Bio 30 - DNA

Page 1: Stages of Protein Synthesis

Stage 1: Transcription

  • Location: Occurs in the nucleus

  • Process:

    • A single DNA gene (a recipe) activates.

    • The enzyme RNA polymerase unzips the DNA, breaking hydrogen bonds between nitrogen bases.

    • RNA nucleotides match with one side of the DNA to copy the gene's information.

    • Adenine (A) pairs with Uracil (U) instead of Thymine (T).

    • RNA polymerase links the RNA nucleotides to form a single-stranded helix (mRNA).

    • Once formed, mRNA leaves the nucleus.

    • The DNA gene rezips, reforming hydrogen bonds.

Key Terminology

  • DNA Code -> mRNA Code -> Amino Acids

  • Triplet: A set of 3 nucleotides in DNA (e.g., TGA).

  • Codon: A set of 3 nucleotides in mRNA (e.g., ACU).

    • Initiator Codon: Starts protein synthesis (AUG).

    • Terminator Codon: Stops protein synthesis.

Stage 2: Translation

  • Process: mRNA -> tRNA -> Polypeptide -> Protein

  • mRNA attaches to a ribosome in the cytoplasm.

  • tRNA, with an anticodon, pairs with the mRNA codon.

  • tRNA Structure:

    • Prongs: Contain the anticodon.

    • Head: Carries the corresponding amino acid.

  • Translation ends when a stop codon is reached, finishing the polypeptide.

  • The polypeptide folds to form the protein coded by the DNA gene.

Initiation Stage

  • Ribosomes clamp on the mRNA.

  • Start Codon (AUG) activates protein synthesis, coding for Methionine.

  • Ribosomes consist of rRNA and proteins and match tRNA anticodons with mRNA codons.

Elongation Stage

  • The ribosome selects a tRNA molecule if the codon and anticodon pair correctly (e.g., mRNA codon UGG pairs with tRNA anticodon ACC).

  • tRNA transports specific amino acids into the ribosome, functioning like a shuttle bus.

  • The ribosome moves along mRNA, adding amino acids to the polypeptide chain.

Termination Stage

  • Racing a stop codon (terminator) signals completion of the polypeptide.

  • The ribosome detaches, and translation is complete.

Page 2: Protein Synthesis Steps

  1. DNA Unwinds and Unzips: The double helix separates to expose genes.

  2. mRNA Formation: Nucleotides attract to exposed DNA, forming a complementary strand.

  3. mRNA Leaves the Nucleus: Newly formed mRNA exits and attaches to a ribosome.

  4. Translation Initiation: Begins when the ribosome recognizes the start codon on mRNA.

  5. tRNA Brings Amino Acid: tRNA molecules carry specific amino acids to ribosome, pairing their anticodons to mRNA codons.

  6. Peptide Bond Formation: Peptide bonds form between adjacent amino acids.

  7. Stop Codon Recognition: Ribosome reads a stop codon signaling the end of translation.

  8. Release of Polypeptide: The completed amino acid chain (polypeptide) is released from the ribosome.

Overview of Protein Synthesis

  • Location: DNA is found in chromosomes within the nucleus.

  • Protein synthesis occurs in the cytoplasm by ribosomes.

  • Messenger RNA (mRNA) facilitates the transfer of information from the nucleus to the cytoplasm.

Types of RNA

  • DNA:

    • Location: In nucleus.

    • Structure: Double stranded; deoxyribose sugar and uses thymine.

  • RNA:

    • Location: Nucleus and cytoplasm (ribosomes).

    • Structure: Single stranded; ribose sugar and uses uracil.

    • Types:

      • mRNA: Carries DNA message from nucleus to ribosome (transcription).

      • tRNA: Delivers amino acids to ribosome (cytoplasm only).

      • rRNA: Makes up the ribosome (cytoplasm only).

Page 3: DNA Mutation and Variation

Overview of Mutations

  • Mutation: Change in the sequence of nitrogen bases in DNA.

  • Mutagen: Any agent causing DNA mutation (e.g., chemicals, radiation, viruses, HPV).

  • Teratogens: Mutagens affecting the developing embryo (e.g., drugs, alcohol).

  • Carcinogens: Mutagens that lead to cancer cell formation.

Types of Mutations

  1. Beneficial Mutation: Provides a selective advantage, contributing to evolutionary change.

  2. Harmful Mutation: Reduces an individual's fitness.

  3. Point Mutations:

    • One base changes (substitution, deletion, insertion).

    • Affects mRNA reading frame.

  4. Deletion, Insertion, Duplication: Mutations of whole segments of DNA.

  5. Translocation (Chromosome Mutation): Part of one chromosome moves and attaches to another.

Structure and Location of DNA

  • Structure: Double helix made of nucleotides containing:

    • Deoxyribose sugar (backbone)

    • Phosphate group (backbone)

    • Nitrogen bases: Adenine, Thymine, Cytosine, Guanine.

  • Strands are held together by hydrogen bonds.

Types of DNA

  • Nuclear DNA: Located in the nucleus; double stranded.

  • Mitochondrial DNA (mtDNA): Circular, present in mitochondria.

  • Chloroplast DNA (cpDNA): Circular, present in chloroplasts.

Page 4: DNA Duplication (Semi-Conservative Replication)

Steps of DNA Duplication

  1. Separating the DNA Strand (Unzipping): DNA helicase breaks hydrogen bonds between nitrogen base pairs.

  2. Attaching Nucleotides: DNA polymerase attaches free-floating nucleotides to the original strand (A = T, C = G).

  3. Gluing Together/Proofreading:

    • DNA ligase connects the sugar-phosphate backbone.

    • Proofreading enzymes fix mismatched base pairs, preventing mutations.

  4. Completion of Duplication: Resulting in two identical sister chromatids, each half old and half new, hence termed semi-conservative replication.