Structure and Function of Nucleic Acids and Proteins: DNA, RNA, and Protein Features

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Macromolecules II: Nucleic Acids and Proteins

Introduction

This study guide covers the structure and function of ribonucleic acids (RNA), deoxyribonucleic acid (DNA), and proteins, focusing on their roles in genetics. Understanding these macromolecules is fundamental to the study of genetic information storage, transmission, and expression.

Nucleic Acids

Structure of Nucleic Acids

Nucleic acids are polymers composed of nucleotide monomers.
Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Both DNA and RNA consist of a sugar-phosphate backbone and nitrogenous bases.

Monomers of Nucleic Acids: Nucleotides

A nucleotide is composed of:
- A nitrogenous base
- A pentose sugar (deoxyribose in DNA, ribose in RNA)
- A phosphate group
Nucleoside: base + sugar (no phosphate)
Nucleotide: base + sugar + phosphate

Nitrogenous Bases: Purines & Pyrimidines

Purines: Adenine (A), Guanine (G) – double-ring structure
Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA) – single-ring structure
DNA contains A, G, C, T; RNA contains A, G, C, U

Pentose Sugars

Deoxyribose (DNA): lacks an oxygen atom at the 2' carbon
Ribose (RNA): has an OH group at the 2' carbon

Nucleotide Structure and Bonds

Nucleotides are linked by phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl of another.
Polynucleotides have directionality: 5' → 3'

DNA Double Helix Structure

DNA is a double-stranded helix (Watson & Crick, 1953).
Strands are antiparallel and held together by hydrogen bonds between complementary bases.
Base pairing: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds; Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.

DNA Complimentary Base-pairing

Ensures accurate replication and transcription.
Example: 5'-AGCTGAT-3' pairs with 3'-TCGACTA-5'

Primary Functions of DNA

Storage of genetic information
Replication & inheritance
Expression of the genetic message

RNA: Types and Functions

Major Classes of RNA

Ribosomal RNA (rRNA): Forms part of ribosomes, essential for protein synthesis.
Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.
Transfer RNA (tRNA): Transfers specific amino acids to the ribosome during protein synthesis; has a characteristic secondary structure.

Other Unique RNAs

Small nuclear RNA (snRNA): Involved in mRNA splicing to form mature mRNA.
Small interfering RNA (siRNA): Post-transcriptional gene silencing.
MicroRNA (miRNA): Regulation of gene expression.
Small nucleolar RNA (snoRNA): Chemical modifications of tRNA, rRNA, and DNA.

Peptides & Proteins

Genetic Programming of Proteins

The sequence of amino acids in peptides and proteins is determined by genes.
Proteins are essential for nearly all cellular functions; about 18% of human body mass is protein.

Functions of Proteins

Structural support
Storage of substances
Transport of substances
Intercellular signaling
Movement
Defense against foreign substances

Type of Protein	Function	Examples
Structural proteins	Support	Keratin, collagen
Storage proteins	Storage of amino acids	Ovalbumin, casein
Transport proteins	Transport of other substances	Hemoglobin, membrane proteins
Hormonal proteins	Coordination of activities	Insulin
Receptor proteins	Response to chemical stimuli	Neuroreceptors
Contractile proteins	Movement	Actin, myosin
Defensive proteins	Protection against disease	Antibodies
Enzymatic proteins	Selective acceleration of chemical reactions	Digestive enzymes

Amino Acids: Monomers of Polypeptides

Each amino acid has a central alpha carbon attached to:
- Hydrogen atom
- Carboxyl group (COO-)
- Amino group (NH3+)
- Variable R group (side chain)
There are 20 different amino acids found in proteins, classified by properties of their R groups (e.g., hydrophobic, charged).

Peptide Bond Formation

Amino acids are linked by peptide bonds formed via condensation reactions (removal of water).
Polypeptides have an N-terminus (amino end) and a C-terminus (carboxyl end).
Sequence of amino acids determines protein structure and function.

Levels of Protein Structure

Primary Structure

Unique sequence of amino acids in a polypeptide chain.
Single amino acid changes can affect protein function (e.g., sickle cell anemia caused by a mutation in hemoglobin).

Secondary Structure

Regular folding patterns stabilized by hydrogen bonds along the polypeptide backbone.
Main types: alpha helices (coils) and beta pleated sheets.

Tertiary Structure

Three-dimensional folding due to interactions between R groups.
Weak interactions: hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces.
Strong interactions: disulfide bridges (covalent bonds between cysteine residues).

Quaternary Structure

Association of two or more polypeptide chains to form a functional protein.
Subunits are held together by non-covalent interactions; some have disulfide bridges.
Example: Hemoglobin (2 alpha and 2 beta subunits, each with a heme group for oxygen transport).

Summary Table: Levels of Protein Structure

Level	Basis of Structure	Kinds of Bonds/Interactions
Primary	Amino acid sequence	Peptide bonds
Secondary	Folding into alpha helix or beta sheet	Hydrogen bonds
Tertiary	Three-dimensional folding	Hydrophobic, ionic, hydrogen bonds, van der Waals, disulfide bridges
Quaternary	Association of multiple polypeptides	Same as tertiary, plus inter-chain interactions

Key Equations and Concepts

Phosphodiester bond formation:

Base pairing:

Directionality of DNA:

Example: Sickle Cell Anemia

A single nucleotide change in the DNA sequence of the beta-globin gene leads to a change in the amino acid sequence, resulting in abnormal hemoglobin and sickle-shaped red blood cells.

Additional info: The notes also briefly mention the role of non-coding RNAs in gene regulation and post-transcriptional gene silencing, which are important in modern genetics research.