140 |
A Handbook of Biology
Cellulose has no complex helices and so cannot hold I2. (AIPMT 2002)
Plant cell walls are made of cellulose. Paper made from plant pulp and
cotton fibre is cellulosic.
CH OH
CH OH
CH
OH
2
2
2
OH
OH
OH
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
Starch forms helical secondary structure. Starch holds I2 molecules in
the helical portion giving blue colour.
Nucleic acids are of 2 types: DNA (Deoxyribonucleic acid) and RNA
(Ribonucleic acid) having deoxyribose and ribose sugar, respectively.
SËÇØÑDÅRÝ STRÜÇTÜRË ØF DÑÅ (WÅTSØÑ - ÇRÏÇK DØÜBLË HËLÏX MØDËL)
DNA is different types such as A, B, C, Z etc. DNA consists of 2 polynucleotide
strands arranged antiparallely as a double helix.
In DNA, a nucleotide consists of nitrogen base, deoxyribose sugar and
phosphate group
Backbone of DNA is formed by the sugar- phosphate-sugar chain.
Steps are formed of Nitrogen base pairs.
Nitrogen bases include Adenine (A), Guanine (G), Thymine (T) and
Cytosine (C). Uracil absent.
A pairs with T (A=T) by 2 hydrogen bonds.
G pairs with C (G ≡ C) by 3 hydrogen bonds.
A phosphate molecule links the 3'-carbon atom of the sugar of one
nucleotide to the 5'-carbon of the sugar of the succeeding nucleotide