DNA STRUCTURE AND RUPLICATIONPRESENTATION OF ZOOLOGY

DNA THE GENATIC METRIAL:

Study of DNA how store information knowns as “MOLECULAR BIOLOGY” DNA is a double stranded molecule which have components: Nitrogenous bases Deoxyribose sugar Phosphates

Phosphate group

Nitrogenous bases

Ribose sugers

• Like ladder, with the rails of the ladder consisting of alternating sugar-phosphate groups

5’3’

3, 5’

• Stands are anti parallel

The entire molecule is twisted into aRight hand helix, with one complete Spinal every 10 base pair.

(a)Nucleotides of one strand of nucleic acid join by linking the

phosphate of one of one nucleotides to the 3’ Carbon of

an adjacent nucleotides. Dashed line between the

nitrogenous bases indicates “HYDROGEN BONDING”.3

dashes are between cytosine and guanine, and 2 between

thymine & adenine .(b) Three-dimentional

representation of DNA. The anti parallel nature of the strands is indicated by the curved arrows.

Nitrogenous bases are specify to attached with each other that is: Cytosine always attached with Guanine & Adenine always attached with Thymine.

AdenineCytosine

ThymineGuanine

The sequence of the nitrogenous bases are responsible for the different trades represent by organisms.

Replication of dna

Replication began simultaneously at many initiation sites along the length of chromosomes in eukaryotic cells and in prokaryotes only one inisiation site is available. there are following Enzymes are evolved of replication of DNA:1. DNA gyrase & Helicase2. Primase3. DNA polymerase I, II, III4. Exonuclease 5. DNA ligase.

Frist step of replication initiation phase

First the enzyme DNA gyrase open the turns of DNA and turn into straight ladder. At the same times unzipping occurs.An enzyme called Helicase unzip the DNA strand and replication bubble and fork is formed at a particular site. The two strands gradually separated from each other and give rise a bubble like appearance

Helicase

After the breakdown of base pairs, single strands of DNA prevented to pair up again by specific proteins called Single stranded binding (SSB).Each side of replication bubble is now termed as replication Fork.A primase enzyme is attached for initiation of the replication which is carried out by DNA polymerase.DNA polymerase can not be functioned unless some nucleotides are arranged on template.so RNA primer is necessary.

RNA primase

After the establishment of primer the DNA polymerase is active.DNA polymerase is of three types: DNA polymerase I ( exonuclease ) DNA polymerase II (involve in repairing of DNA throughout the life) DNA polymerase III (main function in replication)

The DNA polymerase add nitrogenous basses on only 3’ end so replication began from 5’to 3’

The new formed strand is continuous and called leading strand. One subunit of DNA polymerase also posses ability to remove wrong nucleotides, if it is added mistakenly . This ability is called PROOFREADING

The second strand is lagging strand which is formed not continuously. On this strand Okazaki fragments are formed by DNA polymerase

Okazaki fragments are small part of nucleotides which made by DNA polymerase.After every Okazaki fragment new primer is formed. And DNA polymerase jumped next for making new Okazaki fragment. This strands replicate away from fork and called lagging strand

Here formed new Okazaki

Then the RNA nucleotides are removed by an enzyme called Exoneuclase and add DNA nucleotides here.

Okazaki fragments are discontinues so an enzyme called DNA ligase seals the gaps and two daughter strands are formed.

Old strand

New strand

summary

Enzymes used: DNA gyrase Protein SSB (single stranded binding) Helicase Primase DNA polymerase I, II, III Exonuclease (polymerase I)