Molecular Basis of Inheritance

DNA: The Molecule of Inheritance

DNA (deoxyribonucleic acid) is the hereditary material in all living organisms, responsible for storing and transmitting genetic information. Genes are DNA sequences that encode functional RNA or protein products, while a genome is the complete set of genetic material in an organism.

• Nucleotide: The basic building block of DNA and RNA, consisting of a phosphate group, a sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• Chromosome: A long DNA molecule associated with proteins, forming the structure that carries genetic information.

• Histone: Protein around which eukaryotic DNA wraps, aiding in DNA packaging.

• Nucleosome: DNA wrapped around a core of histone proteins, forming the basic unit of chromatin structure.

Nucleotide Structure: Each DNA nucleotide contains a phosphate group, deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). RNA uses uracil (U) instead of thymine.

• Base-pairing rules: A pairs with T (two hydrogen bonds), G pairs with C (three hydrogen bonds).

• Chargaff's rules: In double-stranded DNA, A = T and G = C; the sum of all bases equals 100%.

• Example: If adenine is 18%, thymine is 18%, guanine and cytosine are each 32%.

The Double-Helix Model

James Watson and Francis Crick proposed the double-helix model in 1953, based on X-ray diffraction data (Rosalind Franklin, Raymond Gosling) and base composition studies (Erwin Chargaff).

• DNA consists of two polynucleotide strands twisted into a double helix.

• Sugar-phosphate backbones are on the outside; paired bases face inward.

• Strands are antiparallel: one runs 5′ → 3′, the other 3′ → 5′.

• DNA polymerase synthesizes new DNA in the 5′ → 3′ direction.

Classic Experiments Establishing DNA as Genetic Material

• Griffith's Transformation Experiment (1928): Showed that genetic material from dead S bacteria could transform live R bacteria.

• Avery, MacLeod, and McCarty (1944): Identified DNA as the transforming principle by showing that only DNase treatment prevented transformation.

• Hershey–Chase Experiment (1952): Used radioactive labeling to demonstrate that DNA, not protein, enters bacterial cells during viral infection, confirming DNA as genetic material.

DNA Replication

DNA replication is the process of copying DNA before cell division. It is semiconservative: each daughter DNA molecule contains one parental strand and one newly synthesized strand.

• Meselson–Stahl Experiment: Used nitrogen isotopes to show that DNA replication is semiconservative.

Major Replication Enzymes

• DNA polymerase requires a primer with a free 3′-OH group.

Leading and Lagging Strands

• DNA polymerase reads the template 3′ → 5′ while synthesizing the new strand 5′ → 3′.

Prokaryotic vs. Eukaryotic DNA Replication

Telomeres and Telomerase

• Telomeres: Repetitive DNA sequences at chromosome ends, protecting genetic material.

• Telomerase: Enzyme that extends telomeric DNA using its own RNA template; active in certain cell types and many cancers.

Transcription: DNA to RNA

Transcription is the synthesis of RNA from a DNA template. For protein-coding genes, the product is messenger RNA (mRNA).

• Central dogma: DNA → RNA → Protein.

• Replication: DNA → DNA.

• Transcription: DNA → RNA.

• Translation: mRNA → polypeptide.

Stages of Transcription

• Initiation: RNA polymerase recognizes a promoter, often with help from other proteins.

• Elongation: RNA polymerase reads the template 3′ → 5′ and synthesizes RNA 5′ → 3′.

• Termination: RNA synthesis ends at a termination signal.

Template vs. Coding Strand

• The coding strand matches the mRNA sequence (except T is replaced by U).

• The template strand is complementary to the mRNA.

RNA Processing in Eukaryotes

Pre-mRNA undergoes several modifications before translation:

• Addition of a 5′ cap (protects mRNA and aids translation).

• Addition of a 3′ poly-A tail (stabilizes mRNA).

• RNA splicing removes introns and joins exons; carried out by the spliceosome.

• Alternative splicing: Allows one gene to produce different mature RNA transcripts and protein isoforms.

RNA processing affects mRNA stability, export, translation efficiency, and protein diversity. Most bacterial mRNAs do not undergo this processing.

The Genetic Code

The genetic code is a set of rules by which information in mRNA is translated into proteins. A codon is a sequence of three nucleotides.

• Four bases in triplets yield codons: 61 specify amino acids, 3 are stop codons.

• Example: 5′-AUG GCU UUU UGA-3′ translates to methionine–alanine–phenylalanine; UGA is a stop codon.

