GESCI 101 Section 4.25

Explore 4.25 How Bodies are Continuously Remade

Learning Objectives

By the time you have completed the 4.25. Introduction & Exploration Activities, you should be able to:

Understand and be able to identify each of the following: stages/states (and transitions) of human development--infancy, childhood, adolescence, adulthood, old age; gene regulation, control genes, genetic mutations.
Describe the role of gene regulation in the continuous recreation of bodies, including in the initiation and termination of the postnatal states of human development.
Describe how a cell knows when, where, and for how long to activate, enhance, repress, or deactivate particular genes.
Explain how gene regulation can produce significant differences between organisms that share much of their DNA--like chimpanzees and humans.

Scientific Terms/Concepts

Terms: Post-Natal Stages/States of Human Development (and the transitions that separate them), Infancy (stage/state), Childhood (stage/state), Adolescence (stage/state), Adulthood (stage/state), Old Age (stage/state), Gene Regulation, Control Genes, Genetic Mutations.

Define and give an example of each term:

Term:

Infancy

Definition:

Example:

Term:

Childhood

Definition:

Example:

Term:

Adolescence

Definition:

Example:

Term:

Adulthood

Definition:

Example:

Term:

Old Age

Definition:

Example:

Introduction

In the Introduction Activity, you thought deeply about the amazingly complex system that is your body and you explored the nature of each stage (state) that comprises the normal development of human bodies: infancy, childhood, adolescence, adulthood, and old age. Below, we identify the processes that continuously remake human bodies (and the bodies of all organism) both during developmental stages and during the transitions that separate them. We’ll begin with a thought experiment that will help you understand how amazingly dynamic bodies are—including your body.

Imagine someone you know well, a parent, a friend, a sibling, … choose anyone. Now, imagine you didn’t see them for an hour. Would you still recognize them? Of course! Would you recognize them after a day? A week? A month? Yes! You’d recognize them without a problem. Even if the individual you selected were a young person, you’d probably still recognize them—despite their having changed a lot.

Now, would you recognize this individual after a year or a decade? Record your answer in the space below.

Now, consider that the component parts—atoms, molecules, and cells—of the person you’re thinking of are continuously being replaced. In all bodies, cells continuously divide and die, and atoms and molecules continuously cycle through the body. To become more familiar with the rates of cellular overturn, think about the cells that comprise different parts of human bodies—such as skin, brains, intestines, blood, liver, kidney, heart, teeth, fat, lungs, & bones—and answer the following questions.

Is the person you selected above identical after a week, or have the cells that comprise some of their body parts been replaced?

How about after a month, what body parts are replaced during this period?

And after a year, what body parts are replaced during this period?

A decade?

Are there any parts of the human body that remain essentially unreplaced through most of life?

So, if most of the cells, tissues, or organs that comprise a human body (or the body of any organism) have been replaced, what is that you are recognizing when, after a decade, you recognize someone?

So, if what we recognize as a person is the pattern of their body and not the matter of their body, where does this pattern come from? In other words, what determines the pattern of each individual—be that individual a human, some other animal, or a plant? Briefly describe your thoughts below.

Control Genes

As you’ll recall, the processes by which bodies self-assemble, maintain themselves, and generate new bodies include those described in the table below.

Process	Description
Energy Utilization (metabolism)	Taking in resources, process resources, and expel waste. Includes respiration, digestion, and excretion.
Boundary Maintenance & Internal Environmental Regulation (Homeostasis)	Creating and sustaining organismal conditions that are distinct from the environment
Adaptation to Environment	Responding to external stimuli
Reproduction	Asexually or sexually produce new offspring
Gene Regulation	Governing where and when genes are expressed

We highlighted gene regulation—the orchestration of biological self-assembly—because this process governs the other processes.

Describe what genes do in the box below. (If you need a little review, watch this video.)This is an important answer. We encourage you to think deeply before and during your response. If you do, what we share with you in the answer will be more meaningful. Unless you already thoroughly understand gene regulation, your progress through the rest of this course will be stunted by skimming through this material. In other words, this is really important material—concepts that form the foundation for most of the rest of what we’ll discuss this course.

