WEBVTT - Why Do Most People Have 46 Chromosomes? 0:00:01.920 --> 0:00:06.400 Welcome to brain Stuff, a production of iHeart Radio. Hey 0:00:06.440 --> 0:00:09.639 brain Stuff, Lauren Bogel bum here. You may have heard 0:00:09.640 --> 0:00:11.920 that a hundred and thirty seven is the magic number, 0:00:12.240 --> 0:00:14.720 but if you ask a geneticist, they'll tell you that 0:00:14.760 --> 0:00:18.599 the real magic is in forty six. Why forty six 0:00:19.040 --> 0:00:22.400 because that's the total number of chromosomes found in most 0:00:22.520 --> 0:00:26.160 every human cell, twenty three pairs to be exact, and 0:00:26.400 --> 0:00:29.840 those little red like structures pack all the information about 0:00:29.880 --> 0:00:33.960 who you are and what makes you unique. To understand 0:00:34.000 --> 0:00:37.479 what chromosomes are, first, we have to understand what DNA is. 0:00:37.920 --> 0:00:41.400 Formally known as de oxy ribonucleic acid, DNA is a 0:00:41.400 --> 0:00:45.000 complex molecule found in all plants and animals. It's found 0:00:45.040 --> 0:00:47.760 in almost every cell of an organism's body, and it 0:00:47.800 --> 0:00:51.199 contains all of the information necessary to keep that organism 0:00:51.280 --> 0:00:55.680 up and running and developing and reproducing. DNA is also 0:00:55.720 --> 0:00:59.680 the primary way that organisms pass on hereditary information and 0:00:59.760 --> 0:01:02.600 the access of reproduction. A portion of DNA is passed 0:01:02.640 --> 0:01:06.360 along from parent to offspring. In short, DNA is what 0:01:06.520 --> 0:01:11.080 tells the story of your totally unique biology. As you 0:01:11.120 --> 0:01:14.319 can probably imagine, DNA has to be pretty long to 0:01:14.360 --> 0:01:18.240 hold all of that important information, and it is. If 0:01:18.240 --> 0:01:21.200 you stretched the DNA and just one cell all the 0:01:21.240 --> 0:01:23.280 way to its full length, it would be about six 0:01:23.280 --> 0:01:25.960 and a half feet or two meters long. And if 0:01:26.000 --> 0:01:28.720 you put the DNA and all your cells together, you'd 0:01:28.720 --> 0:01:33.840 have a chain about twice the diameter of the solar system. Luckily, 0:01:34.000 --> 0:01:37.160 cells are pretty smart and have an ingenious way of 0:01:37.200 --> 0:01:42.199 packaging all that info into space efficient parcels. Enter the chromosome, 0:01:43.280 --> 0:01:45.760 with its name rooted in the Greek words for color 0:01:45.880 --> 0:01:50.120 chroma and body soma. The chromosome is a cell structure 0:01:50.360 --> 0:01:53.280 or body that scientists can spot under a microscope by 0:01:53.280 --> 0:01:57.400 staining it with colored dyes and get it. Each chromosome 0:01:57.480 --> 0:02:00.080 is made up of protein and YEP d n A. 0:02:01.280 --> 0:02:05.280 Every chromosome contains exactly one molecule of DNA to be exact, 0:02:05.600 --> 0:02:08.760 and that long string of genetic info is tightly wrapped 0:02:08.760 --> 0:02:11.600 around the protein called a his stone, which acts like 0:02:11.639 --> 0:02:15.640 a spool, efficiently bundling the lengthy info rich molecule into 0:02:15.680 --> 0:02:18.040 the perfect size and shape to fit inside the nucleus 0:02:18.080 --> 0:02:21.520 of a cell. Human cells typically have twenty three pairs 0:02:21.520 --> 0:02:24.800 of chromosomes for a total of forty six aside from 0:02:24.840 --> 0:02:27.440 sperm and egg cells, which each contain only half of 0:02:27.440 --> 0:02:31.600 that that's twenty three chromosomes. The magic number of forty 0:02:31.639 --> 0:02:34.640 six or twenty three pairs of chromosomes per cell isn't 0:02:34.720 --> 0:02:38.639 universal among living things. Of first, humans happen to be 0:02:38.680 --> 0:02:41.839 a diploid species, which means that most of our chromosomes 0:02:41.880 --> 0:02:45.240 come in matched sets called homologous pairs. The two members 0:02:45.280 --> 0:02:48.560 of each pair are called homologus. A lot of animals 0:02:48.560 --> 0:02:51.120 and plants are diploid, but not all of them have 0:02:51.320 --> 0:02:55.040 a total number of forty six chromosomes. Mosquitoes, for example, 0:02:55.240 --> 0:02:59.560 have a diploid chromosome number of six, frogs have twenty six, 0:02:59.800 --> 0:03:03.160 and to shrimp have a whopping five hundred and eight chromosomes. 