Unit 5: DNA Replication, Transcription, and Translation - Comprehensive Study Guide

1. DNA Replication: Types and Mechanism

Types of DNA Replication Models

Before the exact mechanism of DNA replication was confirmed, three primary models were proposed to explain how DNA copies itself. These models represent different theoretical paths for the distribution of parent and daughter DNA strands:

The Meselson-Stahl Experiment (1958): This landmark experiment proved that DNA replication is semi-conservative. By growing Escherichia coli in heavy nitrogen (¹⁵N) and then transferring them to light nitrogen (¹⁴N), they observed density gradient centrifugation bands that matched only the semi-conservative model predictions.

The Mechanism of DNA Replication

DNA replication in prokaryotes is a highly coordinated, multi-step enzymatic process. It occurs in three main stages: Initiation, Elongation, and Termination.

Key Replication Enzymes and Proteins

Step-by-Step Replication Process

1. Initiation: Replication begins at a unique sequence called the Origin of Replication (oriC). DnaA proteins bind to this site, causing the local DNA double helix to destabilize and open. DNA Helicase (DnaB) is loaded onto the single strands, unwinding the DNA in both directions (bidirectionally) to form two active replication forks.
2. Primer Synthesis: DNA Polymerases cannot initiate DNA synthesis de novo; they require a free 3'-OH group. DNA Primase synthesizes a short complementary RNA primer on the single-stranded DNA template.
3. Elongation: DNA Polymerase III reads the template strand in the 3' → 5' direction and synthesizes the new strand in the 5' → 3' direction. Due to the antiparallel nature of DNA, synthesis occurs differently on each strand:
   • Leading Strand: Synthesized continuously toward the replication fork. It requires only a single RNA primer at the beginning.
   • Lagging Strand: Synthesized discontinuously away from the replication fork. It is produced in short segments called Okazaki fragments. Each fragment requires its own RNA primer.
4. Primer Removal and Gap Filling: DNA Polymerase I recognizes the RNA primers, degrades them using its 5' → 3' exonuclease activity, and fills in the resulting gaps with DNA nucleotides using its polymerase activity.
5. Ligation: After DNA Polymerase I completes synthesis, a tiny nick remains in the phosphodiester backbone between the newly synthesized fragments. DNA Ligase seals these nicks to form a continuous, intact DNA strand.
6. Termination: Replication terminates at specific sequences called Ter sites. The Tus protein binds to these sites, blocking the helicase and halting the replication forks, allowing the two completed circular chromosomes to separate.

2. RNA Polymerases and Transcription in Prokaryotes

Structure of Prokaryotic RNA Polymerase

In prokaryotes, a single type of RNA polymerase synthesizes all major types of RNA (mRNA, tRNA, and rRNA). The active enzyme exists in two main forms:

• Core Enzyme (α₂ββ'ω): Composed of five subunits:
  • α (Alpha - two copies): Involved in assembly of the core enzyme and interaction with regulatory factors.
  • β (Beta): Contains the catalytic active site for phosphodiester bond formation.
  • β' (Beta prime): Responsible for binding to the DNA template strand.
  • ω (Omega): Helps in the assembly and stabilization of the core enzyme.
• Holoenzyme (α₂ββ'ω + σ): Formed when the core enzyme associates with the Sigma (σ) factor. The σ factor is essential for recognizing and binding to specific promoter regions on the DNA template. Without the σ factor, the core enzyme cannot initiate transcription accurately or efficiently.

Mechanism of Transcription in Prokaryotes

Transcription is the process by which genetic information encoded in DNA is copied into a complementary RNA strand. It proceeds through three distinct phases: Initiation, Elongation, and Termination.

1. Initiation

Transcription begins when the RNA Polymerase holoenzyme recognizes and binds to specific promoter sequences located upstream of the transcription start site (+1 position):

• -10 Sequence (Pribnow Box): A conserved, AT-rich consensus sequence (TATAAT) located 10 base pairs upstream, which facilitates DNA unwinding due to fewer hydrogen bonds.
• -35 Sequence: A conserved consensus sequence (TTGACA) located 35 base pairs upstream, which serves as the primary recognition site for the σ factor.

