Study Guide: Molecular Structure, Replication, Transcription, and Translation of DNA and RNA

CHAP 9 — MOLECULAR STRUCTURE OF DNA AND RNA

Basic Components and Structural Arrangements

The molecular structure of DNA and RNA is fundamental to understanding genetics. Both molecules are polymers composed of nucleotides, but they differ in several key aspects.

• Nucleotides: The building blocks of DNA and RNA, each consisting of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• Nitrogenous Bases: DNA contains adenine (A), thymine (T), cytosine (C), and guanine (G); RNA contains adenine (A), uracil (U), cytosine (C), and guanine (G).

• Base Pairing: DNA forms double-stranded helices with complementary base pairing (A-T, C-G), while RNA is typically single-stranded but can form secondary structures.

• Major and Minor Groove: The double helix of DNA has major and minor grooves, which are important for protein binding.

• 5' and 3' Ends: Nucleic acid strands have directionality, with a 5' phosphate end and a 3' hydroxyl end.

• Antiparallel Structure: The two strands of DNA run in opposite directions.

• Base Stacking: Stabilizes the double helix through hydrophobic interactions between stacked bases.

Example: The complementary sequence for 5'-ATCG-3' is 3'-TAGC-5'.

Forms of DNA

DNA can exist in several structural forms, with B-DNA being the most common in cells.

• B-DNA: Right-handed helix, most prevalent under physiological conditions.

• Z-DNA: Left-handed helix, less common, may play roles in gene regulation.

Additional info: Z-DNA formation is favored by certain sequences and high salt concentrations.

RNA Secondary Structures

RNA molecules can fold into complex secondary structures, such as hairpins and loops, which are essential for their diverse functions.

• Base Pairing: RNA can form intramolecular base pairs, creating stem-loop structures.

• Function: These structures are critical for RNA's role in catalysis (ribozymes), regulation, and protein synthesis.

CHAP 11 — DNA REPLICATION

Semiconservative Model of DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. The semiconservative model states that each new DNA molecule consists of one old strand and one newly synthesized strand.

• Template Strands: Each original strand serves as a template for a new strand.

• Helicase: Unwinds the DNA double helix.

• Primase: Synthesizes short RNA primers needed for DNA polymerase to begin synthesis.

• DNA Polymerase: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

• Complementary Base-Pairing: Ensures accurate copying of genetic information.

Equation:

$\text{DNA}_{\text{new}} = \text{DNA}_{\text{template}} + \text{nucleotides}$

Leading vs. Lagging Strand Synthesis

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

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, which are later joined by DNA ligase.

Okazaki Fragments and Primers

• Okazaki Fragments: Short DNA segments synthesized on the lagging strand.

• Primers: Short RNA sequences required to initiate DNA synthesis.

• DNA Ligase: Joins Okazaki fragments to form a continuous strand.

Fidelity and Limitations of DNA Polymerases

• Proofreading: DNA polymerases have proofreading activity to correct errors.

• Limitations: DNA polymerases cannot initiate synthesis without a primer and cannot synthesize the ends of linear chromosomes (telomeres).

Telomeres and Telomerase

• Telomeres: Repetitive DNA sequences at chromosome ends that protect genetic information.

• Telomerase: An enzyme that extends telomeres, especially in germ cells and some cancer cells.

CHAP 12 — GENE TRANSCRIPTION AND RNA MODIFICATION

Transcription Process

Transcription is the synthesis of RNA from a DNA template. It involves initiation, elongation, and termination phases.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• RNA Polymerase: Enzyme that synthesizes RNA.

• Template Strand: The DNA strand used for RNA synthesis.

• Coding Strand: The non-template strand, which has the same sequence as the RNA (except T is replaced by U).

• Terminator: Sequence signaling the end of transcription.

Transcription in Prokaryotes vs. Eukaryotes

• Prokaryotes: Transcription and translation occur simultaneously; fewer regulatory elements.

• Eukaryotes: Transcription occurs in the nucleus; involves complex regulation with enhancers, silencers, and transcription factors.

RNA Processing and Modification

• Splicing: Removal of introns and joining of exons in eukaryotic pre-mRNA.

• Alternative Splicing: Allows a single gene to produce multiple protein variants.

• 5'-Capping: Addition of a methylated guanine cap to the 5' end of mRNA.

• Polyadenylation: Addition of a poly(A) tail to the 3' end of mRNA.

• RNA Editing: Chemical modification of RNA nucleotides after transcription.

Regulatory Elements in Eukaryotic Transcription

• Core Promoter / TATA Box: Essential for transcription initiation.

• Regulatory Elements: Enhancers and silencers modulate transcription levels.

• Transcription Factors: Proteins that bind DNA and regulate transcription.

• Mediator: Protein complex that facilitates interaction between transcription factors and RNA polymerase.

• Poly-A Signal: Sequence that signals addition of the poly(A) tail.

Ribozymes

• Definition: RNA molecules with catalytic activity.

• Example: The ribosome's peptidyl transferase activity is a ribozyme function.

CHAP 13 — TRANSLATION OF mRNA

The Genetic Code

The genetic code is the set of rules by which nucleotide sequences are translated into amino acid sequences in proteins.

• Codons: Triplets of nucleotides that specify amino acids.

• Universality: The genetic code is nearly universal among organisms.

• Wobble: Flexibility in base pairing at the third position of the codon allows for fewer tRNAs.

Equation:

$\text{Amino Acid Sequence} = \text{Codon Table}(\text{mRNA Sequence})$

Translation Process

• Initiation: Assembly of the ribosome on the mRNA and the first tRNA.

• Elongation: Addition of amino acids to the growing polypeptide chain.

• Termination: Release of the completed polypeptide when a stop codon is encountered.

Structural Features of tRNA and rRNA

• tRNA: Adaptor molecule that brings amino acids to the ribosome; contains an anticodon that pairs with mRNA codons.

• rRNA: Structural and catalytic component of ribosomes.

Shine-Dalgarno Sequence

• Definition: Ribosome binding site in prokaryotic mRNA, important for translation initiation.

• Contrast: Eukaryotes use the 5' cap structure for ribosome binding.

Directionality and Function

• Information Flow: DNA → RNA → Protein (Central Dogma of Molecular Biology).

• Structure-Function Relationship: Protein structure is determined by the sequence of amino acids encoded by genes.

Summary Table: Key Differences Between DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strandedness	Double-stranded	Single-stranded
Function	Genetic storage	Protein synthesis, regulation, catalysis