BIOL1000 Lecture #10

Lecture #8 Molecular Genetics

Nucleic Acids

  • Definition: Genetic material that carries information; includes DNA and RNA.

  • Genes: Encode the primary amino acid sequence of proteins.

  • DNA vs RNA:

    • DNA (Deoxyribonucleic acid): Contains the information for protein synthesis, must first be transcribed into RNA.

    • RNA (Ribonucleic acid): Acts as the intermediary between DNA and protein synthesis.

  • Processes:

    • Transcription: DNA → RNA

    • Translation: RNA → Protein

  • Result: RNA formation leads to the creation of the primary protein structure.

Structure of Nucleic Acids

  • Components of Nucleotides:

    1. Five carbon sugar (Deoxyribose in DNA, Ribose in RNA)

    2. Phosphate group

    3. Nitrogenous base (contains carbon and nitrogen)

  • Nitrogenous Bases in DNA and RNA:

    • DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

    • RNA: A, G, C, Uracil (U) replaces T

  • Formation of Nucleic Acids:

    • Linked via dehydration reactions forming a sugar-phosphate backbone.

Structure of DNA and RNA

  • Backbones:

    • Sugar-phosphate repeating structure.

    • RNA: single-stranded

    • DNA: double-stranded (double helix)

  • Base Pairing Rules:

    • Adenine pairs with Thymine (2 hydrogen bonds)

    • Cytosine pairs with Guanine (3 hydrogen bonds)

Genetic Variation

  • Length of Nucleic Acids:

    • Ranges from thousands to hundreds of thousands of nucleotides.

  • Pyrimidines vs Purines:

    • Pyrimidines (1 ring): Cytosine, Thymine (DNA) and Uracil (RNA)

    • Purines (2 rings): Adenine, Guanine

DNA Structure and Discovery

  • Discovery: Watson and Crick, 1953.

  • Structure: Double-stranded helix prevents bulges through base pairing rules.

  • Base Pairing: A pairs with T, C pairs with G ensures equal amounts of A=T and G=C.

  • Visualization: DNA appears as a ladder:

    • Sides: Sugar-phosphate backbone

    • Rungs: Paired nitrogenous bases

DNA Replication

  • Process:

    • Strands must separate for replication.

    • Each strand serves as a template for a new strand using free nucleotides.

    • Enzymes facilitate linking of nucleotides; results in semi-conservative replication (each daughter strand contains one original parent strand).

  • Duration and Accuracy:~3 hours, highly accurate with errors in one in several billion.

Enzymes and Regulation in Replication

  • Enzymes Required: >12 enzymes and proteins required for replication.

  • Origin of Replication: Specific sites on DNA where replication starts, proceeding bi-directionally.

  • Multiple Origins: Eukaryotic DNA with many origins increases replication speed.

Strands and Directionality

  • Orientation: Each strand has a 3’ and 5’ end.

    • Strand #1: 3’→5’ and Strand #2: 5’→3’.

  • DNA Polymerases: Enzymes that synthesize new strands by adding nucleotides to the 3’ end.

Lagging Strand Synthesis

  • Discontinuous Synthesis: Opposite strand must be synthesized in small pieces (Okazaki fragments) linked by DNA ligase.

  • Repair Functions: DNA polymerase and ligase also repair damaged DNA.

Genotype vs Phenotype

  • Genotype: Genetic makeup encoded in DNA.

  • Phenotype: Observable traits, influenced by protein synthesis.

  • Central Dogma of Biology:

    • DNA → RNA → Protein

  • Processes:

    • Transcription in nucleus, translation in cytoplasm.

Genetic Code and Amino Acids

  • Codon: Set of three nucleotides encodes one amino acid.

  • Example of Codon Translation: 3’ CCT GGG TCA GGT AAG 5’ results in RNA 5’ GGA CCC AGU CCA UUC 3’.

  • 64 Codons: 61 code for amino acids; 3 are stop codons.

Transcription Process

  • Location: Occurs in the nucleus.

  • Steps:

    1. Initiation: RNA polymerase binds to promoter on DNA.

    2. Elongation: RNA strand grows as nucleotides added one by one.

    3. Termination: RNA polymerase reaches terminator sequence, detaches from DNA.

  • Three Forms of RNA: mRNA, tRNA, rRNA.

RNA Processing in Eukaryotes

  • mRNA Modifications:

    • Addition of a G cap and a polyA tail for stability and transport.

  • Intron vs Exon: Intron (non-coding) segments are removed; exons (coding) are spliced together.

Transfer RNA (tRNA)

  • Function: Convert mRNA codons to amino acids; carries specific amino acids.

  • Structure: 80 nucleotides long; forms loops; contains anticodons matching mRNA codons.

Ribosomes and Protein Synthesis

  • Ribosome Composition: Consist of rRNA and protein; large and small subunits.

  • Functionality: Both prokaryotic and eukaryotic ribosomes build proteins; basis for antibiotic targeting.

Translation Phases

  1. Initiation: mRNA binds to ribosome, tRNA with Methionine (start codon) binds.

  2. Elongation: Amino acids added, peptide bonds formed between growing polypeptide.

  3. Termination: Stop codons signal the end of protein synthesis.

Review of Genetic Processes

  • Transcription: RNA synthesized from DNA template using RNA polymerase.

  • Translation: tRNA attaches correct amino acids to growing polypeptide via ribosome.

  • Termination: Release of completed polypeptide.

Mutations

  • Types of Mutations:

    1. Base Substitutions: Replacing one nucleotide may be neutral or harmful.

    2. Insertions/Deletions: Disrupt reading frame; can cause significant changes to proteins.

  • Causes of Mutations: Spontaneous during replication or induced by mutagens (e.g., UV light).