2.4 Chromosomes, DNA Replication & Protein Synthesis

2.4 Chromosomes, DNA Replication & Protein Synthesis

Molecular genetics dictates how genetic information is encoded, duplicated, and expressed in cellular systems. The Central Dogma of Molecular Biology ($DNA \rightarrow RNA \rightarrow Protein$) is a major focal point in the AMC Initial Test.

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1. Chromosome Structure & Chromatin Packaging

Eukaryotic genomic DNA is organized into complex structures within the nucleus:

• Nucleosome: The fundamental structural repeating unit of chromatin. Consists of approximately 146 base pairs of double-stranded DNA wrapped $1.65$ turns around a histone octamer core (two molecules each of histones H2A, H2B, H3, and H4). Histone H1 acts as a linker histone securing DNA at the entry/exit site.
• Chromatin Types:
  • Euchromatin: Lightly stained, loosely condensed, transcriptionally active chromatin.
  • Heterochromatin: Darkly stained, highly condensed, transcriptionally inactive chromatin (e.g., Barr body in female X-inactivation).
• Chromosome Morphology: During metaphase, chromosomes consist of two identical sister chromatids joined at the centromere. Protein structures called kinetochores assemble at the centromere for spindle fiber attachment.

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2. DNA Structure & Semi-Conservative Replication

Described by Watson and Crick (1953), DNA is a double-stranded antiparallel helix. Adenine pairs with Thymine ($A=T$ via 2 hydrogen bonds) and Guanine pairs with Cytosine ($G \equiv C$ via 3 hydrogen bonds). Nucleotides within a strand are linked by $3'\text{--}5'$ phosphodiester bonds.

Meselson & Stahl Experiment (15N / 14N Isotope Labeling):
Parental Strand (15N-15N) ──> Generation 1: 100% Hybrid (15N-14N)
                         ──> Generation 2: 50% Hybrid (15N-14N) + 50% Light (14N-14N)
(Proved Semi-Conservative Replication: Each daughter helix retains 1 original & 1 new strand)

Key Enzymes of DNA Replication:

Asymmetric Replication Fork Mechanics:

• Leading Strand: Continuous synthesis moving toward the replication fork in the $5' \rightarrow 3'$ direction using one RNA primer.
• Lagging Strand: Discontinuous synthesis moving away from the replication fork, forming short fragments called Okazaki fragments ($1000\text{--}2000$ nucleotides in prokaryotes; $100\text{--}200$ in eukaryotes), each requiring its own RNA primer.

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3. Transcription: DNA to RNA

Transcription is the enzymatic synthesis of RNA from a DNA template by RNA Polymerase.

A. Transcription Steps:

1. Initiation: RNA Polymerase binds to specific promoter sequences on DNA (e.g., TATA box / Goldberg-Hogness box at $-25$ in eukaryotes; $-10$ Pribnow box in prokaryotes). Transcription factors facilitate binding.
2. Elongation: RNA Polymerase reads the template strand ($3' \rightarrow 5'$) and synthesizes pre-mRNA in the $5' \rightarrow 3'$ direction using ribonucleotides ($A, U, G, C$).
3. Termination: Reaches a termination signal (Rho-dependent or Rho-independent hairpin loop in bacteria; polyadenylation signal in eukaryotes).

B. Post-Transcriptional Modifications (Eukaryotic pre-mRNA Processing):

• $5'$ Capping: Addition of a 7-methylguanosine cap to the $5'$ end to protect against exonucleases and aid ribosome binding.
• $3'$ Polyadenylation: Addition of a Poly-A tail ($100\text{--}250$ adenine nucleotides) to the $3'$ end for nuclear export and stability.
• RNA Splicing: Removal of non-coding intervening sequences (introns) and ligation of coding sequences (exons) carried out by snRNPs forming the spliceosome.

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4. Translation & The Genetic Code

Translation converts the nucleotide sequence of mature mRNA into a linear amino acid polypeptide sequence.

Ribosomal Binding Sites during Translation Elongation:
                   ┌──────────────────────────────┐
                   │   [ A Site ] ──> [ P Site ] ──> [ E Site ]   │
                   │ (Aminoacyl)    (Peptidyl)     (Exit)     │
                   └──────────────────────────────┘

A. Characteristics of the Genetic Code:

1. Triplet Code: 3 consecutive mRNA nucleotides form a codon coding for 1 amino acid ($4^3 = 64$ possible codons).
2. Degenerate / Redundant: Multiple codons can code for the same amino acid (61 sense codons for 20 amino acids).
3. Unambiguous: Each specific codon codes for only one particular amino acid.
4. Non-Overlapping & Universal: Codons are read sequentially without punctuation and shared across virtually all organisms.
5. Start & Stop Codons: AUG is the universal start codon (codes for Methionine). UAA, UAG, and UGA are stop codons (nonsense codons).

B. Steps of Translation:

• tRNA Charging: Aminoacyl-tRNA synthetase attaches specific amino acids to the $3'$-CCA stem of matching tRNA molecules using ATP.
• Initiation: Small ribosomal subunit (40S) binds mRNA at the $5'$ cap, scans for start codon AUG, and positions initiator tRNA ($Met-tRNA_i$). The large subunit (60S) joins to form the functional 80S complex.
• Elongation: Incoming aminoacyl-tRNA enters the A site (Aminoacyl). Peptidyl transferase (a ribozyme component of the large subunit) forms a peptide bond between the amino acid at the A site and the growing chain at the P site (Peptidyl). The ribosome translocates 1 codon down mRNA, shifting uncharged tRNA to the E site (Exit) for release.
• Termination: A stop codon (UAA, UAG, UGA) enters the A site. Release factors bind, hydrolyzing the bond between the polypeptide and tRNA, causing complete ribosomal complex dissociation.

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Biotechnology Essentials for AMC / MDCAT Overlap

Even though the AS&RC academic paper is short, FSc biotechnology items recur because they overlap NUMS MDCAT vocabulary:

• Recombinant DNA: cutting a gene with a restriction endonuclease, inserting it into a plasmid vector, and transforming a host (often E. coli).
• PCR (Polymerase Chain Reaction): in vitro amplification using denaturation, annealing, and extension cycles with a thermostable polymerase (classically Taq).
• Gel electrophoresis: separates DNA fragments by size in an agarose gel under an electric field (smaller fragments migrate farther).
• Gene therapy / GMOs: know the definition-level distinction between somatic and germline modification; exam items usually stay conceptual.

Treat these as definition + one-step process questions rather than laboratory protocols.