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DNApart 4
Prep by : Mohamed Abdukadir kalifMphil in Optometry
DNA chain
• In real life even alone man can not produce child , and alone women can not give birth to child without men .
• So as DNA , a single molecule of DNA can not live alone it needs another molecule of DNA .
• So how is single chain DNA arranges another chain of DNA ?
• To produce a child there is a need of man and woman which are different living organisms
• so as the two chains of the DNA molecules , they are different from each other .
• Because mostly identical things don’t attract each other except in some circumstances
• To arrange another chain of DNA • Adenine bonds with thymine in upside down
way • And the bond between them is called
hydrogen bonds • Adenine and thymine have two hydrogen
bonds , and it is a weak bond
Adenine bond with thymine
• Guanine bonds with Cytosine in upside down way , with three hydrogen bonds , and it is a stronger bond
Guanine bond with Cytosine
Complete DNA chain
• The DNA molecules are not straight , they are twisted
, their nitrogenous bases are held together by hydrogen bonds , and their wall is made of phosphate and sugar , which is held together by phosphodiester bonds .
• Most of the DNA are right hand twisted • But also in some points it becomes left hand
twist• So There are different forms that the DNA
twist.
B-form of DNA
• This is right handed twist • It is only a DNA molecule twisting with
another DNA molecule • There are ten (10) pairs of nucleotides in
complete 360’ degree turn • And every pair is perpendicular to the central
axis
A-form of DNA
• Right handed twist • It can be one molecule of DNA twisted with
another molecule of RNA , or it can be DNA twisted with another DNA
• There are 11 pairs of nucleotide in complete 360’ degree turn
• It is slightly dehydrated , because of the more pairs of nucleotide within 360’ degree turn
Z-form of DNA
• With in right handed long chain of DNA , suddenly some areas appear which are left hand twisted , this sudden left hand twist creates zigzag area , that area of DNA which has gone in to reverse twisting is called Z-FORM of DNA .
• There are 12 pairs of nucleotides in complete 360’ degree turn
To be continued
