DNA Structure and Replication

DNA Structure

• Double helix model: two complementary strands of nucleotides twisted together
• Each nucleotide composed of:
  • Sugar molecule (deoxyribose)
  • Phosphate group
  • Nitrogenous base (A, C, G, or T)
• Hydrogen bonds between nitrogenous bases hold strands together

DNA Replication

• Semi-conservative process: each new DNA molecule contains one old strand and one new strand
• Steps:
  1. Unwinding of DNA double helix
  2. Binding of primers to template strands
  3. Synthesis of new DNA strands by DNA polymerase
  4. Proofreading and editing to correct errors
  5. Ligation of Okazaki fragments

Genetic Code

• Sequence of nitrogenous bases determines amino acid sequence of proteins
• Codons: sequences of three nucleotides that code for one amino acid
• Degeneracy: multiple codons can code for the same amino acid
• Start codon (AUG): initiates protein synthesis
• Stop codons (UAA, UAG, UGA): terminate protein synthesis

Gene Expression

• Transcription: DNA → RNA
  • Initiation: RNA polymerase binds to promoter region
  • Elongation: RNA polymerase reads template DNA and adds nucleotides to RNA
  • Termination: RNA polymerase releases RNA transcript
• Translation: RNA → protein
  • Initiation: ribosome binds to mRNA
  • Elongation: ribosome reads mRNA and adds amino acids to protein
  • Termination: ribosome releases completed protein

Mutation and Variation

• Point mutations: change in a single nucleotide
  • Substitution: replacement of one nucleotide with another
  • Insertion: addition of one or more nucleotides
  • Deletion: removal of one or more nucleotides
• Frameshift mutations: insertion or deletion of nucleotides, leading to altered reading frame
• Chromosomal mutations: changes in number or structure of chromosomes
• Gene duplication: duplication of a gene or chromosomal segment, leading to increased gene expression

DNA Structure

• DNA is organized as a double helix, consisting of two complementary strands of nucleotides twisted together
• Each nucleotide is composed of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base (A, C, G, or T)
• Hydrogen bonds between nitrogenous bases hold the two strands together

DNA Replication

• DNA replication is a semi-conservative process, resulting in two new DNA molecules each containing one old strand and one new strand
• The process involves unwinding of the DNA double helix, binding of primers to template strands, synthesis of new DNA strands by DNA polymerase, proofreading and editing to correct errors, and ligation of Okazaki fragments

Genetic Code

• The sequence of nitrogenous bases in DNA determines the amino acid sequence of proteins
• Codons are sequences of three nucleotides that code for one amino acid
• The genetic code is degenerate, meaning multiple codons can code for the same amino acid
• The start codon (AUG) initiates protein synthesis, while stop codons (UAA, UAG, UGA) terminate protein synthesis

Gene Expression

• Transcription is the process of DNA → RNA, involving initiation, elongation, and termination
  • Initiation: RNA polymerase binds to the promoter region
  • Elongation: RNA polymerase reads the template DNA and adds nucleotides to RNA
  • Termination: RNA polymerase releases the RNA transcript
• Translation is the process of RNA → protein, involving initiation, elongation, and termination
  • Initiation: the ribosome binds to mRNA
  • Elongation: the ribosome reads mRNA and adds amino acids to the protein
  • Termination: the ribosome releases the completed protein

Mutation and Variation

• Point mutations involve a change in a single nucleotide, resulting in substitution, insertion, or deletion
• Frameshift mutations occur when the insertion or deletion of nucleotides leads to an altered reading frame
• Chromosomal mutations involve changes in the number or structure of chromosomes
• Gene duplication results in the duplication of a gene or chromosomal segment, leading to increased gene expression