The initiation process proceeds through these steps:

1. Closed Complex Formation: The holoenzyme binds to the promoter sequence, keeping the DNA strands intact and closed.
2. Open Complex Formation: The polymerase unwinds approximately 12 to 14 base pairs of DNA around the -10 region to expose the template strand, forming an open promoter complex.
3. Abortive Initiation: The enzyme synthesizes short, abortive RNA transcripts (fewer than 10 nucleotides) before successfully clearing the promoter.
4. Promoter Clearance: Once a transcript exceeds 10 nucleotides in length, the σ factor dissociates from the core enzyme, allowing the transition to the elongation phase.

2. Elongation

The core RNA polymerase moves downstream along the DNA template. It maintains a transcription bubble of unwound DNA (about 12-15 base pairs long) and synthesizes RNA in the 5' → 3' direction using ribonucleoside triphosphates (rNTPs: ATP, UTP, CTP, GTP) as substrates.

Transcription Formula: (RNA)_n + rNTP → (RNA)_n+1 + PP_i

The DNA template strand is read 3' → 5', and the RNA synthesized is complementary to the template strand and identical (except for U instead of T) to the coding (non-template) strand.

3. Termination

Prokaryotes utilize two different mechanisms to signal the end of transcription and release the completed RNA transcript:

• Rho-Independent (Intrinsic) Termination: This relies on specific sequences in the newly transcribed RNA. The RNA contains an inverted repeat sequence followed by a run of uracil (U) residues. The inverted repeats fold into a stable hairpin loop structure (GC-rich), which physically pauses the moving RNA polymerase. The weak hydrogen bonding between the consecutive U-A base pairs in the transcription bubble destabilizes the RNA-DNA hybrid, causing the transcript to detach.
• Rho-Dependent Termination: This requires an auxiliary protein called the Rho (ρ) factor. The Rho factor is an ATP-dependent helicase that binds to a specific, cytosine-rich region on the growing RNA transcript known as the rut (rho utilization) site. Rho moves along the RNA behind the polymerase. When the polymerase pauses at a termination site, Rho catches up to it and uses its helicase activity to unwind the RNA-DNA hybrid, releasing the transcript.

3. The Genetic Code and Translation in Prokaryotes

The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of an mRNA is translated into the amino acid sequence of a protein. Its key characteristics include:

• Triplet Nature: Every codon (coding unit) consists of a sequence of three nucleotides. Since there are four nucleotides (A, U, C, G), there are 4³ = 64 possible codons, which code for the 20 standard amino acids.
• Universal: The same codons specify the same amino acids in almost all organisms, from simple prokaryotes to complex multicellular eukaryotes.
• Degenerate (Redundant): Most amino acids are specified by more than one codon. For example, Leucine is coded by six different codons.
• Non-overlapping: The translation machinery reads the nucleotides in consecutive groups of three, without sharing any bases between adjacent codons.
• Commaless: The genetic code is read continuously from start to finish without pauses, punctuation, or skipped bases.
• Start and Stop Codons:
  • Start Codon: AUG is the universal start signal. In prokaryotes, it codes for N-formylmethionine (fMet).
  • Stop Codons: UAA (ochre), UAG (amber), and UGA (opal) do not code for any amino acids. They act as termination signals.

The Wobble Hypothesis: Proposed by Francis Crick, this hypothesis states that while the first two base pairings between the codon on mRNA and the anticodon on tRNA are highly specific, the third base pairing (at the 3' end of the codon) is less stringent. This allows a single tRNA anticodon to recognize and pair with multiple degenerate codons, reducing the total number of distinct tRNAs required by the cell.

