 Watson and Crick were the first to make a double-helical modle for the strucure of deoxyribonucleic acid or DNA.  DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria

 The information in DNA is stored as a code made up of four nitrogeneous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).  DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide.

 An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

They transmit heredity information and the amino acid sequence is programmed by a gene to form a polypeptide. There are 2 types of nucleic acids: Deoxyribose sugar ( one O is missed on carbon 4) Strands: Doubled Bases: A, G, C & T Ribose sugar ( no missed O) Strands: single Bases: A, G, C & U

A nucleotide is consist of Phosphate group Phosphate group Pentose sugar Nitrogen base 5 3 Hydrogen bonds Attaches to sugar to result in a backbone  It carries the heredity information.  It control protein synthesis by directing mRNA synthesis.  Antiparallel.  Complementary.

Nitrogenous bases Sugar-phosphate backbones The Nitrogenous bases are : They have Hydrogen Bonds between them

 They are synthesized by The first nucleotide The 2 nd nucleotide Sugar P group Phosphodiester link  The sequence of nitrogen bases in the polymers is unique for each gene.  The flow of this genetic material is DNA mRNA Protein.  As mentioned before, RNA is a single ( strand ) and DNA is double  The backbone of sugar and phosphate is on the outside, leaving the nitrogen bases on the inside to form hydrogen bonds with each other. ( Adenine is always with Thymine, and Guanine is always with Cytosine.)

 Copying the genetic material

 When a DNA molecule is copied each strand is served as a template for ordering nucleotides into a new complementary strand  Nucleotides line up along the template according to the base-pairing rules and they are linked to form new strands.

Separation of the two strands Each strand works as a template to produce new strand According to the base-pairing rules, new strands are produced to link with the original templates That’s why we say DNA is complementary

 There are there types of DNA replication the most common replication and accepted by Watson and Crick is SEMICONSERVATIVE replication the other two types are Conservative and dispersive.

Semiconservative The daughter molecules will have one old strand and one new. Semiconservative The daughter molecules will have one old strand and one new. Conservative The parental double helix remains & a newly copy is made Conservative The parental double helix remains & a newly copy is made Dispersive Each daughter molecule contains a mixture of old & new part. Dispersive Each daughter molecule contains a mixture of old & new part. Most commo n

The replication starts with a bubble called the origin site Elongated by polymeras e Replication fork The replication process happens in both directions. The new copied strand is created from 5 -> 3 in the direction of 3 -> 5 according to the old strand. That’s why we say DNA is antiparallel Binding protein to keep them apart

 Several enzymes and other proteins carry out DNA replications such as : 1. Helicase 2. Primase 3. Polymerase 4. Ligase The ends of the DNA molecules are replicated by a special mechanism.

Creating the leading strand A.One primer starts the new chain. B.Links nucleotides from RNA to matching nucleotides of DNA. C.The polymerase add more nucleotides to the 3 end of the primer’s chain. D.Another polymerase replace the primer’s with DNA nucleotide to link with the template. Primer: Short RNA segment Primer: Short RNA segment The leading strand’s direction is 5->3 going to 3->5 into the fork. Creating the lagging strand Primase: DNA polymerase Primase: DNA polymerase More than 1 primer is used. Form Okazaki fragments which are joined by ligase to form the strand. ligase: Joins Okazaki fragments ligase: Joins Okazaki fragments Okazaki’ fragmentsDNA polymerase III must work in the direction away from the replication fork

 Before a cell can divide, it must duplicate its DNA.

 PLZ,OPEN THE VIDEO DNA Structure

 PLZ,OPEN THE VIDEO

DNA Replication Process

 Q1: Write whether each of the following statements is True (T) or False (F):  1-In the double helix the two strands of DNA are complimentary.  2-The role of DNA polymerase in replication is to link the nucleotides one by one in the 5’ 3’ direction.  3-DNA polymerase II is used for mRNA synthesis.  4- Elongation of new DNA is catalyzed by an enzyme called RNA polymerase.  5-During the replication of DNA, _____. both strands of a molecule act as templates. 

 Q 2: Choose the correct answer (one answer only):  1-The pentose sugar in RNA is;-  a-Ribose.  b-Deoxyribose.  c-Fructose.  2-The short RNA segment that is complementary to DNA segment and is necessary to begin replication is known as;-  a-Primer.  b-Promotor.  c-Polymerase.  3-In DNA,adenine A always pairs with ;-  a-G.  b-C.  c-T.  4-To replicate the leading strand primer is required.  a-Many.  b-Two.  c-One.

 5-The Okazaki fragments are joined by;-  a-DNA polymerase.  b-DNA ligase.  c-Primase.  6-Which is not a nucleotide base in DNA?  a-Uracil.  b-Adenine.  c-Cytosine.  7-A DNA strand having the sequence of C-G-A-T-T-G would be complementary to the sequence  a-G-C-T-A-A-C  b-G-C-T-A-A-G  c-C-G-A-T-T-G  8-Each codon calls for a specific  a-Amino acid.  b-Protein.  c-Polypeptide.  9-Anticodons pair with  a- mRNA codons.  b-DNA codons.  c-tRNA anticodons.  10-An RNA molecule is  a-A double helix.  b-Usually single- stranded.  c-Always double -stranded