Structure of DNA

includes structure, triplet code and semi conservative replication

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  • Created by: malu
  • Created on: 04-07-13 10:48
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STRUCTURE OF DNA
DNA is the chemical that determines inherited characteristics and it contains vast
amount of information in the form of genetic code.
DNA is made up of three basic components that
combine to form a nucleotide.
NUCLEOTIDE STRUCTURE
It is made up of three components:-
DEOXYRIBOSE SUGAR
A PHOSPHATE GROUP
AN ORGANIC BASE belonging to one of
two different groups:-
1. Pyrimidine (single ring bases) - cytosine and thymine..... they are small
molecules
2. Purine (double ring bases) ­ adenine
and guanine.... They are longer
molecules
The deoxyribose sugar, phosphate sugar and
organic base combine to form a mononucleotide in
condensation reaction. Two mononucleotides
combine in condensation reaction between
deoxyribose and phosphate group of another to
form a dinucleotide.
DNA is sequence of organic bases and
proteins are sequence of amino acids. Nucleotides join to form
polynucleotide and amino acids join to form polypeptide
DNA STRUCTURE
In 1953, James Watson and Francis Crick worked out the structure of DNA.
DNA is made up of two strands of nucleotides. Each of the two strands is
extremely long and they are joined together by hydrogen bonds formed between
certain bases.
PAIRING OF THE BASES
Purines are longer molecules and pyrimidines are smaller molecules. The rungs of
the DNA ladder have to be same
length, so the base pair must

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Adenine always pairs with thymine and has two
hydrogen bonds
Guanine always pairs with cytosine and has 3
hydrogen bonds
As a result of this adenine is said to be complementary
to thymine and guanine is complementary to cytosine
THE DOUBLE HELIX
The sequence of bases along one chain determines the
sequence along the other. The chains run in opposite
directions or are antiparallel. Backbone is made up of
alternating sugar and phosphate groups held together
by PHOSPHODIESTER BONDS forming a
twisted ladder.…read more

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It is extremely large molecule therefore carries an immense amount of genetic
information.
By having the base pairs within the helical cylinder of the
deoxyribose-phosphate backbone, the genetic information is protected from
corruption by outside chemicals and physical forces.
DNA IS NOT MADE UP OF AMINO ACIDS IT IS MADE UP OF
NUCLEOTIDES
What is a gene?
Genes are sections of DNA that contain the coded information for making
polypeptides.…read more

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More recently the region of DNA that carries the information for a polypeptide
has been called a cistron instead of a gene
A cistron is made up of regions called introns and exons.
Exons are sections of DNA that code for amino acids
Introns are sections of nonsense DNA which do not code for amino acids
GENE MUTATIONS
Any change in the arrangement of nucleotide bases in DNA is known as a gene
mutation. Gene mutations usually occur during DNA replication.…read more

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REPLICATION OF DNA
Cell divides in two main stages:-
Nuclear division- it is the process by which nucleus divides. There are two
types of nuclear division, mitosis and meiosis
Cell division- it follows nuclear division and is the process by which the
whole cell divides.
Before a nucleus divides its DNA must be replicated (copied). This is to ensure
that all daughter cells have the same genetic information to produce the proteins
and enzymes.…read more

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