nucleic acids and proteins synthesis
DESCRIPTION
DNA RNA protein synthesisTRANSCRIPT
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BIOSYNTHESIS OF NUCLEIC
ACIDS AND PROTEINS
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The flow of genetic information in a typical cell
DNA
RNA
protein
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Primary structure of nucleic acids
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Two-stranded structure of DNA
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Watson JamesCrick Francis
The double helix of DNA was discovered in 1953 by Crick F. and Watson J. Nobel prize in 1962.
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When replication takes place?
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What are the principles of replication?
• Based on template;• Complementary;• Antiparallel;• Two directions;• Semi-conservative;• Very complex.
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Components required for replications
• DNA molecule - template• Origin of replication – point ORI• Enzymes• Nucleotides (dNTP and NTP) • SSB proteins
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Main enzymes required for replication• DNA-polymerase (I, II,
III – in prokaryotes,
ᵅ,ᵞ,ᵋ – in eukaryotes)
• Primase • DNA-helicases • Topoisomerase • DNA-ligase• Telomerase
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Topoisomerase
Protein complexes of the replication fork
DNA replication
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Reiji Okazaki provided experimental evidence for discontinuous DNA synthesis
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Details of lagging strand synthesis
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DNA
mRNARibosome
Polypeptide
TRANSCRIPTION
TRANSLATION
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
DNA
mRNA
Pre-mRNARNA PROCESSING
(a) Bacterial cell (b) Eukaryotic cell
Nuclearenvelope
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TRANSCRIPTIONDNA
mRNA
(a) Bacterial cell
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TRANSCRIPTIONDNA
mRNA
(a) Bacterial cell
TRANSLATIONRibosome
Polypeptide
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Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
TRANSCRIPTION
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RNA PROCESSING
Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION
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RNA PROCESSING
Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION
TRANSLATION Ribosome
Polypeptide
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Nontemplatestrand of DNA
RNA nucleotides
RNApolymerase
Templatestrand of DNA
3
35
5
5
3
Newly madeRNA
Direction of transcription
A
A A A
AA
A
T
TT
T
TTT G
GG
C
C C
CC
G
C CC A AA
U
U
U
end
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Initiation and elongationsteps of transcription
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Biochemistry For Medics- Lecture Notes
26
Post Transcriptional Post Transcriptional modifications of modifications of
pre m- RNApre m- RNA• In prokaryotic organisms, the primary
transcripts of mRNA-encoding genes begin to serve as translation templates even before their transcription has been completed.
• In all eukaryotes the primary transcripts of mRNA-encoding genes undergo extensive processing before they are converted to mature functional forms
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Post Transcriptional modifications of Post Transcriptional modifications of pre m- RNApre m- RNA
a) 5' Capping• Mammalian mRNA molecules contain a 7-methylguanosine cap
structure at their 5' terminal. • The cap structure is added to the 5' end of the newly transcribed
mRNA precursor in the nucleus prior to transport of the mRNA molecule to the cytoplasm.
• The 5' cap of the RNA transcript is required both for efficient translation initiation and protection of the 5' end of mRNA from attack by 5-'3' exonucleases.
• Eukaryotic m RNAs lacking the cap are not efficiently translated.
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Post Transcriptional modifications of Post Transcriptional modifications of pre m- RNApre m- RNA
•The addition of the Guanosine triphosphate (part of the cap is catalyzed by the nuclear enzyme guanylyl transferase.
•Methylation of the terminal guanine occurs in the cytoplasm. and is catalyzed by guanine-7-
methyl transferase.•S-Adenosyl methionine is the methyl group donor. •Additional methylation steps may occur.The secondary methylations of mRNA molecules, those on the 2'-hydroxy and the•N6 of adenylyl residues, occur after the mRNA molecule has appeared in the cytoplasm
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Post Transcriptional modifications of Post Transcriptional modifications of pre m- RNApre m- RNA
b) Addition of poly A tail• Poly(A) tails are added to the 3' end of mRNA molecules in a
posttranscriptional processing step. • The mRNA is first cleaved about 20 nucleotides downstream from an
AAUAA recognition sequence • Another enzyme, poly(A) polymerase, adds a poly(A) tail which is
subsequently extended to as many as 200 A residues. • The poly(A) tail appears to protect the 3' end of mRNA from 3' 5'
exonuclease attack. • Histone and interferon's mRNAs lack poly A tail.• After the m-RNA enters the cytosol, the poly A tail is gradually shortened.