Mechanism of Translation in Prokaryotes

Translation is the ribosomally directed process of protein synthesis. In prokaryotes, translation is coupled with transcription, meaning translation can begin while the mRNA is still being transcribed. The machinery consists of the 70S ribosome (comprising a small 30S subunit and a large 50S subunit), tRNAs, mRNA, and various protein factors.

The Ribosome Active Sites

• A Site (Aminoacyl site): Accommodates the incoming charged aminoacyl-tRNA.
• P Site (Peptidyl site): Holds the tRNA carrying the growing polypeptide chain.
• E Site (Exit site): Holds the uncharged tRNA before it is released from the ribosome.

Step-by-Step Translation Process

Step 1: Activation and Charging of tRNA (Aminoacylation)

Before translation begins, amino acids must be attached to their corresponding tRNAs. This is catalyzed by aminoacyl-tRNA synthetase enzymes in a two-step reaction:

1. Amino Acid + ATP → Aminoacyl-AMP + PP_i
2. Aminoacyl-AMP + tRNA → Aminoacyl-tRNA + AMP

Step 2: Initiation

Translation initiation sets the correct reading frame for protein synthesis:

1. The 30S ribosomal subunit binds to the mRNA. It is positioned correctly at the start codon (AUG) by the pairing of the 16S rRNA within the subunit with a complementary sequence on the mRNA called the Shine-Dalgarno sequence (located upstream of the start codon).
2. Initiation Factors (IF1, IF2, and IF3) assist the assembly. IF3 prevents the 50S subunit from binding prematurely to the 30S subunit.
3. The initiator tRNA, charged with N-formylmethionine (fMet-tRNA^fMet), complexed with IF2-GTP, binds directly to the P site at the AUG start codon.
4. IF1 and IF3 are released. The 50S ribosomal subunit binds to the 30S initiation complex, causing GTP hydrolysis on IF2, leading to the dissociation of IF2-GDP. This leaves a fully functional 70S ribosome ready for elongation.

Step 3: Elongation

Elongation is a cyclic process that adds amino acids to the growing polypeptide chain:

1. Codon Recognition: An incoming charged aminoacyl-tRNA, complexed with Elongation Factor Tu (EF-Tu) and GTP, enters the vacant A site of the ribosome. If the anticodon pairs correctly with the mRNA codon, GTP is hydrolyzed, and EF-Tu-GDP is released. Elongation Factor Ts (EF-Ts) recycles EF-Tu-GDP back to EF-Tu-GTP.
2. Peptide Bond Formation: The enzyme peptidyl transferase (which is actually a ribozyme, the 23S rRNA of the 50S subunit) catalyzes the formation of a peptide bond between the amino acid in the A site and the carboxyl terminus of the peptidyl-tRNA in the P site. This transfers the peptide chain to the tRNA in the A site.
3. Translocation: Elongation Factor G (EF-G) uses energy from GTP hydrolysis to move the ribosome exactly one codon along the mRNA in the 5' → 3' direction. This shifts:
   • The uncharged tRNA from the P site to the E site (where it is released).
   • The peptidyl-tRNA from the A site to the P site.
   • The A site becomes empty and ready to accept the next codon's corresponding tRNA.

Step 4: Termination

Translation continues until one of the three stop codons (UAA, UAG, or UGA) enters the A site:

1. No tRNA corresponds to a stop codon. Instead, protein molecules called Release Factors (RFs) recognize these sequences:
   • RF1: Recognizes UAA and UAG stop codons.
   • RF2: Recognizes UAA and UGA stop codons.
   • RF3-GTP: Promotes the release of RF1 and RF2 from the ribosome after chemical cleavage.
2. The binding of a Release Factor to the A site triggers peptidyl transferase to cleave the ester bond holding the completed polypeptide chain to the terminal tRNA, releasing the polypeptide.
3. The entire translation apparatus is disassembled by Ribosome Recycling Factor (RRF) and EF-G through GTP hydrolysis, preparing the ribosomal subunits to start translating another mRNA.