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Genetics
Through
microscopy we are able to view and study the unseen world of micro organisms. We have come a long way since the ancient
Greek culture. It is evident the study of reproduction and heredity
began during the writings of the Hippocratic school of medicine (500 -
400 B.C.) and philosopher and naturalist Aristotle (384 - 322 B.C.).
Although the practices of the discovers before us may seem quite primitive compared to
today's knowledge of heredity, we need to keep in mind that prior to
the 1800's sperm and eggs had not been observed in mammals. And for this
reason,
genetics is considered a relatively new science.To better understand genetics you will need to be familiar with basic vocabulary.
Eukaryotic cell literally means "true
nucleus". The term applies to all protists, plants, animals, and
fungi. Eukaryotic cells have internal membranes that separate them into
regions for different functions, such as mitochondria, plastids, the Endoplasmic
Reticulum, Golgi Body, etc. They also possess a cytoskeleton that helps
them control their shape.
 | The center of heredity in a eukaryotic cell is the nucleus, whereas in a prokaryotic cell (bacteria) the genetic information is stored in an area of the cell called the nucleoid region. |
| In eukaryotes and prokaryotes, DNA serves as the molecule that stores the genetic information. In viruses either DNA or RNA contains the genetic information. DNA stands for deoxyribonucleic acid and RNA stands for ribonucleic acid. These two types of nucleic acids found in organisms, along with carbohydrates, lipids, and proteins, make up the four major classes of molecules found in living things. |
| Within each DNA molecule are the units of heredity, called genes, which are part of the bigger picture that make up chromosomes. The genetic material of all organisms is composed of two complementary chains of nucleotides wound in a double helix. |
| Chromosomes contain the genetic information that dictates what characteristics the daughter cells will possess. It helps to visualize a chromosome as a continuous strand of DNA. |
|  Meiosis (my-oh’sis)
is the division of cells during sexual reproduction. Meiosis is the process
in which a 2n cell undergoes two successive nuclear divisions (meiosis
I and meiosis II), potentially producing four n nuclei; which leads to
the formation of gametes in animals and spores in plants.
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| Mitosis (my-toh’sis) is the division of the cell nucleus, resulting in two daughter nuclei, each with the same number of chromosomes as parent nucleus. Mitosis consists of four phases: prophase, metaphase, anaphase, and telophase. |
| Cytokinesis (division of the cytoplasm to form two separate cells) usually overlaps the telophase stage. This process is the division of body (somatic) cells during reproduction. |
| Chromosomes are most easily seen during mitosis (division of somatic/body cells) or meiosis (reduction, division of sex [egg & sperm] cells) through an electron microscope during interphase. |
"The Father of Genetics"
Gregor Mendel
There were many scientists who discussed the idea of a descent of species, however no one was successful in proving the hypothesis of heredity. Gregor Johann Mendel is considered the "Father of Genetics", and through his experiments with garden pea plants came the basis for what we now call genetics. This new science was not accepted until many years later when several scientists stumbled upon Mendel's papers and found that all his postulates of transmission genetics answered their questions of heredity.
Gregor Mendel attempted to study heredity through his experimentation with plants and animals. Mendel first experimented with the cross breeding of mice. From his prior farm background, he knew that crossbreeding stock animals resulted in the presence, or lack of, various traits. He then attempted to find any common ground that existed in future generations, which was later termed F1 and F2 generations. Due to the public opinion of the time, experimenting with animals was frowned upon. After much thought and consideration, Mendel chose to use garden pea plants for his future experiments. His decision to use the garden pea worked well, as it reproduced many generations in a short time with remarkable and measurable results. He went on to record and explain that "inheritance was like beads on a string" and described that these traits repeatedly combined in mathematical ratios. Through his persistence and use of scientific methods, Mendel succeeded where others had failed in achieving scientific data regarding the inheritance of traits.
