Exam 2 (Lecture 9) Microbial Genetics: Central Dogma and DNA Replication

GENETICS-PART 1: The Central Dogma

Overview of the Central Dogma

The Central Dogma of molecular biology describes the flow of genetic information within a cell, from DNA to RNA to protein. This process is fundamental to all living organisms and underpins gene expression and regulation.

• DNA: The hereditary material, usually present as two complementary strands.

• Replication: The process by which DNA is copied to produce identical DNA molecules.

• Transcription: The synthesis of RNA from a DNA template.

• Translation: The process by which ribosomes synthesize proteins using the information in mRNA.

Example: In the provided diagram (FIG 6.1 & 6.5), DNA is replicated, transcribed into RNA, and then translated into a protein sequence (Phe-Val-Asn-Gln-His-Leu).

DNA Replication

Part 1: Mechanism and Enzymes

DNA replication is a highly conserved process in all cells, ensuring genetic continuity. It is described as semiconservative, meaning each new DNA molecule contains one parental and one newly synthesized strand.

• Semiconservative Replication: Each daughter DNA molecule consists of one old (parental) strand and one new strand.

• Supercoiling: DNA in prokaryotes is supercoiled by enzymes such as DNA gyrase and other topoisomerases. These enzymes relieve torsional strain during replication.

• Histones: In Archaea and Eukarya, DNA is associated with histone proteins, forming nucleosomes and aiding in DNA packaging.

Example: The diagram shows nucleosome structure in eukaryotes and the action of DNA gyrase in bacteria.

Part 2: DNA Polymerase and Fidelity

DNA synthesis is catalyzed by DNA polymerase, which adds nucleotides in the 5' to 3' direction. All DNA polymerases require an RNA primer to initiate synthesis.

• Directionality: DNA polymerase synthesizes new DNA from 5' phosphate to 3' hydroxyl ends.

• RNA Primer: Short RNA sequences synthesized by primase provide a starting point for DNA polymerase.

• Proofreading: Most DNA polymerases have proofreading activity to ensure high fidelity during replication.

• DNA Repair: Specialized DNA polymerases and repair mechanisms correct damaged DNA.

Equation:

$\text{(dNMP)}_n + dNTP \xrightarrow{DNA\,polymerase} \text{(dNMP)}_{n+1} + PP_i$

Example: The diagram shows the role of primase and DNA polymerase in synthesizing new DNA strands.

Part 3: Origins of Replication (ORI)

DNA replication begins at specific sites called origins of replication (ORI). The number and structure of ORIs vary among domains of life.

• Bacteria: Typically have 1-2 circular chromosomes, each with a single ORI.

• Archaea: Have circular chromosomes with 1-3 ORIs.

• Eukaryotes: Possess multiple linear chromosomes, each with multiple ORIs and telomeres.

Example: The diagram illustrates circular chromosomes in prokaryotes and linear chromosomes with telomeres in eukaryotes.

Domain	Chromosome Structure	Number of ORIs
Bacteria	Circular	1 per chromosome
Archaea	Circular	1-3 per chromosome
Eukaryotes	Linear	Multiple per chromosome

Part 4: Replication Forks

Replication forks are formed at each ORI, where DNA synthesis proceeds bidirectionally. Synthesis always occurs in the 5' to 3' direction.

• Replication Fork: The Y-shaped region where new DNA strands are synthesized.

• Bidirectional Synthesis: Replication proceeds in both directions from the ORI.

Example: The diagram shows replication forks forming at the ORI in circular and linear chromosomes.

Transcription and Translation

Transcription: DNA to RNA

Transcription is the process by which RNA is synthesized from a DNA template, catalyzed by RNA polymerase.

• RNA Polymerase: Enzyme responsible for synthesizing RNA; consists of multiple subunits.

• Promoters: Specific DNA sequences recognized by RNA polymerase to initiate transcription.

• Directionality: RNA synthesis occurs in the 5' to 3' direction.

• Termination: Transcription stops at terminator sequences in the DNA or RNA.

Equation:

$\text{DNA} \xrightarrow{RNA\,polymerase} \text{RNA}$

Example: The diagram shows transcription of the bottom DNA strand to produce an RNA molecule.

Translation: RNA to Protein

Translation is the process by which ribosomes synthesize proteins using the sequence information in mRNA.

• Ribosomes: Complexes of rRNA and proteins; consist of two subunits (30S + 50S = 70S in prokaryotes, 40S + 60S = 80S in eukaryotes).

• Reading Frame: Defined by the start codon (AUG or GUG) and ends at a stop codon (UAA, UAG, UGA).

• tRNA: Adaptor molecules that decode mRNA codons into amino acids.

• Genetic Code: Universal triplet code specifying amino acids.

Equation:

$\text{mRNA} \xrightarrow{ribosome} \text{Protein}$

Example: The diagram shows translation of mRNA into a polypeptide sequence (Phe-Val-Asn-Gln-His-Leu).

Summary Table: Key Enzymes and Structures

Process	Key Enzyme(s)	Directionality	Major Features
Replication	DNA polymerase, primase, gyrase/topoisomerase	5' to 3'	Semiconservative, requires primer, proofreading
Transcription	RNA polymerase	5' to 3'	Promoter recognition, termination sequences
Translation	Ribosome, tRNA	N-terminus to C-terminus	Start/stop codons, reading frame, genetic code

Additional info: These notes expand on the provided diagrams and handwritten annotations, clarifying the roles of enzymes, directionality, and structural features in microbial genetics. The content is suitable for college-level microbiology students preparing for exams on molecular genetics and gene expression.