Week 8 - Oct 27

The Continuity of Life

Introduction

The continuity of life is maintained through the faithful transmission of genetic information from one generation to the next. This process relies on the structure and replication of DNA, the molecule that encodes hereditary information in all living organisms. The principle that "all cells come from pre-existing cells" underpins modern cell theory and highlights the importance of DNA replication in cell division.

Overview of Key Topics

• DNA Structure

• DNA Replication and Repair

• Gene Expression

• Cell Reproduction

• Mendelian Genetics

DNA Structure

Discovery of DNA Structure

In 1953, James Watson and Francis Crick proposed the double helix model for DNA structure, based on experimental data including Rosalind Franklin's X-ray diffraction images. Their work revealed the molecular basis for genetic inheritance.

• Watson and Crick Model: DNA is a double helix composed of two antiparallel strands held together by complementary base pairing.

• Historical Significance: The discovery explained how genetic information could be copied and passed on.

• Key Reference: Watson, J.D. & Crick, F.H.C. (1953) Nature 171, 737-738.

Nucleic Acids: DNA and RNA

Nucleic acids are polymers that store and transmit genetic information. There are two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA: Usually double-stranded, forms a stable double helix.

• RNA: Usually single-stranded, can fold into complex secondary and tertiary structures.

The Nucleotide: Basic Unit of Nucleic Acids

Nucleic acids are polymers of nucleotides. Each nucleotide consists of three components:

• Phosphate group

• Pentose sugar: Ribose in RNA, deoxyribose in DNA

• Nitrogenous base: Purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; uracil in RNA)

The nucleotide is structurally polar, with a 5' (phosphate) end and a 3' (hydroxyl) end, due to the orientation of the pentose sugar. (Polarity here refers to structural direction, not electrical charge.)

Component	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strandedness	Double-stranded	Single-stranded (often)

Phosphodiester Bonds

Nucleotides are joined together by phosphodiester bonds, which link the 3' hydroxyl group of one nucleotide to the 5' phosphate group of the next. This forms the sugar-phosphate backbone of nucleic acids.

• Condensation Reaction: The formation of a phosphodiester bond releases a molecule of water ($H_2O$).

Polarity of Nucleic Acid Strands

Single strands of nucleic acid have a sugar-phosphate backbone and are structurally polar, with distinct 5' and 3' ends. This polarity is crucial for processes such as DNA replication and transcription.

Double- and Single-Stranded Nucleic Acids

• DNA: Typically double-stranded, forming a double helix.

• RNA: Typically single-stranded, but can form double-stranded regions through base pairing.

Complementary Base Pairing and the Double Helix

The double helix structure of DNA is stabilized by hydrogen bonds between complementary bases:

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Only purine-pyrimidine pairs fit within the double helix, ensuring a uniform diameter.

Antiparallel Strands

The two strands of DNA run in opposite directions (antiparallel): one strand runs 5' to 3', the other 3' to 5'. This orientation is essential for replication and other cellular processes.

RNA Structure: Secondary and Tertiary Folding

Although RNA is usually single-stranded, it can fold back on itself to form secondary structures (such as hairpins and loops) and complex tertiary structures. These structures are stabilized by complementary base pairing and are critical for RNA function (e.g., in ribosomes).

• Secondary Structure: Local base pairing forms stems and loops.

• Tertiary Structure: Further folding creates complex three-dimensional shapes, as seen in rRNA and tRNA.

DNA Replication (Introduction)

How is DNA Replicated?

DNA replication is the process by which a cell copies its DNA before cell division. The double helix unwinds, and each strand serves as a template for the synthesis of a new complementary strand. This ensures genetic continuity between generations of cells.

• Key Principle: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand (semiconservative replication).

Example: During mitosis, DNA replication ensures that each daughter cell receives an identical set of chromosomes.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strandedness	Double-stranded	Single-stranded (often)
Function	Genetic information storage	Information transfer, catalysis

Key Terms

• Nucleotide: The monomer unit of nucleic acids, consisting of a phosphate, sugar, and nitrogenous base.

• Phosphodiester bond: The covalent bond linking nucleotides in a nucleic acid strand.

• Antiparallel: The opposite orientation of the two strands in a DNA double helix.

• Complementary base pairing: Specific hydrogen bonding between A-T (or A-U in RNA) and G-C.

Additional info: The provided materials focus on the foundational aspects of nucleic acid structure and the principles underlying DNA replication, which are essential for understanding genetics, cell division, and molecular biology.