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DNA , RNA and Fl owofGeneti c Inf ormat ion
(Chapter 5)
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James Watson (L) and Francis Crick (R),and the model they built of the structure
of DNA.
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Nucleoside- a base is bonded to asugar ( N1 of pyrimidineC1 of
sugar or N9 of purineC1
Nucleotideis a nucleoside joined to one or
more phosphate s by an ester bond
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A Pair of Nucleic Acid
Chains with Complementary
Sequences Can Form Double
Helical Structure
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Rosalind Franklin and Maurice Wilkins-
discovered the existence of specific base-
pairing interaction.
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Supercoiling of Prokaryotic DNA
Schematic Representation of circular and supercoiled DNA
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Electron micrograph of supercoiled DNA
2 Types of Supecoiling
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2 Types of Supecoiling
Negatively supercoiled- a right handed helix acquires an extra-
helical twist
Positively supercolied- there is overwinding of the closed double
helix
2 Enzymes which are involved in the regulation of
conversion
Topoisomerase- an enzyme that hydrolyze a phosphodiester
bond in one strand, relax the supercoiling by rotating in onestrand, then reseal the break
DNA gyrase- induce a negative supercoling in relaxed, closed-
circular DNA
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Supercoiling in Eukaryotic DNA
Eukaryotic DNA is complex with DNA
(chromatin)Histones- principal proteins, H1, H2A, H2B, H3
and H4
Chromatin resembles beads on a string.
Nucleosome- consist of a DNA wrap around a
histone core
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The Double Helix Facilitates the
Accurate Transmission of Hereditary
Information
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The Double Helix Can Be
Reversibly Melted
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M RNA ( RNA)
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Messenger RNA (m-RNA)
least abundant of the three RNAs
The sequences of bases in mRNA specify the order
of amino acids in proteins
turn over most rapidly in the cell
Heteregenous in size
No intrachain folding, very likely an open chain
structure
Synthesized in the nucleus during the process of
transcription
After transcription, mRNA passes to the ribosomes
where it serves as the template for the seqeuntial
ordering of a.a. during synthesis
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TRANSFER DNA
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smallest of the three RNAs
Single stranded polynucleotide chain, usually about 80 nucleotides
and generally having a MW of about 25,000 daltons
INTRACHAIN H-bonding- occurs in tRNA, forming A-U and G-C
base pairs.
Molecule folds back on itself to form a cloverleaf structure
STEMS- hydrogen bonded structure, LOOPS- non-hydrogen
bonded position, contain modified bases
At one end of the polynucleotide chain, all t-RNAs contain a
guanylic acid residue, linked by its 3-hydroxyl group to the
penultimate nucleotide residue.
The other side of the chain contains a common trinucleotide
sequence namely pCpCpA.
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Ribosomal RNA
forms a complex with protein to form ribosome
Prokaryotic ribosome : 70 S
50 S and 30 SEukaryotic ribosome: 73S to 80S 60S and40S
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QUIZ onWEDNESDAY!
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