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Post Transcriptional modifications of Post Transcriptional modifications of Pre m RNAPre m RNA
Removal of introns (Splicing)• Introns or intervening sequences are the
RNA sequences which do not code for the proteins.
• These introns are removed from the primary transcript in the nucleus, exons (coding sequences) are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm.
• The steps of splicing are as follows-32
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Post Transcriptional modifications of Post Transcriptional modifications of Pre m RNAPre m RNA
• Introns are removed from the primary transcript in the nucleus, exons (coding sequences) are ligated to form the mRNA molecule
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Figure 17.12-1RNA transcript (pre-mRNA)
5Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
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Figure 17.12-2RNA transcript (pre-mRNA)
5Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
Spliceosome
5
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Figure 17.12-3RNA transcript (pre-mRNA)
5Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
Spliceosome
5
Spliceosomecomponents
Cut-outintronmRNA
5Exon 1 Exon 2
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Nirenberg Marshall decoded the genetic code. Nobel prize, 1968
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GENETIC CODE - sequence of mononucleotides in mRNA that specifies the sequence of amino acids in peptide chain
CODON – mRNA triplet base sequence responsible for 1 amino acid
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PROPERTIES OF GENETIC CODE
1. Degenerate
2. Specific
3. Nonoverlapping
4. Without punctuation
5. Universal
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TRANSLATION
• 1. Recognition
• 2. Initiation
• 3. Elongation
• 4. Termination
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Formation of aminoacyl tRNAs by aminoacyl tRNA synthetase.
RECOGNITION
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
Figure 17.16-1
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
Figure 17.16-2
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Aminoacid
Aminoacyl-tRNAsynthetase
Figure 17.16-3
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Aminoacid
Aminoacyl-tRNAsynthetase
Aminoacyl tRNA(“charged tRNA”)
Figure 17.16-4
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Components of a 70S prokaryotic ribosome
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tRNAmolecules
Growingpolypeptide Exit tunnel
E PA
Largesubunit
Smallsubunit
mRNA5
3
(a) Computer model of functioning ribosome
Exit tunnel Amino end
A site (Aminoacyl-tRNA binding site)
Smallsubunit
Largesubunit
E P AmRNA
E
P site (Peptidyl-tRNAbinding site)
mRNAbinding site
(b) Schematic model showing binding sites
E site (Exit site)
(c) Schematic model with mRNA and tRNA
5 Codons
3
tRNA
Growing polypeptide
Next aminoacid to beadded topolypeptidechain
Figure 17.17
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Figure 17.17a
tRNAmolecules
Growingpolypeptide Exit tunnel
E P A
Largesubunit
Smallsubunit
mRNA5
3
(a) Computer model of functioning ribosome
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Figure 17.17b
Exit tunnel
A site (Aminoacyl-tRNA binding site)
Smallsubunit
Largesubunit
P A
P site (Peptidyl-tRNAbinding site)
mRNAbinding site
(b) Schematic model showing binding sites
E site (Exit site)
E
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Figure 17.17c
Amino end
mRNAE
(c) Schematic model with mRNA and tRNA
5 Codons
3
tRNA
Growing polypeptide
Next aminoacid to beadded topolypeptidechain
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Initiation of protein biosynthesis in prokaryotes.
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Elongation of the Polypeptide Chain
• During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain
• Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation
• Translation proceeds along the mRNA in a 5′ to 3′ direction
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Amino end ofpolypeptide
mRNA
5
E
Psite
Asite
3
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Amino end ofpolypeptide
mRNA
5
E
Psite
Asite
3
E
GTP
GDP P i
P A
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Amino end ofpolypeptide
mRNA
5
E
Psite
Asite
3
E
GTP
GDP P i
P A
E
P A
Elongation1) Positioning of aminoacyl-tRNA in the A site 2) Formation of the peptide bound (enzyme – peptidyl transferase)3) Translocation
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Amino end ofpolypeptide
mRNA
5
E
Asite
3
E
GTP
GDP P i
P A
E
P A
GTP
GDP P i
P A
E
Ribosome ready fornext aminoacyl tRNA
Psite
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Termination of translation in prokaryotes
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POSTTRANSLATIONAL MODIFICATION
1) Preparing of proteins for different functions
2) Direction of proteins to different locations (targeting)
1. Proteolytic cleavage
2. Hydroxylation
3. Glycosilation
4. Phosphorilation
5. Lipophilic modification
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The operon model (by Jacob and Monod)
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Some inhibitors of transcription
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Some antibiotics that act by interfering with protein biosynthesis