1. Dental Biochemistry 1- (11)
Genetic code and Protein
biosynthesis
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2. Ribonucleic acid (RNA)
• RNA is also a polymer of purine and
pyrimidine nucleotides linked by
phosphodiester bonds
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3. Differences between RNA and DNA
RNA DNA
Mainly seen in cytoplasm Mostly inside nucleus
Usually 100-5000 bases Millions of base pairs
Generally single stranded Double stranded
Sugar is ribose Sugar is deoxyribose
Purins: Adenine, Guanine Purins: Adenine, Guanine
Pyrimidines: Cytosine, Uracil Pyrimidines: Cytosine, Thymine
Guanine content is not equal Guanine content is equal to
to cytosine and adenine is cytosine and adenine is equal
not equal to uracil to thymine
Easily destroyed by alkali
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Alkali resistant

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5. Types of RNA
• Messenger RNA (mRNA).
• Transfer RNA (tRNA) or (sRNA).
• Ribosomal RNA (rRNA).
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6. Messenger RNA or mRNA
• It acts as a messenger of the information in the
gene in DNA to the protein synthesizing machinery
in cytoplasm. It carries the message to be
translated to a protein.
• The template strand of DNA is transcribed into a
single stranded mRNA. The is accomplished by the
DNA dependent RNA polymerase.
• The mRNA is a complementary copy of the template
strand of the DNA.
• However, thymine is not present in RNA; instead
uracil will be incorporated.
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7. Transfer RNA (tRNA) or (sRNA)
• They transfer amino acids from cytoplasm to the
ribosomal protein synthesizing machinery; hence
the name transfer RNA.
• Since they are easily soluble, they are also referred
to as soluble RNA or sRNA.
• They are RNA molecules present in the cytoplasm.
• Each molecule is only 73-93 nucleotides in length;
much shorter than mRNA molecules.
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8. Transfer RNA
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9. Ribosomal RNA (rRNA)
• Ribosomes provide necessary infrastructure
for the mRNA, tRNA and amino acids to
interact with each other for the translation
process.
• Thus, ribosomal assembly is the protein
synthesizing machinery.
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10. Central Dogma of Molecular Biology:
The information available in the DNA is passed to
messenger RNA, which is then used for synthesis of a
particular protein.
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11. Steps of protein synthesis
1. Transcription:
• It is taking place all the time. Only certain
areas of the DNA are copied (selected region
on the sense strand).
• The genetic information (code) of DNA is
transcribed (copied) to the messenger RNA
(mRNA).
• During transcription, the message from the
DNA is copied in the language of nucleotides
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12. 2. Translation:
• The mRNA then reaches the cytoplasm
where it is translated into functional
proteins.
• During translation, the nucleotide sequence
is translated to the language of amino acid
sequence.
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14. Inhibitors of RNA synthesis
• Actinomycin D and Mitomycin
intercalate with DNA strands, thus
blocking transcription. They are used
as anticancer drugs.
• Rifampicin is widely used in the
treatment of tuberculosis and leprosy.
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15. Genetic code
A triplet sequence of nucleotides
on the mRNA is the codon for
each amino acid.
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16. Salient features of genetic code
• 1. Triplet codons:
Each codon is a consecutive sequence of three bases
on the mRNA,
e.g. UUU codes for phenylalanine.
• 2. Non overlapping:
The codes are consecutive. Therefore, the starting
points is extremely important. The codes are read
one after another in a continuous manner, e.g.
AUG, CAU,CAU, GCA, etc.
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17. • 3. Non punctuated:
There is no punctuation between the codons. It is
consecutive or continuous.
• 4. Degenerate:
When an amino acid has more than codon, this called
degeneracy of the code.
E.g. serine has 6 codons while glycine has 4 codons.
• 5. Unambiguous:
Through the codons are degenerate, they are
unambiguous: or without any doubtful meaning.
That is, one codon codes only one amino acid.
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18. • 6. Universal:
The codons are the same for the same amino acid in all
species; the same for “Elephant and E.coli”.
The genetic code has been highly preserved during
evolution.
• 7. Terminator codons:
There are three codons which do not code for any
particular amino acids. They are “nonsense codons”,
more correctly termed as punctuator codons or
terminator codons. They put “full stop” to the protein
synthesis. These three codons are UAA, UAG, and UGA.
• 8. Initiator codon:
• In most of the cases, AUG acts as the initiator codon.
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19. Inhibitors of protein synthesis
• The modern medical practice is heavily dependent
on the use of antibiotics.
• They generally act only on bacteria and are nontoxic
to human beings.
• Some antibiotics act as irreversible inhibitors to
bacteria:
• E.g. Streptomycin which causes misreading of
mRNA.
• Others act as reversible inhibitors to bacteria
• E.g. Erythromycin prevents translation process
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