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STRUCTURE OF DNA

Biology

Biology

STRUCTURE OF DNA

What is structure of DNA? The structure of DNA was first explained in 1953 by Watson and Crick. DNA was shown to be a double helix that coils around itself. The two strands are parallel and the distance between the two is constant.

 

Components of DNA

What are the components of DNA?  DNA is made up of repeating units called nucleotides.

Each nucleotide is composed of:

The bases are represented by their initials as A, G, C and T respectively.

The sugar alternates with the phosphate, and the two form the backbone of the strands. The bases combine in a specific manner, such that Adenine pairs with Thymine and Guanine pairs with Cytosine.

The bases are held together by hydrogen bonds. A gene is the basic unit of inheritance consisting of a number of bases in linear sequence on the DNA.

Genes exert their effect through protein synthesis.

The sequences of bases that make up a gene determine the arrangement of amino acids to make a particular protein.

The proteins manufactured are used to make cellular structures as well as hormones and enzymes.

The type of proteins an organism manufactures determines its characteristics.

For example, albinism is due to failure of the cells of an organism to synthesise the enzyme tyrosine required for the formation of the pigment melanin.

 

First Law of Heredity

 

Monohybrid Inheritance

Examples

 

Mendel’s Procedure

 

Results

 

Terms used in Genetics Genotype:

Phenotype:

Alleles:

Homozygous:

Heterozygous:

Hybrid:

Hybrid vigour or Heterosis:

The hybrid develops the best characteristics from both parents i.e. it is stronger or healthier, or yields more than either parent.

Use of Symbols

Fertilization-using checker board or Punnet square

F1 genotype Tt

F1 Phenotypic ratio = All tall.

F2 Genotype TT, 2Tt, tt

F2 Phenotypic ratio: 3 Tall; 1 short

 

Test Cross or Back Cross

This is made between the F 1 bearing the dominant trait with the homozygous recessive parent. It is called a back cross because of using the first parent. It is also a test cross because it tests the genotype of the individual.

 

See also

STRUCTURE AND PROPERTIES OF CHROMOSOMES

GENETICS

Growth and Development in Animals

ROLE OF GROWTH & HORMONES IN PLANTS

PRIMARY AND SECONDARY GROWTH

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