Structure of DNA and RNA: Foundations of Genetic Material

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Structure of DNA and RNA

Genetic Material: Essential Properties

The genetic material must fulfill four key criteria to serve its biological function: information, transmission, replication, and variation. These properties ensure that DNA can encode the instructions for life, be inherited, copied accurately, and allow for diversity.

Information: Contains instructions necessary to build and maintain an organism.
Transmission: Passed from parent to offspring during reproduction.
Replication: Must be duplicated before cell division.
Variation: Capable of change, enabling evolution and adaptation.

Historical Experiments Identifying DNA as Genetic Material

Early 20th-century research established chromosomes as carriers of genetic traits. Key experiments demonstrated that DNA, not protein, is the genetic material.

Griffith's Experiment: Showed transformation in Streptococcus pneumoniae using mice, suggesting a 'transforming principle' (later identified as DNA).
Avery, MacLeod, and McCarty: Fractionated bacterial cells and demonstrated that only DNA could transform non-virulent bacteria into virulent forms.
Hershey and Chase: Used bacteriophage T2 and radioactive labeling to show that DNA, not protein, enters bacteria and directs viral replication.

Griffith's experiment: mouse with live type R bacteria Bacteriophage T2 structure Hershey and Chase experiment: phage infection cycle

Additional Evidence for DNA as Genetic Material

Organelle Genetics: DNA is present in mitochondria and chloroplasts, which perform genetic functions.
UV Mutagenesis: DNA and RNA absorb UV light at 260 nm, the wavelength most effective for inducing mutations, while proteins absorb at 280 nm.
DNA Content Correlation: Amount of DNA correlates with chromosome set number in cells.

DNA and RNA: Chemical Structure

Nucleotides: Building Blocks

Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide consists of a phosphate group, a five-carbon sugar, and a nitrogenous base.

DNA: Contains deoxyribose sugar; bases are adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA: Contains ribose sugar; bases are adenine (A), guanine (G), cytosine (C), and uracil (U).

Structure of DNA and RNA nucleotides DNA and RNA nucleotide components

Types of Nitrogenous Bases

Purines: Double-ring structures; adenine (A) and guanine (G).
Pyrimidines: Single-ring structures; cytosine (C), thymine (T, in DNA), and uracil (U, in RNA).

Purines and pyrimidines in DNA and RNA

Phosphodiester Bonds and Strand Directionality

Nucleotides are covalently linked by phosphodiester bonds, connecting the 5′ carbon of one nucleotide to the 3′ carbon of the next. This gives nucleic acid strands directionality, from 5′ to 3′.

Phosphodiester bond: Essential for the stability and directionality of DNA and RNA strands.

Discovery of DNA Structure

Key Contributors and Experimental Evidence

The structure of DNA was elucidated through the combined efforts of several scientists:

Linus Pauling: Proposed the alpha helix structure in proteins, influencing DNA modeling.
Rosalind Franklin: Used X-ray diffraction to reveal DNA's helical structure and dimensions.
Erwin Chargaff: Discovered base pairing rules (A=T, G=C) across species.
Watson and Crick: Built the first accurate model of DNA's double helix, integrating all prior findings.

Linus Pauling and alpha helix model $Rosalind Franklin and X-ray diffraction of DNA$ Chargaff's table of base content in DNA Watson and Crick with DNA model

Watson and Crick Model: Double Helix

The Watson and Crick model describes DNA as two antiparallel strands forming a right-handed double helix. The strands are held together by hydrogen bonds between complementary bases.

Base Pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds).
Antiparallel Strands: One strand runs 5′ to 3′, the other 3′ to 5′.
Major and Minor Grooves: The helix has alternating grooves, important for protein binding.
Helical Parameters: 10 base pairs per turn, 2 nm diameter, 0.34 nm between base pairs.

Watson and Crick DNA model Key features of DNA double helix Ball-and-stick and space-filling models of DNA

DNA Forms: B-DNA and Z-DNA

B-DNA is the predominant form in cells, while Z-DNA can form under certain conditions. These forms differ in helical twist and backbone structure.

B-DNA: Right-handed helix, bases perpendicular to axis.
Z-DNA: Left-handed helix, zigzag backbone, bases tilted.

B-DNA and Z-DNA molecular structures B-DNA and Z-DNA space-filling models

RNA Structure and Function

RNA: Primary and Secondary Structure

RNA is typically single-stranded, but can form double-stranded regions through complementary base pairing. The primary structure is similar to DNA, but uses uracil (U) instead of thymine (T) and ribose instead of deoxyribose.

Secondary Structure: Includes hairpins, bulge loops, internal loops, and multibranched junctions.
Base Pairing: A pairs with U, C pairs with G.

RNA bulge loop RNA internal loop RNA multibranched junction RNA stem-loop

Major Classes of RNA

rRNA (Ribosomal RNA): Structural component of ribosomes, essential for protein synthesis.
mRNA (Messenger RNA): Carries genetic information from DNA to ribosomes for translation.
tRNA (Transfer RNA): Brings amino acids to ribosomes during protein synthesis; has a distinctive secondary and tertiary structure.

Ribbon model of tRNA structure

Central Dogma of Molecular Biology

Information Flow

The central dogma describes the flow of genetic information within cells: DNA is transcribed into RNA, which is then translated into protein.

Transcription: DNA → RNA
Translation: RNA → Protein

Summary: Properties of Genetic Material

Information: Encodes all instructions for life.
Transmission: Inherited from parent to offspring.
Replication: Accurately copied for cell division.
Variation: Allows for genetic diversity and evolution.