In what two ways do chromosomes pair up in prophase 1?

Expert Answers
ashley227 eNotes educator| Certified Educator

Prophase I is the first step in meiosis. During meiosis a diploid cell (46 chromosomes) divides twice to produce 4 haploid (23 chromosome) cells. The body utilizes this process to produce gametes (oocytes/eggs or spermatozoa).  

When the cell is in interphase, the DNA is in a stringy form called chromatin. This gives enzymes access to the nucleotides so that DNA can be copied and/or transcribed into RNA for protein synthesis. When the cell is going to divide, the nuclear envelope disappears so actions must be taken to protect the DNA from damage (the job of the envelope when it is present). During prophase (I or II) the DNA within the nucleus of the cell condenses around histones. This forms the X-shaped chromosomes that you are used to seeing in text books (in karyotypes). Each X-shaped chromosome is actually 2 sister chromatids that are held together at the center of the X-the centromere. Each sister chromatid makes up either the left or right side of the X-shaped chromosome. These sister chromatids are exact copies of each other. This is one way that chromosomes pair up in prophase I. Sister chromatids do not separate until anaphase II of meiosis.

In prophase I, you also have homologous chromosomes that align for a process called "crossing over." Remember that most of your cells are diploid (46 chromosomes). The 46 chromosomes come from the 23 that you received from your mother and the 23 that you received from your father. Usually, instead of saying that we have 46 chromosomes, we say that we have 23 pairs of chromosomes. For example, you receive a chromosome 1 from your mom and one from your dad. The difference isn't in the genes that are received on each, but in the alleles (or versions of the genes) that you get. Because they contain information regarding the different versions of the same genes, they are called homologous chromosomes. These chromosomes align (chromosome 1 from your mom pairs up with chromosome 1 from your dad and so on) and corresponding pieces from the chromosomes from your mom and the chromosomes from your dad will actually be interchanged. This ends up creating new allele combinations and resulting in additional genetic diversity available in the gametes (haploid cells that result from meiosis). Homologous chromosomes separate into separate cells during anaphase I of meiosis.