chpt9
Chapter 9: Structure of DNA and RNA
Overview
This chapter focuses on the molecular structure of DNA and RNA, highlighting their roles as genetic material. It discusses key experiments and discoveries that led to the understanding of nucleic acids as the carriers of genetic information, as well as the structure and function of nucleotides, the building blocks of these molecules.
1. The Genetic Material
Definition of Genetic Material: The genetic material must fulfill four key criteria:
Information: Necessary to make an entire organism.
Transmission: Passed from parent to offspring.
Replication: Must be copied accurately.
Variation: Capable of change due to mutations and variations.
2. Historical Context
By the early 1900s, researchers such as Gregor Mendel discovered that certain traits were correlated with the inheritance of chromosomes, which are composed of both DNA and proteins.
The question arose: Which component (proteins or DNA) acts as the genetic material?
3. Key Experiments
Griffith's Experiment
Studied Streptococcus pneumoniae to determine the identity of the genetic material. He demonstrated that living type R bacteria could be transformed into type S bacteria when mixed with heat-killed type S bacteria, suggesting a transformation occurring through genetic material.
Avery, MacLeod and McCarty Experiment
Fractionated type S bacterial cells into four classes of macromolecules (lipids, carbohydrates, proteins, and nucleic acids). Through their experiments, they concluded that DNA was essential for transformation because only DNA could cause type R to transform into type S.
Hershey and Chase Experiment
Provided further evidence that DNA is the genetic material using bacteriophage T2, which consists of only DNA and protein. They used isotopes to label DNA and proteins:
The phage injects its DNA into a bacterial host cell, which leads to the production of new phages only when DNA, not protein, is present.
Significant findings were that the majority of labeled DNA remained with the bacterial cells after blending, while labeled proteins were found in the solution.
4. Properties of DNA
Nucleotides are the building blocks of nucleic acids (DNA and RNA).
Structure: Consists of deoxyribose (in DNA) or ribose (in RNA), a phosphate group, and nitrogenous bases (purines: A, G; pyrimidines: C, T (in DNA), U (in RNA)).
Nucleotides are linked by phosphodiester bonds, creating a backbone that runs in a 5' to 3' directionality.
5. Structure of DNA
Double Helix: DNA exists as a right-handed double helix, with two strands held together by hydrogen bonding between complementary bases (A-T, G-C).
Chargaff’s Rule: Observations by Erwin Chargaff indicated that the amount of adenine (A) is always equal to thymine (T), and the amount of guanine (G) is always equal to cytosine (C).
In Watson and Crick’s model:
The two strands are antiparallel, and there are 10 nucleotides per turn of the helix.
6. RNA Structure
RNA shares similarities with DNA but has uracil instead of thymine and ribose instead of deoxyribose.
RNA molecules are typically single-stranded but can form complex structures due to base pairing (A-U and C-G) leading to secondary structures.
7. Types of RNA
rRNA (ribosomal RNA): Structural components of ribosomes for protein synthesis during translation.
mRNA (messenger RNA): Template for protein synthesis.
tRNA (transfer RNA): Carries amino acids to the ribosome for protein synthesis, featuring a complex secondary and tertiary structure.
8. The Central Dogma of Molecular Biology
This concept describes the flow of genetic information from DNA to RNA to proteins, emphasizing the role of DNA as a template for RNA, which in turn directs the synthesis of proteins.
9. Summary
To reiterate, the genetic material must provide:
Information necessary to construct an entire organism.
Transmission of genetic information from parent to offspring.
Capability for replication.
Potential for variation, allowing for genetic diversity.