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Introduction to
Molecular Biology
MOLECULAR BIOLOGY
1- Nucleotides
2- DNA
3- RNA
1- NUCLEOTIDES
1- Importance of nucleotides
2- Structure of nucleotides
3- Metabolism of nucleotides
i. synthesis
ii. degradation
Importance of nucleotides
1- Building units for nucleic acids (DNA & RNA)
2- Other rules in metabolism & energy storage
(e.g. ATP is a nucleotide)
Structure of nucleotides
Nucleotides = nitrogenous base + sugar + phosphate (1,2 or 3)
Nitrogenous base =
Purine OR Pyrimidine
Sugar =
Ribose
Purine =
Adenine
OR
OR
Deoxyribose
Guanine
Pyrimidine = Thymine, Cytosine OR
Uracil
PURINE RING
C
N
6
7
N
C
1
5
C
8
C
2
C
N
3
4
N
9
Purine ring in adenine & guanine
Pyrimidine RING
C
4
N
C
3
5
C
2
C
N
6
1
Pyrimidine ring in thymine, cytosine & uracil
b
• Purines :
• Pyrimidines:
Adenine & Guanine
Cytosine, Thymine & Uracil
• DNA contains
Adenine & Guanine (purines)
Cytosine & Thymine (pyrimidines)
• RNA contains
Adenine & Guanine (purines)
Cytosine & Uracil
(pyrimidines)
Metabolism of nucleotides
1- Synthesis (anabolism)
i. sources of purine ring atoms
ii. sources of pyrimidine ring atoms
2- Degradation (catabolism)
i. end products of purine ring
ii. end product of pyrimidine ring
Synthesis of purines:
Sources of atoms of purine ring
Synthesis of pyrimidines:
Sources of atoms of pyrimidine ring
Degradation (catabolism):
End products of purine ring degradation
• In human cells purine nucleotides is finally degraded to URIC ACID
• Uric acid is transported in blood to kidneys
• Finally, Uric acid is excreted in urine
• If uric acid is increased in blood, the case is called HYPERURICEMIA
• Hyperuricemia may lead to GOUT
• GOUT is a disease affects joints (arthritis) & kidneys (kidney stones)
caused by deposition of uric acid in these tissues
Degradation (catabolism):
End products of pyrimidine ring degradation
Pyrimidine nucleotides are degraded to highly
soluble products :
b-alanine & b-aminoisobutyrate
DNA
1- Importance of DNA
2- Location of DNA in human cells
3- Structure of DNA molecule
- Structure of a single strand of DNA
- Structure of double stranded DNA
- Linear & circular DNA
Importance of DNA
1- Storage of genetic material & information
(material of GENES)
2- Transformation of genetic information to new cells
(template for REPLICATION)
i.e. synthesis of new DNA for new cells
3- Transformation of information for protein synthesis
in cytosol (template for TRANSCRIPTION)
i.e. synthesis of mRNA in nucleus
Structure of DNA molecule
DNA molecule is formed of double helical strands.
(Dounble helix)
The two strands are held together by hydrogen bonds
Each single strand is formed of polynucleotides
Polyncleotides are mononucleotides bound to
each other by phosphodiester bonds
Structure of Single strand of DNA
Building Units: Polynucleotide sugar: deoxyribose
Base: Purine: A or G OR Pyrimidine: T or C
Phosphoric acid
Mononucleotides are bound together by phosphodiester bonds
In linear DNA Strand : two ends (5` = phosphate & 3` = OH of deoxyribose)
In circular strand: no ends
Structure of Single strand of DNA
Sequence
of DNA
Structure of double stranded DNA
Hydrogen bonds link
the two single strands together
Two strands are anti-parallel (in opposite directions)
Hydrogen bonds between bases of opposite strands (A & T , C & G)
Denaturation
breakdown (loss) of hydrogen bonds between two strands leading to formation of two
separate single strands)
Causes of denaturation : heating or change of pH of DNA
Linear & Circular DNA
1- Linear DNA
in nucleus of eukaryotes (including human cells)
i.e. DNA of chromosomes
2- Circular DNA
i. in eukaryotes: mitochondria
ii. in prokaryotic chromosomes (nucleoid of bacteria)
iii. in plasmids of bacteria (extrachromosomal element)
iv. in plant chroroplasts
DNA Synthesis (Replication)
DNA synthesis (replication) is the synthesis
of new DNA (daughter) duplexes using a
template of old (parental) DNA duplex
The two strands of the parental DNA
double helix are separated, each can serve
as a template for the replication of a new
Complementary (daughter) strand.
Each of the individual parental strands
remains intact in one of the two new
Duplexes
i.e. one of the parental strands is
conserved in each of the two new dublexes
OLD
DNA
DUPLEX
(PARENTAL)
DAUGHTER
DNA
DUPLEXES
RNA
1- Structure (differences from DNA)
3- Types
4- Importance of each type
Structure of RNA
• Building units:
Polynucleotides (bound together by PDE)
• Single strand
• Linear (but may fold into complex structure)
• with two ends:
5`(phosphate) & 3`(-OH end)
• Sugar:
Ribose
• Purine bases:
Adenine & Guanine
• Pyrimidine bases:
Cytosine & Uracil
Types & Functions of RNA
Ribosomal RNA (rRNA)
80% of total RNA in the cell (most abundant RNA)
Location: cytosol
Function: machine for protein biosynthesis
Types:
Transfer RNA (tRNA)
• Smallest of RNAs in cell: 4S
• Location: cytosol
• At least one specific tRNA for each
of the 20 amino acids found in
proteins
• with some unusual bases
• with intrachain base-pairing (to
provide the folding structure of
tRNA)
• Function:
1- recognizes genetic code word on
mRNA
2- then, carries its specific amino
acid for protein biosynthesis
Messenger RNA (mRNA)
•
synthesized in the nucleus (by transcription):
DNA (the gene) is used a template for mRNA synthesis
mRNA is synthesized complementary to DNA but in RNA
language i.e. U instead of T
So, if A in DNA it will be U in RNA , if T in DNA it will be A in
mRNA….etc
•
•
Carries the genetic information from the nuclear DNA (gene) to the
cytosol
In the cytosol, mRNA is used as a template for protein biosynthesis by
ribosomes (with help of tRNA)…. This is called Translation or Protein
Biosynthesis)
Transcription + Translation
=
GENE EXPRESSION
complementary base-pair between DNA & RNA
in transcription
Types of mRNA
• Polycistronic mRNA:
One single mRNA strand carries information from more
than one gene (in prokaryotes)
• Monocistronic mRNA:
one single mRNA strand carries information from only one
gene (in eukaryotes)
Eukaryotic mRNA
5`-end: cap of 7-methylguanosine
3`-end: poly-A tail
The Genetic Code
• is a dictionary that identifies the correspondence between a
sequence of nucleotide bases & a sequence of amino acids
• Each individual word of the code
is called a codon
a codon is composed three nucleotide bases
in mRNA language (A, G, C & U)
in 5`-3` direction e.g. 5`-AUG-3`
• The four bases are used by three at a time to produce 64 different
combinations of bases
61 codons: code for the 20 common amino acids
3 codons UAG, UGA & UAA: do not code for amino acids but are
termination (stop) codons
Genetic Code Table