Speaker 1: Welcome to brain Stuff, a production of iHeart Radio. Hey
Speaker 1: brain Stuff, Lauren Bogel bum here. You may have heard
Speaker 1: that a hundred and thirty seven is the magic number,
Speaker 1: but if you ask a geneticist, they'll tell you that
Speaker 1: the real magic is in forty six. Why forty six
Speaker 1: because that's the total number of chromosomes found in most
Speaker 1: every human cell, twenty three pairs to be exact, and
Speaker 1: those little red like structures pack all the information about
Speaker 1: who you are and what makes you unique. To understand
Speaker 1: what chromosomes are, first, we have to understand what DNA is.
Speaker 1: Formally known as de oxy ribonucleic acid, DNA is a
Speaker 1: complex molecule found in all plants and animals. It's found
Speaker 1: in almost every cell of an organism's body, and it
Speaker 1: contains all of the information necessary to keep that organism
Speaker 1: up and running and developing and reproducing. DNA is also
Speaker 1: the primary way that organisms pass on hereditary information and
Speaker 1: the access of reproduction. A portion of DNA is passed
Speaker 1: along from parent to offspring. In short, DNA is what
Speaker 1: tells the story of your totally unique biology. As you
Speaker 1: can probably imagine, DNA has to be pretty long to
Speaker 1: hold all of that important information, and it is. If
Speaker 1: you stretched the DNA and just one cell all the
Speaker 1: way to its full length, it would be about six
Speaker 1: and a half feet or two meters long. And if
Speaker 1: you put the DNA and all your cells together, you'd
Speaker 1: have a chain about twice the diameter of the solar system. Luckily,
Speaker 1: cells are pretty smart and have an ingenious way of
Speaker 1: packaging all that info into space efficient parcels. Enter the chromosome,
Speaker 1: with its name rooted in the Greek words for color
Speaker 1: chroma and body soma. The chromosome is a cell structure
Speaker 1: or body that scientists can spot under a microscope by
Speaker 1: staining it with colored dyes and get it. Each chromosome
Speaker 1: is made up of protein and YEP d n A.
Speaker 1: Every chromosome contains exactly one molecule of DNA to be exact,
Speaker 1: and that long string of genetic info is tightly wrapped
Speaker 1: around the protein called a his stone, which acts like
Speaker 1: a spool, efficiently bundling the lengthy info rich molecule into
Speaker 1: the perfect size and shape to fit inside the nucleus
Speaker 1: of a cell. Human cells typically have twenty three pairs
Speaker 1: of chromosomes for a total of forty six aside from
Speaker 1: sperm and egg cells, which each contain only half of
Speaker 1: that that's twenty three chromosomes. The magic number of forty
Speaker 1: six or twenty three pairs of chromosomes per cell isn't
Speaker 1: universal among living things. Of first, humans happen to be
Speaker 1: a diploid species, which means that most of our chromosomes
Speaker 1: come in matched sets called homologous pairs. The two members
Speaker 1: of each pair are called homologus. A lot of animals
Speaker 1: and plants are diploid, but not all of them have
Speaker 1: a total number of forty six chromosomes. Mosquitoes, for example,
Speaker 1: have a diploid chromosome number of six, frogs have twenty six,
Speaker 1: and to shrimp have a whopping five hundred and eight chromosomes.
Speaker 1: But why do humans have twenty three pairs? It happened
Speaker 1: during evolution. For the article, this episode is based on
Speaker 1: How Stuff Works. Spoken by email with Berlin Hurley, PhD,
Speaker 1: a research fellow at the National Human Genome Research Institute
Speaker 1: at the National Institutes of Health. She said humans have
Speaker 1: twenty three pairs of chromosomes, while all other great apes, chipanzees,
Speaker 1: Bonnabo's guerrillas, and orangutans have twenty four pairs of chromosomes.
Speaker 1: This is because in the human evolutionary lineage, two ancestral
Speaker 1: ape chromosomes fused at their telomeres or tips, producing human
Speaker 1: chromosome two. Thus, humans have one fewer pair of chromosomes.
Speaker 1: This is one of the main differences between the human
Speaker 1: genome and the genome of our closest relatives. Now let's
Speaker 1: get back to that sperm and egg issue. These cells
Speaker 1: only have one homologous chromosome from each pair and are
Speaker 1: therefore considered haploid. Here's why. When a sperm and egg fuse,
Speaker 1: they combine their genetic material to form one complete diploid
Speaker 1: set of chromosomes. And if you think about it, that
Speaker 1: makes perfect sense. It means each parent contributes one set
Speaker 1: to a pair of chromosomes in their child cells. Consider
Speaker 1: blood type as a clear example. People with a B
Speaker 1: blood type inherited two different gene variations on their two
Speaker 1: homologous chromosomes, one for A and one for B that
Speaker 1: when combined, produced a B Okay, So twenty three pairs
Speaker 1: of chromosomes is typical for humans. But life doesn't always
Speaker 1: go by the book. So what happens if something causes
Speaker 1: more or fewer chromosomes to develop. A gain or loss
Speaker 1: of chromosomes from these standard forty six is called an euploidy,
Speaker 1: and it can occur either during the formation of reproductive
Speaker 1: cells the sperm in the egg, or in early fetal development,
Speaker 1: or in any other cell of the body after birth.
Speaker 1: One of the most common forms of antiploidy is trisome,
Speaker 1: which is the presence of an extra chromosome in the cells.
Speaker 1: One well known result of trisome is Down syndrome, which
Speaker 1: is a condition caused by three copies of chromosome twenty
Speaker 1: one in each cell. This extra chromosome leads to a
Speaker 1: total of forty seven chromosomes per cell rather than forty six.
Speaker 1: The loss of one chromosome in a cell is called monosomey.
Speaker 1: That's when people have just one copy of a specific
Speaker 1: chromosome per cell as opposed to two. For example, Turner syndrome,
Speaker 1: in which women have only one copy of the X
Speaker 1: chromosome per cell versus the regular two, is a form
Speaker 1: of monosomey. There are other variations of ante eploidy as well,
Speaker 1: and in extreme cases, they may compromise a person's life. Additionally,
Speaker 1: cancer cells also have alterations in their number of chromosomes.
Speaker 1: Unlike the variations that happen in reproductive cells, these changes
Speaker 1: cannot be inherited. Today's episode is based on the article
Speaker 1: why do most humans have three pairs of chromosomes? On
Speaker 1: how stuff works dot Com written by Michelle Konstantinovski. Brain
Speaker 1: Stuff is production of I Heart Radio in partnership with
Speaker 1: how stuff works dot Com and is produced by Tyler Clang.
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