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Essential Questions and Answers: Genetics
How does the storage of DNA ensure that DNA is replicated consistently each time?
DNA cannot leave the nucleus of the cell where it is stored because it is too large to pass through the
pores of the nuclear membrane. During replication, enzymes “unzip” the two strands of the DNA
molecule by breaking the hydrogen bonds that hold the base pairs together. Each strand serves as a
template for the attachment of complementary bases. Then, another enzyme zips up the two new DNA
molecules. The two new molecules are identical, each containing one of the parent strands and one newly
made daughter strand. When the cell replicates, the nuclear membrane disintegrates and each new
daughter cell receives an identical copy of the parent cell’s DNA.
How is the DNA code transferred to the ribosome for protein synthesis?
Proteins are synthesized at the ribosomes, from the DNA code, but since DNA is stored in the nucleus, a
message with the DNA’s code is transcribed and sent instead. During transcription an enzyme bind to a
promoter site and “unzips” the two strands of the DNA molecule by breaking the hydrogen bonds that
hold the base pairs together. One strand serves a template for the attachment of complementary bases.
However, not all the DNA is transcribed; just the instructions for the protein that is being synthesized.
The single stranded copy of the DNA called messenger RNA (mRNA) exits the nucleus through the pores
in the nuclear membrane and makes its way to the ribosome. mRNA is constructed of nucleotides just as
DNA is except the base uracil is used instead of the base thymine.
How are proteins synthesized?
The mRNA strand serves as the instructions for the sequence of amino acids that make the protein. Each
three base sequence of mRNA is called a codon. Each codon specifies a particular amino acid. A transfer
RNA (tRNA) molecule “transfers” the specified amino acid from the cytoplasm to the ribosome. Because
tRNA molecules have the complementary bases to the codon (called an anti codon) on one end of it, they
can bind with the mRNA codons. The other end of the molecule carries the amino acid that is specified
by the codon. The next amino acid is brought by another tRNA molecule and as the tRNA anti-codons
bind with the codons on the mRNA the amino acids are bonded to each other forming the protein chain.
When a stop codon is reached, the protein is released and coils up in the structure that will allow it to
perform its function.
What is the role of DNA in gene expression?
An expressed gene is a gene that has been transcribed into RNA. Only a fraction of the genes in a cell are
expressed at any given time. Certain DNA sequences serve as promoters and binding sites for the RNA
polymerase (enzyme). Regulation of gene expression is important in shaping the way a complex
organism develops from a single fertilized egg.
What is crossing over and how does it affect genetic variability of the offspring?
During prophase I of meiosis, each chromosome pair lines up beside its corresponding homologous pair
to form a structure called a tetrad. There are 4 chromatids in a tetrad. The end of a chromatid from one
parent may “flop” over a chromatid from the other parent and the alleles below that point are exchanged
resulting in new combinations of alleles. This is extremely significant because it generates genetic
diversity which helps subsequent generations withstand environmental change.
What is the role of DNA in cellular replication prior to mitosis
DNA replicates during interphase of the cell cycle. At this point, the DNA is in the form of chromatin
which coils and thickens forming district structures known as chromosomes. These chromosomes move
to the center of the cell during metaphase and the sister chromatids (homologous copies of parental genes)
are pulled to opposite poles during anaphase. At the end of telophase, two identical cells are formed;
thus maintaining the continuity of body cells from one generation to the next. The new cells are diploid
Describe the roles of DNA in meiosis.
DNA replicates during interphase as explained above. However, in meiosis, the cell divides two times so
the number of chromosomes can be reduced in half. Four haploid cells are formed; each containing a
different set of alleles for the same traits. Meiosis generates diversity which is essential in order for the
species to adapt to changing environments.
Evaluate the role of genetic variation in successive generations
The more genetic diversity that exists within individuals of the same species, the more variation there is
for nature to select as environments change. A species with a genetically diverse population has the
strongest ability to respond to environmental change, increasing the chance that the species will continue
to exist.
What are mutations?
Mutations are changes to the genetic code caused by errors in DNA replication. They can occur in
somatic (body) cells or gamete (reproductive) cells. When mutations occur in gamete cells, they can be
passed on to subsequent generations.
Describe the causes and consequences of gene mutations.
