Comprehensive Study Notes on MT 203 Cytogenetics: The Central Dogma

Core Institutional Values & Overview of the Central Dogma

• Institutional Vision-Mission (St. Alexius College): St. Alexius College is an educational prime mover in Allied Health and Basic Education, forming ethical, competent, and development-oriented graduates for a meaningful life in a globalizing society.

• Core Values (H.I.S.S.E.):

  • H: Humility

  • I: Integrity

  • S: Simplicity

  • S: Service

  • E: Excellence

• The Central Dogma Flow of Genetic Information:

  • DNA Replication: Process by which DNA is copied/doubled within dividing cells.

  • Transcription: Process of synthesizing single-stranded messenger RNA (mRNA) from a double-stranded DNA template.

  • Reverse Transcription: Synthesis of DNA from an RNA template.

  • Translation: Synthesis of a polypeptide chain (protein) by decoding mRNA at the ribosome.

DNA Replication

Overview and Models of Replication

• Definition: DNA replication is the process by which DNA is essentially doubled within a dividing cell to ensure accurate transmission of genetic information to daughter cells.

• Models for DNA Replication:

  • Conservative Model: The parental double helix remains intact, and a completely new double-stranded DNA molecule is synthesized.

  • Semiconservative Model: The double-stranded DNA unwinds, and each parental strand serves as a template for a new complementary strand. This results in daughter double-stranded DNA containing one parental strand and one new daughter strand.

  • Dispersive Model: Parental and newly synthesized segments are interspersed within both strands of the resulting DNA molecules.

• Semi-Conservative Method Characteristics:

  • Yields double-stranded DNA with one parental strand and one new daughter strand.

  • Always follows strict complementary base-pairing rules ($A\text{--}T$ and $G\text{--}C$).

Basic Requirements for DNA Replication

• Substrates:

  • Four deoxyribonucleoside triphosphates (dNTPs):

  • $dATP$ (deoxyadenosine triphosphate)

  • $dCTP$ (deoxycytidine triphosphate)

  • $dGTP$ (deoxyguanosine triphosphate)

  • $dTTP$ (deoxythymidine triphosphate)

• Template:

  • Both strands of the DNA double helix serve as templates for the synthesis of new daughter DNA strands.

• Enzymes and Accessory Proteins:

  • DNA A protein: Recognizes and binds to the origin of replication ($Ori$) and denatures/opens the DNA duplex.

  • Helicase (DNA B protein): Known as the "unzipping enzyme"; unwinds the complementary DNA double helix.

  • Single-Strand Binding (SSB) Proteins: Bind to and stabilize separated single-stranded DNA, preventing premature re-association or annealing.

  • DNA Topoisomerases (I & II): Relieve torsional strain and supercoiling created by unwinding by cutting and rejoining single or double strands of DNA.

  • Primase: A DNA-dependent RNA polymerase that synthesizes short RNA primers ($5' \rightarrow 3'$ direction) using DNA as a template.

  • Primosome: A functional complex formed by the association of the enzyme primase with SSB proteins.

  • DNA Polymerase: Responsible for DNA chain elongation, proofreading, and gap filling.

  • DNA Ligase: Joins polynucleotide chains by catalyzing phosphodiester bond formation to seal Okazaki fragments.

  • Ter Binding Protein: Binds to termination sequences ($ter$) to prevent helicase from further unwinding and facilitate replication termination.

Catalog and Functions of DNA Polymerases

• General Catalytic Roles:

  • Catalyzed by DNA-dependent DNA polymerases.

  • Requirement 1: DNA chain elongation.

  • Requirement 2: DNA repair via $5' \rightarrow 3'$ exonuclease activity.

  • Requirement 3: Proofreading via $3' \rightarrow 5'$ exonuclease activity.

