Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Meiosis is a reduction division of a diploid nucleus () to form four haploid nuclei ().
DNA replication occurs during the phase of interphase prior to meiosis, resulting in chromosomes consisting of two identical sister chromatids.
During Prophase I, homologous chromosomes pair up (synapsis) to form bivalents. Crossing over occurs between non-sister chromatids at points called chiasmata, leading to the exchange of genetic material.
Random orientation of bivalents occurs during Metaphase I, where the orientation of one pair of chromosomes is independent of other pairs. This leads to independent assortment.
In Meiosis I, homologous chromosomes are separated, reducing the chromosome number by half. In Meiosis II, sister chromatids are separated.
Genetic variation is promoted by: 1. Crossing over in Prophase I; 2. Random orientation in Metaphase I; 3. Random fusion of gametes during fertilization.
Nondisjunction is the failure of chromosomes to separate properly during Anaphase I or II, which can result in gametes with an extra or missing chromosome, such as in Down Syndrome (Trisomy ).
📐Formulae
💡Examples
Problem 1:
A cell from an organism with a diploid number of undergoes meiosis. Calculate the number of possible chromosome combinations that can be produced in the gametes due to independent assortment, excluding the effects of crossing over.
Solution:
Explanation:
The formula for the number of possible combinations due to independent assortment is . Given , we find that . Substituting this into the formula gives , which equals possible genetic combinations.
Problem 2:
Explain the chromosome count in a human cell following Meiosis I and Meiosis II, starting from a primary spermatocyte ().
Solution:
After Meiosis I: chromosomes (each with chromatids). After Meiosis II: chromosomes (each with chromatid).
Explanation:
Meiosis I is the reduction division where homologous pairs separate, so the number of chromosomes drops from () to (). Meiosis II is an equational division where sister chromatids separate, so the chromosome count remains in each of the four daughter cells.