0:03:04.200 --> 0:03:07.360 But why do humans have twenty three pairs? It happened 0:03:07.440 --> 0:03:11.200 during evolution. For the article, this episode is based on 0:03:11.320 --> 0:03:14.639 How Stuff Works. Spoken by email with Berlin Hurley, PhD, 0:03:14.919 --> 0:03:17.919 a research fellow at the National Human Genome Research Institute 0:03:18.000 --> 0:03:21.720 at the National Institutes of Health. She said humans have 0:03:21.800 --> 0:03:25.520 twenty three pairs of chromosomes, while all other great apes, chipanzees, 0:03:25.560 --> 0:03:30.000 Bonnabo's guerrillas, and orangutans have twenty four pairs of chromosomes. 0:03:30.200 --> 0:03:34.080 This is because in the human evolutionary lineage, two ancestral 0:03:34.120 --> 0:03:38.440 ape chromosomes fused at their telomeres or tips, producing human 0:03:38.520 --> 0:03:43.000 chromosome two. Thus, humans have one fewer pair of chromosomes. 0:03:43.320 --> 0:03:45.320 This is one of the main differences between the human 0:03:45.360 --> 0:03:50.160 genome and the genome of our closest relatives. Now let's 0:03:50.200 --> 0:03:53.480 get back to that sperm and egg issue. These cells 0:03:53.520 --> 0:03:56.960 only have one homologous chromosome from each pair and are 0:03:57.000 --> 0:04:02.120 therefore considered haploid. Here's why. When a sperm and egg fuse, 0:04:02.320 --> 0:04:05.960 they combine their genetic material to form one complete diploid 0:04:06.080 --> 0:04:08.880 set of chromosomes. And if you think about it, that 0:04:08.960 --> 0:04:12.280 makes perfect sense. It means each parent contributes one set 0:04:12.360 --> 0:04:16.679 to a pair of chromosomes in their child cells. Consider 0:04:16.720 --> 0:04:19.880 blood type as a clear example. People with a B 0:04:19.880 --> 0:04:23.280 blood type inherited two different gene variations on their two 0:04:23.279 --> 0:04:27.159 homologous chromosomes, one for A and one for B that 0:04:27.480 --> 0:04:32.360 when combined, produced a B Okay, So twenty three pairs 0:04:32.360 --> 0:04:35.679 of chromosomes is typical for humans. But life doesn't always 0:04:35.720 --> 0:04:38.679 go by the book. So what happens if something causes 0:04:38.720 --> 0:04:42.360 more or fewer chromosomes to develop. A gain or loss 0:04:42.360 --> 0:04:45.640 of chromosomes from these standard forty six is called an euploidy, 0:04:45.920 --> 0:04:48.720 and it can occur either during the formation of reproductive 0:04:48.760 --> 0:04:51.920 cells the sperm in the egg, or in early fetal development, 0:04:52.360 --> 0:04:54.679 or in any other cell of the body after birth. 0:04:55.839 --> 0:04:58.640 One of the most common forms of antiploidy is trisome, 0:04:58.920 --> 0:05:01.560 which is the presence of an extra chromosome in the cells. 0:05:02.200 --> 0:05:05.400 One well known result of trisome is Down syndrome, which 0:05:05.480 --> 0:05:08.279 is a condition caused by three copies of chromosome twenty 0:05:08.320 --> 0:05:11.640 one in each cell. This extra chromosome leads to a 0:05:11.680 --> 0:05:15.239 total of forty seven chromosomes per cell rather than forty six. 0:05:16.600 --> 0:05:19.479 The loss of one chromosome in a cell is called monosomey. 0:05:19.920 --> 0:05:22.280 That's when people have just one copy of a specific 0:05:22.320 --> 0:05:26.880 chromosome per cell as opposed to two. For example, Turner syndrome, 0:05:26.960 --> 0:05:29.160 in which women have only one copy of the X 0:05:29.240 --> 0:05:32.359 chromosome per cell versus the regular two, is a form 0:05:32.440 --> 0:05:37.960 of monosomey. There are other variations of ante eploidy as well, 0:05:38.279 --> 0:05:42.800 and in extreme cases, they may compromise a person's life. Additionally, 0:05:42.920 --> 0:05:46.320 cancer cells also have alterations in their number of chromosomes. 0:05:46.960 --> 0:05:50.400 Unlike the variations that happen in reproductive cells, these changes 0:05:50.480 --> 0:05:58.440 cannot be inherited. Today's episode is based on the article 0:05:58.560 --> 0:06:01.360 why do most humans have three pairs of chromosomes? On 0:06:01.520 --> 0:06:05.360 how stuff works dot Com written by Michelle Konstantinovski. Brain 0:06:05.400 --> 0:06:07.640 Stuff is production of I Heart Radio in partnership with 0:06:07.640 --> 0:06:10.320 how stuff works dot Com and is produced by Tyler Clang. 0:06:10.800 --> 0:06:13.200 Four more podcasts my heart Radio, visit the I heart 0:06:13.240 --> 0:06:15.800 Radio app, Apple Podcasts, or wherever you listen to your 0:06:15.800 --> 0:06:16.480 favorite shows.