• Prokaryotic vs. Eukaryotic Polymerase Functions:

  | Function / Role | Prokaryotic Polymerase | Eukaryotic Polymerase |   | :--- | :--- | :--- |   | Gap filling & synthesis between Okazaki fragments of lagging strand | DNA Polymerase I | DNA Polymerase $\beta$ |   | DNA proofreading & DNA repair | DNA Polymerase II | DNA Polymerase $\rightarrow$ |   | Primary DNA repair | DNA Polymerase II | DNA Polymerase $\beta$ |   | Mitochondrial DNA synthesis | N/A | DNA Polymerase $\rightarrow$ |   | Primary leading and lagging strand synthesis | DNA Polymerase III | DNA Polymerase $\rightarrow$ |

  Detailed Breakdown:

• Prokaryotic Types: DNA Polymerase I, II, III.

• Eukaryotic Types: DNA Polymerase $\rightarrow$, $\beta$, $\rightarrow$, $\rightarrow$, $\rightarrow$.

RNA Primer Characteristics

• Length: Short piece of RNA ($5\text{--}50$ nucleotides in length).

• Synthesis: Synthesized in a $5' \rightarrow 3'$ direction by primase using DNA as a template.

• Function: Provides a free $3'\text{-OH}$ group required by DNA polymerase to initiate chain growth.

Step-by-Step Process of DNA Replication

1. Initiation

• Origin of Replication ($Ori$):

  • In prokaryotes: Single origin of replication.

  • In eukaryotes: Multiple origins of replication composed almost exclusively of $A\text{--}T$ base pairs (consensus sequences).

• Process:

  • DNA A protein recognizes and binds to $Ori$, denaturing the DNA helix.

  • Helicase (DNA B protein) attaches to the denatured region and unwinds the double strand, forming a "V" or "Y" shaped replication fork / replication bubble.

  • SSB proteins coat the single strands to prevent re-annealing.

  • Topoisomerase relieves upstream supercoiling strain.

  • Primase binds to form the primosome and synthesizes the RNA primer.

• Kinetics: One round of synthesis in prokaryotes involves over 4 million nucleotides in each strand, completed in approximately 40 minutes.

2. Elongation

• Mechanism: DNA Polymerase III adds deoxyribonucleotides sequentially to the $3'$ end of the RNA primer. DNA synthesis occurs strictly in the $5' \rightarrow 3'$ direction.

• Leading Strand Synthesis:

  • Synthesis proceeds continuously toward the replication fork.

  • Template strand orientation is $3' \rightarrow 5'$.

  • Requires only a single RNA primer.

• Lagging Strand Synthesis:

  • Synthesis proceeds discontinuously away from the replication fork.

  • Template strand orientation is $5' \rightarrow 3'$.

  • Requires multiple RNA primers synthesized at specified intervals.

  • Okazaki Fragments: Short, newly synthesized DNA fragments formed on the lagging strand. Discovered by Reiji Okazaki ($1930\text{--}1975$).

• Processing Lagging Fragments:

  • DNA Polymerase I removes RNA primers via exonuclease activity and fills the resulting gaps with deoxyribonucleotides, leaving a single-stranded nick.

  • DNA Ligase seals the nicks by forming phosphodiester bonds to connect Okazaki fragments into a continuous strand.

3. Termination

• Specific termination sequences ($ter$) direct the end of replication.

• Ter binding protein binds to $ter$ sites, preventing helicase from unwinding further DNA and releasing the replication machinery.

4. Proofreading

• High fidelity DNA synthesis is maintained by proofreading mechanisms.

• All three prokaryotic DNA polymerases possess $3' \rightarrow 5'$ exonuclease activity, which allows them to excise incorrectly incorporated mismatched bases prior to continuing chain extension.

Inhibitors of DNA Replication

• Prokaryotic Topoisomerase (DNA Gyrase) Inhibitors:

  • Agents: Novobiocin, Nalidixic acid, Ciprofloxacin.

  • Application: Widely used as antibiotics to treat urinary tract infections (UTIs) and other systemic bacterial infections.

