The Magic of Chemistry
Before we talk about genes, let's make sure we all know a little bit about molecules in general.
Cast your eyes skywards, to the periodic table of elements...
Or just look here:
All living things are composed of ATOMS and MOLECULES.
A molecule is two or more ATOMS (composed of protons (+) and neutrons (0) to make
the nucleus, and orbited by electrons (-) bonded together. They may be the
same type of atom (which means that the molecule is an ELEMENT), or they may
be different types of atoms (which means that the molecule is a
Examples of elements: O2, H2, N2, etc.
Examples of compounds: H2O, CO2, CH4, etc.
An organic molecule is one which has a basic "skeleton" made of CARBON (C)
and HYDROGEN. They are so named because these types of molecules usually
are made only by LIVING ORGANISMS. (That's changed with the advent of
modern chemistry--but originally, organic molecules were made almost
exclusively by living organisms.)
Examples of organic molecules: CH4 (methane),
benzene (C6H6), the sugar glucose
An inorganic molecule is one that lacks that CARBON-HYDROGEN backbone.
Examples are among the elements and compounds I listed above:
O2, H2, N2, H2O, and CO2
No one does it better than They Might Be Giants
The Organization of Life
From smallest unit to largest, we sometimes categorize life this way:
-->macromolecule (BIG molecule)
-->cell (the smallest LIVING unit possible)
-->tissue (cells working together for a common function)
-->organ (tissues working together for a common function)
-->organ system (organs working together for a common function)
-->organism (all the organ systems together!)
Molecules combine to form the next level of complexity in biological
systems, the biological macromolecules.
The structural components of the bodies of all living things are mostly
made up of these very large, compounds (though
there are lots of other inorganic and smaller organic molecules
throughout the body, too). The four main types of biological
- nucleic acids, polymers of nucleotides (A, T, C, G, U)
- proteins, polymers of amino acids (22 flavors!)
- carbohydrates, polymers of sugars (monosaccharides or disaccharides)
- lipids, polymers of fatty acids
Biological macromolecules listed above are polymers: long chains of
What are the main functions of each type of macromolecule?
All form components of cellular structures and organelles.
In multicellular organisms...
It is the last of these macromolecules, the nucleic acids (DNA and RNA) that make up the genes.
- carbohydrate functions
- short-term energy storage
- physical structure of the organism (e.g., cellulose in plants)
- lipid functions
- long term energy storage
- physical structure/cushioning in animals
- protein functions
- structural (e.g., keratin, silk)
- functional (e.g., enzymes)
- both structural and functional (e.g., myosin and actin)
- nucleic acid (DNA and RNA) functions
- DNA - permanent "blueprint" of genetic instructions in the cell nucleus
- RNA - intermediate copy of the blueprint, a mobile nucleic acid that is read by the cell when it makes proteins
What is a GENE?
A gene is a unit of inheritance. It is composed of DNA.
One specific gene controls the manufacture
of one specific protein by the cell.
What is a protein?
- a protein is a long molecule composed of repeating
subunits called amino acids.
- There are 21 different amino acids, and the order in which they
occur in the protein strand (the "primary" structure of the protein)
- which way the protein folds and loops in three dimensions (its
- which way it curls into complex, globular structures (its
- which way various individual proteins link together to form
complex proteins (quaternary structure)
- Let's have a LOOK.
- protein may be structural, composing the physical body of the
- hair and nails
- protein may be functional, existing as a three-dimensional
molecule that acts as a catalyst to drive chemical reactions. These either build up
or break down the physical molecules that make up the body of the
organism and allow it to function. These are called enzymes and they are the protein "machinery" that
helps the cell make carbohydrates, nucleic acids, lipids, other proteins, and
more complex structures composed of those macromolecules plus other "building
materials" in the cell. Let's have a LOOK.
What is DNA?
Deoxyribonucleic Acid is the permanent "blueprint" of instructions for how
the cell must build its structural and functional protein. A Gene is
composed of DNA.
Put together into a long, informational code that looks like this:
- DNA is a long molecule composed of repeating subunits called
- Each of these four nucleotides is abbreviated by a single letter
which stands for the nitrogenous base on the nucleotide
How do we go from DNA to protein?
The cell uses enzymes to read the code on the DNA.
The enzymes translate the DNA code into a temporary form called messenger RNA or mRNA, which is carried out of the nucleus into the cell's cytoplasm.
The RNA combines with protein/nucleic acid organelles in the cytoplasm called ribosomes.
Special molecules of RNA (called transfer RNA or tRNA) bring amino acides to the ribosome, which can then construct a long chain of amino acids by reading the code on the RNA molecule.
One small difference between DNA and RNA is that the DNA letter "T" has changed to a "U" for uracil in RNA. But U is chemically very similar to T, so no big deal.
The cell follows a very specific genetic code.
For example, if a section of a gene reads AUG ACC UUC GGU UAA, the order
of the amino acids the cell uses to build that protein in that
section would be:
"start" - thr - phe - gly - "stop"
("begin protein strand" - threonine - phenylalanine - glycine - "end of protein
Each long strand of triplet codes that begins with a "start" signal and ends with one or more "stop" signals is a gene.One gene codes for one polypeptide.
The nucleus of an animal cell contains tens of thousands of
genes, each in charge of storing the code for a specific protein with
its own unique order of amino acids (and hence, its own special
physical and chemical properties).
The unique complement of proteins any individual organism makes determines its identity and how it functions.
The average mammal (including the human mammal) probably has somewhere between 30,000 - 60,000
genes comprising its genome (i.e., the complete genetic instructions for
building and operating the organism itself).
