Why Do Most People Have 46 Chromosomes?

Chromosomes contain the DNA that codes our very existence, and most people have 23 pairs. Learn how they work in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/life/genetic/23-pairs-chromosomes.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of iHeart Radio. Hey
brain Stuff, Lauren Bogel bum here. You may have heard
that a hundred and thirty seven is the magic number,
but if you ask a geneticist, they'll tell you that
the real magic is in forty six. Why forty six
because that's the total number of chromosomes found in most

(00:22):
every human cell, twenty three pairs to be exact, and
those little red like structures pack all the information about
who you are and what makes you unique. To understand
what chromosomes are, first, we have to understand what DNA is.
Formally known as de oxy ribonucleic acid, DNA is a
complex molecule found in all plants and animals. It's found

(00:45):
in almost every cell of an organism's body, and it
contains all of the information necessary to keep that organism
up and running and developing and reproducing. DNA is also
the primary way that organisms pass on hereditary information and
the access of reproduction. A portion of DNA is passed
along from parent to offspring. In short, DNA is what

(01:06):
tells the story of your totally unique biology. As you
can probably imagine, DNA has to be pretty long to
hold all of that important information, and it is. If
you stretched the DNA and just one cell all the
way to its full length, it would be about six
and a half feet or two meters long. And if
you put the DNA and all your cells together, you'd

(01:28):
have a chain about twice the diameter of the solar system. Luckily,
cells are pretty smart and have an ingenious way of
packaging all that info into space efficient parcels. Enter the chromosome,
with its name rooted in the Greek words for color
chroma and body soma. The chromosome is a cell structure

(01:50):
or body that scientists can spot under a microscope by
staining it with colored dyes and get it. Each chromosome
is made up of protein and YEP d n A.
Every chromosome contains exactly one molecule of DNA to be exact,
and that long string of genetic info is tightly wrapped
around the protein called a his stone, which acts like

(02:11):
a spool, efficiently bundling the lengthy info rich molecule into
the perfect size and shape to fit inside the nucleus
of a cell. Human cells typically have twenty three pairs
of chromosomes for a total of forty six aside from
sperm and egg cells, which each contain only half of
that that's twenty three chromosomes. The magic number of forty
six or twenty three pairs of chromosomes per cell isn't

(02:34):
universal among living things. Of first, humans happen to be
a diploid species, which means that most of our chromosomes
come in matched sets called homologous pairs. The two members
of each pair are called homologus. A lot of animals
and plants are diploid, but not all of them have
a total number of forty six chromosomes. Mosquitoes, for example,

(02:55):
have a diploid chromosome number of six, frogs have twenty six,
and to shrimp have a whopping five hundred and eight chromosomes.
But why do humans have twenty three pairs? It happened
during evolution. For the article, this episode is based on
How Stuff Works. Spoken by email with Berlin Hurley, PhD,
a research fellow at the National Human Genome Research Institute

(03:18):
at the National Institutes of Health. She said humans have
twenty three pairs of chromosomes, while all other great apes, chipanzees,
Bonnabo's guerrillas, and orangutans have twenty four pairs of chromosomes.
This is because in the human evolutionary lineage, two ancestral
ape chromosomes fused at their telomeres or tips, producing human

(03:38):
chromosome two. Thus, humans have one fewer pair of chromosomes.
This is one of the main differences between the human
genome and the genome of our closest relatives. Now let's
get back to that sperm and egg issue. These cells
only have one homologous chromosome from each pair and are
therefore considered haploid. Here's why. When a sperm and egg fuse,

(04:02):
they combine their genetic material to form one complete diploid
set of chromosomes. And if you think about it, that
makes perfect sense. It means each parent contributes one set
to a pair of chromosomes in their child cells. Consider
blood type as a clear example. People with a B
blood type inherited two different gene variations on their two

(04:23):
homologous chromosomes, one for A and one for B that
when combined, produced a B Okay, So twenty three pairs
of chromosomes is typical for humans. But life doesn't always
go by the book. So what happens if something causes
more or fewer chromosomes to develop. A gain or loss
of chromosomes from these standard forty six is called an euploidy,

(04:45):
and it can occur either during the formation of reproductive
cells the sperm in the egg, or in early fetal development,
or in any other cell of the body after birth.
One of the most common forms of antiploidy is trisome,
which is the presence of an extra chromosome in the cells.
One well known result of trisome is Down syndrome, which
is a condition caused by three copies of chromosome twenty

(05:08):
one in each cell. This extra chromosome leads to a
total of forty seven chromosomes per cell rather than forty six.
The loss of one chromosome in a cell is called monosomey.
That's when people have just one copy of a specific
chromosome per cell as opposed to two. For example, Turner syndrome,
in which women have only one copy of the X

(05:29):
chromosome per cell versus the regular two, is a form
of monosomey. There are other variations of ante eploidy as well,
and in extreme cases, they may compromise a person's life. Additionally,
cancer cells also have alterations in their number of chromosomes.
Unlike the variations that happen in reproductive cells, these changes

(05:50):
cannot be inherited. Today's episode is based on the article
why do most humans have three pairs of chromosomes? On
how stuff works dot Com written by Michelle Konstantinovski. Brain
Stuff is production of I Heart Radio in partnership with
how stuff works dot Com and is produced by Tyler Clang.

(06:10):
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