Download Creating A Hypothesis At Breakfast: An Evolutionary Biologist`s Story

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Creating A Hypothesis At Breakfast: An Evolutionary Biologist’s
Story
Directions: You are an evolutionary biologist. On a Saturday morning like any
other, you walk downstairs for your morning cup of coffee. While sipping away
slowly at the breakfast table, you open up the newspaper. Immediately, you notice
the story headline:
Individuals In Certain Populations Found To Be Resistant To HIV
What?!?! This is the story of a lifetime! (You happen to be an evolutionary biologist
that specializes in HIV transmission.) Before you even read the article, your mind
starts brainstorming all the possible ways that somebody could be naturally
resistant to HIV infection. Knowing what you already know about the viral spread of
HIV in the body, brainstorm three possible mutations that could lead to natural
resistance. (Hint: This is just to get your creative juices flowing! You do not need to
be too specific! But think of how HIV spreads naturally in the body, and how a body
might mutate to stop that spread. Below are a few pictures that illustrate how HIV
invades the CD4 cell that might help with some inspiration!)
Hypotheses
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Genetic HIV Resistance Deciphered
Randy Dotinga
01.07.05
Throughout the history of the AIDS epidemic, a few lucky people have avoided infection
despite being exposed again and again. Now, researchers are traveling back in
evolutionary time to understand why some people are resistant -- and in some cases
virtually immune -- to the AIDS virus.
Studies released this week and last year suggest that the roots of AIDS immunity extend
back for centuries, long before the disease even existed. Our ethnic backgrounds and the
illnesses suffered by our distant ancestors appear to play a crucial role in determining
whether our genes will allow HIV to take hold in our bodies.
Genetic resistance to AIDS works in different ways and appears in different ethnic
groups. The most powerful form of resistance, caused by a genetic defect, is limited to
people with European or Central Asian heritage. An estimated 1 percent of people
descended from Northern Europeans are virtually immune to AIDS infection, with
Swedes the most likely to be protected. One theory suggests that the mutation developed
in Scandinavia and moved southward with Viking raiders.
STOP AND ANSWER QUESTIONS 1-2
All those with the highest level of HIV immunity share a pair of mutated genes -- one in
each chromosome -- that prevent their CD4 cells from developing a normal "receptor"
that lets the AIDS virus break in. Receptors are proteins found on nearly every cell of the
body. They function like a door: if other cells have a key, a receptor can let that cell dock
and share information. Typically, HIV enters a CD4 cell by binding to a receptor on the
cell’s surface known as the CCR5 receptor. However, if the CCR5 receptor is too mutated,
the virus can’t attach to the CD4 cell and take it over.
An estimated 10 percent to 15 percent of those descended from Northern Europeans
have some sort of resistance against HIV infection because they carry a mutated gene
coding for the CCR5 receptor. Because their receptors are slightly mutated, HIV cannot
properly bind and enter the CD4 cell. Using formulas that estimate how long genetic
mutations have been around, researchers have discovered that the mutation dates to the
Middle Ages. (Similar research in mitochondrial DNA -- passed along by women -- has
suggested that Europeans are all descended from seven Ice Age matriarchs.)
STOP AND ANSWER QUESTIONS 3-5
Name: _____________________________________________________________ Pd: _____ Date: __________
Reading Questions
1. How would you describe the difference between resistance and immunity?
Immunity means that someone cannot contract a disease. Pathogens, for
some reason, cannot get past the person’s immune system. If a person is
resistant to an infection, they still could possibly get the disease. Their
chances of developing it, however, are notably less than the average person.
2. Where do scientists think the genetic resistance to HIV first developed in the
world?
Scandinavia
3. What is the function of a receptor?
Proteins found on cell membranes that allow things to pass in and out of the
cell, given that the external object has the appropriate “key” to bind to the
receptor.
4. Why does a mutated CCR5 gene help prevent HIV spread in the body?
HIV cannot properly dock to the mutated CCR5 receptor, so it cannot enter
the cell.
5. When do scientists think this mutation originated and in what population?
This mutation may have originated in the Middle Ages from the seven Ice Age
Matriarchs.
Critical Thinking
According to the last article, the CCR5 mutation has been around for thousands of
years. In fact, it was spreading in the population even before the HIV virus existed!
Why might this genetic mutation have lasted for so long in the population?
Some possible student answers:
 There was some other benefit that it created for the cell
 There used to be a similar virus to HIV back in the Middle Ages that the CCR5
receptor mutation protected against
 Genetic Drift