAG
GAT, GAC
AAA, AAG
CGT, CGC, CGA, CGG, AGA, AGG
o
o
o
o
Some aspects of gene structure, such as the
existence of the introns and exons.
Most human genes are divided into exons and
introns.
The exons are the sections that are found in the
mature transcript (messenger RNA),
while the introns are removed from the
primary transcript by the process called
splicing.
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While DNA is formed and replicated in the cell nucleus,
protein synthesis takes place in the cytoplasm .The
information contained in DNA must be transported to
the cytoplasm and then used to dictate the composition
of proteins.
This involves two processes, transcription and translation.
Briefly, the DNA code is transcribed into mRNA, which
then leaves the nucleus to be translated into proteins.
There is an important difference between replication and
transcription. During replication the entire chromosome
is copied to yield daughter DNA identical to the parent
DNA .But in the transcription process not all of the cell
DNA is necessarily transcribed, usually only individual
genes are transcribed.
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Is the process by which an m-RNA sequence is
formed from a DNA template.
This process is divided into three phases:
1. Initiation.
2. Elongation.
3.Termination.
Involves the binding of the RNA polymerase to a
region on the DNA known as promoter region
(a promoter is a nucleotide sequence that lies up
stream of the gene).
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The RNA polymerase then pulls a portion of the
DNA strands apart from one another, exposing
unattached DNA bases. One of the two DNA strands
provides the template for the sequence of mRNA
nucleotides.
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RNA polymerase begins to synthesize a transcript of
the DNA sequence by adding complementary
ribonucleotide to the new RNA.
Because the m RNA molecule can be synthesized
only in the 5' to 3' direction, the promoter, by
specifying directionality, determines which DNA
strand serves as the template. This template DNA
strand is also known as the antisense strand.
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RNA polymerase moves in the 3' to 5'
directions along the DNA template strand,
assembling the complementary m RNA strand
from the 5' to 3'.
Because of complementary base paring, the
m RNA nucleotide sequence is identical to that
of the DNA strand that does not serve as the
template-the sense strand-except, of course, for
the substitution of uracil for thymine.
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The process of elongation of the RNA chain
continues until a termination signal is reached.
Then the enzyme releases the completed RNA
and detaches from DNA.
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A primary transcript is a linear copy of a
transcriptional unit, the segment of DNA between
specific initiation and termination sequences. The
primary transcript is modified after transcription.
These Modifications include:
1. 5' "Capping“.
2. Addition of a poly-A tail.
3. Removal of introns.
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After RNA synthesis begins, the 5' end of the
growing RNA molecule is capped by the
addition of a guanine nucleotide. To prevent
the RNA molecule from being degraded during
synthesis, and late it helps to indicate the
starting position for translation of m RNA
molecule into protein.
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A series of 40 to 200 adenine bases are added to
the 3' end of the RNA molecule.
This structure involves in stabilizing the
mRNA molecule so that it is not degraded
when it reaches the cytoplasm.
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Maturation of m RNA may involve in the
removal of RNA sequences(introns) that do not
code for protein by nuclear enzymes.
and the remaining coding sequences, the
(exons )are spliced together to form the mature
m RNA that will migrate to the cytoplasm.
Post-Transcriptional
Modification of RNA
1) 5' "Capping“.
2)Addition of a
poly-A tail.
3)Removal of
introns.