Mendel began to detect patterns of inheritance in the garden pea having to do with flower color, internodal stem length, pod color and pea shape. He noticed that when he crossed a certain red flowered plant with a white flowered plant, the offspring were all red. Mendel was puzzled by what had become of the trait for white flowers. He then crossed two of his second generation red flowered plants. The results of this cross were in a ratio of 3:1 (red:white). His conclusions were the factors of inheritance occurred in pairs, the factors become separated in reproduction and the factors for various traits can be inherited independent of any other factor. For example, the traits causing a plant to be tall or short might have no effect on whether its seeds are smooth or wrinkled. This scientific find prepared the way for future finds in the genetic field.
Mendel died in 1882 never realizing the
importance of his work. Eighteen years later, in 1900, three other scientists,
Hugo De Vries in Holland, Karl Correns in Germany, and Erick von Tschermak in
Austria all reached conclusions similar to Mendel's discoveries. In researching
past scientific literature, Mendel's papers were found and he received the
credit for his life long work. And from this point forward Gregor Mendel was honored with the title "Father of Genetics".
| Mendelism Overview | |
| Mendelism Through Corn Snake Genetics | |
| Mouse House | |
| Practice Problems |
Protein Synthesis
DNA- Deoxyribonucleic acid is the genetic material of all organisms. It is composed of two complementary chains of nucleotides wound in a double helix. Chromosomes contain the genetic information that dictates what characteristics the daughter cells will possess. Visualize a chromosome as a continuous strand of DNA. Arrayed along this DNA strand are the genes, specific regions whose sequences carry the genetic code for making specific proteins.
The diagram on the left is a very generalized model of semi-conservative replication of DNA. The new synthesis is represented by the black strands.
A- adenine
T- thymine
G- guanine
C- cytosine
Adenine always pairs with thymine (double bond) and guanine always pairs with cytosine (triple bond). RNA does not contain thymine, therefore uracil takes thymine's place. This differnece is a good way to tell if you are working with DNA or RNA. Cytosine, uracil and thymine are known as pyrimidines (six-member single ring), and guanine and adenine are known as purines (nine-member double ring).
Models are Important
DNA was determined to be a right handed double helix based on x-ray crystallographic data by Maurice Wilkins and Rosalind Franklin. This information was used by James Watson and Francis Crick to assemble the structure of deoxyribonucleic acid (DNA). The 3D model was presented by Watson and Crick in 1953 which later won them the Nobel Peace Prize for this remarkable discovery.
Looking at the diagram on the left of DNA, you will see that it is composed of repeating subunits called nucleotides. Nucleotides are composed of a phosphate group, a sugar, and a nitrogenous base. Four different bases are commonly found in DNA as stated above: adenine (A), guanine (G), cytosine (C), and thymine (T). In their common structural configurations, A and T form two hydrogen bonds while C and G form three hydrogen bonds. Because of the specificity of base pairing, the two strands of DNA are called complementary. This characteristic makes DNA unique and capable of transmitting genetic information.
The diagram below also shows the double and triple
bonds between pyrimidines and purines. Remember, pyrimidines always
bond with purines. In any segment of the molecule, alternating larger major
grooves and smaller minor grooves will be noticeable along the
DNA's axis.
An interesting DNA Question and answer
If RNA came first and DNA came second, is there an evolutionary advantage to DNA have thymine or a disadvantage to RNA having uracil?
DNA is more stable than RNA. RNA, do to the 2' hydroxyl, is hydrolyzed under alkaline conditions. This hydroxyl is important in RNA self-splicing reactions (group II introns).
Thymidine is made from uracil (UTP--> TTP); this step requires glutamine and ATP, therefore energy input. Cytosine is deaminated to uracil in a spontaneous reaction of about 10^7 cytidine residues daily; therefore, the uracil is recognized as foreign in DNA and is rapidly removed by a specific repair system. If DNA contained uracil, recognition of these changes would not be recognized. Unrepaired cytosines would result in mutations, since uracil pairs with adenine.
For a more in-depth discussion, see
http://www.madsci.org/posts/archives/feb2000/950728469.Bp.r.html
Elizabeth A. Cowles, Ph.D. , Associate Professor, Biology, Eastern Connecticut State University
Codon Table (RNA)
There are 64 different kinds of codons but only 20 amino acids.
