Molecular Biology of the Gene: Structure, Function, and Expression

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Molecular Biology of the Gene

The Structure of the Genetic Material

The molecular basis for inheritance was a major mystery in early 20th-century biology. Key experiments demonstrated that DNA, not protein, is the genetic material responsible for heredity.

Frederick Griffith's Experiment (1928): Showed that non-pathogenic bacteria could be transformed into pathogenic forms by exposure to heat-killed pathogenic bacteria, indicating a 'transforming principle' (later identified as DNA).
Hershey-Chase Experiment: Used bacteriophages (viruses that infect bacteria) to show that DNA, not protein, is injected into bacterial cells and directs viral replication.

Structure of a bacteriophage with labeled head, tail, tail fiber, and DNA Steps of the Hershey-Chase experiment showing phage infection and DNA injection Phage DNA directs host cell to make more phages, leading to cell lysis

DNA and RNA Are Polymers of Nucleotides

DNA and RNA are nucleic acids composed of long chains (polymers) of nucleotides. Each nucleotide consists of a nitrogenous base, a five-carbon sugar, and a phosphate group.

DNA Nucleotides: Adenine (A), Cytosine (C), Thymine (T), Guanine (G)
RNA Nucleotides: Adenine (A), Cytosine (C), Guanine (G), Uracil (U) (replaces thymine)
Sugar-Phosphate Backbone: Nucleotides are joined by covalent bonds between the sugar of one nucleotide and the phosphate of the next.
DNA vs. RNA: DNA contains deoxyribose sugar; RNA contains ribose sugar.

DNA double helix and nucleotide structure Structures of pyrimidines: thymine and cytosine Structures of purines: adenine and guanine Structure of an RNA nucleotide with uracil base RNA polynucleotide chain with labeled bases and sugars Comparison table of DNA and RNA components

DNA Is a Double-Stranded Helix

The three-dimensional structure of DNA was elucidated by Watson and Crick, who proposed that DNA consists of two polynucleotide strands wound into a double helix. The strands are held together by hydrogen bonds between complementary bases: A pairs with T, and G pairs with C.

Base Pairing: A–T and G–C pairing ensures accurate replication and information storage.
Genetic Information: Encoded in the sequence of nucleotides along each strand.

DNA double helix ribbon model, partial chemical structure, and computer model

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It relies on the principle of complementary base pairing and follows a semiconservative model.

Semiconservative Model: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.
Enzymes: DNA polymerase synthesizes new DNA strands; DNA ligase joins short fragments on the lagging strand.
Replication Fork: The site where DNA unwinding and synthesis occur.

Semiconservative DNA replication: parental strands serve as templates DNA replication fork with parental and daughter strands Multiple origins of replication and replication bubbles Leading and lagging strand synthesis with DNA polymerase and ligase

The Flow of Genetic Information: DNA → RNA → Protein

Genes control phenotypic traits by directing the synthesis of proteins. The flow of genetic information follows the central dogma: DNA is transcribed into RNA, which is then translated into protein.

Transcription: Synthesis of RNA from a DNA template.
Translation: Synthesis of a polypeptide (protein) from an mRNA template.
Gene: A region of DNA that can be expressed to produce a functional product (polypeptide or RNA molecule).

Flow of genetic information: DNA to RNA to protein Detailed diagram of transcription and translation

Genetic Code and Codons

The genetic code is a set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. Each codon (a sequence of three nucleotides) specifies a particular amino acid.

Triplet Code: Three nucleotides (codon) code for one amino acid.
Universality: The genetic code is nearly universal among organisms.

mRNA codons and corresponding amino acids Translation of codons into amino acids

DNA/RNA Codon	Amino Acid
UUU	Phenylalanine
AUG	Methionine (Start)
UAA, UAG, UGA	Stop
...	...
Additional info: See standard genetic code table for all codons.

Transcription and RNA Processing

Transcription occurs in the nucleus, where RNA polymerase synthesizes a complementary RNA strand from a DNA template. In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes processing before becoming mature mRNA.

Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
Terminator: Sequence signaling the end of the gene.
RNA Splicing: Removal of introns (noncoding regions) and joining of exons (coding regions).
Cap and Tail: Added to protect mRNA and facilitate export from the nucleus.

RNA processing: splicing, cap, and tail addition

Translation and the Role of tRNA and Ribosomes

Translation occurs in the cytoplasm, where ribosomes read the mRNA sequence and, with the help of tRNA molecules, assemble the corresponding amino acid chain.

tRNA: Transfers specific amino acids to the ribosome; contains an anticodon that pairs with mRNA codons.
Ribosome: Composed of rRNA and proteins; facilitates the binding of tRNA and catalyzes peptide bond formation.
Phases of Translation: Initiation, elongation, and termination.

tRNA structure with anticodon and amino acid attachment site Ribosome structure and function in translation Initiation of translation: assembly of ribosome, mRNA, and tRNA Steps of translation initiation Elongation and termination of translation

Mutations and Their Effects

Mutations are changes in the genetic information of a cell or virus. They can result from errors in DNA replication, recombination, or exposure to mutagens. Types of mutations include substitutions, insertions, and deletions, which can alter gene function and protein products.

Point Mutation: Substitution of a single nucleotide.
Frameshift Mutation: Insertion or deletion that shifts the reading frame.
Effects: Can be silent, missense, nonsense, or frameshift, depending on the change.

Types of mutations and their effects on protein synthesis Substitution mutation example Insertion mutation example Deletion mutation example

The Genetics of Viruses and Bacteria

Viruses and bacteria have unique mechanisms for genetic exchange and propagation.

Virus Structure: Consists of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid), sometimes with a membrane envelope.
Lytic Cycle: Viral DNA is replicated, transcribed, and translated, leading to host cell lysis and release of new viruses.
Lysogenic Cycle: Viral DNA integrates into the host genome as a prophage and is replicated along with host DNA.
Prions: Infectious proteins that cause disease by inducing misfolding of normal proteins.

Lytic and lysogenic cycles of a bacteriophage Prion-induced protein misfolding

Bacterial Gene Transfer Mechanisms

Bacteria can exchange genetic material through several mechanisms, contributing to genetic diversity and adaptation.

Transformation: Uptake of free DNA from the environment.
Transduction: Gene transfer mediated by bacteriophages.
Conjugation: Direct transfer of DNA between bacterial cells via a pilus.
Plasmids: Small, circular DNA molecules that can carry genes, including those for antibiotic resistance (R plasmids) or fertility (F factor).

Mechanisms of bacterial gene transfer: transformation, transduction, conjugation Plasmid transfer during conjugation Integration of plasmid DNA into bacterial chromosome

Additional info: This summary covers the core concepts of molecular genetics, including the structure and function of DNA and RNA, the processes of replication, transcription, and translation, the genetic code, mutations, and mechanisms of genetic exchange in viruses and bacteria. These topics are foundational for understanding heredity, gene expression, and biotechnology applications.