Most people understand that genes are instructions for building bodies, and many people know that genes code for proteins—specific strings of amino acids, but few people understand that some genes regulate other genes. Proteins, the large complex molecules produced by genes, fill many critical roles in bodies. Among other things, they build tissue, facilitate the formation of other molecules, and signal & coordinate biological processes.

Protein signaling coordinates biological processes inside and between cells. Animals and plants are coordinated collections of cells, and cells are collections of coordinated biochemical processes. As you can see, coordination is really important in assembling and maintaining bodies, and proteins are the chief signalers. As we’ll see, proteins produced in one region of a body can initiate and terminate processes in other regions of the body. In this way, proteins control gene expression—the turning on and off of genes. We refer to the genes that produce proteins that activate and deactivate other genes as control genes. In other words, a control gene produces a protein that turns other genes on or off.

Humanity first discovered the nature of control genes several decades ago. This is a hugely important area of active research and one that will tremendously benefit the human family. For example, as humanity deepens its understanding of gene control (regulation) our ability to regrow malformed, impaired, or damaged body parts will improve. Seriously! From our present vantage point, what seemed impossible in the recent past now appears within our grasp. Recall that every cell in your body (or the body of any organism) contains all the instructions to make every part of you. After all, those are the instructions that self-assembled your body in the first place. So, if we already know how to read the instructions (DNA) for building body parts, what else do we need to know? We need to know the chemical signals that initiate and terminate specific instructions—we need to know where, when, and for how long to express those genetic instructions; in other words, we need to understand gene regulation.

In case you’re still wondering, gene regulation is a profoundly important process!!! It is responsible for guiding a single cell through the incredibly nuanced and complex process that produced your body during gestation and continuously re-creates your body today. As we’ll see, it also plays an essential role in the adaptation of populations to their environments and, as such, in the development of life on Earth—from the very first single-celled organisms to those we observe today, including humans.

Before proceeding, make certain that you understand the material between the above table and this sentence. You may want to read through it one more time.

Gene Regulation

Describe the role of gene regulation in the continuous recreation of bodies, including in the initiation and termination of the states that comprise the postnatal development of an organism. Also, describe how a cell knows, at a specific time, which genes to activate and which to deactivate.

This video will give you some idea on how gene regulation and expression works:

Explain how gene regulation can produce significant differences between organisms that share much of their DNA (e.g., chimpanzees and humans).

What Causes Differences in Organisms?

Above, we learned how genetic changes (mutations) and the interactions of populations with their environments (inheritance, selection, & adaption) can cause populations to change. Before we explore another cause, let’s take a peek at the genetic similarities and differences between organisms. The table below shows how genetically similar you are to yourself, other humans, Neanderthals, and chimpanzees.

Organism	Shared DNA
You	100%
Other humans	~99.9%
Neanderthals	~99.7%
Monkey	~93%
Fish	~70%
House fly	~60%

Continue exploring the genetic similarities and differences between you and other organisms by answering the questions below.

How similar are your genes to those of monkeys and mice?

How similar are your genes to those of fish and flies?

How similar are your genes to those of single-celled eukaryotes and bacteria?

Now, we’re ready to explore another process that produces differences between organisms. To do this we’ll investigate the similarities and differences between humans, chimpanzees, & bonobos and the similarities and differences between homologous mammal limbs.

The first image below shows the bodies of chimpanzees and bonobos and allows you to compare these to human bodies. The other three images illustrate the genetic similarities between these three species. As you can see, chimps and bonobos are nearly genetically identical (~99.6% identical) and share similar bodies. However, humans and chimpanzees/bonobos have quite different bodies despite their tremendous genetic similarity (nearly 99% identical). How can this be? Don’t genes determine the shape of bodies? Well, the answer to that question is both ‘Yes!’ and ‘yes, but not just in the way most people think’. Here’s what we mean: differences in bodies result from two aspects of genes—which genes are there and how they’re used (expressed). So, two organisms—like humans and chimpanzees/bonobos—can have very similar genes but their bodies can have significant differences because those genes are used differently. Said another way, both genes and gene regulation cause differences in the characteristics of bodies.

Venn diagram comparing chimpanzees and bonobos

Venn diagram comparing chimpanzees, bonobos, and humans.

