Learn how to change a conflicting keyboard shortcut.) (If you use multiple input sources to type in different languages, these shortcuts change input sources instead of showing Spotlight. To perform a Spotlight search from a Finder window, press Command–Option–Space bar. Command–Space bar: Show or hide the Spotlight search field.To close all windows of the app, press Option-Command-W. Command-O: Open the selected item, or open a dialog to select a file to open.To minimize all windows of the front app, press Option-Command-M. Command-M: Minimize the front window to the Dock.To view the front app but hide all other apps, press Option-Command-H. Command-H: Hide the windows of the front app.To find the previous occurrence, press Shift-Command-G. Command-G: Find Again: Find the next occurrence of the item previously found.Command-F: Find items in a document or open a Find window.In some apps, you can undo and redo multiple commands. You can then press Shift-Command-Z to Redo, reversing the undo command. Command-V: Paste the contents of the Clipboard into the current document or app.Command-C: Copy the selected item to the Clipboard.Command-X: Cut the selected item and copy it to the Clipboard.When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy. This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells.īesides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria. When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome. Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. Most bacteria have one or two circular chromosomes. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people.Ĭhromosomes vary in number and shape among living things. If not, the resulting offspring may fail to develop properly. It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes. Still, mistakes do occur on rare occasions.Ĭhanges in the number or structure of chromosomes in new cells may lead to serious problems. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions. During cell division, it is essential that DNA remains intact and evenly distributed among cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet.įor an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. Without such packaging, DNA molecules would be too long to fit inside cells. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones.
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