Question: Introduction It is nals week on college campus and everybody is in the middle of studying for exa…

Introduction

It is nals week on college campus and everybody is in the middle
of studying for exams, just waiting for summer break. Björn,
Chris, Esiankiki, Xiao-Ma, Sanjeet and Linda are studying together
in the international students dorm.

Björn: Chris: Esiankiki:

Xiao-Ma:

Linda: Xiao-Ma:

Sanjeet:

Hey, how are you doing with studying? I’m tired …
Me too. Let’s take a break. I could also use some ice cream after
that last nal I bombed.

DidIhearice-cream?Iloveice-cream!Let’sgodowntownand
ndaspot.Iseverybodycoming? Let’s go!

Hm, I can’t tolerate dairy so I usually don’t get ice-cream or
other milk products. Sorry guys, I’ll stay behind.

What does that mean? Do you just not like milk? I used to hate
milk as a little kid.

No, I actually get really bad stomach cramps, bloating and
diarrhea from milk. It’s not pretty. My whole family is that way.
When we were still living in China, it wasn’t a big deal because we
don’t tradi- tionally eat a lot of milk products there.

Same for me! I get the same thing, and in India we don’t eat a
lot of dairy either, so it’s not a problem if I eat Indian food. My
mom gave me Lactaid® though, so that helps me to eat ice-cream.
Xiao-Ma, I can give you some. Let’s go!

Part I – Geographical Variation in Tolerance to Dairy

I.i – Survey of Dairy Intolerance

Björn: Linda:

Wait, so are you saying that where you’re from, people are
generally not able to digest milk? At home in Sweden, we eat dairy
products like milk and cheese all the time and everyone I know
seems to tolerate it well.

at’s so interesting! I’m curious to see how that maps to the
world and if what you observed could be generally true. Let’s do a
survey in the international dorm and nd out who is tolerant to
dairy and where they are from. I’ll put a black dot for a dairy
tolerant person in his or her home country and a white dot for a
dairy intolerant person. I’ll start with Xiao-Ma—one white dot in
China!

Part II — Lactase Catalyzes the Hydrolysis of Lactose

II.i – Enzymes and Sugars

e group of college students is on their way to the ice-cream
shop and Sanjeet has o ered Xiao-Ma Lactaid so that she could go
with the group and also eat ice cream despite her lactose
intolerance.

Xiao-Ma:

Sanjeet:

Chris: Sanjeet:

I’ve never taken Lactaid. What is it?

My mom told me that you become intolerant to dairy because you
don’t have the enzyme that can digest lactose, which is the sugar
found in milk products. Lactaid is a dietary supplement that
contains the lactase enzyme.

Uh, Sanjeet, remember—I’m a literature major. What’s an enzyme?
I don’t remember what that word really means. Also, is the sugar in
the milk di erent from the other sugar in my food?

Finally my biochem class comes in handy in real life!
An enzyme is a protein which catalyzes a reaction in the cell. ere
are tons of di erent enzymes in your body. e lactase enzyme
catalyzes the degrada- tion of lactose into its subunits. Lactose
is a sugar and there are several other sugars; the one that you
probably know is sucrose. Sucrose consists of two parts: glucose
and fructose. Lactose is a di erent kind of sugar and consists of
glucose and galactose.

You need to digest lactose into the two components because the
cells in your intestine can only take up galactose and glucose but
not lactose. If lactose stays in the intestine and is not digested
into its com- ponents, it will be consumed by gut bacteria which
produce various gases in the large intestine—that’s what leads to
the symptoms Xiao-Ma and I experience when we consume dairy.

Here’s a picture from my biochem book showing how the lactase
enzyme breaks down the milk sugar lactose into its components
glucose and galactose.

It also says in here that you can test if you are intolerant to
dairy (so lactose intolerant) by measuring

the level of glucose in your blood after drinking milk.

II.ii – Blood Glucose Test

Esiankiki:

Xiao-Ma: Sanjeet:

Chris: Esiankiki:

Oh, now I remember—I’ve heard of this test before. My mom’s a
physician with her own o ce here in town. I know that my mom is
participating in a clinical trial for a new lactose intolerance
test that measures blood glucose.

Now I wonder if I’m lactose intolerant. I once had really bad
stomach aches when I ate ice cream. Maybe that’s why? If we go to
her o ce, we could try to participate in the trial and if we ask
nicely, she might give us the data. Wouldn’t it be nice to know for
sure if you are lactose intolerant?

Yes, I would love that!

And you know what’s interesting—when I was a baby, I didn’t have
these issues but I was able to digest milk just ne as I was
breastfed.

Wow that’s fascinating—so as a baby you probably did produce the
lactase enzyme and, if you are right, then your body at some point
just stopped making it. Suddenly biology is starting to be more
interesting…

Well, that would explain why my mom calls people who can eat
dairy without problems “lactase persistent.” I never really
understood what she meant by that. She probably means that the
production of lactase enzyme persists in those individuals when
they are adults. Let’s go and do that blood test; I’m really
curious now.

Here are the results that the group of friends obtained from
Esiankiki’s mother. Each of them had to drink a liquid that
contains lactose. At the time the liquid was consumed, a physician
assistant took a blood glucose measurement and repeated this every
30 minutes for two hours.

NATIONAL CENTER FOR CASE STUDY TEACHING IN
SCIENCE

Minutes after drinking milk:	Blood glucose (mg/dL)
	0	30	60	90	120
Björn	117	128	146	160	152
Chris	97	111	135	154	143
Xiao-Ma	96	99	105	101	98
Esiankiki	108	116	129	141	139
Linda	94	109	128	143	140
Sanjeet	97	96	94	83	88

NATIONAL CENTER FOR CASE STUDY TEACHING IN
SCIENCE

Regulation of Lactase Gene Expression in Eukaryotes

III.i – Introduction to Gene Regulation and SNPs

Some students are lactose intolerant whereas others are not. is
is due to the expression of an enzyme that is required to digest
the discaccharide lactose.

Chris: I’m just blown away. I never thought that people
could be di erent on that level. How come we’re all generally
healthy humans but some of us have a certain enzyme and others
don’t?

Xiao-Ma: Now I can tell you something I learned in my
biology class. Good thing I was just studying that for my nals.
Here’s an excerpt from my lecture notes:

Every cell in an organism has the same genetic material and each
human has an almost identical genome.

– Individuals differ from each other in regions that are
variants of each other.

– The most common genetic variant between individuals are single
nucleotide polymorphisms (SNPs), which is a genetic difference of
the DNA sequence in just a single base. There are millions of SNPs
distinguishing individuals from each other.

-Eukaryotic cells are different from each other by only
expressing (=producing, synthesizing, making) proteins from certain
genes of the genome. The expression of eukaryotic genes can be
regulated at any step along the pathway from DNA to protein .

Questions

1. In which ways can the individual steps of this process be
regulated to lead to higher or lower expression of a particular
protein? Formulate hypotheses using terms such as: RNA,
protein, stability, splicing, transport, translation, and
e ciency.

2. How could SNPs contribute to gene regulation? Formulate
hypotheses.