Meiosis is a specialized form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the formation of gametes in animals. Meiosis is a special type of cell division that occurs during formation of sperm and egg cells and gives them the correct number of chromosomes. Since a sperm and egg unite during fertilization, each must have only half the number of chromosomes other body cells have.
This is not the case, because the chromosomes are reduced to half their normal number in germ cell production. This process is called meiosis. It consists of two successive mitosis-like divisions: in the first division is the number of chromosomes reduced to their half, the second is a normal mitosis. Each germ cell contains a complete set of chromosomes, a haploid set. Accordingly are the cells haploid and zygotes and the body cells that stem from them are diploid, because they contain two equal sets of chromosomes, one from the mother and one from the father.
This helps to create genetic diversity. Anaphase 1 is where the bivalents separate and the homologous chromosomes move to opposite poles of the cell. Telophase 1 is where the nuclear envelope reforms after disintegrating in prophase 1. Then cytokinesis is where the cell divides to create two new cells which are haploid (GENIE, 2010). The next main stage is meiosis 2 and this is where each chromosome is split into 2 sister chromatids.
Down Syndrome is also (or formally) known as mongolism, and is the most frequent chromosomal abnormality in the United States. Over 350,000 people carry this condition. It is caused by an extra 21st chromosome. This is also known as trisomy 21. Instead of 46 chromosomes in each cell it was determined that an extra 21st chromosome affected Down syndrome, this occurs 95% of the time.
Chromosomes come in pairs and these pairs are called homologous pairs. Homologous chromosomes mean that both of the chromosomes in the pair have the same gene sequence. Human somatic cells(diploid) contain 46 chromosomes consisting of 23 homologous pairs; therefore, human gametes, which are haploid, have 23 total chromosomes-one from each homologous pair. Just like in mitosis, meiosis undergoes all of the same phases. However, because meiosis involves two divisions, the cell goes through all the phases twice.
Recently, attention has been turned to another type of genetic variation that of differences in the number of repeated copies of a segment of DNA. These sequences can be classified based on decreasing sizes into satellites, minisatellites and microsatellites (Tautz, 1993). Satellites consist of units of several thousand base pairs, repeated thousands or millions of times. Minisatellites consist of DNA sequences of some 9-100bp in length that is repeated from 2 to several 100 times at a locus. Minisatellites discovered in human insulin gene loci with repeat unit lengths between 10 and 64 bp were also referred to as ‘Variable Number of Tandem Repeats’ (VNTRs) DNA (Nakamura et al., 1987).
Human cells normally contain 23 pairs of chromosomes to make a total of 46 chromosomes in each cell. When sex cells (eggs and sperm) divide through the process of meiosis, one cell splits into two parts so that each of the resulting cells only has 23 chromosomes, rather than 46. But many errors can occur during cell division. During meiosis, the chromosomes are supposed to split and go to different areas of the cell. This step in the process of meiosis is called disjunction.
Meiosis is where the daughter cell has one half the numbers of chromosomes as the parent cells and involves the divisions of two cells and therefore produces four daughter cells. Meiosis undergoes the same phases as Mitosis but undergoes two times. “In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated” (Genome).
Humans are estimated to have about 20,000 to 30,000 genes in their genome. Chromosomes contain these genes and DNA. Humans have 23 pairs of chromosomes or a total of 46 chromosomes. One pair of these chromosomes determines the sex of a person while the other 22 are autosomal, meaning that they determine the rest of the body’s traits, both genetic and phenotypic. Sometimes mutations occur in the genes of the chromosomes which could lead to a genetic disorder or could be perfectly harmless.
Most DNA is found in the nucleus and a small amount is found in the Mitochondria. When DNA is found in the nucleus it is considered to be Nuclear DNA and when DNA is found in the Mitochondria it is considered Mitochondrial DNA (“U.S. National”, 2009). Humans always get 50% of DNA from your mom and 50% of DNA from your dad through reproduction; although only Mitochondrial DNA is from your mom because during fertilization egg cells only keep Mitochondria (“National Human” 2008). Sometimes DNA can be found in bacterial cells and in viruses (Rubenstein, 2006).