Point mutations affect one nucleotide. If one nucleotide is substituted for another, only one amino acid is
altered. This may or may not affect the functioning of the protein. This is because several codons can
code for the same amino acid. Even the substitution of one amino acid for another may not affect the
function of the protein. However, if a nucleotide is deleted or inserted every amino acid after that point
will be affected because every codon will be changed. This is called a frame shift mutation. Such
mutations often alter a protein so that it is unable to perform its normal functions.
Describe the causes and consequences of chromosomal mutations.
If chromosomes fail to separate during meiosis II, one cell will have two copies of the same homologous
chromosome and the other will have none. This failure to separate is called non-disjunction. If a gamete
with one chromosome fuses with another that has 2 copies of the same chromosome, the resulting zygote
will have 3 copies of that chromosome. This is called trisomy. Such zygotes rarely survive because
chromosomes contain many genes which together have a significant impact on the viability of the
organism. However, if an individual receives 3 copies of chromosome 21, that individual will be born
with down syndrome.
What mutagenic factors exist in the environment and what effect do they have?
A mutagen is an agent that can cause a change in the DNA sequence that in turn affects the expression of
a gene. Environmental mutagens are chemicals such as those found in tobacco and radiation such as
nuclear or UVB. Only if such mutations occur in gamete cells can they be passed down from one
generation to another.
Essential Questions and Answers:
How is water cycled through an ecosystem?
All living things need water to survive. Water enters the atmosphere from bodies of liquid
(lakes, oceans) by evaporation, which is the process by which water changes from liquid to gas.
Water also enters the atmosphere from plants in a process called transpiration. In transpiration
water evaporates out of the leaves of plants. Air containing moisture rises and eventually cools
and is converted back into liquid water through the process of condensation (gas to liquid).
These tiny droplets of water form clouds and eventually release it back to earth as precipitation
(droplets merge to form larger droplets). On land water runs off the surface into lakes and
streams or is taken into the soil where it is either taken up by the roots of plants or seeps into the
groundwater.
Why do living things need carbon?
Carbon is important to living things because it is the key element in the biological molecules that
make up living things. These biological molecules can also be broken down and the energy from
their catabolism used to power or build (grow) organisms.
How is carbon cycled through an ecosystem?
Carbon is present in the atmosphere as carbon dioxide gas. Carbon dioxide enters the
atmosphere through diffusion from ocean surfaces, through the respiration by all living
organisms (mostly soil micro-organisms and insects), volcanic activity and the burning of
biomass and fossil fuels. Carbon leaves the atmosphere by two processes; absorption into the
oceans and photosynthesis. Photosynthesis, performed by plants and algae, converts carbon
dioxide gas into carbon molecules such as glucose. The amount being absorbed by the oceans is
relatively the same as the amount diffusing out of the oceans. The amount plants take in by
photosynthesis is relatively the same as the amount given off by the respiration of soil microorganisms and insects. While in the ocean the carbon is used to make shells for sea
invertebrates.
When animals eat plants they use the carbon from the plants to build their own tissues and to
obtain energy through respiration. The waste of respiration, carbon dioxide, is released into the
atmosphere. Decomposition converts organic carbon back into atmospheric carbon dioxide.
When humans burn these substances they release carbon dioxide that has been stored for long
periods of time back into the atmosphere. There is nothing to balance this.
Why do living things need nitrogen?
All organisms need nitrogen to make amino acids, which are in turn used to build proteins.
How is nitrogen cycled through an ecosystem?
Nitrogen gas makes up 78% of the atmosphere. The gaseous nitrogen is converted into solid
nitrogen in the soil where it may be taken up by plants. Plants make proteins from the nitrogen.
All nitrogen obtained by animals can be traced back to the eating of plants. The nitrogen is
passed up the food chain. At each trophic level, nitrogen is excreted as liquid which is converted
back into forms plants can use. Some nitrogen is converted back into gas and re-enters the
atmosphere.
Why are bacteria essential to the nitrogen cycle?
Only bacteria can convert nitrogen from gas to solid forms and back into gas form. Certain
bacteria that live in the root nodules of certain plants called legumes (peas & beans) convert
nitrogen gas solid forms plants can use in a process called nitrogen fixation. This is a symbiotic
relationship because the bacteria get a place to live and an energy source and the plants get
nitrogen. Other soil bacteria convert nitrates into nitrogen gas.
Why do living things need phosphorus?
Phosphorus is essential to living organisms because it is a component of DNA and RNA which
form our genetic code and ATP which we use for energy.