• Human Topoisomerase Inhibitors:

  • Agents: Camptothecin, Adriamycin, Etoposide, Doxorubicin.

  • Application: Widely used as antitumor and anticancer chemotherapeutic drugs.

• Chain Elongation Inhibitors:

  • Agents: Nucleotide analogs such as $2',3'$ ext{-dideoxyinosine}.

  • Application: Inhibit DNA chain extension; used as anticancer and antiviral agents.

Comparative Summary: Prokaryotes vs. Eukaryotes in Replication

Transcription

Overview and Fundamental Concepts

• Definition: The process of synthesizing a complementary, single-stranded mRNA molecule from a double-stranded DNA template strand.

• Key Features:

  • Represents the first stage of protein biosynthesis and the first step leading to gene expression.

  • Synthesis proceeds exclusively in the $5' \rightarrow 3'$ direction.

  • Catalyzed by RNA Polymerase, which reads the DNA template strand and builds an antiparallel, complementary RNA sequence.

  • Unlike DNA polymerase, RNA polymerase does not require a primer to initiate synthesis.

  • Only one strand of DNA (the template / antisense strand) is transcribed.

Functional Types of RNA

• mRNA (Messenger RNA):

  • Carries genetic code transcribed from DNA in the form of three-nucleotide sequences called codons.

  • Serves as the protein-synthesis template during translation.

• rRNA (Ribosomal RNA):

  • Combines with structural proteins and enzymes in the cytoplasm to assemble ribosomes.

  • Forms the catalytic core and physical site for protein synthesis.

• tRNA (Transfer RNA):

  • Acts as an adapter molecule during translation.

  • Reads mRNA codons via complementary anticodons and transfers specific amino acids to the growing polypeptide chain.

Prokaryotic vs. Eukaryotic Transcription Differences

• Prokaryotes:

  • Location: Cytoplasm.

  • Coupling: Coupled transcription-translation is the standard rule (translation begins while transcription is still ongoing).

  • Cell Cycle: No specific phase timing.

  • Polymerases: A single RNA polymerase synthesizes all three major RNA types (mRNA, tRNA, rRNA).

• Eukaryotes:

  • Location: Nucleus (separated from cytoplasm by the nuclear membrane).

  • Coupling: Coupled transcription-translation is impossible due to compartmentalization.

  • Cell Cycle: Occurs primarily during $G_1$ and $G_2$ phases.

  • Polymerases:

  • RNA Polymerase I: Synthesizes rRNA.

  • RNA Polymerase II: Synthesizes mRNA (and pre-mRNA).

  • RNA Polymerase III: Synthesizes tRNA (and 5S rRNA).

  • Chromatin State Constraints: Eukaryotic DNA stored as condensed heterochromatin wrapped around histones in nucleosomes must be unwound into relaxed euchromatin to become accessible for transcription.

Chromatin States in Eukaryotes

• Heterochromatin:

  • Tightly packed, highly condensed structure; appears dark and dense under microscopic staining.

  • Transcriptionally inactive (contains permanently silenced genes).

  • Rich in non-coding repetitive sequences (e.g., satellite DNA).

• Euchromatin:

  • Less condensed, relaxed structure; appears lighter and less dense.

  • Transcriptionally active (rich in protein-coding genes and regulatory sequences).

  • Dynamic state (can open or condense in response to cellular cues).

Steps of Transcription

1. Initiation

• RNA Polymerase binds to a specific region on DNA called the promoter site, signaling the start of transcription.

• Requires a sigma factor ($\rightarrow$) in prokaryotes for target promoter recognition.

• Prokaryotic RNA Polymerase Core/Holoenzyme Subunits: Contains 5 core subunits:

  • $2\rightarrow$ subunits

  • $\beta$ subunit

  • $\beta'$ subunit

  • $\rightarrow$ subunit

• Complex Formation:

  • Closed Complex: Polymerase bound to the promoter double-stranded DNA.

  • Open Complex: Polymerase unwinds the double helix, creating a transcription bubble ($10\text{--}20$ base pairs melted) exposing the template strand.

