DNA Structure and Nucleic Acids

• Lesson 17.1 focuses on DNA structure.
• The objectives are to describe nucleic acids and differentiate between DNA and RNA.

Nucleic Acids

• Nucleic acids are biomolecules with roles in the storage and expression of genetic information.
• Genetic information resides in the cell nucleus.

Cell Structure

• Key cell parts include the cytoplasm, cell membrane, endoplasmic reticulum, nucleus, Golgi body, mitochondria, and ribosomes

Nucleus Structure

• The nucleus contains the nucleolus, nucleoplasm, nuclear envelope, nuclear pores, chromosomes, and chromatin.

Chromatin

• Chromatin is composed of DNA and protein.
• Chromatin forms during the early stage of cell division.
• In 1973, scientists Don Olins, Ada Olins, and Christopher Woodcock found chromosomes' DNA coils around special proteins called histones.

Discovery of Nucleic Acids

• Friedrich Miescher isolated nucleic acids from white blood cells in 1869, calling it "nuclein".
• This material is now known as DNA.

Nucleotides

• Nucleotides are the monomers that are the building blocks of nucleic acids.
• Key components of nucleotides are a phosphate group, a 5-C sugar, and a nitrogenous base that determines the sequence.

Bonds in a Nucleotide

• Glycosidic bonds attach the base to the first carbon of the sugar.
• Ester bonds attach the phosphate to the first carbon of the sugar.
• Both ester and glycosidic bonds occur within a nucleotide.

Nucleotides: Pentose Sugar

• Deoxyribose is found in DNA, while ribose is found in RNA
• The structural difference between deoxyribose and ribose occurs at carbon 2', where deoxyribose lacks an oxygen atom.
• Lacking a functional group makes DNA more stable than RNA.

Types of Nucleic Acids

• Nucleic acids are either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid).
• They differ in terms of nitrogenous bases and the 5-C sugar component.

Nucleotides: Nitrogenous Bases

• Purines are nitrogenous bases with a two-ringed structure.

Nucleotides: Nitrogenous Bases

• Pyrimidines are nitrogenous bases with a single-ringed structure.

Nucleotide Chain

• Phosphodiester linkages connect the nucleotides in a chain.
• A phosphate group is bonded to the 3' carbon of the sugar.

Nucleotide Chain - Primary Structure

• The primary structure of a nucleic acid backbone is an alternating sugar-phosphate pattern.

Reading the Primary Structure

• The 5' end indicates where the phosphate group is located, and the 3' end indicates where the sugar molecule is located.
• A sequence is read starting from the free 5' end using the letters of the bases.

The Chargaff Rule

• Erwin Chargaff established rules about DNA composition.
• The amount of adenine (A) is almost equal to thymine (T).
• The amount of guanine (G) is almost equal to cytosine (C).
• There is a 1:1 ratio between A and T, and C and G.

Secondary Structure: The Double Helix

• The two strands are connected by hydrogen bonds.
• The strands are antiparallel; one runs in the 5'-to-3' direction, while the other is oriented in the 3'-to-5' direction.

DNA Structure

• DNA strands run in opposite directions, making them antiparallel.
• The coding strand runs in the 5' to 3' direction and the template strand (which is expressed) runs in the 3' to 5' direction.

DNA Structure

• A DNA double helix is a twisted, right-handed staircase.
• Sugar-phosphates form the sides of the ladder, while base pairs form the rungs.

DNA as The Genetic Material

• DNA stores genetic information in nucleotide sequences.
• Base pairings in DNA help protect and conserve genetic information.
• DNA is contained in a cell nucleus.

Discovery of DNA Structure

• James Watson and Francis Crick first proposed the double helical structure of DNA.
• Watson and Crick used data sets from Rosalind Franklin and Erwin Chargaff.

Discovery of DNA Structure

• Rosalind Franklin, with Raymond Gosling, obtained Photograph 51 via X-ray crystallography.
• The symmetric patterns in the photograph were interpreted as the DNA structure.

Key Points

• Nucleotides, monomers of nucleic acids, comprise a five-carbon sugar, a phosphate group, and a nitrogenous base.
• Nucleic acid strands form as nucleotides link via phosphodiester bonds.
• DNA strands have a 5' end where the phosphate group is and a 3' end where the sugar molecule is.
• The double helix is right-handed and twists clockwise, forming major and minor grooves.
• A DNA sequence is represented by its nitrogenous bases.
• Ribonucleic acid (RNA) is involved in protein synthesis and uses uracil instead of thymine.