Unit 7

BIO.3A.1 - Meiosis and Fertilization

Need to know

• Meiosis begins with one diploid cell and produces four genetically different haploid cells.
• DNA is copied once before Meiosis I. Homologous chromosomes separate in Meiosis I; sister chromatids separate in Meiosis II.
• Crossing over and independent assortment create new combinations of alleles.
• Fertilization joins two haploid gametes to restore the diploid chromosome number in a zygote.
• Remember the cycle: body cell (2n) → meiosis → gametes (n) → fertilization → zygote (2n) → mitosis.

Be able to do

• Order the stages of meiosis and identify the stage shown in a diagram.
• Distinguish homologous chromosomes from sister chromatids.
• Calculate the chromosome number in gametes or body cells.
• Use a model to explain how meiosis and fertilization maintain chromosome number across generations.

BIO.3A.2 - Mitosis and Meiosis

• Mitosis uses one nuclear division to produce two genetically identical cells for growth and repair.
• Meiosis uses two nuclear divisions to produce four genetically different gametes for sexual reproduction.
• Mitosis maintains chromosome number (2n → 2n); meiosis halves it (2n → n).
• Sexual reproduction increases variation; asexual reproduction usually produces genetically identical offspring.

BIO.3A.3 - Nondisjunction and Karyotypes

• A karyotype displays chromosomes in matched pairs. Count chromosomes to identify an extra or missing chromosome.
• Nondisjunction occurs when chromosomes fail to separate during meiosis.
• Nondisjunction in Meiosis I affects all four gametes; nondisjunction in Meiosis II affects half.
• Trisomy means an extra chromosome; monosomy means a missing chromosome.
• A chromosome-number abnormality differs from a gene mutation, which changes the DNA sequence within a gene.

Be able to do

• Identify chromosome-number abnormalities from a karyotype.
• Use a meiosis model to determine when nondisjunction occurred and predict the resulting gametes.
• Support a conclusion with both the chromosome count and the location of the extra or missing chromosome.