DNA, Chromosomes, Genes, and Traits: An Intro to Heredity


Captions are on! Click CC button at bottom right to turn off. Follow us on Twitter (@AmoebaSisters) and Facebook for updates! The week before the 1st day of my first year
of teaching…wow, over a decade ago…was an exciting time. It was also stressful because I’m a bit
of a perfectionist—but a bit of a messy, unable to decorate my classroom kind of perfectionist—actually
maybe I’m not a perfectionist. I wanted to make my classroom inviting and
exciting and…I really wanted my students to walk in and think, “This is AMAZING.” I could never make it look like what I imagined
up. But, the most amazing thing did happen in
the week to follow. I knew a colleague who had a friend who had
a son who was going to college-confusing- but anyway that son could not take his pet
snake to college with him. Some rule about dorms and snakes. My colleague asked, “Would you like it? They said they’d give you the cage and everything;
it could be a classroom pet.” I didn’t even have to think about it. Of course. So Spike, a Texas rat snake, became a classroom
pet, an amazing, popular one at that. He was friendly—well, for a snake… he’d
let you hold him and not bite. That year, I was the only one in the science
wing with a pet snake and I’m pretty sure there were some people that wondered what
was the point. Well, I’m big on a relevance so anytime
I would teach a biology concept, I found some way to work in Spike. Predation? Well, let’s talk about Spike’s appetite
for rats. Mitosis? Let’s talk about why Spike even needs to
make more cells. One day in tutorials, a student asked me,
“Since Spike’s parents were bred in captivity, did you ever see Spike’s parents? Do you think Spike’s parents looked like
him?” You can’t ask a question like that and not
expect an answer! It was a GREAT question because we were getting
close to our heredity unit. Heredity is about how traits are passed down
from parent to offspring. We’ve made a playlist of our videos that
focus on heredity including reproduction, how to track inheritance in pedigrees, how
to solve genetic problems in Punnett squares, and understanding different Mendelian and
non-Mendelian inheritance. But you really can’t delve into those and
study heredity without understanding DNA, chromosomes, genes, and traits- and that’s
what this introductory video is going to focus on. So back to the student question. Spike has traits. The patterns on his body, his size—these
are all traits. These traits are coded for in his DNA. Some of the traits he inherited can be influenced
by the environment. For example, if Spike had not had the nourishment
he needed, that could affect his size. Spike’s DNA—the whole DNA code—is actually
found in nearly all of his body cells. DNA is not just one big code buried deep down
in an organism like some treasure. Spike’s DNA is in the nuclei of nearly all
of his body cells. He inherited his DNA from his mother and father. I can’t know for sure what Spike’s parents
looked like, but I do know that Spike inherited his DNA from them. One fun fact: many snake species can reproduce
asexually. Had that been the case for Spike, he would
have inherited all of his DNA from only one parent. But it would still be DNA coding for traits. Just like Spike, your DNA codes for your traits,
and your cells can’t function without it. DNA determines how tall you are, what color
your eyes are, what color your hair is, or even if you’re at risk for certain diseases. Also like Spike, your ENTIRE DNA code is in
most of your body cells. That’s why in those crime solving shows,
which we may or may not absolutely love, a criminal can sometimes be caught by just leaving
a cell from a hair follicle behind. DNA has a beautiful structure and that structure
will help you understand how inheritance works. DNA stands for deoxyribonucleic acid. It’s a type of nucleic acid. If you remember from our biomolecules video,
nucleic acids are a type of biomolecule. Nucleic acids are made up of building blocks
called nucleotides. Nucleotides have 3 parts. One of them is a sugar called deoxyribose. One of them is a phosphate. We sometimes say DNA has a sugar-phosphate
backbone. But the most important part of the nucleotide
is the base, because the sequence of the bases actually code for traits. So as far as the bases go, there are four
types of bases in DNA. A lot of times they’ll just use the letters
A, T, C, G. The a is for adenine. The T is thymine. The C is for cytosine, and the G is for guanine. These bases actually pair in a specific way,
and there is a popular mnemonic that can help you remember which of them pair together:
apples in the tree; that tells you that A for apples, T for trees, because the bases
A and T go together. The other verse is: car in the garage; that
can help you remember that the base C always goes with the base G. Regardless of whether
we’re talking about Spike, or a plant, or a protist, or a human like you…these are
the DNA bases in living organisms. But the amount of DNA bases overall—and
the sequence of those bases we mentioned—-will vary among different species, and also, among
different individuals. But…it’s likely to infer that Spike has
a sequence of DNA bases that is more similar to his parents than he would to, say, a rattlesnake. DNA has two strands so there are nucleotides
running up one side and there’s nucleotides running up the other side. The bases are what pair in the middle. The bases are held together by hydrogen bonds. The DNA is also twisted in something we call
a double-helix shape. Portions of DNA make up genes. We can say, for example, that this part of
the DNA here makes up a gene. Genes can code for proteins. Proteins can have a huge role in expressing
a trait. For example, let’s consider your own eye
color. Human eye color is a pretty complex trait
that is actually determined by many genes. The genes can code for proteins involved in
producing the eye color pigment. But proteins coded for by genes play a wide
variety of roles besides just your eye color. Proteins are involved in transport, in structure,
in acting as enzymes that can make all kinds of materials, in protecting the body…and
so much more. We do want to mention that not all genes are
used to make protein. And there are parts of DNA that are noncoding. And even though nearly all of your body cells
have your entire DNA code—your body cells may only use certain portions of those genes. Genes can be turned on or turned off by a
variety of mechanisms. We call that gene regulation; check out our
video on that. Now you have a lot of DNA. When it is compacted, it can be organized
into a unit called a chromosome. Very helpful when you’re trying to make
more cells and need to get the DNA into those new cells. Chromosomes in your body involve DNA wrapped
around protein structures. Humans have 46 chromosomes. That means nearly every body cell in your
body has 46 chromosomes. Human sperm and egg cells, on the other hand,
each contain 23 chromosomes. So you received 23 chromosomes from your mother
and 23 chromosomes from your father to give you your 46 chromosomes. Your genetic code. So let’s do a recap of the big picture:
here’s a single chromosome. You see genes on this chromosome. These genes consist of portions of DNA. DNA is made up of nucleotides, and it’s
these bases here—the sequence of them—that makes the difference in coding traits. Phew. Understanding this foundation is essential
for understanding heredity—whether you’re talking about you—-or Spike. So this may bring up more questions now like
where do dominant and recessive traits come in? What about alleles and how do you arrange
those around a Punnett square? You can explore those concepts and more in
our heredity playlist. Well that’s it for the Amoeba Sisters and
we remind you to stay curious!

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