What type of division in meiosis involves the separation of chromatids to produce 4 haploid cells?
Learning Objectives
The Cell Division Cycle Show
Cell division cycle, figure from Wikipedia. Cells that stop dividing exit the G1 phase of the cell cycle into a so-called G0 state. Cells reproduce genetically identical copies of themselves by cycles of cell growth and division. The cell cycle diagram on the left shows that a cell division cycle consists of 4 stages:
ChromosomesChromosomes were first named by cytologists viewing dividing cells through a microscope. The modern definition of a chromosome now includes
the function of heredity and the chemical composition. A chromosome is a DNA molecule that carries all or part of the hereditary information of an organism. In eukaryotic cells, the DNA is packaged with proteins in the nucleus, and varies in structure and appearance at different parts of the cell cycle. In G1, each chromosome is a single chromatid. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister. The sister chromatids are joined at their centromeres, as shown in the image below. A pair of sister chromatids is a single replicated chromosome, a single package of hereditary information. Human karyotype “painted” using fluorescent DNA probes. These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs (e.g., 2 copies of chromosome 1) have been lined up next to each other. Image from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157 Ploidy
Here is a simplified diagram illustrating the overall process and products of mitosis: Source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:MajorEventsInMeiosis_variant_int.svg) Questions or points to ponder or note about the figure above (answers at bottom of page):
This animation below shows the packaging of DNA and condensation of chromosomes as a cell undergoes mitosis. The video narration has a major error at time 1:22: chromosomes exist throughout the entire cell cycle (at all times in a cell’s life); they are visible in their condensed form only during mitosis and meiosis. MeiosisThis is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells. It starts with a diploid cell that has undergone chromosomal DNA replication: 2N chromosomes, 4X DNA content. Two successive divisions, with no additional DNA replication, results in 4 haploid gametes: 1N chromosomes, 1X DNA content.
The last point appears to be the most difficult for students to grasp. Consider the X and Y chromosomes. They pair in prophase I, and then separate in the first division. The daughter cells of the first meiotic division have either an X or a Y; they don’t have both. Each cell now has only one sex chromosome, like a haploid cell. Meiosis Overview from Wikipedia by Rdbickel And here is a video
that walks through the steps of meiosis: It is very important that you recognize how and why cells become haploid after meiosis I. To confirm for yourself that you understand meiosis, work through one or more of these interactive tutorials:
Chromosomes, chromatids, what is the difference and how many chromosomes are there at different times of the cell cycle and after mitosis and meiosis?Chromosomes by
definition contain the DNA that makes up the fundamental genome of the cell. In a prokaryote, the genome is usually packaged into one circular chromosome consisting of a circular DNA molecule of a few million base pairs (Mbp). In eukaryotes, the genome is packaged into multiple linear chromosomes, each consisting of a linear DNA molecule of tens or hundreds of Mbp. Chromosomes exist at all different phases of the cell cycle. They condense and become visible to light microscopy in prophase of
mitosis or meiosis, and they decondense during interphase, in the form of chromatin (DNA wrapped around nucleosomes, like “beads on a string”). Meiosis Overview from Wikipedia by Rdbickel The video below is geared toward a high school audience, but it does present a helpful way for recognizing how many chromosomes are present in a cell (and thus the ploidy level of that cell). While watching, see if you can recognize why the products of meiosis 1 are haploid cells: Answers to questions about the mitosis figure:
Which phase of meiosis produces 4 haploid cells?During prophase II, sister chromatids align at the center of the cell in singular chromosome structures. These sister chromatids are separated during anaphase II, resulting in a total of four haploid cells.
What type of cell division produces 4 haploid cells?During meiosis, a diploid germ cell undergoes two cell divisions to produce four haploid gamete cells (e.g., egg or sperm cells), which are genetically distinct from the original parent cell and contain half as many chromosomes.
|