Key Terms
Chromatin: consists of DNA proteins that form the chromosomes.
Chromosomes: composed of 2 threads which are chromatids.
Chromatid: are two identical halves of chromosome that has been replicated
in preparation for cell division.
Tetrads: paired of homologous chromosomes, composed of sister
chromatids.
Sister Chromatids: pairs of replicated chromosomes remain joined at
centromere.
Crossing over/Recombination: process that breaks, recombines, & rejoins
to produce new combinations of genes.
Spindle Fibers: threads of protein that will attach themselves to the
chromosomes.
MEIOSIS
It is a cell division that produces our sex cells (female: egg cells, male: sperm
cells) for our sexual reproduction. Meiosis process simply reduces the chromosome
numbers into half. It begins with one diploid (2 sets of chromosomes) parent cell and
ends with 4 haploids (1 set of chromosome) daughter cells. Each round of division,
cells go through 4 stages: prophase, metaphase, anaphase, telophase and
cytokinesis.
MEIOSIS I
1. Prophase 1 – Nuclear envelope dissolves and chromatin condenses to
form chromosomes. Pairs of homologous chromosomes come together
to form tetrad or also called synapsis. Then, crossing
over/recombination occurs wherein enzymes rearrange portions of
homologous chromosomes which creates genetic variation.
2. Metaphase 1 – Spindle fibers pull the tetrads joining at their
centromeres aligned along the equator (middle line) of the cell.
3. Anaphase 1 – Spindle fibers pull the chromosomes apart, splitting the
tetrads while sister chromatids stay together (One chromosome of
tetrads pulled left and the other one pulled right).
4. Telophase 1 & Cytokinesis – Cytokinesis divide the one cell into two
cells, chromosomes complete move to opposite poles of the cells. The
spindle fibers disappear and nuclear envelope reforms.
MEIOSIS II
1. Prophase 2 – New set of spindle fibers forms and nuclear envelope
dissolves releasing chromosomes.
2. Metaphase 2 – Individual chromosomes, no more tetrads, aligned
along again the equator of the both cell.
3. Anaphase 2 – Chromatids separate at the centromere. Spindle fibers
contracts and pull the sister chromatids apart toward opposite ends of
the cell (one pulled upward, one pulled downward).
4. Telophase 2 & Cytokinesis – Cytokinesis divides two cells into four
cells. Chromatids uncoil into strands of chromatin. Nuclei reforms
from2 diploid (2n) cells into 4 haploid (n) daughter cells.
4 haploid cells, 23 strands of DNA every cells. At the conclusion
of meiosis, there are 4 haploid daughter cells that go on to develop into
either sperm or egg cells.
References:
O’Connor, C. (2008). Meiosis. Genetic Recombination, and Sexual Reproduction.
Nature Education 1(1):174. https://www.nature.com/scitable/topicpage/meiosis-
genetic-recombination-and-sexual-reproduction-210/
BYJU’S. (n.d.). Definition, Cell Division, And Stages of Meiosis.
https://byjus.com/biology/stages-of-meiosis/
Beverly Biology (2016, November 21). Meiosis in Human Cells [Video]. Youtube.
https://www.youtube.com/watch?v=YmXc3r6-Ma8
(Reference photo from iStock.com)
