Lec 12 level 3-nu (gene expression and synthesis of protein)
1. Nursing - 12
Gene expression and
synthesis of protein
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2. Nucleotides
• Nucleotides are precursors of the nucleic
acids, deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA).
• The nucleic acids are concerned with the
storage and transfer of genetic information.
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3. Composition of nucleotides
A nucleotide is made up of three
components:
1. Nitrogenous base, (a purine or a
pyrimidine).
2. Pentose sugar, either ribose or
deoxyribose.
3. Phosphate groups esterified to the
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sugar
4. • Nucleoside: Formed when a base combines with
a pentose sugar.
• Nucleotide: Formed when nucleoside is esterified
to phosphate group. It also called nucleoside
monophosphate.
• The nucleic acids (DNA and RNA) are polymers of
nucleoside monophosphates.
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5. Bases present in the nucleic acids
1- Purine bases:
• The purine bases present in RNA and DNA are the
same; adenine and guanine.
• Uric acid is formed as the end product of the
catabolism of other purine bases.
• 2- Pyrimidine bases
• The pyrimidine bases present in nucleic acids
are cytosine, thymine and uracil.
• Cytosine is present in both DNA and RNA.
• Thymine is present in DNA and uracil in RNA.
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7. 2- Pyrimidine bases
• The pyrimidine bases present in
nucleic acids are cytosine, thymine
and uracil.
• Cytosine is present in both DNA and
RNA.
• Thymine is present in DNA and uracil
in RNA.
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12. Structure of DNA
• Deoxyribonucleic acid (DNA) is composed of four
deoxyribonucleotides:
Deoxyadenylate (A) - Deoxyguanylate (G)
Deoxycytidylate (C) - Deoxythymidylate (T)
• These units are combined through 3’ to 5’
phosphodiester bonds to polymerize into a long
chain.
• The nucleotide is formed by a combination of base
+ sugar + phosphoric acid.
• The 3’-hydroxyl of one sugar is combined to the 5’-
hydroxyl of another sugar through a phosphate
group.
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15. • In the DNA, the base sequence is of
paramount importance.
• The genetic information is coded in the
specific sequence of bases; if the base is
altered, the information is also altered.
• The deoxyribose and phosphodiester linkages
are the same in all the repeating nucleotides.
• Therefore, the message will be conveyed,
even if the base sequences alone are
mentioned as e.g.
5’P-Thymine-Cytosine-Adenine-3’OH
or 5’-T-C-A-3’.
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16. Watson-Grick model af DNA structure
1. Right handed double helix:
• DNA consists of two polydeoxy ribonucleotide
chains twisted around one another in a right
handed double helix similar to a spiral staircase.
• The sugar and phosphate groups comprise the
handrail and the bases jutting inside represent the
steps of the staircase. The bases are located
perpendicular to the helix axis, whereas sugars are
nearly at right angles to the axis.
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17. 2- The base pairing rule:
• Always the two strands are complementary to each
other.
• So, the adenine of one strand will pair with thymine of
the opposite strand, while guanine will pair with
cytosine.
• The base pairing (A with T; G with C) is called
Chargaff’s rule, which states that the number of
purines is equal to the number of pyrimidines.
3- Hydrogen bonding:
• The DNA strands are held together mainly by hydrogen
bonds between the purine and pyrimidine bases.
• There are two hydrogen bonds between A and T while
there are three hydrogen bonds between C and G.
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18. 4- Antiparallel:
• The two strands in a DNA molecule run antiparallel,
which means that one strand runs in the 5’ to 3’
direction, while the other is in the 3’ to 5’ direction.
• This is similar to a road divided into two, each half
carrying traffic in the opposite direction.
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21. Replication of DNA
• During cell division, each daughter cell gets an
exact copy of the genetic information of the
mother cell.
• This process of copying the DNA is known as
DNA replication.
• In the daughter cell, one strand I derived
from the mother cell; while the other strand is
newly synthesized.
• This is called semi-conservative type of DNA
replication.
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23. Steps of Replication
1. Each strand serves as a template or mold, over
which a new complementary strand is synthesized.
2. The base pairing rule is always maintained. The
new strand is joined to the old strand by hydrogen
bonds between base pairs (A with T and G with C).
3. Polymerization of the new strand of DNA is taking
place from 5’ to 3’ direction. This means that the
template is read in the 3’ to 5’ direction. So, tha 3’
end of the last nucleotide is free.
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24. Ribonucleic acid (RNA)
• RNA is also a polymer of purine and
pyrimidine nucleotides linked by
phosphodiester bonds
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26. 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
28. Types of RNA
• Messenger RNA (mRNA).
• Transfer RNA (tRNA) or (sRNA).
• Ribosomal RNA (rRNA).
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29. 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.
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30. 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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32. 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.
• They contain different rRNAs and specific proteins.
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33. Overview on protein synthesis
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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34. Steps of protein synthesis
1. Replication:
• DNA replication is like printing a copy of all
the pages of a book.
• The replication process occurs only at the
time of cell division.
2. Transcription:
• It is taking place all the time. Only certain
areas of the DNA are copied (selected region
on the sense strand).
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35. • The genetic information of DNA is transcribed
(copied) to the messenger RNA (mRNA).
• During transcription, the message from the DNA is
copied in the language of nucleotides.
3. 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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