Biology MCAT: Meiosis Lecture Notes

Introduction

• Presented by Medicosus Perfectionellis
• Part of the biology MCAT playlist
• Previous video: Mitosis
• Today's focus: Meiosis - critical for producing gametes in ovaries/testicles

Cell Cycle Overview

• Consists of Interphase and M Phase (Mitosis/Meiosis)
  • Interphase:
    • G1: Growth
    • S: DNA synthesis
    • G2: Another growth phase
  • M Phase:
    • Mitosis or Meiosis occurs

Mitosis vs. Meiosis

• Mitosis:
  • Occurs in somatic cells
  • Not involved in sexual reproduction
  • Produces two identical diploid (2n) daughter cells
  • Single division cycle
• Meiosis:
  • Occurs in gametes (sex cells)
  • Involved in sexual reproduction
  • Produces four non-identical haploid (n) cells
  • Two rounds of division:
    • Reduction Division: Reduces 2n to n
    • Equatorial Division: Similar to mitosis, but starts with haploid cells

Phases of Meiosis

• Just like mitosis, meiosis involves phases but has two rounds:
  • Meiosis I:
    • Prophase I, Metaphase I, Anaphase I, Telophase I
  • Meiosis II:
    • Prophase II, Metaphase II, Anaphase II, Telophase II

Key Differences Between Mitosis and Meiosis Phases

• Prophase I vs. Prophase (Mitosis):
  • Crossing over occurs - homologous chromosomes exchange genetic material
  • Responsible for genetic diversity
• Metaphase I vs. Metaphase (Mitosis):
  • Alignment differences - each chromosome connected only to one spindle fiber in meiosis
• Anaphase I and Telophase I:
  • Disjunction occurs, separating homologous pairs (not sister chromatids)
  • Results in two haploid cells

Crossing Over and Genetic Diversity

• Occurs in Prophase I
• Involves homologous chromosomes (not sister chromatids)
• Creates recombinant chromosomes, contributing to genetic variety

Disjunction vs. Non-disjunction

• Disjunction: Normal separation of homologous chromosomes during Anaphase I
  • Basis for Mendel's First Law of Segregation
• Non-disjunction: Abnormal separation
  • Leads to conditions like Turner Syndrome (45 chromosomes) or Down Syndrome (47 chromosomes)
  • Can occur in sex chromosomes (Klinefelter Syndrome: 47, XXY)

Mendelian Laws and Impacts

• First Law (Segregation): Based on disjunction
• Second Law (Independent Assortment): Due to crossing over
• Gene distance measurement is in centimorgans

Closing Remarks

• Mitosis creates identical cells; meiosis contributes to genetic diversity
• Importance of crossing over in meiosis
• Link to further resources and courses available on Medicosus' website

Additional Resources

• Anti-cancer pharmacology, antibiotics, and electrolytes courses available
• Free downloadable biology notes

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Tips for Studying:

• Understand the differences between meiosis and mitosis
• Focus on the impact of crossing over and genetic diversity
• Pay attention to the phases of meiosis I and meiosis II

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