WATSON AND CRICK's MODEL OF DNA - IDEA OF SELF REPLICATION

WATSON AND CRICK's MODEL OF DNA

Watson and crick suggested a model of DNA in 1953, which was based on X-ray, photographic and chemical analysis of DNA:

• Watson and Crick suggested a ladder type organization of DNA. Each DNA molecule is made up of two polynucleotide chains which are twisted around each other and form a DOUBLE HELIX.
• The upright of the ladder are made up of sugar and phosphate part of nucleotide and rungs are made up of paired nitrogenous bases.
• ADENINE always pair with THYMINE and CYTOSINE always pair with GUANINE.

• Two polynucleotides chains, which are complimentary to each other, are held together by HYDROGEN BONDS.

REPLICATION OF DNA

When the formation of new DNA molecule takes place in the cell without any change, it is known as replication of DNA.

• DNA is able to duplicate itself in the chromosome.such duplication of DNA is called as REPLICATION of DNA.
• During this process it makes an exact copy of itself.
• The Watson and Crick model of DNA ideally proposed the idea of self replication.
• The hydrogen bond that hold together the double helix of DNA are broken up.
• Starting from the end like a zip,the double spiral gradually 'UNZIP' along its length and each new nucleotide of the proper kind would be added to the separated chain. 

                                         

BASIC STRUCTURE OF DNA                                              

 

 

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DNA (Deoxyribo Nucleic Acid)- Structure of DNA

BASIC STRUCTURE OF DNA-Introduction:

You must have heard  about DNA or in the full form Deoxyribo Nucleic Acid which is related to Genes or which encodes the Genetic instructions used in the functioning and development of all known living organisms.
Here is a short introduction of  DNA:
DNA is a nucleic acid, produced in the nucleus, and found in Genes and Chromosomes.DNA acts as a genetic material,because it takes part to transfer the heredity characters from parents to their offspring and from one generation to another.Each DNA molecule is composed of small components, called NUCLEOTIDES.These nucleotides are joined together to form POLYNUCLEOTIDES.

COMPONENTS OF DNA:

As I have told you that a single molecule of DNA is composed of small organic molecules of nucleotides Each nucleotide consists of basic three components:

1. DEOXYRIBOSE SUGAR. (5 carbon sugar)
2. PHOSPHORIC ACID.(One phosphate group)
3. ORGANIC BASES.(Nitrogenous basis)

There are 4 different types of Organic basis:

i.Adenine.(A)        ii.Guanine.(G)        iii.Thymine.(T)        iv.Cytosine.(C)

 TYPES OF NUCLEOTIDES:

On the basis of Nitrogen group there are four types of Nucleotides:

i. Adenine Nucleotides.
ii.Guanine Nucleotides.
iii.Thymine Nucleotides.
iv.Cytosine Nucleotides.

Billions of these nucleotides join together to form a single molecule of DNA.These  nucleotides are joined on specific pairs i.e.(A-T, G-C) with one another through phosphoric acid.

GENETIC TERMS

INHERITANCE:

The process by which characters are transmitted from parents to offspring(child), is called INHERITANCE.

HEREDITY: 

The way by which genes transmit traits(a genetically determined characteristic) from  parents to offspring is called HEREDITY.

GENE: 

The small bodies found on the chromosomes are called GENES. These are the UNITS OF INHERITANCE. Genes take part to transfer one or more characters from parents to the new generation, and responsible for biological expressions i.e. appearance of characters. each gene has a specific function.

For example: A gene determine the height of the pea plants, another gene color of their flower. 
                

                         

LOCUS:

The position of a gene on the chromosomes is called LOCUS.

GENE LOCUS:

The place on the chromosomes where the gene resides is called GENE LOCUS.
 

                                            

ALLELE:

The alternate form for the same gene is called ALLELE. 
For example: All genes controlling a  flower color of plant in a similar or dissimilar manner are called ALLELES.

GENOME:

The total amount of genetic material in a total chromosome set of an individual is called GENOME.

KARYOTYPE:

The entire chromosome complement of an individual or cell is called KARYOTYPE.

HYBRID:

The organism that arise from mating between two genetically different parent is called HYBRID.

ALLELOMORPH:

A pair of contrasting characters is called ALLELOMORPH.
For example: Tall and dwarf stem of the plant.

HOMOZYGOUS:

When an individual possesses two alleles controlling a common character in a similar manner then it is called as HOMOZYGOUS.
For example: TT (gene for tallness), RR (gene for red color). Such plants breed true. 

HETEROZYGOUS:

An individual is said to be HETEROZYGOUS,if the two members of a pair of genes alleles control a common character in a different manner.
For example: (Tt), (rr).

                                   

DOMINANT CHARACTER:

The allele which masks the appearance of other allele in heterozygous condition  is said to be DOMINANT CHARACTER.

RECESSIVE CHARACTER:

The allele which is unable to express in heterozygous condition is said to be RECESSIVE CHARACTER.

PHENOTYPE:

The physical appearance "outward, physical manifestation" of an individual on the basis of inherent traits are called as PHENOTYPE.

GENOTYPE:

The genetic makeup of an individual or the entire set of genes in an organism are called as GENOTYPE.