Structure of DNA & Information Flow: Genes to Proteins

Structure of DNA

Primary, Secondary, and Tertiary Structure of DNA

The structure of DNA is organized into three hierarchical levels: primary, secondary, and tertiary. Each level contributes to the molecule's function as the carrier of genetic information.

• Primary Structure: The linear sequence of nucleotides (adenine, thymine, cytosine, guanine) joined by phosphodiester bonds forms the backbone of DNA. The order of these bases encodes genetic information.

• Secondary Structure: DNA forms a double helix through complementary base pairing. The two strands are held together by hydrogen bonds between bases: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). The strands are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5').

• Tertiary Structure: DNA is further compacted by winding around histone proteins to form chromosomes in eukaryotic cells. This allows efficient storage and regulation of genetic material.

• Space-filling Model: The three-dimensional arrangement of atoms in DNA can be visualized using space-filling models, which show the double helix's major and minor grooves.

Example: The classic Watson-Crick model of DNA illustrates the double helix with specific base pairing and antiparallel orientation.

Base Pairing Rules and Chemical Structure

DNA's stability and information storage depend on specific base pairing and the chemical structure of its nucleotides.

• Purines: Adenine (A) and guanine (G) are larger, double-ringed bases.

• Pyrimidines: Cytosine (C) and thymine (T) are smaller, single-ringed bases.

• Base Pairing: A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds.

• Chargaff's Rule: In double-stranded DNA, the percentage of A equals T, and the percentage of G equals C.

Example: In a double-stranded DNA sample, if %A = 30%, then %T = 30%, and if %G = 20%, then %C = 20%.

Antiparallel Strands and Directionality

The two strands of DNA run in opposite directions, which is essential for replication and transcription.

• 5' and 3' Ends: Each DNA strand has a 5' phosphate group and a 3' hydroxyl group.

• Antiparallel Orientation: One strand runs 5' to 3', while the complementary strand runs 3' to 5'.

• Base Pairing: The antiparallel arrangement allows proper alignment for hydrogen bonding between bases.

Example: For the DNA sequence TAGCTAG 5', the complementary strand is CTAGCTA 3'.

Information Flow: Genes to Proteins

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein. This process is fundamental to gene expression and cellular function.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

• Reverse Transcription: In some viruses, RNA can be reverse-transcribed into DNA.

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated into the hemoglobin protein.

Transcription: DNA to RNA

Transcription is the process by which genetic information in DNA is copied into RNA.

• Template Strand: Only one strand of DNA is used as a template for RNA synthesis.

• Base Pairing: RNA uses uracil (U) instead of thymine (T); A pairs with U, and C pairs with G.

• Directionality: RNA is synthesized in the 5' to 3' direction.

• RNA Structure: RNA is typically single-stranded and contains ribose sugar.

Example: If the DNA template strand is 3'-TACGGA-5', the mRNA sequence will be 5'-AUGCCU-3'.

Translation: RNA to Protein

Translation is the process by which the sequence of bases in mRNA is converted into a sequence of amino acids, forming a protein.

• Codons: mRNA is read in sets of three bases (codons), each specifying an amino acid.

• Start Codon: AUG codes for methionine and signals the start of translation.

• Redundancy: The genetic code is redundant; multiple codons can code for the same amino acid.

• Universality: The genetic code is nearly universal across all organisms.

• Non-overlapping: Codons are read one after another without overlap.

Example: The mRNA sequence AUG GCC AAT GAT translates to the amino acids Methionine-Alanine-Asparagine-Aspartic acid.

Features of the Genetic Code

The genetic code has several important properties that ensure accurate translation of genetic information.

• Redundant: Most amino acids are encoded by more than one codon.

• Unambiguous: Each codon specifies only one amino acid.

• Non-overlapping: Codons are read sequentially, without sharing bases.

• Universal: The same codons specify the same amino acids in nearly all organisms.

• Conservative: When multiple codons specify the same amino acid, the first two bases are usually the same.

Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double-stranded (usually)	Single-stranded (usually)
Bases	A, T, C, G	A, U, C, G
Location	Nucleus (eukaryotes)	Nucleus & cytoplasm

Key Equations

• Chargaff's Rule:

$\%A = \%T$ $\%G = \%C$

• Number of Codons in a DNA Strand:

$\text{Number of codons} = \frac{\text{Number of bases}}{3}$

Additional info: These notes expand on the original slides by providing definitions, examples, and a comparison table for DNA and RNA, as well as key equations relevant to the topics.