Fundamentals of DNA Structure and Function: Replication, Transcription, and Translation

Central Dogma of Molecular Biology

Overview of the Central Dogma

The central dogma of molecular biology describes the flow of genetic information within a biological system. It outlines the processes by which genetic information in DNA is transcribed into RNA and then translated into protein.

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used as a template to synthesize proteins.

Diagram:

• DNA → (Replication) → DNA

• DNA → (Transcription) → RNA

• RNA → (Translation) → Protein

DNA Structure

Nucleotide Composition and Double Helix

DNA is composed of two antiparallel strands forming a double helix. Each strand consists of nucleotides, which are made up of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• Nitrogenous Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

• Base Pairing: A pairs with T, G pairs with C

• Strand Directionality: One strand runs 5' to 3', the other 3' to 5'

• 5' End: Has a phosphate group

• 3' End: Has a free hydroxyl (-OH) group

Base Pairing Rules

Purines and Pyrimidines

Base pairing in DNA follows specific rules based on the chemical structure of the bases:

• Purines: Adenine (A) and Guanine (G)

• Pyrimidines: Thymine (T) and Cytosine (C)

• Pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds)

• Strength: G-C pairs are stronger than A-T pairs due to the extra hydrogen bond

Example: In a DNA strand, if the sequence is 5'-ATGC-3', the complementary strand will be 3'-TACG-5'.

DNA Replication

Mechanism and Enzymes

DNA replication is the process by which DNA makes a copy of itself during cell division. Several key enzymes and steps are involved:

• Template: DNA polymerase requires a template strand, which is read 3' to 5', and synthesizes new DNA in the 5' to 3' direction.

• Primer: DNA polymerase also needs a primer to provide a free 3'-OH group for the addition of nucleotides.

• Proofreading: DNA polymerase can remove incorrectly paired bases through proofreading activity.

• Leading Strand: Synthesized continuously in the direction of the replication fork.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, which are later joined together.

Equation:

$ext{(DNA)}_{n} + ext{dNTP} ightarrow ext{(DNA)}_{n+1} + ext{PP}_i$

Transcription Initiation in Bacteria

Promoter Recognition and Consensus Sequences

Bacteria initiate transcription at specific DNA sequences called promoters. Promoters signal the beginning of a gene and are recognized by RNA polymerase and associated factors.

• Promoter Sequence: A region upstream of the gene that signals where transcription should begin.

• Consensus Sequences: Promoters are recognized more efficiently if they resemble consensus sequences, typically located at -10 and -35 positions relative to the transcription start site.

• Example: The -10 (Pribnow box) and -35 regions are common in bacterial promoters.

Additional info: The consensus sequence for the -10 region is typically TATAAT, and for the -35 region is TTGACA.

Summary Table: DNA Base Pairing Properties

Base	Type	Pairs With	# of Hydrogen Bonds	Strength
Adenine (A)	Purine	Thymine (T)	2	Weaker
Thymine (T)	Pyrimidine	Adenine (A)	2	Weaker
Guanine (G)	Purine	Cytosine (C)	3	Stronger
Cytosine (C)	Pyrimidine	Guanine (G)	3	Stronger