DNA Replication and Structure

• DNA consists of four nitrogenous bases (adenine, thymine, guanine, cytosine), deoxyribose sugar, and a phosphate group, according to Levene's Nucleotides (1920s)
• Chargaff's Rule (1940s) states Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C)
• Rosalind Franklin discovered that phosphate units are located externally on the DNA molecule
• Watson-Crick Model describes the double-helix structure, where two strands wind around each other, forming a twisted ladder, where the backbone is alternating sugar and phosphate molecules.
• Nitrogenous base pairs are held together by hydrogen bonds (A-T and G-C)
• Central dogma of molecular biology flow: DNA → RNA → Protein
• DNA replication is the process of making identical copies of DNA before cell division

Overview of DNA Replication

• DNA replication is semiconservative, meaning one strand from the parent DNA is retained in each daughter strand

Detailed Enzyme Roles in DNA Replication

• Helicase unzips the DNA by breaking hydrogen bonds between base pairs, creating two single strands
• Single-Strand Binding Proteins (SSBPs) prevent the two single strands from reannealing
• DNA Topoisomerase (DNA Gyrase) relieves supercoiling and tension in the DNA strands at the replication fork
• Primase synthesizes RNA primers to provide a starting point for DNA synthesis
• DNA Polymerase III adds complementary nucleotides to the parent strand in the 5' to 3' direction, and is responsible for elongating the new DNA strand
• DNA Polymerase I removes RNA primers and replaces them with DNA
• . DNA Ligase seals gaps between Okazaki fragments on the lagging strand
• Proofreading involves DNA polymerase enzymes correcting errors during replication to ensure fidelity

Summary of DNA Replication Process

• Helicase separates the DNA strands through unwinding
• Primase creates primers, and DNA polymerase adds nucleotides during base pairing
• Ligase seals nicks and completes the new DNA strands during joining

Transcription

• Transcription is the process of creating mRNA from a DNA template

Steps in Transcription:

• RNA polymerase binds to the promoter region near the start of a gene, separating the DNA strands to expose the template strand
• During elongation, RNA polymerase synthesizes an RNA strand using the DNA template, and nucleotides are added in the 5' to 3' direction
• Transcription ends when terminator sequences are transcribed, and the mRNA strand is released and exits the nucleus during termination

Key factors in Transcription

• RNA Polymerase catalyzes the synthesis of the mRNA strand
• A DNA sequence that signals the start of transcription is called a promoter
• Transcription Factors bind to the promoter and help RNA polymerase initiate transcription
• Terminator Sequence signals the end of transcription

Translation

• Translation is the process of synthesizing proteins from mRNA

Steps in Translation

• mRNA binds to the ribosome during initiation, and the start codon (AUG) signals the beginning of translation
• During elongation, tRNA molecules bring amino acids to the ribosome, each tRNA anticodon pairs with the corresponding mRNA codon, and peptide bonds form between amino acids to create a growing polypeptide chain
• A stop codon (UAA, UAG, or UGA) signals the end of translation and the completed protein is released during termination

Key components in Translation

• Ribosomes facilitate the binding of mRNA and tRNA
• tRNA (Transfer RNA) delivers the appropriate amino acids to the ribosome
• Peptidyl Transferase catalyzes the formation of peptide bonds
• Codon is a three-nucleotide sequence on mRNA that specifies an amino acid
• Anticodon is a complementary three-nucleotide sequence on tRNA that pairs with the codon

Mutations

• Mutations result from nondisjunction (failure of chromosomes to separate)

Examples of Mutations

• Klinefelter's Syndrome exhibits an XXY chromosome pattern
• Turner's Syndrome exhibits a single X chromosome
• Down Syndrome exhibits an extra chromosome 21 (Trisomy 21)

Structural Mutations

• Point mutations change a single nucleotide
  • Nonsense mutations cause early termination of protein synthesis
  • Missense mutations code for a different amino acid
  • Same-sense mutations cause no change in the amino acid sequence
• Frameshift mutations are insertions or deletions that shift the reading frame of the mRNA

Cell Types Affected by Mutations

• Somatic mutations occur in body cells (not passed to offspring)
• Germline mutations occur in sex cells (heritable)

Explore DNA replication, structure, and key experiments. Learn about nitrogenous bases, Chargaff's rule, and the Watson-Crick model. Understand the roles of enzymes like helicase.