Molecular Basis of Inheritance and Gene Expression (Chapters 16-17) – Study Guide

Chapter 16: The Molecular Basis of Inheritance

Structure of DNA

The majority of DNA molecules are double-helical structures. The discovery of DNA's structure was a pivotal moment in biology, involving several key scientists:

• James Watson and Francis Crick: Proposed the double helix model of DNA.

• Rosalind Franklin: Used X-ray crystallography to produce images of DNA, revealing its helical structure.

• Maurice Wilkins: Assisted in X-ray diffraction studies and shared Franklin's data with Watson and Crick.

X-ray crystallography was crucial in determining the 3D structure of DNA by analyzing the patterns formed when X-rays are diffracted through DNA fibers.

Bacteriophages and the Hershey-Chase Experiment

Bacteriophages are viruses that infect bacteria. The Hershey-Chase experiment demonstrated that DNA, not protein, is the genetic material by using radioactive labeling to trace the fate of DNA and protein during viral infection of bacteria.

Chargaff’s Rule

Erwin Chargaff discovered that in any given species, the amount of Adenine (A) equals Thymine (T), and the amount of Cytosine (C) equals Guanine (G). This is summarized as:

• A = T

• C = G

Origins of Replication

DNA replication begins at specific sites called origins of replication:

• Prokaryotes: Typically have a single origin of replication.

• Eukaryotes: Have multiple origins of replication on each chromosome.

Replication Fork and Directionality

The replication fork is a Y-shaped region where new DNA strands are synthesized. DNA polymerase can only add nucleotides in the 5’ to 3’ direction, meaning new DNA strands elongate in this direction.

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.

• Antiparallel configuration: The two DNA strands run in opposite directions (5’ to 3’ and 3’ to 5’).

Major Players in DNA Replication

• Helicase: Unwinds the DNA double helix.

• Single-strand binding proteins: Stabilize unwound DNA.

• Primase: Synthesizes RNA primers.

• DNA polymerase: Adds nucleotides to the growing DNA strand.

• Ligase: Joins Okazaki fragments on the lagging strand.

Telomeres

Telomeres are repetitive nucleotide sequences at the ends of eukaryotic chromosomes that protect genes from being eroded during successive rounds of DNA replication.

Euchromatin vs. Heterochromatin

• Euchromatin: Less condensed, transcriptionally active chromatin.

• Heterochromatin: Highly condensed, transcriptionally inactive chromatin.

DNA and Histones

DNA wraps around histone proteins to form nucleosomes, which further condense to form chromosomes during cell division.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

• DNA → mRNA (transcription) → Protein (translation)

Chapter 17: Gene Expression – From Gene to Protein

Transcription and Translation

Transcription is the process by which a DNA sequence is copied into messenger RNA (mRNA). Translation is the synthesis of a polypeptide using the information in mRNA, occurring at the ribosome.

• Ribosomes: Sites of protein synthesis.

• tRNA (transfer RNA): Brings amino acids to the ribosome; contains an anticodon that pairs with mRNA codons.

• E-, P-, and A-sites: Sites on the ribosome where tRNA binds during translation (Exit, Peptidyl, Aminoacyl).

Major Players in Transcription and Translation

• RNA polymerase: Synthesizes RNA from a DNA template during transcription.

• Transcription factors: Proteins that help RNA polymerase bind to DNA.

• Ribosomal subunits: Assemble with mRNA and tRNA to begin translation.

mRNA Processing in Eukaryotes

Before leaving the nucleus, eukaryotic mRNA undergoes several modifications:

• 5’ cap: Added to the beginning of the mRNA.

• Poly-A tail: Added to the end of the mRNA.

• Splicing: Removal of non-coding sequences (introns); coding sequences (exons) are joined together.

Prokaryotes vs. Eukaryotes in Gene Expression

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm; no mRNA processing.

• Eukaryotes: Transcription occurs in the nucleus, mRNA is processed, then exported to the cytoplasm for translation.

Genetic Code and Codons

The genetic code is universal and redundant:

• Codons: Triplets of nucleotides in mRNA that specify amino acids.

• Redundancy: Multiple codons can code for the same amino acid.

Amino Acids, Polypeptides, Exons, and Introns

• Amino acids: Building blocks of proteins.

• Polypeptide chains: Linear chains of amino acids.

• Exons: Coding regions of a gene that are expressed.

• Introns: Non-coding regions removed during mRNA processing (not present in prokaryotes).

Alternative Splicing

Alternative splicing allows a single gene to code for multiple proteins by varying the combination of exons included in the final mRNA.

Key Equations and Concepts

• Chargaff’s Rule:

$[A] = [T]$ $[C] = [G]$

• Central Dogma:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info: This study guide covers the foundational concepts of DNA structure, replication, and gene expression, which are essential for understanding molecular genetics and biotechnology.