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DNA Strand

Chromatin material is basically the DNA strand

wrapped around Histones forming nucleosomes.

This coiled strand is further coiled to formchromatin filament.

DNA itself consists of two filaments or strands

helically coiled with each other.

It is also called a macromolecule because it is a

very large single molecule.The detailed structure of DNA was worked out by

Watson & Crick in 1953.

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Watson and Crick DNA

model- Each DNA strand has a

complex chemical structure.- It is made up small building

units called nucleotides.

- Each spiral of DNA has 10nucleotides.

- Each nucleotide consists of 3

subunits (one molecule each

of pentose sugar, phosphate

and nitrogenous base).

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-The Phosphate and Sugar molecule are

arranged one above the other alternately whilethe base is oriented inwards, attached to ribosesugar.

- The base of one strand joins with the base ofthe complementary strand with Hydrogen-bondformin a s iral staircase where the bases

forms the steps.-The nitrogenous base join only with itscomplementary base thus Adenine (A) joins

with Thymine (T) through double bond &Cytosine (C) joins with Guanine (G) throughtriple bond i.e. A=T and CG

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Main Features of DNA

There are 2 strands running anti-parallel to each other &are spirally arranged around an axis so it is called a doublehelical arrangement. 1 strand runs from 5 3 and othervice versa.

Each DNA strand is made up ofsugar, base & phosphate. The length of a helix is 3.4 nm and the distance between 2

consecutive base pair in a helix is 0.34 nm.

e ame er or e s ance e ween s ran s s nm. SugarSugar are pentose type and bases are of 2 types:

1). Purines: Adenine (A) & Guanine (G).

2). Pyrimidines: Cytosine (C) & Thymine (T).

Bases are connected with each other through HydrogenBonds i.e. A=T and GC.

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Total number ofPurines = Pyrimidines

i.e. A + G = T + CBase ratio i.e. (A + T) / (G + C) is constant for a

given species. This ratio is called Dissymmetry

Ratio.

Bases are inside and sugars are outside and 2

sugars are connec e w osp or c c .DNA has a property of Duplication. Each replicates

and from each replicated DNA another new DNA is

formed. This is called Replication of DNA.

Before Replication of DNA, Hydrogen bonds

between the bases are broken.

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Importance of DNAVery important because it is Hereditary

material.It acts as director ofProtein Synthesis.

Some DNA of chromosome formsnucleolus.

DNA in presence of enzymes formmRNA which acts as messenger.

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DNA Replication

During Cell Cycle chromatin materialduplicated during inter-phase in order to

prepare for mitosis.In this process, DNA double helix open up at

.

When the 2 strands become free at one end,new nucleotides start forming

complementary pairs and thus new strandsstart forming around each unraveling oldstrand sequentially.

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DNA Replication

In the replication process the parent DNA molecule

unwinds and unzips.

Then each of the old strands serves as thetemplate for the new strands.

strand and a newly synthesized strand.

This type of DNA replication is commonly called as

semi-conservative replication, because here eachdaughter DNA molecule receives one parental

strand.

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DNA replication requires following three steps:

Unwinding: - The old strand that makes up the parent DNAmolecule is unwound and unzipped (weak hydrogen bondsbetween the paired bases are broken). The hydrogen bondsbetween the molecules are broken with the help of Helicase

enzyme.

Complementary base pairing:- With the help of enzyme DNA

polymerase new complementary nucleotides (that arealways present in the nucleus) are positioned adjacent toeach other opposite to the parent DNA template.

Joining:-This step also requires DNA polymerase enzyme for joining the complementary nucleotides. Each daughtermolecule contains an old and a new strand.

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Replication of DNA strand has an origin point at which thereplication is initiated. It may also have a terminus point wherethe replication of DNA is terminated. A ' Y ' shaped structure isformed at the point of replication which is called as "replicationfork ".

Replication of DNA may be unidirectional or bidirectional. DuringDNA replication, one nucleotide is joined with another. Eachnucleotide already has a phosphate group at the 5' carbon atomand it is joined to 3' carbon atom of the sugar molecule.

Thus the synthesis of DNA molecule takes place in the 5'->3' direction with the help of DNA polymerase enzyme. But thiscauses a problem at the replication fork where only one of thenew strands run in the 5'->3' direction ( the template for this

strand runs in the 3'->5' direction). This strand is called asLEADING STRAND.

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The template for the other strand runs in the 5'->3'direction, but DNA synthesis could only take place in

5'->3' direction. Thus, this poses a problem and due tothis reason synthesis has to begin in the replicationfork.

Replication of the 5'->3' parental strand begins assoon as the DNA molecule unwinds and unzipsre lication of this strand is discontinuous. The

replication of this strand results in segment calledOKAZAKI FRAGMENTS.

Discontinuous replication takes more time thancontinuous replication therefore the new strand inthis case is called the LAGGING STRAND.

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DNA Replication

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Summary Chromosomes are thread like structures in nucleus.

They are studied generally at Metaphase stage of Mitosis.

Chromosomes are made up ofDNA & proteins.

Chromosome number (2n) of species is constant.

Chromosomes may be rod shaped / V / J / L shaped.

DNA is the hereditary material.

.

There are 2 nucleic acids DNA & RNA.

Man has 46 chromosomes i.e. 23 pairs.

Chromosomes Common to males and females are referredto as Autosomes (22 pairs in human).

Chromosome determining the sex of an individual are calledsex chromosomes (XY in males & XX in females).