WATSON AND CRICKS MODEL OF DNA

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WATSON AND CRICKS MODEL OF DNA

• The double stranded helical structure of DNA was proposed for the first time by James Watson and Francis Crick in 1953.
• In Eukaryotic, DNA molecule is present in each chromosomes if nucleus.
• The two strands of DNA molecule are spirally twisted around each other as well as a central axis to form tight handed double helix.
• The coiling of two strand is plectonemic i.e. the two strands can’t be easily separated from each other.
• The backbone of strand is formed by alternately arranged Deoxyribose sugar and phosphate molecular which are joined together by Phosphodiester bonds.
• Each strand is long polypeptide chain composed of many basic units or monomers known as Deoxyribonucleotides, hence called polymer compounds.

Each transverse step is made up of nitrogen bases which are purines and pyrimidines.

1. Nitrogen bade is attached horizontally at carbon number one of Deoxyribose sugars.
2. The nitrogen bases of two DNA strands are present in pairs joined by weak hydrogen bonds. This is called as base-pairing.
3. The base pairing is very specific and complementary to each other.
4. Adenine always pairs with Thymine with two hydrogen bonds i.e. A=T.
5. While Guanine always pairs with Cytosine with three hydrogen bonds i.e. G≡C.
6. The two strands of DNA molecules are not similar but complementary to each other.
7. The amount of Adenine is equal to that of Thymine and amount of Guanine is equal to Cytosine (A=T and G≡C) , hence A+C=G+T.
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         In each strands, one end of the stand has one free phosphate group at carbon-5 of the sugar molecule, the end is known as C-5 or 5‟ end. The other end has a free –OH at carbon -3 of the sugar molecule, the end is called as C-3 or 3‟ end.

                           

        The two strands runs parallel but in oposite direction i.e.one strand runs in 3‟→5 direction and another in 5‟→3‟direction hence called antiparallel.

The helical DNA strand shows two types of groove such as shallow or minor grooves and large major grooves. Major grooves is the binding site for histone proteins.

Dimensions of DNA molecule-

1. The DNA molecule has three dimensional structure.
2. The diameter of DNA molecule is 20 Ǻ.
3. Each complete spiral or turn of DNA strand has a distance about 34 Ǻ which consist of 10 base pairs.
4. The distance between two successive base pairs is 3.4 Ǻ.
5. The angle between the successive base pairs is 36°

FUNCTION OF DNAbiologicalinform.blogspot.com

1. DNA contains genetic information on coded form .
2. It carries genetic information from one generation to next generation by the property of replication.
3. It controls all metabolic activities by synthesizing specific proteins.
4. It helps in the synthesis of RNA which carry out protein synthesis.
5. It plays an important role in variation and evolution because of mutations.

 ERWIN CHARGAFF’S RULE:-

1. In a DNA double helix, total quantity of purines is equal to the total quantity of pyrimidines.
2. Ratio of Purines & Pyrimidines is always 1: 1.
3. Total quantity of Adenine (A) = Total quantity of Thymine (T).
4. Total quantity of Guanine (G) = Total quantity of Cytosine (C).

 REPLICATION OFDNA

“Replication is the process by which DNA molecule produces daughter DNA molecules which are exact copies of parent DNA”.

• One of the most important property of DNA is that it can make exact copies of itself, as a result of which the amount of DNA becomes double.
• The process of DNA replication in eukaryotes was discovered by Watson and Crick in 1953.
• The process of replication tales places in S-phase of interphase between two mitotic cycles.
• Replication is the semiconservative process on which each of the double strands formed from the parental DNA having one old strand and one new strand.

 SEMICONSERVATIVEMETHOD OF DNA REPLICATION

• It is the most common method of DNA replication.
• It takes place in the nucleus where the DNS is present in chromosomes.
• The Deoxyribose nucleotides needed for the formation of new DNA strands are present in the nucleoplasm. 

v Various steps involved in the process of replication-

1. Replication of DNA starts at a specific point of DNA molecule called initiation point.
2. The DNA strand breaks at the origin because of break made by incision enzyme called Endonuclease.
3. DNA replicates either by uni- or bi-directional methods.
4. The double stranded DNA molecule starts uncoiling or unwinding at the point of origin with the help of enzyme Helicase.
5. The hydrogen bonds present between the complimentary nitrogen bases are broken by the action of enzyme Topoisomerase and two strands get separated from each other.
6. The point where the two DNA strands get separated appear like a fork or „Y‟ shaped structure known as Replication fork.
7. Each old strand which is separated with its exposed nitrogen bases acts as Template for the synthesis of new DNA strand.
8. Initiation of DNA synthesis required an RNA primer synthesized by the DNA template by the activity of enzyme RNA primase.
9. The RNA primer attaches itself to the old strand and the complementary nucleotides are selected from the nucleoplasm are attached by Hydrogen bonds to their respective complementary nucleotides on template strand (A=T and G≡C).
10. The selection of nucleotides and their arrangement in proper sequence takes place with the help of enzymes DNA polymerase III.
11. Thus the nucleotides get linked with each other by Phosphodiester bonds and a new DNA strand is constructed opposite to old DNA strand.
12. The two original strands, when separates from each other, one template strand opens from 3‟→5‟ direction and replicates first is called Leading templete.
    
13. Other strand opens from 5‟→3‟ direction and replicates later is called Lagging templete.
14. The new strand formed on leading template continuously is called Leading strand which is formed in 5‟→3‟direction.
15. The new strand from on Lagging template discontinuously is called Lagging strand which is formed in 3‟→5‟ direction.
16. The replication of lagging strand takes place in small fragments of DNA are called Okazaki fragments.
17. These Okazaki fragments are then connected with each othr with the help of DNA ligase enzymes to form continuous strand.
18. Watson and Crick called this method of self replication as semiconservative method of replication because in each of the newly formed DNA molecule, one molecule of the parent DNA is conserved.

PACKAGING OFDNA-

      Length of DNA double helix molecule, in mammalian cell is approximately 2.2 meters. It is condensed and coiled and super coiled to fit in the nucleus. In eukaryotes, this packaging of DNA is complex.

1. The Histone are required for the packaging of DNA.
2. Histone are protein that rich in the basic amino acid residues Lysines and Arginines which carry positive charge in their side chains.
3. Eight molecules of histones (two each of H2A, H2B, H3, H4) get organized to form histone octamer.
4. DNA is negatively charged and it is wrapped around the positively charged histone octamer to form nucleosome.
5. The nucleosomes in chromatin are seen as „beads-on-string’.
6. Around the octamer, DNA molecule is wrapped called core DNA. It consists of about 146 bp.
   
7. Adjacent nucleosomes are linked with small segments of DNA called linker DNA (54bp).
8. This „beads-on-string‟ structure gets condensed into nucleosome fiber (10 nm).
9. H1 histone is present in the linker region where the DNA starts wrapping to octamer and leaves, and as DNA makes two complete turns. Each nucleosome contains 200 bp of DNA helix.
10. The thin and long nucleosome fiber is coiled like a telephone wire to make solenoid fiber with diameter 30 nm.
11. Nucleosome and solenoid fibres are characteristic of nucleus at interphase.
12. The packaging of chromatin at higher levels need additional set of proteins that are collectively called Non-Histone chromosomal Protein (NHC).
13. A loosely packed region of chromatin that stains light, is called Euchromatin . And densely packed region that stains dark, is called as Heterochromatin.
14. Euchromatin is considered as transcriptionally active chromatin, while heterochromatin is inactive.