Lecture 10

Lecture 10 Objectives

• Acquire ability to describe DNA's components and features of the Watson-Crick model like monomeric unit orientation, geometry, and base pairing.
• Gain ability to outline semiconservative DNA replication and its role in passing on genetic information.

DNA Function:

• DNA acts as the genetic material.

Discovery of DNA Components

• The identification of nucleic acids and their components occurred from the late 18th to early 19th century.
• DNA and RNA represent the two types of nucleic acids.
• Chromosomes were found to carry 'heritable factors’.
• Chromosomes consist of proteins and deoxyribonucleic acid, also known as DNA.

The search for the genetic material

• DNA and RNA are two types of nucleic acids.
• Nucleotide units join via phosphate groups to form the backbone of the DNA molecule.
• Nucleotides were initially believed to be present as tetranucleotide repeats.
• The ratio of A, T, C, and G nucleotides was thought to be 1:1:1:1 which led to problems.
• DNA/RNA contain only four bases and were believed to be too simple to encode life's complexity.

Chargaff's Discovery

• DNA varies significantly between species.
• This suggests DNA could be the genetic material.

Chargaff’s Rules

• The amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C).
• The composition of DNA varies between species.

Discovery of DNA as genetic material

• The process involved many experiments and scientists.

DNA Structure

• DNA features a double-stranded helical structure.

Structure of DNA

• Intense efforts in the early 1950s aimed to determine DNA's structure.
• Key scientists involved were Rosalind Franklin, Maurice Wilkins, Francis Crick, and James Watson.

X-Ray Diffraction Pattern of DNA

• The structure is helical.
• Bases are arranged perpendicular to DNA molecule length.
• DNA is double-stranded.

Bases in a Double-Stranded Helical Structure

• Chargaff's rules state that [A] = [T] and [G] = [C].

DNA Component Quick Reminder

• Nucleotides are the single-unit building blocks of the macromolecule DNA.

Polynucleotide Formation

• Nucleotide monomers are joined by phosphodiester bonds to create a polynucleotide, known as nucleic acid or deoxyribonucleic acid (DNA).

Formation of the Phosphodiester Bond

• The hydroxyl group (OH) on the 3' carbon of one nucleotide reacts with the phosphate group attached to the 5' carbon of another nucleotide, forming a phosphodiester bond.
• DNA and RNA strands are synthesized in the 5' to 3' direction.

DNA Strand Direction

• Each DNA strand possesses a specific direction.
• The two strands run antiparallel to each other.

Antiparallel DNA Strands

• Antiparallel DNA strands form a double helix structure.

Structure Summary

• DNA structure has a useful diagram.

The Watson-Crick Model of DNA Structure

• DNA contains a double-stranded helical structure.
• The sugar-phosphate backbone is located on the exterior.
• The bases are on the inside.
• Stabilized by hydrogen bonds.
• Two polynucleotide strands are oriented in opposite directions.

Model of DNA

• Provided stimulus for deciphering the genetic code.
• Provided a possible mechanism for DNA replication, as mentioned in Nature, April 25 1953.

Semiconservative Replication

• Each DNA strand of the double helix acts as a template for synthesizing new strands.

DNA Replication

• Cells replicate DNA because the body needs to make more cells.

Lecture 10 Summary

• Nucleotide monomers are linked by phosphodiester bonds, forming a DNA strand (polynucleotide).
• The DNA molecule is a double helix of two linear DNA strands in an antiparallel orientation.
• Nucleotide bases reside inside the DNA helix.
• Two hydrogen bonds between A=T and three between G≡C nucleotides stabilize the structure.
• The sugar-phosphate backbone is on the exterior of the DNA helix.
• Semiconservative DNA replication enables the transfer of genetic information to subsequent generations.