Introduction It is nals week on college campus and everybody

Introduction

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

Bjorn: 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

Bjorn: 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

Bjorn

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.

Minutes after drinking milk:	Blood glucose (mg/dL)
0	30	60	90	120
Bjorn	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

Solution

the hypothesis is always an if ... then statement, the proteins can regulate the expression of a particular gene by choosing to make its RNA or not. the proteins can control the splicing of the particular gene and make different products from same gene by even using alternative splicing. if the gene translation efficiency is not great then protein formation and transport is not possible.

If a protien like a transcription promoter or enhancer is present at the time of transcription of a gene, then the expression and transcription efficiency of that gene increases leading to greater splicing and protein translation. once these proteins are translated, they can then be transported to different cells.