Chapter 10: DNA Duplication

Processes of DNA Duplication

DNA duplication, or replication, is the process by which a cell makes an identical copy of its DNA. This is a semiconservative process, meaning each new DNA molecule consists of one old (template) strand and one newly synthesized strand. The process involves three main phases:

• Initiation: The DNA double helix is unwound and separated at specific points called origins of replication.

• Elongation: New DNA strands are synthesized by DNA polymerase using the original strands as templates.

• Termination: Replication is completed and the two new DNA molecules are separated.

Central Dogma of Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• DNA: Stores genetic instructions in a stable, double-helical structure. It functions as the blueprint for the cell.

• RNA: Acts as messenger and regulatory molecule. It is typically single-stranded and has roles in transcription and translation.

• Proteins: Perform most of the work in a cell, serving as enzymes, structural components, and signaling molecules. Their function is crucial to all cellular activities.

• Transcription: The process of synthesizing an RNA molecule from a DNA template.

• Translation: The process of synthesizing a protein from an mRNA template.

Four Critical Characteristics of Genetic Material

1. Replication: It must be able to be copied accurately.

2. Storage: It must contain all the information to build and maintain an organism.

3. Expression: It must be able to change over time through mutation as a phenotype.

4. Variation: It must be able to change over time during evolution.

Evidence for DNA as Genetic Material

• Griffith's Experiment: Showed that a "transforming principle" from heat-killed pathogenic bacteria could make non-pathogenic bacteria deadly.

• Avery–MacLeod–McCarty Experiment: Demonstrated that DNA, not protein, was the transforming principle by showing that only DNA-degrading enzymes prevented the transformation.

• Hershey–Chase Experiment: Used radioactive sulfur to label protein and radioactive phosphorus to label DNA in bacteriophages, proving that DNA was transferred into the host to direct viral replication.

Chemistry of Nucleic Acids

• Nucleic acids are polymers made of repeating units called nucleotides.

• Each nucleotide consists of three parts: a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (A, T, C, G, or U).

• The sugar and phosphate groups form the sugar-phosphate backbone, while the nitrogenous bases extend inward.

Structure of DNA and Its Function

• DNA is a double helix composed of two antiparallel strands.

• The strands are held together by hydrogen bonds between complementary base pairs: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• This structure allows DNA to store information stably and to be easily replicated by separating the two strands.

Structure of RNA

• RNA is typically a single-stranded molecule.

• It contains the sugar ribose instead of deoxyribose.

• RNA uses uracil (U) instead of thymine (T). This means A pairs with U in RNA.

• The single-stranded nature allows RNA to fold into complex 3D structures with diverse functions.

Chapter 11: DNA Replication

Central Dogma (revisited)

The central dogma is the foundational concept that genetic information flows from DNA to RNA to protein.

Semiconservative DNA Replication

In this model, each new DNA molecule is composed of one parental strand and one newly synthesized strand.

Origin of Replication, Replication Fork, and Bidirectional Replication

• Origin of replication: A specific sequence where DNA replication begins.

• Replication fork: The Y-shaped structure formed when the DNA double helix is unwound for replication.

• Bidirectional replication: Replication proceeds in both directions away from the origin of replication.

Enzymes Involved in DNA Replication

• DNA helicase: Unwinds and separates the double helix.

• DNA primase: Synthesizes a short RNA primer to start synthesis.

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

• DNA ligase: Joins Okazaki fragments on the lagging strand.

• Topoisomerase/gyrase: Relieves tension ahead of the replication fork.

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously in the 5' to 3' direction, following the replication fork.

• Lagging strand: Synthesized discontinuously as a series of short fragments (Okazaki fragments), later joined by DNA ligase.

Prokaryotic vs. Eukaryotic DNA Replication

• Prokaryotes: Have a single, circular chromosome with one origin of replication.

• Eukaryotes: Have multiple, linear chromosomes with many origins of replication.