2. Sense vs. Antisense Strands

• Sense Strand (Non-Template Strand / Coding Strand):

  • Runs $5' \rightarrow 3'$.

  • Has the same sequence as the newly synthesized mRNA (except DNA has Thymine where mRNA has Uracil).

• Antisense Strand (Template Strand):

  • Runs $3' \rightarrow 5'$.

  • Used directly as the physical template for RNA synthesis.

  • Joined to the sense strand by weak hydrogen bonds.

3. Elongation

• RNA Polymerase advances downstream along the template strand ($3' \rightarrow 5'$ direction on template), synthesizing RNA in the $5' \rightarrow 3'$ direction.

• Sequentially incorporates complementary ribonucleoside triphosphates (rNTPs):

  • Adenine ($A$) pairs with Uracil ($U$)

  • Thymine ($T$) pairs with Adenine ($A$)

  • Guanine ($G$) pairs with Cytosine ($C$)

  • Cytosine ($C$) pairs with Guanine ($G$)

• Forms phosphodiester linkages between adjacent rNTPs.

• Unwinds $10\text{--}20$ DNA bases at a time; nascent RNA peels away as the DNA double helix rewinds behind the enzyme.

4. Termination

• Polymerase reaches a specific DNA sequence called a terminator site (e.g., sequence $AAUAAA$ or hairpin loop structure).

• The secondary hairpin structure impairs the progress of RNA polymerase.

• RNA Polymerase detaches, and the completed primary RNA transcript is released.

Post-Transcriptional Processing of mRNA (Eukaryotes)

• Pre-mRNA Transcript: The initial unmodified RNA product containing both non-coding and coding sequences.

• Introns vs. Exons:

  • Introns: Non-coding "nonsense" sequences interspersed between coding regions. Must be excised.

  • Exons: Expressed coding sequences that contain the functional information for protein synthesis.

• Splicing: Process carried out in the nucleus where introns are precisely cut out and exons are spliced together.

• 5' Cap Addition: Modified guanine nucleotide attached to the $5'$ end of pre-mRNA to protect against degradation and assist ribosome binding.

• 3' Poly-A Tail Addition: Polyadenylation sequence consisting of multiple adenine nucleotides added to the $3'$ end for stability and nuclear export.

• Pre-mRNA remains inside the nucleus; only fully processed mature mRNA exits into the cytoplasm.

Final Product Structural Organization

• Prokaryotic mRNA Structure: $5' \text{ Untranslated Region} \rightarrow \text{Ribosome-Binding Site} \rightarrow \text{Coding Sequence} \rightarrow \text{Untranslated Region } 3'$.

• Eukaryotic mRNA Structure: $5' \text{ Cap} \rightarrow \text{Untranslated Region} \rightarrow \text{Start Codon (AUG)} \rightarrow \text{Coding Sequence} \rightarrow \text{Stop Codon (UAG/UAA/UGA)} \rightarrow \text{Untranslated Region} \rightarrow \text{Poly(A) Tail } 3'$.

Translation

Overview and Location

• Definition: Decoding of mRNA sequence into a specific amino acid sequence to construct a polypeptide chain.

• Location: Ribosomes located in the cytoplasm.

• Key Player: Transfer RNA (tRNA) acts as the deciphering interpreter of codons.

Key Components and Ribosomal Binding Sites

• Components: Small and large ribosomal subunits, mRNA, charged initiator tRNA carrying Methionine ($Met$), initiation factors, and $GTP$ (energy source).

• Charged tRNA: A tRNA molecule covalently bonded to its specific amino acid.

• Codons and Anticodons:

  • Start Codon: $AUG$ (codes for Methionine).

  • Start Anticodon: $UAC$ (complementary on tRNA).

  • Stop Codons: $UAA$, $UAG$, $UGA$ (do not code for amino acids).

• Ribosomal Functional Sites:

  • A-site (Acceptor Site): Accepts incoming aminoacyl-tRNA molecules.

