4Why don’t all of our cells have 92 chromosomes? 46 from male + 46 from female = 92?
5Gametes – sex cellssome cells (namely the sex cells, or gametes - sperm and egg cells) have half that number, 23 chromosomes.
6Where do these cells come from? If mitosis always produces two identical daughter cells from a mother cell, how do the sex cells develop?
7MEIOSIS is the answer! It occurs only in the gonads Testes for men Ovaries for womenIt allows the chromosomes to reduce their number by half so that when the egg and sperm unite, they create a life-form with the same number of chromosomes as the parent.
9Cells that possess the full 23 pairs of chromosomes are called diploid cells They have chromosome sets from both mother and fatherDiploid cells are also referred to as 2n because they contain two sets of chromosomes
10Sperm cells and egg cells, the sex cells or gametes, are n or haploid cells because they only contain one set of chromosomesA sex cell is set to join with another haploid sex cell to produce a 2n or diploid cell
14Meiosis is quite similar to mitosis in that they are both divided into the same phases. The first phase of meiosis I, like mitosis, is always preceded by an interphase.During this interphase, both chromosomes of each pair (1 through 23) make an exact copy of themselves.
15Like the interphase that occurred before mitosis, the one in meiosis is also not visible to the eye. If we could see something, it would possibly resemble the pictures below:
16Each chromosome is now double-stranded, and each chromosome consists of a pair of sister chromatids The two homologous chromosomes now consist of four chromatids, an identical pair from your mother and an identical pair from your father. We are now ready for meiosis to begin!
17Prophase 1The meiotic prophase I begins much like the prophase of mitosiscentrioles migrate to the poles of the cellspindle fibres become visiblethe nuclear membrane breaks down
18But… there is a big difference The two homologous chromosomes move together and pair up.This is known as synapsis .The pairs intertwine their sister chromatids and become one tangled unit known as a tetrad .
19These intertwined chromatids swap genetic information. It is at this point in prophase I that each chromatid is no longer either from your mother or from your father.Each chromatid, after exchanging with its homologue, is a blend of information from both your parents. The actual exchange of genetic information is referred to as crossing-over.
20These blended homologous pairs are now uniquely yours These blended homologous pairs are now uniquely yours. When you have children of your own, you will not simply pass along a chromosome from either your mother's side or your father's side of the family. The genetic information you pass along will be a unique blend from both sides.
22The diagram below illustrates two crossovers The diagram below illustrates two crossovers. In reality, each pair of homologues crossover in at least two or three different locations.
23Metaphase 1 The chromosomes line up along the equatorial plane. They are still in homologous pairs and may well still be intertwined into a tetrad.The spindle fibres are attached to the kinetochores of each homologous pair.
24This is quite different from what happened in the same phase of mitosis. In mitosis metaphase, the spindle fibres attached to each individual sister chromatid.
25Anaphase 1Anaphase I is about the same as anaphase in mitosis. However, the sister chromatids remain attached together.
26The homologous pairs are separated from one another The homologous pairs are separated from one another. Each homologue is drawn to an opposite pole using the spindle fibres as guide wires.Each pole will receive one of the newly jumbled (crossed-over) homologues.
27Telophase 1 & Cytokinesis Each pole has now received one of the homologues.The number of chromosomes has been reduced to half, or the haploid (n) number.Each homologue is double-stranded. The cells have gone from 2n to n, from diploid to haploid.
29In animal cells, a cleavage furrow will now form In animal cells, a cleavage furrow will now form. In plant species, an endplate grows between the two new nuclei.
30Meiosis IIMeiosis I ended with two new cells containing half the chromosome number of the original.Each of those chromosomes is double-stranded.In meiosis II the sister chromatids will be separated.The resulting cells at the end of this phase will also be haploid, however, all the chromosomes will be single-stranded and ready for fertilization.
31Interphase IIThere is no need for an interphase between meiosis I and meiosis II. The chromosome strands are already doubled so there is no need to make another copy. Some organisms, however, will go through a stage that can only be called interphase II. The chromosomes unwind, the nuclear membrane forms, and the cell produces proteins just as in any other interphase. However, this is the exception as most cells go from telophase I and cytokinesis directly into prophase II.
32Prophase IIThere is no synapsis, and there are no tetrads, nor crossing-over in prophase II.There is simply a forming of a set of spindle fibres. It is interesting to note that the spindle fibres in this phase is set up at 90 degrees to where they did during prophase I.
33Metaphase IIThe chromosomes are aligned along the equatorial plane, just as in mitosis and meiosis I.Here, the spindle fibres are attached to each kinetochore of each chromatid.
34Anaphase IIThe centromere region between the sister chromatids now widens and each chromatid is drawn towards an opposite pole of the cell.
35Telophase II & Cytokinesis Nuclear membranes form around the haploid, single-stranded nuclei. There are now four cells that were derived from the original single cell.
38NondisjunctionWe know a little about what happens when mitosis goes wrong. Does meiosis take a wrong turn sometimes too? Yes, it does.Imagine what might happen if two sister chromatids got "stuck" together and did not separate during anaphase I.
39The spindle fibers would eventually break under the strain and both of the homologues would be pulled towards the same pole of the same "new" cell.There would be an extra chromosome in one of the cells and one less chromosome in the other cell.This occurrence is known as nondisjunction.
40Nondisjunction can occur with any chromosome. The first 22 pairs of human chromosomes are called autosomes.The 23rd pair of chromosomes is called the sex chromosomes, since they determine the gender of the person.Women possess 2 X chromosomes for their 23rd pair, while men have an X chromosome and a Y chromosome.Nondisjunction can occur with autosomes as well as with the sex chromosomes.
41Edward’s Syndrome One extra 18th chromosomes Severe birth defects with a life expectancy less than three months
42One extra 21st chromosome Down SyndromeOne extra 21st chromosomeIndividuals have rounded faces with well spaced eyes and an enlarged tongue. Often these individuals have lower academic abilities.
43Turner’s Syndrome One missing 23rd chromosome These women have only one X chromosome for their sex chromosomes. They are often small in size and do not fully develop sexually.
44Patau Syndrome One extra 13th chromosome Severe defects, very short life expectancy.