chikcen and rice
It is the failure of chromosome pairs to separate properly during cell division.
Since gametes divide by a special process, meiosis, they will always have half the number of chromosomes as regular body cells. This is because the chromosomes only copy themselves once, however they are split apart twice. Think of it as a math problem. If the original cell count is, for example, 2 chromosomes, and each chromosome makes a copy of itself, the cell will have 4 chromosomes. During meiosis 1, the cell divides, and the chromosomes split, resulting in each daughter cell having 2 chromosomes, the same as the original. However, in meiosis 2, they divide again without making another copy of themselves, so that by the time meiosis is done, each one of the 4 cells that results have 1 chromosome, half the number of the original cell. 2x2=4 4/2=2 2/2=1
Meiosis creates gametes (sperm and ova/eggs) which are involved in sexual reproduction. The result of meiosis is cells with half the number of chromosomes as a somatic (non-sex) cell. This ensures that when the gametes join during sexual reproduction, the resulting organism has the correct number of chromosomes (half from each gamete). Thus meiosis maintains the chromosome number.
There are three sources of genetic variation in meiosis: 1) Independent assortment = like shuffling a deck of cards and dealing out half the deck 2) Crossing over = homologous chromosomes swap parts, making new combinations 3) Random fertilization = only one sperm cell makes it into the egg
Meiosis is the process by which gametes or sex cells (ie sperm and ova) are made. Gametes have only one copy of each chromosome (haploid) where as the progenitor cells have two copies (diploid).
It is the failure of chromosome pairs to separate properly during cell division.
Meiosis
technically the chromosomes copy during interphase right before metaphase I of meiosis I so during Meiosis I the chromosomes are duplicated and not until metaphase II during meiosis II are the sister chromatids separated.
Mitosis occurs in almost all eukaryotic cells. The duplication of chromosomes only occurs during the S phase of interphase in mitosis to form chromatids.
People with Down syndrome have an extra copy of chromosome 21 in every cell. Down syndrome most often occurs when chromosomes fail to separate properly during meiosis, when sex cells form. Hope this helps!!
In Down syndrome, a person's cells have an extra copy of chromosome 21. In other words, instead of a pair of chromosomes, a person with Down syndrome has three of that chromosome. Down syndrome most often occurs when chromosomes fail to separate properly during meiosis. People with Down syndrome have a degree of mental retardation. Heart defects are also common, but can be treated.
Since gametes divide by a special process, meiosis, they will always have half the number of chromosomes as regular body cells. This is because the chromosomes only copy themselves once, however they are split apart twice. Think of it as a math problem. If the original cell count is, for example, 2 chromosomes, and each chromosome makes a copy of itself, the cell will have 4 chromosomes. During meiosis 1, the cell divides, and the chromosomes split, resulting in each daughter cell having 2 chromosomes, the same as the original. However, in meiosis 2, they divide again without making another copy of themselves, so that by the time meiosis is done, each one of the 4 cells that results have 1 chromosome, half the number of the original cell. 2x2=4 4/2=2 2/2=1
Some gametes may have an extra copy of some genes.
Down Syndrome is known as trisomy 21 because someone with the syndrome has 3 chromosomes of the same type when we are only supposed to have 2 (one from each parent). Chromosomes are paired and numbered so Down Syndrome is the result of having 3 of the #21 chromosome. The body will not know what to do with the extra chromosome so as a precautionary measure it will shut down all 3 and the genes that are within the chromosomes can no longer be accessed. So as you can see, Down Syndrome is not from being inherited by a specific gene. It is due to an error in meiosis in either the mother or the father.
During _______, a cell grows, makes a copy of its chromosomes, or ___________ material, and prepares for cell division.
Each chromosome makes a copy The homologous chromosomes form pairs. Chromosomes line up in the middle of the cell. Chromosomes condense or shorten up before cell division begins 1 of those 4
Crossing over begins early in prophase I of meiosis. At the time, homologous chromosomes Are paired along their lengths. Each gene on one homologue ia alined precisely with the corresponding gene on the other homologue. Source: my science book copy right 2006