Types of chromosomes

In the  human beings, there are two main types of chromosomes, allosomes and autosomes  There are 22 pairs of autosomes and one pair of allosomes in our genome, resulting in a total of 46 chromosomes.


These are nucleoprotein bodies, which are dark-staining with basic dyes, microscopically

observable in the cell during cell division. They carry the genes that are arranged in linear order.

Each species has a characteristics chromosome number.


Onion, contains 16 chromosomes human beings contains 46 chromosomes.

The chromosome s composed of DNA and basic protein, called hi stones, DNA and hi stone

together form a structure called  nucleoside. The chromatin material! is formed by a series of

repeating chromosomes, which arc closely attached to each other to form a continuous fiber.

Chromosomes wiÆimi1ar size, shape and position of centromere arc said to be homologous





An exchange of segments between non sister chromatids of homologous chromosomes occurs at

chiasmata is known as crossing over.

Process of crossing over during meiosis


In synapsis during prophase-! of Meiosis-! the homologous chromosomes pair up length wise,

point to point and locus to locus. Each homologue consists of two sister chrornatids with

identical genes. Chamita or x-shaped crop links are formed by non-sister chromatids.

Significance of crossing over

Crossing over is very important as ii helps in the variations of genes. Actually during crossing

over the exchange of genes takes place and this results in the formation of new combinations of

genes (alleles). So the resulting zygote would be an individual onc (of its own kind). This is

because of crossing over that every individual in this world is some what different from Lh41 of







Deoxyribonucleic acid (DNA) is the chemical substance responsible for transmission of hereditary information. In its structure information is coded for synthesis of cell proteins



Structures’ of DNA

  • DNA is long rope like very complex molecule.
  • The structural units of this molecule are called


  • These nucleotides are joined together in long chain

called polynucleotides.

  • It is composed of two strands, each wounds around the

other o form double helix.

  • Each Deoxyribonucleic is made up of three


  1. Deoxyribose sugar
  2. Phosphoric Acid

iii. Organic bases (Nitrogenous bases)

There arc four different types of organic bases.

  1. a) Adenine b)Thymine c) Guanine
  2. d) Cytosine
  • Billions of these nucleotide combine to form a single

molecule of DNA

  • These molecules are joined in specific pairs (A-T G -C)

with one another through phosphoric acid.




Duplication of DNA is called replication. During this process it makes an exact copy of itself.


  • Watson and Crick model of DNA ideally proposed the mechanism of self replication.
  • The hydrogen bonds (weak forces) that hold together the double helix of DNA are broken


  • 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 chains.

  • Through there steps a new upright would be supplied for each ladder.
  • By the lime the end of the spiral is reached and the two original strands of the DNA ill

have separated each strands will have the nucleotide partners it has lost, substituted with

new ones of the exactly the same kind.

. This each newly formed daughter DAN molecule has one parental strand and one daughter

strand in a double standard structure.






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