Is every copy of a particular gene exactly the same in every individual? (For example, are all the genes coding for hair color exactly the same in every person in this room?)
Different "versions" of the same gene are called alleles.
Inheritance and the Chromosome
Recall that inside the nucleus of each cell, the genes are located on long strands of DNA called chromosomes. Each chromosome is a long strand of genes.
Every animal receives one set of chromosomes from mom, and one set from
dad. You can see this in a karyotype.
Humans have 23 chromosomes in each set: 22 autosomes and 2 sex chromosomes. The two sets are comprised of homologous pairs of chromosomes, each of which carry matching genes.
Most cells in the body contain two copies of the genome: one from each parent. Such a cell is said to be diploid.
Some cells in the body contain only one copy of the genome. Such a cell is said to be haploid.
A diploid cell carries two alleles of each gene.
A haploid cell carries only one allele of each gene.
A cell may reproduce asexually, meaning that the two cells produced by dividing a single progenitor cell are genetically identical (they have exactly the same DNA in the same quantity). This process is known as mitosis.
A cell may also divide in such a way as to allow sexual reproduction. In sexual reproduction, two members of the same species each make cells that have half the original amount of DNA (one complete copy of the genome in each new cell). This process is known as meiosis.
After meiosis, each new haploid cell is processed further to become either
in a process called gametogenesis.
- sperm (male)
- ovum (female)
Sperm and egg unite during sexual reproduction to form a new, genetically unique cell called a zygote, a "fertilized egg". This will divide via mitosis in an orderly fashion, with various genes turning on and off at specific times in the embryo's growth in order to direct its development into a new, diploid member of its species that will express (show) genetic traits passed on to it by its parents.
Genes and Alleles
Some human traits are controlled by only ONE gene (monogenic traits). Examples:
- Eye "base" color (blue or brown)
- Hitchhiker's Thumb
- Tongue rolling
- Widow's Peak
But most traits are controlled by many genes (polygenic traits) interacting not only with
each other, but also with the environment, as the organism grows and
- shape of body
- total eye color (brown, blue, green, hazel, grey, etc.)
- skin tone
- athletic ability
- outlook on life!
The scientist who studies the relative contributions of "Nature" (genes)
and "Nurture" (environment) to any given trait is known as a
Recall that different versions of the same gene are called alleles. Every person carries two genes for every trait.
Does it matter whether you're homozygous or heterozygous? Yes!
- Sometimes the two alleles are the same (the organism is homozygous for that gene).
- Sometimes the two alleles are different (the organism is heterozygous for that gene.
Example: Eye color in humans is controlled by at least four different genes. But one of those codes for the "background" color of the iris, and will cause the iris to be either brown or blue:
Brown (B) and Blue
(b). Brown eyes are brown because a gene causes a dark brown pigment
(melanin) to be deposited in the iris tissues. In blue eyes, melanin is
not deposited in the iris tissues, and the blue color results from
refraction of blue light by the iris tissues.
- The brown allele (B) is dominant to the blue allele (b), which is recessive.
- This means that Brown masks the expression of blue, if the two different versions of the gene (alleles) are present.
- BB = brown eyes
- Bb = brown eyes
- bb = blue eyes
- Other genes contribute other pigments, and blue eyes vary from blue depending on those other pigments: green, hazel, grey, etc.
Since you have two copies of each gene, you could have two "identical twin"
copies (you're homozygous for that gene) or you might have "sibling" copies
that are slightly different from each other (you're heterozygous for that
The sex cells of any organism--sperm or ova (eggs)--are haploid. Each one
contains only half the number of genes of the original diploid germ cell
from which it was derived during meiosis.
The Vocabulary of Genetics
- gene: a unit of inheritance; a sequence of DNA that codes for a particular polypeptide chain (protein)
- allele: alternate forms of a particular gene. For example, one gene coding for eye color in humans has two different alleles, blue or brown.
- mutation: any change in a gene. (its effects may or may not be apparent in the physical being of the organism in which it occurs)
- locus: (plural = loci) they physical location of a gene on its chromosome.
- phenotype: the physical appearance/expression of a given trait in an
- genotype: the genetic coding of a particular trait in an organism.
- dominant allele: one which masks the expression
of another at the same locus
- recessive allele: one whose expression is masked
by another at the same locus.
- diploid: having two complete sets of chromosomes (one from each parent)
- haploid: having only one complete set of chromosomes (sperm or egg)
- homozygous: the two alleles of a gene at a particular locus are the
same in one individual.
- heterozygous: the two alleles of a gene at a particular locus are
different in one individual.
Genes and Development
Every living multicellular organism begins life as a zygote.
It is equipped with all the DNA information it needs to divide (via mitosis) and become whatever its genes tell it to be.
This is done by means of a series of orderly cell divisions (cleavages). In animals, it looks something like this:
How does each cell know what to do and what to become?
The genes in each cell are either turned on or off at any given stage in development, and as the embryo gets older and more differentiated, each cell has different genes being expressed.
This is why your liver cells are different from your brain cells, and so on.
Environment can play a significant role in embryo development and survival!
In egg-laying animals (such as your butterflies), temperature, humidity, pollution, chemicals present in the environment can sometimes affect the proper turning on and off of genes.
This means that environmental factors can affect the development of the embryo!
Environment can also strongly affect the development of an animal that develops inside its mother's body.
So in our caterpillar manipulations, we will be exploring how external factors might influence DNA and the final outcome of a caterpillar's fate.
A Guide to Testing Environmental Variables' Effects on Caterpillar Development.