Meiosis I is unique in that genetic diversity is generated through crossing over and random positioning of homologous chromosomes bivalent chromosomes.
The steps leading up to meiosis are similar to those of mitosis — the centrioles and chromosomes are replicated. In figure below, following crossing over, the blue and red chromosomes, which originally carried AA and aa alleles, respectively, now carry Aa alleles in both chromosomes at the end of prophase I.
Each cell is haploid , containing only half the number of chromosomes as the original cell. Each of the two daughter cells is now haploid n , with half the number of chromosomes per nucleus as in meiosis I.
This means that homologous chromosomes will attach at their centromere regions to spindle fibers extending from only one of the two cell poles. Sister chromatids are separated in anaphase II when spindle fibers pull them toward opposite poles.
A centromere is a region on a chromosome that joins sister chromatids. Kinetochores are composed of a number of protein complexes that generate kinetochore fibers, which attach to spindle fibers. Sister chromatids are formed that are joined at their centromeres.
These fibers help to manipulate and separate chromosomes during cell division. In late prophase I, homologous chromosomes also called bivalent chromosomes, or bivalents pair laterally, or side-by-side.
Kinetochores have protein complexes that generate kinetochore fibers. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chromosomes to spindle polar fibers. Regina Bailey is a science writer and educator who has covered biology for ThoughtCo since 1997.
This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homolog for each chromosome see figure below.
Prior to the start of mitosis, the cell enters a stage known as interphase where it replicates its DNA in preparation for cell division.
In metaphase I, each pair of bivalents two chromosomes, four chromatids total align on the metaphase plate. Heterochromatin is very condensed and is therefore not transcribed. In addition, in meiosis I, the chromosomal number is reduced from diploid 2 n to haploid n during this process. Key Takeaways Centromeres are regions on a chromosome that join sister chromatids whose primary function is for the attachment of spindle fibers in cell division.