  • P-site (Donor / Peptidyl Site): Holds the tRNA linked to the growing polypeptide chain.

  • E-site (Exit Site): Releases uncharged tRNAs leaving the ribosome.

Four Main Phases of Translation

1. Initiation

• Initiation factors $IF_1$ and $IF_2$ bind to the free prokaryotic 30S small ribosomal subunit.

• $IF_3$ complexed with $GTP$ binds the small subunit, promoting binding of the mRNA and charged initiator tRNA ($Met\text{-tRNA}$).

• Small subunit attaches at the $5'$ end of mRNA, recognizing a specific ribosome-binding sequence upstream of the start codon.

• Initiator tRNA ($UAC$ anticodon) pairs with the start codon ($AUG$) at the P-site.

• Arrival of the large ribosomal subunit completes assembly of the translation initiation complex (powered by $GTP$ hydrolysis).

2. Elongation

• Codon Recognition: Incoming aminoacyl-tRNA binds to the complementary mRNA codon at the A-site (facilitated by elongation factors $EF\text{-Tu}$, $EF\text{-Ts}$, $EF\text{-G}$ / $EF\text{-T4}$, $EF\text{-T5}$, $EF\text{-G}$ and $GTP$).

• Peptide Bond Formation: Ribosomal RNA (peptidyl transferase activity) catalyzes peptide bond formation between the amino acid in the A-site and the carboxyl terminal of the growing polypeptide chain in the P-site. The polypeptide chain is transferred to the tRNA in the A-site.

3. Translocation

• The ribosome moves along the mRNA chain by one codon in the $5' \rightarrow 3'$ direction.

• The tRNA carrying the polypeptide chain in the A-site is moved to the P-site.

• The empty tRNA in the P-site shifts to the E-site and exits the ribosome.

• The A-site is now clear to receive the next incoming aminoacyl-tRNA.

4. Termination

• The ribosome reaches a stop codon ($UAA$, $UAG$, or $UGA$) on the mRNA.

• Release Factors enter the A-site instead of a tRNA:

  • $RF_1$: Recognizes $UAA$ and $UAG$.

  • $RF_2$: Recognizes $UAA$ and $UGA$.

  • $RF_3$: Assists $RF_1$ and $RF_2$ in mediating reaction efficiency.

• The release factor hydrolyzes the bond between the completed polypeptide chain and the tRNA in the P-site.

• The freed polypeptide is released, and ribosomal subunits dissociate.

Key Eukaryotic Translation Differences

• Initiation: Involves at least 9 distinct eukaryotic initiation factors ($eIFs$). Eukaryotic initiator tRNA is not formylated (unlike prokaryotic $fMet\text{-tRNA}$).

