BIL 106 - Lecture 3 - A Genetics Primer

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What is a GENE?

A GENE is a unit of inheritance. One specific gene controls the manufacture of one specific PROTEIN by the cell.

There are four basic types of biological macromolecules, each of which is a : polymer--a long chain of repeating, similar subunits. A polymer can be likened to a string of "pearls," known as monomers.

CARBOHYDRATES are long chains of sugars
PROTEINS (polypeptides) are long chains of amino acids of which there are 20 known.
LIPIDS are long chains of fatty acids
NUCLEIC ACIDS are long chains of nucleotides.
- DNA (DeoxyriboNucleic Acid) is the famous twisted double helix: the instructions for life present in the nucleus of all cells that have a nucleus. DNA never leaves the nucleus.
- RNA (RiboNucleic Acid) structure is similar to that of DNA, except that it is single stranded. It is manufactured by the cell from the DNA template (a process called transcription) only when a particular gene is being expressed. The RNA version of that gene is transported out of the nucleus where its code is read by ribosomes, which make a specific protein (a process known as translation).
  There are three types of RNA, all of which are involved in protein synthesis:
  - messenger RNA (mRNA) is the transcribed gene that travels from DNA to ribosome.
  - transfer RNA (tRNA) is a three-dimensional type of RNA that carries amino acids to the ribosomes for assembly into protein chains.
  - ribosomal RNA (rRNA) is a structural component of the ribosome; it helps line up mRNA and tRNA in their proper locations and orientations for protein synthesis.
- In short, DNA is the permanent, nucleus-based "blueprint" for assembling and running the metabolism of the cell. RNA is the "courier" that brings the instructions of that blueprint to the workers outside the nucleus, who assemble protein from the DNA code (temporarily transcribed as RNA).

What is a PROTEIN?

Physically...
- a protein is a long molecule composed of repeating subunits called AMINO ACIDS.
- The specific order of the amino acids is the proteins primary structure This 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)
- Functionally...
  - protein may be STRUCTURAL (i.e. compose the physical body of the organism)
  - protein may be FUNCTIONAL (i.e., an enzyme: a three-dimensional protein that acts as a catalyst to drive chemical reactions that build up or break down the molecules that make up the body of the organism and allow it to function.) Enzymes are the "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.

What is DNA?

Physically...

DNA is a long molecule composed of repeating subunits called NUCLEOTIDES.
Recall that each of the four nucleotides is abbreviated by a single letter which stands for the nitrogenous base on the nucleotide
- Adenine
- Guanine
- Cytosine
- Thymine

Functionally...

The letters A, G, C, and T can occur in ANY ORDER on the DNA strand.
When the cell needs to manufacture the protein product encoded by a particualr gene, the DNA of that gene opens (the double helix "unzips") and a strand of RNA is made from the DNA template.
every three RNA "letters" represents one amino acid to the cell.
The cell transports RNA out of the nucleus to the ribosomes read the RNA message (the "T" has changed to a "U" for uracil in RNA) and manufacture protein.
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")
Here's a short animation of the Wonders of Translation.
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 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 animal itself).

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:

stature
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.

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)
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.
Almost all animals (and plants) are DIPLOID: Every cell contains TWO copies of every gene. One copy came from mom, and the other came from dad. Therefore...
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.

Illustration: One of the genes controlling eye color in humans has two alleles: 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 absent from the iris tissues, and the blue color results from differential refraction of blue light striking the iris from the outside.

Brown is dominant, blue is recessive
This means that Brown (B) masks the expression of blue (b), if both versions of the gene are present in the same individual.
BB = brown eyes Bb = brown eyes bb = blue eyes
Other genes contribute other pigments to make eyes variations on blue: green, hazel, grey, etc.

The genes are located on long strands of DNA called CHROMOSOMES, each of which has many genes separated by "spacer DNA" that doesn't code for anything.

Every animal receives one set of chromosomes from mom, and one set from dad. You can see this in a karyotype.

The sex cells of an animal--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 via the cell division processes known as MEIOSIS.

Notice the difference between males and females! This will become very important later, when we discuss why male animals seem to have a less discriminatory sex drive than females.