On Genetic Flow of Information

8/14/2019 On Genetic Flow of Information

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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!

On Genetic Flow of Information

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