Telomeres and Chromosome Stability

• Telomeres: Protective DNA sequences at the ends of eukaryotic chromosomes. They prevent the loss of genes during replication and protect chromosomes against degradation.

• Telomerase: An enzyme that extends telomeres, allowing cells to divide for longer periods.

• Loss of telomere length is associated with aging and cancer.

Holliday Structure

A branched nucleic acid structure that forms during genetic recombination and/or certain types of DNA repair. It is an intermediate where two DNA molecules are joined together, facilitating the exchange of genetic information.

Chapters 13 & 14: Gene Expression

Central Dogma (re-visited)

The flow of genetic information from DNA (transcription) to RNA (translation) to protein.

Directionality of Genetic Information

The flow is unidirectional, meaning information from a protein cannot be transferred back to a nucleic acid.

Genetic Code and Its Uses

• The genetic code is a set of rules that translates nucleotide sequences in mRNA into amino acid sequences.

• Triplet nature: Three bases (codon) specify one amino acid.

• Degeneracy: Most amino acids are specified by more than one codon.

• Wobble hypothesis: The third base of a codon can sometimes pair non-conventionally with the first base of an anticodon, allowing fewer tRNAs to recognize all codons.

Transcription

• RNA polymerase binds to a promoter region to initiate transcription.

• It then moves along the DNA, synthesizing a complementary RNA strand (elongation), until it reaches a terminator sequence (termination).

Transcription in Prokaryotes vs. Eukaryotes

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm.

• Eukaryotes: Transcription occurs in the nucleus, and translation occurs in the cytoplasm. Eukaryotic transcripts undergo processing before translation.

RNA Processing in Eukaryotes

• Splicing: Non-coding segments (introns) are removed from the mRNA, and coding segments (exons) are joined together.

• Poly-A tail addition: A long chain of adenine nucleotides is added to the 3' end, also for protection and transport.

Post-transcriptional Processing by RNA Editing

• RNA editing can change the nucleotide sequence of an mRNA after transcription, which can change the resulting protein sequence.

Ribosomes and mRNA in Translation

• Ribosomes: Molecular machines composed of rRNA and proteins. They serve as the site of protein synthesis.

• tRNA (transfer RNA): Small RNA molecules that carry specific amino acids and have an anticodon that recognizes a specific mRNA codon.

Three Phases of Translation

1. Initiation: The ribosome assembles about mRNA, tRNA, and initiator tRNA; one recognizes an initiation codon.

2. Elongation: The ribosome moves along the mRNA, adding amino acids one by one to the growing polypeptide chain.

3. Termination: A stop codon is reached, and a release factor binds, causing the polypeptide to be released and the ribosomal complex to dissociate.

Key Characteristics of the Functional Ribosome

• Composed of two subunits (small and large).

• Has binding sites for mRNA and tRNA.

• Catalyzes the formation of peptide bonds.

Eukaryotic Regulation of Translation

• Often involves eukaryotic initiation factors (eIFs) that control the binding of the ribosomal complex to the mRNA.

Defective Genes and Genetic Disorders

• A mutation in a gene can lead to a defective or non-functional protein or enzyme. This defective protein can disrupt a metabolic pathway or cellular process, resulting in a genetic disorder (e.g., cystic fibrosis, which is caused by a mutation in the CFTR gene).

One-gene-one polypeptide chain hypothesis

This concept states that a single gene typically codes for a single polypeptide chain. Many proteins are made of more than one polypeptide or "gene product"; however, these proteins are composed of multiple polypeptide chains coded by different genes.

Four Levels of Protein Structure

• Primary: The linear sequence of amino acids.

• Secondary: Local folding into structures like alpha-helices or beta-pleated sheets.

• Tertiary: The overall 3D shape of a single polypeptide chain.

• Quaternary: The arrangement of multiple polypeptide chains into a functional protein complex.

Example Table: DNA vs. RNA

Key Equations and Concepts

• Chargaff's Rule: In double-stranded DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine:

• Central Dogma:

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard genetics curriculum.