Think about it: identify a body part that chimps/bonobos have that humans don’t, or vice-versa. Do all of these organisms have a heart? Eyes? A brain? Arms & legs? A spleen? …. Yep! What causes the differences between these body parts across these species? Not just genes! Identical genes turned on for different periods in different locations can produce different shapes/aspects of the same body parts!!! This is an important idea; don’t continue until you understand that last sentence.

Alright, let’s think about this same thing—the role of gene regulation—in another context. The image below shows the now-familiar homologous limbs of several mammals. Look carefully at each limb. Compare the parts and pieces that comprise each limb. Are they all pretty much made of the same parts and pieces organized in the same ways? Yes, they sure are! So, ask yourself this question: could all of these limbs be produced by the same genes? If you’re like most people, you really want to say, “No way! That’s impossible! To produce such different body parts (wings, arms, fins, and legs) requires vastly different genes!” But knowing what you now know about gene regulation should cause you to answer the question differently, perhaps as follows, “The limbs in the image—and, in fact, those of all limbed animals—are made using pretty much identical genes that are controlled differently.” Pretty cool, right?!

Showing homologous structures between humans, cats, horses, bats and dolphins. We all share something similar to a humerus, radius, ulna, carpals, metacarpals, and phalanges.

Perhaps this analogy will help you understand the role of gene regulation in producing homologous structures more deeply: the limbs of these organisms are like an orchestral performance. The instruments in the orchestra are like the genes that build homologous elements (limbs, hearts, brains, skulls, skin, kidneys, gonads, backbones, etc.) and the differences between homologous body parts are like the conductor signaling the instruments (genes) to play a particular note or sequence longer or shorter, louder or softer, etc. Make sense? We hope so. It’s a really important concept, if you want to understand how bodies are built.

Incidentally, one of the barriers to understanding evolution for many people is the notion that most differences between organisms require new genes. And that is simply not true!!! Small changes in the expression of control genes can produce large changes in the form and function of a body! (That’s another really important sentence. Read it again.)

For each bone in each organism, describe the result of gene expression (what it ultimately looks like).

	Human (Example)	Cat	Horse	Bat	Dolphin
Humerus	Bone is longer bones of forearm and as thick as the radius and ulna together	Answer - Bone is much shorter and thicker in proportion to the radius and ulna Bone is much shorter and thicker in proportion to the radius and ulna	Answer - Bone is much shorter and thicker in proportion to the radius and ulna Bone is much shorter and thicker in proportion to the radius and ulna	Answer - The bone is very delicate and not as long as the radius and ulna The bone is very delicate and not as long as the radius and ulna	Answer - The bone is very short but very thick The bone is very short but very thick
Radius/Ulna	Bones are nearly equal length and half as thick as the humerus	Answer - Bones are much longer in proportion to humerus Bones are much longer in proportion to humerus	Answer - The two bones have fused into one bone The two bones have fused into one bone	Answer - The bones are narrow and longer than the humerus The bones are narrow and longer than the humerus	Answer - The bones are also very short and very thick The bones are also very short and very thick
Carpals	Bones are small and squarish	Answer - Two of the carpals have fused into one. Also small and squarish. Two of the carpals have fused into one. Also small and squarish.	Answer - The bones are small and squarish The bones are small and squarish	Answer - The bones are small and squarish The bones are small and squarish	Answer - The bones are larger but still squarish The bones are larger but still squarish
Metacarpals	Bones are long and narrow to form the palm	Answer - Bones are long and narrow. Bones are long and narrow.	Answer - There is only one metacarpals There is only one metacarpals	Answer - Bones are quite long to form part of the wing Bones are quite long to form part of the wing	Answer - Bones are short and very thick Bones are short and very thick
Phalanges	Each finger has three bones of decreasing length.	Answer - Each digit has three bones. Each digit has three bones.	Answer - There is only one digit with three phalanges There is only one digit with three phalanges	Answer - The bones are very long to form the wing The bones are very long to form the wing	Answer - There are only four digits with an increased number of short phalanges in the second and third digits to form the flipper There are only four digits with an increased number of short phalanges in the second and third digits to form the flipper