Meiosis I and II

Question 1

what happens in interphase?

Answer 1

♣ same as mitosis, chromosomes are replicated

Question 2

what happens in prophase I?

Answer 2

♣ Chromatin condenses into chromosomes
♣ Spindle apparatus forms
♣ Nucleoli and nucleus membrane disappears
♣ Homologous chromosomes come together and intertwine in a process called synapsis
• These are chromosomes which code for the same traits, one inherited from each parent
• Each chromosome contains two sister chromatids, so each synaptic pair of chromosome contains 4 sister chromatids, known as a tetrad
♣ Some chromatids of homologous chromosomes break in cooresponding places and exchange equivalent pieces of DNA
• This is called crossing over
• The points of contact where crossing over occurs is called the chiasmata
• This is important for genetic diversity
o Therefore, crossing over between two sister chromatids would not yield any genetic changes

Question 3

what happens in metaphase I?

Answer 3

♣ Homologous pairs (tetrads) align at the equator and each pair of sister chromatids (no longer genetically identical after crossing over) attaches to a spindle fibre at the kinetochore

Question 4

what happens in anaphase I?

Answer 4

♣ The tetrad separates into two and each pair is pulled to opposite pole of the cell
• This is referred to as disjunction
• It accounts for a fundamental Mendelian law
• During disjunction each paternal chromosome disjoins from the maternal homologue, and either chromosome can end up in either daughter cell
o Therefore, distribution of homologous chromosomes to the two intermediate daughter cells is random with respect to paternal origin
Each daughter cell will have a unique pool of genes from a random mixture of maternal and paternal origin

Question 5

what is nondisjunction?

Answer 5

♣ Occurs when the chromosomes do not separate properly

♣ Results in final daughter cells having an incorrect number of chromosomes

Question 6

what happens in telophase I?

Answer 6

♣ A nuclear membrane forms around the new diploid nucleus

♣ Each chromosome still consist of two sister chromatids joined at the centromere

Question 7

what happens during the second meiotic division?

Answer 7

♣ Very similar to mitosis
• Without preceding chromosome replication
• Chromosomes align at the equator, spindle fibres attach to kinetochore, move to opposite poles, and new nuclear membrane forms
• Now they are haploid
• Note that in women only one of the four gametes survives, with the others being destroyed by the body