• Elongation: Utilizes eukaryotic elongation factors $eEF1\rightarrow$, $eEF1\beta$, and eEF2$.\n- **Termination**: Contains a single eukaryotic release factor (eRF$) that recognizes all three stop codons ($UAA$,$UAG$,$UGA).\n\n# Practical Exercises & Post-Test Assessment\n\n## Unscramble Exercises\n1. **EDYRIOLXCEUBINCO DCIA** ightarrow **DEOXYRIBONUCLEIC ACID**\n2. **ERILCUBINCO DCIA** ightarrow **RIBONUCLEIC ACID**\n3. **MTATELEP ARDNTS** ightarrow **TEMPLATE STRAND**\n4. **NAR OEESRMAYPL** ightarrow **RNA POLYMERASE**\n\n## DNA to RNA Transcription Exercises\n\n- **Rules**: DNA base pairing (A ext{--}T, G ext{--}C$) translates to RNA base pairing ($A ightarrow U$,$T ightarrow A$,$G ightarrow C$,$C ightarrow G).\n\n- **Exercise #1**:\n - Non-template strand (5' ightarrow 3'): `5' A T C G A C G A T C 3'`\n - Template strand (3' ightarrow 5'): `3' T A G C T G C T A G 5'`\n - **Transcribed mRNA (5' ightarrow 3')**: `5' A U C G A C G A U C 3'`\n\n- **Exercise #2**:\n - Non-template strand (5' ightarrow 3'): `5' C T G C A T C A A G 3'`\n - Template strand (3' ightarrow 5'): `3' G A C G T A G T T C 5'`\n - **Transcribed mRNA (5' ightarrow 3')**: `5' C U G C A U C A A G 3'`\n\n- **Exercise #3**:\n - Given DNA strand (3' ightarrow 5'): `3' C T G C A T C A A G 5'`\n - Complementary DNA strand (5' ightarrow 3'): `5' G A C G T A G T T C 3'`\n - **Transcribed mRNA (from 3' ightarrow 5' template)**: `5' G A C G U A G U U C 3'`\n\n- **Exercise #4**:\n - Given DNA strand (3' ightarrow 5'): `3' A T C A G C A T G G 5'`\n - Complementary DNA strand (5' ightarrow 3'): `5' T A G T C G T A C C 3'`\n - **Transcribed mRNA (from 3' ightarrow 5' template)**: `5' U A G U C G U A C C 3'`\n\n- **Exercise #1.1**:\n - Given DNA strand (3' ightarrow 5'): `3' T A G C A A A G T C 5'`\n - Complementary strand (5' ightarrow 3'): `5' A T C G T T T C A G 3'`\n - **Transcribed mRNA (from 3' ightarrow 5' template)**: `5' A U C G U U U C A G 3'`\n\n- **Exercise #1.2**:\n - Given non-template strand (5' ightarrow 3'): `5' C A G T G G T C A C 3'`\n - Template strand (3' ightarrow 5'): `3' G T C A C C A G T G 5'`\n - **Transcribed mRNA (5' ightarrow 3'$$)**: 5' C A G U G G U C A C 3'

Post-Test Review Questions & Verified Answers

1. What is the primary function of transcription in cells?

   • A) DNA replication

   • B) Protein synthesis

   • C) RNA synthesis

   • D) Cell division

   • Answer: C) RNA synthesis

2. Which enzyme is primarily responsible for synthesizing RNA during transcription?

   • A) DNA polymerase

   • B) RNA polymerase

   • C) Ligase

   • D) Helicase

   • Answer: B) RNA polymerase

3. During transcription, the DNA strand that is used as a template is called the:

   • A) Coding strand

   • B) Non-template strand

   • C) Sense strand

   • D) Template strand

   • Answer: D) Template strand

4. In eukaryotic cells, where does transcription take place?

   • A) Cytoplasm

   • B) Ribosome

   • C) Nucleus

   • D) Mitochondria

   • Answer: C) Nucleus

5. What is the process called that modifies the primary RNA transcript in eukaryotes?

   • A) Translation

   • B) Splicing

   • C) Translocation

   • D) Replication

   • Answer: B) Splicing

6. What is the primary function of transcription in cells?

   • A) DNA replication

   • B) Protein synthesis

   • C) RNA synthesis

   • D) Cell division

   • Answer: C) RNA synthesis

7. Which of the following is NOT a component of RNA?

   • A) Ribose

   • B) Thymine

   • C) Uracil

   • D) Phosphate

   • Answer: B) Thymine

8. What is the role of the promoter in transcription?

   • A) It terminates transcription.

   • B) It signals the start of transcription.

   • C) It codes for the protein.

   • D) It unwinds the DNA.

   • Answer: B) It signals the start of transcription.

9. Which type of RNA carries the genetic information from DNA to the ribosome?

   • A) rRNA

   • B) tRNA

   • C) mRNA

   • D) snRNA

   • Answer: C) mRNA

10. What is the term for the sequence of nucleotides in DNA that signals the end of transcription?

    • A) Enhancer

    • B) Terminator

    • C) Promoter

    • D) Exon

    • Answer: B) Terminator