Bio 9/29

Administrative Notes

  • Homework was due the following Wednesday, but the instructor was unsure of the due dates and advised students to refer to the weekly module for updates.

  • Students are required to submit specific B grades for review and exam fractions for those scoring 50 or below.

Review of Previous Class

  • The previous class focused on proteins mentioned in the lecture slides, specifically regarding their roles relating to DNA.

  • The telomerase and DNA gyrase are two enzymes that are not essential for the exam; students can disregard them in their studies.

Proteins Discussed in Class

  • DNA Topoisomerase

    • Function: Prevents supercoiling during DNA unwinding.

    • Works alongside DNA helicases, which unwind the DNA strands.

    • If topoisomerase is absent, excessive supercoiling occurs due to torsional stress from rapid unwinding.

    • Topoisomerase alleviates this by making transient single-strand cuts (not double-strand), providing relief to torsional strain.

    • The term transient indicates that the single-strand breaks are temporary and do not last through the entire unwinding process.

  • Emphasis on the importance of a composite list of proteins that may appear on the exam.

Introduction to DNA Repair

  • Importance: DNA repair mechanisms are essential to correct mismatched bases and prevent mutations from being passed on through replication cycles.

  • Mutations can be:

    • Deleterious: Harmful to organism.

    • Beneficial: Provide an advantage.

    • Neutral: Have no observable effect on function.

  • DNA Polymerase has a proofreading capacity, which detects and repairs mistakes during replication.

    • If it misses errors, the DNA mismatch repair system serves as a backup, correcting nearly 99% of errors.

    • The error rate of DNA polymerase is one mistake per 10710^7 nucleotides, while the mismatch repair system lowers it to one mistake per 10910^9 nucleotides.

DNA Mismatch Repair System

  • Components: DNA polymerase, ligase, and nucleases collaborate during repair processes.

    • Process:

    • Detects mismatches and nicks in newly synthesized DNA.

    • Nucleases remove the erroneous DNA segment.

    • DNA polymerase fills in the gap with correct nucleotides.

    • DNA ligase seals the final phosphodiester bond.

  • Nucleases cut the DNA strand, and DNA polymerase synthesizes the correct strand, while ligase completes the repair.

  • The focus is on repairing mistakes only in the newly synthesized strand, not the template strand, to ensure accuracy.

Types of DNA Damage

  1. Depurination: Removal of a purine (adenine or guanine), leading to a gap in the DNA.

  2. Deamination: Removal of the amine group from cytosine, converting it to uracil, leading to improper base pairing during replication.

  3. Thymine Dimer Formation: UV radiation can cause two adjacent thymine bases on the same strand to bond, leading to replication issues.

Consequences of DNA Damage

  • If deamination or depurination occurs without repair:

    1. Deamination: The deaminated cytosine (that appears as uracil) can lead to mutations in subsequent replication cycles.

    2. Depurination: A missing purine facilitates mispairing during replication, shortening the resultant DNA strand or creating frameshift mutations.

    3. Thymine Dimer: Causing one base pair in replication leading to skipping of a sequence or mispairing.

Amplification Concept in Gene Functioning

  • Amplification: A single gene can produce multiple mRNA copies.

  • Example: One double-stranded DNA may yield five mRNA copies through transcription, emphasizing regulatory mechanisms in gene expression.

General Structure of DNA vs. RNA

  • DNA: Double-stranded, stores genetic information, contains deoxyribonucleotides, and forms a double helix structure.

  • RNA: Single-stranded, can have various structural configurations, is involved in protein synthesis, and consists of ribonucleotides.

Eukaryotic vs. Prokaryotic Processes

  • In eukaryotes, DNA replication and transcription occur in the nucleus, while translation takes place in the cytoplasm.

  • In prokaryotes, all processes occur in the cytoplasm, reflecting their lack of organelles.

Summary of Central Dogma

  • The flow of genetic information proceeds from DNA to RNA to Protein.

  • Amplification can occur at the levels of transcription and translation.

Upcoming Topics

  • Next topics will include deeper analysis of transcription in both prokaryotic and eukaryotic cells, RNA processing, and transcription machinery.

  • Emphasis will also be placed on comparing DNA replication with transcription and understanding the specifics of mRNAs, including the roles of various RNA polymerases.

Conclusion

  • Students encouraged to ask questions regarding DNA replication, repair, or the central dogma before transitioning to upcoming chapters.

  • A reminder to check the syllabus for coverage and exam data as the course progresses.