The Structure of DNA

by Janine Elijah Hermoso

DNA (Deoxyribonucleic Acid)

• It is often called the blueprint of life.

• It contains the instructions for making proteins within the cell. 

Why do we study DNA?

• Its central importance to all life on Earth

• Medical benefits such as cures for diseases

• Better food crops

Chromosomes and DNA

• Chromosomes are pieces of DNA that carry the genetic instructions, or genes, of an organism.
• Genome is the complete set of genetic information in all the chromosomes of an organism.
• Genes contain DNA (Deoxyribonucleic acid). DNA is the chemical basis of heredity.

Watson & Crick

            The double helix structure of the DNA which was discovered by James Watson and Francis Crick in 1953 explains why in the process of DNA replication, the resulting daughter DNA molecules are identical to the parental DNA helix.

The Shape of the Molecule

• DNA is a very long polymer.

• The basic shape is like a twisted ladder or zipper.

• This is called a double helix.

The Double Helix Molecule

 The DNA double helix has two strands twisted together.

One Strand of DNA

The backbone of the molecule is alternating phosphates and deoxyribose sugar.

One strand of DNA is a polymer of nucleotides.

One strand of DNA has many millions of nucleotides.

   

Four nitrogenous bases

       DNA has four different bases:

•            Cytosine   C
•            Thymine   T
•            Adenine   A   
•            Guanine   G

Two Kinds of Bases in DNA

         Pyrimidines are single ring bases.

        Purines are double ring bases.

Thymine and Cytosine are pyrimidines

            Thymine and cytosine each have one ring of carbon and nitrogen atoms.

Adenine and Guanine are purines

            Adenine and guanine each have two rings of carbon and nitrogen atoms.

                                       

                                            

Two Stranded DNA

• Remember, DNA has two strands that fit together something like a zipper.
• The teeth are the nitrogenous bases but why do they stick together?

Hydrogen Bonds

• The bases attract each other because of hydrogen bonds.
• Hydrogen bonds are weak but there are millions and millions of them in a  single molecule of DNA.
• The bonds between cytosine and guanine are shown here with dotted lines.
• Cytosine (C) and Guanine (G) always join together
• Adenine (A) and Thymine (T) always join together.   

                                                      

          The pairing of bases is specific and complementary because the sequence of bases on one automatically determines the sequence of bases on the other.

Information flow in Biological Systems

         The central dogma of molecular biology states that in cells the flow of genetic information contained in DNA is one-way street that leads from DNA to RNA to protein.

         Ribonucleic acid (RNA) is one of the three major biological macromolecules that are essential for all known forms of life (along with DNA and proteins). A central tenet of molecular biology states that the flow of genetic information in a cell is from DNA through RNA to proteins: “DNA makes RNA makes protein”.

Transciption

• It is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language.
• The process by which a single strand of DNA serves as a template for the synthesis of an RNA molecule.
• Part of the information in the DNA is copied into a strand of RNA

Translation

• The synthesis of a protein from the genetic code carried on the mRNA.
• The genetic information in the linear sequence of nucleotides is being translated into a protein, a linear sequence of amino acids.
• The expression of the information contained in DNA is fundamental to the growth, development and maintenance of all organisms.
• RNA to protein

Three Classes of RNA  (RNA molecules are produced by transcription)

• mRNA

              - messenger RNA

              -Copy genetic information from DNA

• tRNA

              - transfer RNA

              - transfer/carry genetic information

• rRNA 

              - ribosomal RNA

              - synthesis of protein