Inside the Nucleus all of the genetic information of a eukaryotic cell is stored in DNA. DNA is extremely long and must be wound together tightly to fit in the nucleus. The entire genome, the genetic instructions for an organism, is stored the DNA in the nucleus of each of that organism's cells.
Just How Small are we Talking?
The number of chromosomes an organism has is different in every species. This number has no correlation to complexity or size.
Humans are diploid, so they have two sets of chromosomes, one from their mom and one from their dad. We have 23 pairs of chromosomes as seen in the karyotype below.
Each pair is called "homologous" meaning they have the same traits. One chromosome came from your mom, and one from your dad, but they both code for the same traits, such as eye color. So for example, if the trait for eye color is on chromosome 1, your mom's chromosome 1 may code for blue eye color and your dad's chromosome 1 might code for brown eye color. Homologous chromosomes have the same genes, just different versions of the genes.
KaryotypeWhen a cell is dividing, its DNA is packed tightly in chromosomes. Scientists are able to extract the chromosomes from a dividing cell and take a picture of them. From this pictures, the chromosomes are pared up with their homologous chromosome and placed in order from largest to smallest. The sex chromosomes (X and Y) are placed at the end of the karyotype, after the smallest pair. From this picture, scientists are able to detect genetic disorders from chromosomal mutations before a child is even born.
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Practice Karyotyping
MutationsA chromosomal mutation can be detected early by looking at a karyotype. For example, this picture shows a karyotype where an individual has a missing or incomplete X chromosome. This causes a disorder called Turner's Syndrome. People who have Turner syndrome develop as females. Some of the genes on the X chromosome are involved in growth and sexual development, which is why girls with the disorder are shorter than normal and have incompletely developed sexual characteristics.
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The Cell Cycle
Check Points
Cells are not always in an environment that is favorable for cell division. Single-celled organisms may be surrounded by too many competitors or not enough resources. Multicellular organisms, such as humans, might have the optimal amount of cells they need to function. During the cell cycle, there are "check points" where enzymes are able to detect if a cell should replicate.
Cells are not always in an environment that is favorable for cell division. Single-celled organisms may be surrounded by too many competitors or not enough resources. Multicellular organisms, such as humans, might have the optimal amount of cells they need to function. During the cell cycle, there are "check points" where enzymes are able to detect if a cell should replicate.
G1/S Checkpoint
During the first growth phase, a checkpoint is reach where the cell "decides" to prepare for replication or not based on chemical interactions with its surroundings. |
Mitosis Checkpoint
During mitosis, once the DNA has separated, a third set of enzymes is able to detect if any mistakes were made. |
G2/S Checkpoint
After DNA is replicated during the synthesis stage of the cell cycle, enzymes are able to check the work done in that stage during the second growth stage. |
After the cell has cleared every checkpoint, it is able to finish dividing and begin the cell cycle all over again.
Cancer
When there are problems with these checkpoints, cells can divide uncontrollably. The cell either continues to tell itself to reproduce or no longer tells itself to stop reproducing when it should stop, due to damage on to proteins at each checkpoint. This uncontrollable cell division can lead to tumors growing and eventually spreading through one's body. If untreated, these masses of cells will grow enough to stop the function of crucial organs.
When there are problems with these checkpoints, cells can divide uncontrollably. The cell either continues to tell itself to reproduce or no longer tells itself to stop reproducing when it should stop, due to damage on to proteins at each checkpoint. This uncontrollable cell division can lead to tumors growing and eventually spreading through one's body. If untreated, these masses of cells will grow enough to stop the function of crucial organs.
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Mitosis
When a cell prepares to divide, it has to separate its genetic material so that each new cell, or daughter cell, will be genetically identical. The process by which a somatic cell (body cell) divides is called mitosis. Mitosis involve four phases. To remember the order of these phases, remember "PMAT," Prophase, Metaphase, Anaphase, and Telophase.
When a cell prepares to divide, it has to separate its genetic material so that each new cell, or daughter cell, will be genetically identical. The process by which a somatic cell (body cell) divides is called mitosis. Mitosis involve four phases. To remember the order of these phases, remember "PMAT," Prophase, Metaphase, Anaphase, and Telophase.
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Need more practice? Try the FREE mitosis app in the app store.
Meiosis
We all started as a sperm cell and an egg cell that came together to form a zygote. These sex cells, or gametes, each had half of our parents chromosomes. So when mom's 23 chromosomes met dad's 23 chromosomes, they formed the 46 chromosomes that are in all of your somatic cells that began as that one zygote. In order to make those original sex cells, your parents' cells had to undergo a specific cell division called meiosis. The phases of meiosis are very similar to mitosis, but what makes them different is also what make you different from your siblings.
We all started as a sperm cell and an egg cell that came together to form a zygote. These sex cells, or gametes, each had half of our parents chromosomes. So when mom's 23 chromosomes met dad's 23 chromosomes, they formed the 46 chromosomes that are in all of your somatic cells that began as that one zygote. In order to make those original sex cells, your parents' cells had to undergo a specific cell division called meiosis. The phases of meiosis are very similar to mitosis, but what makes them different is also what make you different from your siblings.
Prophase I in meiosis involves a step called genetic crossover where homologous match up and their genes can switch and recombine. For example, if your mom's chromosome 1 has traits for blue eyes and blonde hair and your dad's chromosome 1 has traits for brown eyes and brown hair, these traits could switch places and one of your chromosome 1s could now code for blue eyes and brown hair and the other brown eyes and blonde hair.
Metaphase I in meiosis involve the chromosomes lining up with their homologous chromosomes, not just their sister chromatids.
Anaphase I separates the homologous chromosomes, leave the sister chromatids connected.
Telophase I occurs when the nuclear membrane forms, just like in mitosis. and is followed by cytokinesis.
Anaphase I separates the homologous chromosomes, leave the sister chromatids connected.
Telophase I occurs when the nuclear membrane forms, just like in mitosis. and is followed by cytokinesis.
After cytokinesis, it's like mitosis all over again, only with less DNA. The first dividing cell (if human) had 46 pair of chromosomes, so 92 total. After Meiosis I, the new cells have 23 pairs of chromosomes (46 total). Each pair in these cells is a pair of sister chromatids, not homologous chromosomes. After Meiosis II, the new cells will have 23 chromosomes that were previously chromatids.
Prophase II directly follows cytokinesis without the DNA replicating.
Metaphase II proceeds jut like in mitosis, only there are 23 pairs of chromatids, not 46.
Anaphase II also proceeds the same as mitosis, as doe Telophase II
After Cytokinesis, we now have for haploid cells from the one original diploid cell. Each of these cells is a unique combination your genes that were once your parents' genes.
Metaphase II proceeds jut like in mitosis, only there are 23 pairs of chromatids, not 46.
Anaphase II also proceeds the same as mitosis, as doe Telophase II
After Cytokinesis, we now have for haploid cells from the one original diploid cell. Each of these cells is a unique combination your genes that were once your parents' genes.
Oogenesis - the Eggception to the rule
When human egg cells are made, instead of making four equal sized cells, one large egg cell and three small cells called polar bodies. This is why egg cells are the largest cells.
When human egg cells are made, instead of making four equal sized cells, one large egg cell and three small cells called polar bodies. This is why egg cells are the largest cells.
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