DNA ( Deoxyribonucleic acid ) Site: Human DNA is present in the nucleus and mitochondria. Function: carry genetic information. Structure: Human DNA consists of two strands of polydeoxyribonucleotides. The nucleotides are arranged in chains linked together by 3'→5′ phosphodiester bond between C3′ of deoxyribose of one nucleotide and C5′ of the next one. phosphodiester bond means one phosphate is linked to 2 sugars. 1

2 This is a tetranucleotide GCTA

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4 Each DNA strand has two ends; 5′ end ( the end with free phosphate group attached to C5′ of the first pentose) and 3′ end ( the end with free OH group on C3′ of the last pentose. The nucleotides in the polynucleotide chain is always read from 5′ → 3′ direction This part of polynucleotide is read ATC

5 Base pairing: The two strands are linked together through hydrogen bonding formed between purine bases in one strand with pyrimidine bases in the other resulting in two types of base pairing: - Adenine is always paired with thtmine by 2 hydrogen bonds (A=T) - Guanine is always paired with cytosine by 3 H-bonds. _

6 So in the double stranded DNA: 1- The content of adenine equals to that of thymine and the content of guanine is equal to that of cytosine 2- also, each strand must be complementary to the other i.e. each base of one strand is matched by a complementary hydrogen bonding base on the other strand.

7 DNA double helix: The two strands of DNA wind arround each other forming double helix which stabilize DNA. Also abundance of hydrogen bonds between bases help to stabilize DNA. Watson and Crick model

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9 Functions of DNA: 1- it is the storage site of genetic information. 2- DNA replication (reproduction): the stored information are transmitted from parent DNA to daughter DNA during cell division by a process called: replication. 3- transcription of mRNA for protein synthesis DNA replication ( DNA synthesis): DNAs have the ability to reproduce themselves by the process of replication, thus ensuring the transfer of genetic information from one generation to the next. DNA synthesizes replicate of itself by using its own structure as a template. Each strand of the double helix serves as a template for constructing a complementary daughter strand. The resulting double helix contains one parental and one daughter, and the mode of replication is thus called: Semiconservative