Translation: RNA to Protein

Translation is the process by which mRNA is decoded to synthesize a polypeptide at the ribosome.

• Initiation: Ribosome assembles on mRNA and positions initiating tRNA.

• Elongation: Amino acids are added to the growing polypeptide.

• Termination: Stop codon is recognized; polypeptide is released.

Mutations

A mutation is a change in genetic material. Effects depend on the location and nature of the change.

• Insertions or deletions of one nucleotide shift the reading frame; multiples of three do not.

• Mutations can be harmful, neutral, or beneficial.

• Germ-cell mutations are heritable; somatic-cell mutations are not.

Gene Regulation: The lac Operon

The lac operon is a classic bacterial gene-regulation system for lactose utilization, responding to lactose and glucose levels.

• Without lactose: lac repressor binds operator, inhibiting transcription.

• With lactose: allolactose binds repressor, reducing its binding to operator, allowing transcription.

• Low glucose: cAMP levels rise, cAMP binds CAP/CRP, activating transcription.

• Lactose-derived allolactose relieves repression; low glucose favors activation.

The Human Genome Project

The Human Genome Project was an international effort to sequence and map the human genome, completed in 2003.

• A haploid human genome contains ~3 billion base pairs.

• Humans have ~20,000 protein-coding genes.

• Most of the genome is noncoding, but noncoding DNA can have regulatory, structural, or RNA-related functions.

• The project advanced disease-gene discovery, genomic medicine, genetic testing, evolutionary research, and DNA sequencing.

DNA Fingerprinting and Profiling

Modern forensic DNA profiling examines variable genomic regions such as short tandem repeats (STRs).

• Applications: forensic investigations, kinship testing, identification of human remains, population genetics, wildlife conservation.

• DNA evidence must be interpreted carefully; a match does not establish circumstances of deposition.

High-Yield Facts to Memorize

Common Exam Mistakes

• Transcription is DNA → RNA; translation is mRNA → protein.

• DNA polymerase synthesizes DNA 5′ → 3′, not 3′ → 5′.

• The coding DNA strand matches mRNA except T replaces U; template strand is complementary.

• Not every mutation changes a protein; some are silent or affect regulation.

• Semiconservative replication: each daughter DNA has one parental and one new strand.

• Ribosomes translate mRNA, not DNA.

• Not every gene encodes a protein; some produce functional RNA.

Practice Test (Selected Questions)

• Which enzyme separates the two strands of DNA during replication? Helicase

• A double-stranded DNA molecule contains 24% adenine. What percentage is guanine? 26%

• Which experiment supported semiconservative DNA replication? Meselson–Stahl

• Which enzyme synthesizes RNA during transcription? RNA polymerase

• What is the usual function of a stop codon? Signals the end of translation

• Which mutation changes the reading frame when one nucleotide is inserted? Frameshift

• Which RNA carries amino acids to the ribosome? tRNA

• What does DNA ligase do during replication? Seals breaks in the DNA backbone

• Which statement about eukaryotic RNA processing is correct? Splicing can remove introns

• In the lac operon, what happens when lactose is present and glucose is scarce? Expression is relatively high

Short Answer Concepts

• Semiconservative replication: Each daughter DNA molecule contains one original parental strand and one newly synthesized strand.

• Template vs. coding strand: RNA polymerase reads the template strand to synthesize complementary RNA; coding strand matches RNA sequence except T is replaced by U.

• Roles of mRNA, tRNA, rRNA: mRNA carries codons; tRNA carries amino acids and recognizes codons via anticodon; rRNA forms structural and catalytic part of ribosome.

• Frameshift mutation: A one-nucleotide insertion can shift the reading frame, changing many downstream codons and potentially creating a premature stop codon.

• Hershey–Chase experiment: Labeled phage DNA with ³²P and protein with ³⁵S; DNA label entered bacterial cells, supporting DNA as genetic instructions.

Final Revision Strategy

• Prioritize: DNA structure and base pairing; replication enzymes and direction; transcription, RNA processing, and translation; codons and mutations; classic experiments and the lac operon.

• Mastery check: Convert a DNA template sequence into mRNA, identify codons, and determine the resulting amino acid sequence.

Additional info: This study guide covers the core concepts of Chapter 16 (The Molecular Basis of Inheritance) and related topics from Chapters 17 and 18 (Gene Expression and Regulation), as outlined in standard General Biology college courses.