Mitosis vs. Meiosis

Diploid Number of Chromosomes In eukaryotes, such as us, a cell has two sets of chromosomes.  In sexually reproducing organisms, one of these sets was inherited from each of the parents.  Any cell that has two sets of chromosomes is said to be a diploid cell.   A diploid cell has a 2N number of chromosomes, where N equals the number of chromosomes in the cells of that species.  In human cells that have 46 chromosomes, N = 23.  All of our somatic cells are diploid.

Diploid vs. Haploid

Sperm & Eggs The only exception to this occurs during the production of gametes (sperm or eggs).  Before fertilization can take place, the number of chromosomes must be reduced to half their normal number. This is the N#, otherwise referred to as Haploid.  Why does this occur?  Because otherwise the number of chromosomes would double in the cells of each new generation of individuals! Sperm and eggs are gamete cells, which are haploid.

Meiosis Mature sperm and eggs are haploid cells (1N) that have half the normal number of chromosomes.    The union of sperm and egg then creates a new fertilized egg cell (zygote) that has the diploid number of chromosomes.  All cell divisions after fertilization are mitotic divisions The process that creates haploid cells is called meiosis. 

Mitosis vs. Meiosis The process that creates haploid cells is called meiosis.  The process that creates diploid cells is called mitosis A comparison of mitosis and meiosis can be seen in the next slide.

Mitosis

Meiosis

Genetic Variation There are four main sources of variability in sexually reproducing organisms.  Three of these occur during meiosis and are as follows: 1) the independent assortment of chromosomes during metaphase I 2) the presence of different information on the two chromosomes of a homologous pair 3) crossing over to create new gene combinations. The fourth source of genetic variation is mutation.  This will be covered in a later lesson.

Genetic Errors Life is not perfect. Sometimes a chromosomal separation during meiosis doesn’t go right and errors result . This can be a simple source of genetic variety. But this can also present major problems.  Chromosomal aberrations may be fatal to a developing embryo or may lead to profound physical or mental problems.  Genetic Errors

Karyotype A karyotype can identify chromosomal aberrations.  This is a photographic record of an individual’s chromosomes.  After photographing the metaphase chromosomes, the individual chromosome images are cut out and matched up by homologous pairs. The resulting karyotype should show 22 pair of autosomes and 2 sex chromosomes (X or Y).  This procedure can be seen below:

Karyotype

Amniocentesis & Chorionic Villi Sampling In older women or women determined to be at high risk for conceiving babies with genetic abnormalities, procedures called Amniocentesis and Chorionic Villi Sampling can be conducted to get the karyotype of the developing fetus. Thus chromosomal abnormalities can be determined early in development.

Amniocentesis & CVS

Down’s Syndrome Extra or absent autosomes often lead to spontaneous abortion of the fetus.  Some defects, however, are not as life-threatening and the fetus survives.  One such condition is trisomy 21 (Down’s syndrome).  Note that the karyotype will show an extra chromosome # 21.  Nondisjunction or a failure of daughter chromosomes to separate during meiosis can cause this to occur. (see the next slide)

Trisomy of Chromosome # 21

Turner and Klinefelter Syndromes There can also be abnormal numbers of sex chromosomes, however these don’t affect the survival of the individual.  In some cases they may cause infertility or interfere with the development of secondary sexual characteristics.  The frequency of some of these abnormalities is shown in the next slide.  Note that several of these conditions, such as XYY, have no known effect on the individual.

Chromosomal Mutations Remember, anything that changes the structure of a chromosome can lead to birth defects or cancer.  Besides adding or deleting entire chromosomes, there can be deletions, duplications, or inversion of parts of the message.  These changes may not be seen in the karyotype of a fetus, but will certainly alter how the genetic message is read and interpreted.