biochem week 12 part 2

Test Results and Reactions

• Test results were announced yesterday.

• One student (Steph) scored 94.

• Another student expressed disappointment, scoring 90 despite getting all multiple choice questions wrong and zero points on free response questions.
      - Specific example discussed involved a misconception related to glycolysis, specifically regarding product formation.
      - Student incorrectly thought that the reaction needed to be calculated twice, which led them to misunderstand the outcome.

• Highlights issues with understanding test formats and potential misconceptions in core concepts.

Understanding DNA Structure and Bonding

• Discussion about the fixed nature of bases in relation to pentose sugars and phosphate groups in DNA; there is no rotation, making it essential to understand hydrogen bonding interactions.

• The connection patterns of hydrogen bonds in DNA were reviewed:
      - 3 hydrogen bonds available in total.
      - Specific pairs discussed:
          - Adenine (A) and Thymine (T) form 2 hydrogen bonds.
          - Guanine (G) and Cytosine (C) form 3 hydrogen bonds.
      - Mismatch consequences discussed, emphasizing the instability caused by incorrect pairing, which can lead to errors during DNA replication.

• Mechanism of DNA polymerase in recognizing mismatched pairs and the implication for enzyme activity were explored.

The Role of DNA Polymerase

• DNA polymerase structure discussed as resembling a right hand:
      - Palm: Main active site for nucleotide synthesis.
      - Fingers and Thumb: Assist in positioning the nucleotides correctly.
      - Functionality includes proofreading to ensure fidelity in DNA replication.

• Mechanism of mismatch recognition and correction:
      - A mismatched pair causes instability, leading DNA polymerase to pause, allowing the error to be corrected.
      - DNA polymerase also has exonuclease activity, which allows it to digest mismatched nucleotides by moving backwards on the strand (3' to 5').

• The video demonstrated DNA polymerase's process and fidelity, noting error rates of about every 1 in 10 million (-7 to -8 errors).

Mechanisms of DNA Repair

• Mismatch repair systems are essential for correcting replication errors.

• Flow of mismatch repair:
      1. Irregularity is identified.
      2. Incorrect bases are removed.
      3. Correct bases are synthesized by DNA polymerase III.
      4. Situations discussed where additional proteins (like MutS, MutL, and MutH) come into play to recognize and bind to mismatched bases for repair.
          - MutS identifies mismatches, utilizing ATP hydrolysis for scanning.
          - MutH is an endonuclease that cuts DNA to assist in repair.

• The importance of distinguishing between parent and daughter strands during repair was highlighted, noting that methylation plays a critical role (parent strand being methylated, daughter strand being unmethylated).

Features of Mismatch Recognition Proteins

• The distinction between minor and major grooves of DNA was discussed concerning the binding of recognition monomers in the repair process:
      - Mismatch Recognition Monomer: Binds to the minor groove, identifying mismatches.
      - Non-Mismatch Binding Monomer: Binds to the major groove and assists in stabilizing the structure.

• Clamping domains on the proteins enable sequence-independent interaction with the negatively charged DNA backbone, utilizing electrostatic interactions.
      - The significance of shape recognition around mismatches, leading to successful binding and repair, was elaborated.

Comparative Overview of Repair Mechanisms

• DNA proofreading occurs during replication, correcting errors immediately through exonuclease activity in DNA polymerase.

• Mismatch repair operates after DNA replication, utilizing various proteins to identify and fix errors systematically.
      - Key proteins include MutS for recognition, MutH for cutting, and DNA polymerase III for synthesizing corrections.

• Importance of understanding both mechanisms for maintaining genetic fidelity was reinforced.

Closing Thoughts and Further Activities

-Students were encouraged to engage with case studies to apply learned concepts, focusing on problems regarding mismatch detection and repair mechanisms.

• A brief break was taken, where opportunities for peer discussions and clarifications were provided on the material covered.