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First stage
Biology
Lec-2
2/12/2015
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The flow of genetic information
It is a remarkable fact that an organism’s characteristics are encoded by a four-letter
alphabet, defining a language of three-letter words. The letters of this alphabet are the
nitrogenous bases adenine (A), guanine (G), cytosine (C), and thymine (T). So how do these
bases enable cells to function? The expression of genetic information is achieved ultimately
via proteins, particularly the enzymes that catalyse the reactions of metabolism. Proteins
are condensation heteropolymers synthesized from amino acids, of which 20 are used in
natural proteins. Given that a protein may consist of several hundred amino acid residues.
The flow of genetic information is unidirectional, from DNA to protein, with messenger RNA
(mRNA) as an intermediate. The copying of DNA-encoded genetic information into RNA is
known as transcription (TC), with the further conversion into protein being termed
translation (TL). This concept of information flow is known as the Central Dogma of
molecular biology and is an underlying theme in all studies of gene expression.
Transcription and translation
These two processes are the critical steps involved in producing functional proteins in the
cell. Transcription involves synthesis of an RNA from the DNA template provided by the non-
coding strand of the transcriptional unit in question. The enzyme responsible is RNA
polymerase. In prokaryotes there is a single RNA polymerase enzyme, but in eukaryotes
there are three types of RNA polymerase (I, II, and III). These synthesize ribosomal,
messenger, and transfer/5S ribosomal RNAs, respectively. When the RNA molecule is
released, it may be immediately available for translation (as in prokaryotes) or it may be
processed and exported to the cytoplasm (as in eukaryotes) before translation occurs.
Translation requires an mRNA molecule, a supply of charged tRNAs (tRNA molecules with
their associated amino acid residues), and ribosomes (composed of rRNA and ribosomal
proteins). The ribosomes are the sites where protein synthesis occurs; in prokaryotes,
ribosomes are composed of three rRNAs and some 52 different ribosomal proteins. The
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ribosome is a complex structure that essentially acts as hold the mRNA in place so that the
codons may be matched up with the appropriate anticodon on the tRNA, thus ensuring that
the correct amino acid is inserted into the growing polypeptide chain. The mRNA molecule is
translated in a 5'→3' direction.
Transcription ( RNA synthesis ) :
Transfer of genetic information from DNA by the synthesis of a complementary RNA
molecule under the direction of RNA polymerase .
Transcription occurs in three stages :
1- Initiation :
Transcription begins when transcription factors help RNA polymerase bind to the
promoter ( which is a special sequence that signals the start of the gene). Transcription
factor regulate which genes are transcriped in a particular cell type .
Figure: Setting the stage for transcription to begin: a- Proteins that initiate recognize
specific sequences in the promoter region of a gene. b-A binding protein recognizes the
TATA region and binds to the DNA. This allows other transcription factors to bind. c- the
bound transcription factors form a pocket that allows RNA-polymerase to bind and begin
making RNA.
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2- Elongation :
RNA polymerase unwind the DNA double helix locally, RNA polymerase then adds RNA
nucleotides to a growing chain, in a sequence complementary to the DNA template strand .
RNA is transcribed from the template strand of DNA .
The other DNA strand is called the coding strand .
3- Termination :
A termination sequence in the gene signals the end of transcription .
Figure: Transcription: Initiation is the control point that determines which genes are
transcribed. RNA nucleotides are added during elongation. A termination sequence in the
gene signals the end transcription.
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mRNA Processing in Eukaryotes
In the nucleus a gene composed of Exons ( coding sequences ) and Introns (non coding
sequences). Both of these are transcriped to pre- mRNA or primary transcripts (primary
mRNA). Pre mRNA undergo 3 major modification prior to their transport to the cytoplasm
for translation :
1- 7-Methyl guanosine caps are added to the 5' ends of the primary transcripts.
2- Poly (A) tails (a series of adenine molecules ) are added to the 3 ends of the transcripts .
It is important for RNA stability and translation of polypeptide .
3- Processing involves spliceosomes consist of snRNAs and protein subunits in the nucleus
to remove the intron and splice together the exon into mRNA.
Mature mRNA transmits to the cytoplasm where it directs protein synthesis.
Transcription and RNA Processing occur in the nucleus while translation occurs in the
cytoplasm.
Translation (Protein Synthesis )
The Process which the genetic information (which is stored in the sequence of nucleotides
in an mRNA molecule ) is translated , according to the specification of the genetic code into
the sequence of amino acids in the polypeptide gene product. A ribosome has two subunits
(each composed of rRNA and various proteins)
small subunit and large subunit . Eukaryotic ribosome (80 S ) has small subunit (40 S )
and large subunit ( 60 S ) .
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Protein synthesis consist of 3 phases :
1- Initiation Phase :
A small ribosomal subunit binds to mRNA : an initiator tRNA with the anticodon UAC
pairs with the start codon AUG.
The large ribosomal subunit completes the ribosome , initiator tRNA carry
methionine occupies the P-site. The A-site is ready for the second tRNA
The small and large subunit together form two tRNA binding sites P(Peptidyl) site
and A ( Aminoacyl ) site .
2- Elongation Phase : Elongation consist of 3 steps :
arged tRNA with an anticodon complementary to the second codon
A second ch
st
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on mRNA binds to A-site .
( part of the large ribosomal
ransferase
t
Peptidyl
Peptide bond formation :
-
nd
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subunit ) can catalyze formation of peptide bond between the amino acids carried by
the two tRNAs . This bond forming reaction connects the methionine at the P-site to the
amino acid carried by the tRNA at the A-site . It also disconnects methionine from the
initiation tRNA as a result the tRNA at the A-site now carries two amino acids .
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Translocation :
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rd
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Three concerted movements occur , collectively called translocation :
1- Uncharged -tRNA leave the P-site .
2- The dipeptide-tRNA in the A-site moves to the P-site .
3- The ribosome moves a long the mRNA by three nucleotides (codon ) to place the next
codon in the A-site .
The empty A-site now receives another tRNA whose identity is determined by the next
codon in the mRNA and the ( peptide bond formation and translocation ) occurs once again
.
3- Termination Phase :
Termination of protein synthesis occurs when one of 3 stop codons ( UAG , UAA ,
UGA ) appears in A-site of the ribosome . A protein called release factor recognize stop
codons and hydrolysis the bond between the last tRNA at the P-site and the polypeptide
releasing them . The ribosomal subunits dissociate .
The resultant polypeptide chain may be enzyme , hormone , antibody , or structural
proteins .
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Features of the genetic code :
1- Genetic code is Triplet : AAA code for lysine .
2- Unambiguous : each Triplet codon has only one meaning .
3- The genetic code has start and stop signals . There is one start signals ( AUG ) . and there
is 3 stop signals ( UAA , UAG , UGA ) .
4- Universal : the code is the same and stable in all living organisms.
5- The genetic code is nonoverlapping and degenerate.