Download What is a GENE? - West East University

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 COMPOUND).
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 (C6H12O6), etc.
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: atom
-->molecule
-->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 macromolecules are...
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 repeating subunits.
What are the main functions of each type of macromolecule?
All form components of cellular structures and organelles.
In multicellular organisms...
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
It is the last of these macromolecules, the nucleic acids (DNA and RNA) that make up the genes.
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?
Physically...
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) determines...
which way the protein folds and loops in three dimensions (its secondary structure)
which way it curls into complex, globular structures (its tertiary structure)
which way various individual proteins link together to form complex proteins (quaternary structure)
Let's have a LOOK.
Functionally...
protein may be structural, composing the physical body of the organism. Examples:
muscle
hair and nails
silk
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.
Physically...
DNA is a long molecule composed of repeating subunits called nucleotides.
Each of these four nucleotides is abbreviated by a single letter which stands for the nitrogenous base on the
nucleotide
Adenine
Guanine
Cytosine
Thymine
Put together into a long, informational code that looks like this:
Functionally...
A, G, C, and T are the "letters" of the DNA alphabet.
They can occur in any order on the DNA strand, and different orders of the letters make different
"sentences."
Every three DNA letters represents one amino acid ("word") to the cell.
A long strand of three-letter amino acids can be considered a "sentence" made up of individual amino acid
words. And that sentence is a protein, also known as a polypeptide.
The DNA is packaged in the form of chromosomes:
The physical location of a gene on the chromosome is called its locus (plural = loci). We often use the terms
"gene" and "locus" interchangeably.
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 strand")
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.
Cell Reproduction
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
sperm (male)
ovum (female)
in a process called gametogenesis.
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 develops. Examples:
height
shape of body
total eye color (brown, blue, green, hazel, grey, etc.)
skin tone
athletic ability
intelligence
outlook on life!
The scientist who studies the relative contributions of "Nature" (genes) and "Nurture" (environment) to any given
trait is known as a quantitative geneticist.
~~~~~~~~~~~~~~~~~~~
Recall that different versions of the same gene are called alleles. Every person carries two genes for every trait.
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.
Does it matter whether you're homozygous or heterozygous? Yes!
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 gene).
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 organism
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.