The Codon table
link illustrates how
the different triplets are named.
AUG encodes methionine, which initiates
most polypeptide chains, referred to as the "start" codon.
All other amino acids except tryptophan,
which is encoded only by UGG, are represented by two to six triplets.
The triplets UAA, UAG, and UGA are
termination signals, referred to as "stop" codons, and do not encode
any amino acids.
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Viruses & Bacteria
Some tumor viruses cause cancer in animals
| Viral Group | Cancer Type | ||||||||||||||||
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Plant Viruses
RNA viruses spread by
Horizontal transmission
from external source
insects, freezing, injury
Vertical transmission
from parent
Asexual: propagation
Sexual: via infected seeds
Genetic Engineering
Karyotype
The complete set of chromosomes in the cells of an organism is its karyotype.
All species have a characteristic number
of homologous pairs of
chromosomes in their cells called the diploid
(or 2n) number.
Below are examples of various species.
Homo sapiens (human) 46
Mus musculus (house mouse) 40
Zea mays (corn or maize) 20
Drosophila melanogaster (fruit fly) 8
Xenopus laevis (South African clawed frog) 36
Caenorhabditis elegans (microscopic roundworm) 12
Equisetum arvense (field horsetail, a plant) 216
Saccharomyces cerevisiae (budding yeast) 32
Canis familiaris (domestic dog) 78
Arabidopsis thaliana (plant in the mustard family) 10
Myrmecia pilosula (an ant) 2
Parascaris equorum (parasitic roundworm) 2
Procambarus clarkii formally named Cambarus clarkii (crayfish) 192
Pacifastacus trowbridgii formally named Astacus trowdridgii (crayfish) 372
Human Karyotype
(haploid)
The karyotype of a normal human female contains
23 pairs of homologous chromosomes:
22 pairs of autosomes
1 pair of X chromosomes
The karyotype of a normal human male contains:
the same 22 pairs of autosomes
one X chromosome
one Y chromosome
Punnett devised the "Punnett Square" to depict the number and variety of genetic combinations, and had a role in shaping the Hardy-Weinberg law. Punnett and Bateson co-discovered "coupling" or gene linkage. William Bateson brought Mendel's laws to the attention of English scientists.
Punnett Square
Male: Bbrr (Br Br br br)
Female: BbRR (BR BR bR bR)
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Br |
Br |
br |
br |
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| BR |
BBRr |
BBRr |
BbRr |
BbRr |
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BR |
BBRr |
BbRr |
BbRr |
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bR |
BbRr |
BbRr |
bbRr |
bbRr |
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bR |
BbRr |
BbRr |
bbRr |
bbRr |
This matrix reveals the F2 from a breeding pair of corn snakes, Pantherophis guttatus.
Hardy Weinberg Equilibrium
p2 + 2pq + q2 = 1
We will use two alleles, A and a, with the dominant allele represented by the letter p and the recessive allele by the letter q.
p = the frequency of the dominant allele (represented here by
A)
q = the frequency of the recessive allele (represented here by
a)
p2 = frequency of AA (homozygous dominant)
2pq = frequency of Aa (heterozygous)
q2 = frequency of aa (homozygous recessive)
Mathematical Definitions:
Allele Frequencies = p (A) + q (a) = 1
Genotypic Frequencies = (p+q)2
p2 (AA) + 2pq (2Aa) + q2 (aa) = 1
This law assumes random mating in each generation and no disruption of allele frequencies or genotypic frequencies. If the end result is not 1, there is no equilibrium.
Possible reasons for population diversity (or lack of equilibrium): genetic drift, mutation, migration, and meiotic drive.
Chi-Square
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Category |
O |
E |
O-E |
(O-E)2 |
(O-E)2/E |
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Totals |
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c 2= |
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S |
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Df = |
Circle one |
Reject or Accept |
S = sum of
Df = degrees of freedom
c2 = Chi Square
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