How is phosphorus cycled through an ecosystem?
Phosphorus is not very common in the biosphere and unlike carbon, hydrogen, oxygen and
nitrogen, is never found in the atmosphere. On land it is found mostly in rock and in the ocean in
sediment. Phosphate is released as rock and sediment are eroded. It cycles on land from the soil
to the plants to the animals and back to the soil and in water from the water to the algae to the
zooplankton to fish and back to the water. There is exchange between the land and sea’s cycles
as phosphorus from land washes into rivers and flows to the sea and the phosphate that has been
stored in sediments for thousands of years is eventually weathered and released back to land.
How does energy move through an ecosystem?
The sun’s energy first enters an ecosystem through its producers: photosynthesizing plants, algae
and bacteria. They are the only living things that can harness the sun’s energy directly. Other
organisms called consumers, including animals, fungi and other bacteria, obtain their energy
from the producers. Herbivores eat the plants and carnivores eat the herbivores. Second level
carnivores eat the carnivores and are eaten by third level carnivores. Each time an organism is
eaten the energy in it is passed up the next level. Any level can be consumed by a decomposer
and at every level the energy that is not passed up is released as heat. The energy transferred
from one organism to another in the food chain is not 100% of the energy – much is lost by the
life processes and inefficiencies of the organisms.
What do we call each level in a food chain, such as producer, herbivore and first level
consumer?
Each level is called a trophic level, or literally “feeding step”.
How does an energy pyramid demonstrate what happens to energy as it moves up a food
chain?
An energy pyramid shows the relative amount of energy available at each trophic level. Only
about 10% of the energy available at each level is passed up to the next. The rest is used for life
processes, such as respiration, movement and reproduction. The more links in a food chain the
less energy that remains of the original amount.
How does a biomass pyramid show us?
A pyramid of biomass shows the amount of living organic matter (weight of living things) at
each trophic level. It is usually expressed in grams of organic matter per unit of area. It
represents the amount of potential food available for each trophic level in an ecosystem.
What are the stages of primary succession?
 Primary succession begins when lichens populate bare rock. Because lichens are the first
species to live where no life has lived in the past, they are called pioneer species. As the
lichens grow, they help break up the rocks and when they die their organic material helps
to form soil in which plants can grow.
 Mosses may grow next and then grasses and small weeds will root in the soil trapping
soil and organic matter that blows their way. In addition, the decomposition of grasses
and weeds will accelerate the soil formation process.
 Small shrubs and pine trees can now grow amongst the grasses and weeds.
 The community then moves from a scrubby one to a pine dominated forest.
 Hardwood trees such as Maple, Beech and Oak grow in the shade of the pines and
eventually grow taller than the pines shading them and ultimately resulting in their death
and replacement by the hardwood forest. This is called the climax community. If
nothing changes, which in nature is highly unlikely, this community will be stable.
What happens if a community is disturbed at some stage of succession?
If a community is disturbed, without removing the soil, it will be set back to an earlier stage and
succession will start over from there. This is called secondary succession. Secondary succession
proceeds much quicker than primary succession, because there is soil present and often even
seeds. Soil formation is a very slow process. Secondary succession results after a disturbance
such as abandoning a plowed field, a wildfire or clear cutting a forest. The pioneer species in
this case are grasses and weeds. The intermediate steps are similar to primary succession and the
climax community, in Georgia, is a hardwood forest.
What are some general trends in succession?
Early successional plants are opportunistic species such as weeds and grasses that are small, fast
growing, short lived and widely dispersed. Late successional plants are larger, slow growing and
highly competitive. The total biomass increases while productivity (rate of conversion of
sunlight into glucose) decreases throughout the sequence. Species composition changes rapidly
at first and then slower later. Species diversity increases initially then stabilizes or declines in
late stages. Early communities consist of simple food chains, whereas later stage communities
have complex food webs.
What are some human activities that influence and modify the environment?
Humans influence the environment by using renewable and non-renewable resources, by using
herbicides and pesticides and by using carbon based fuels for energy.
What are some indicators that human activities have influenced and modified the
environment?
Water and air pollution, land degradation/desertification acid rain, global warming, the ozone
hole and loss of habitat and biodiversity.
Essential Questions and Answers:
What are some human activities that influence and modify the environment?
Humans influence the environment in many ways: the use of nonrenewable resources, especially
fossil fuels; pollution of air, water, and land; the use of herbicides and pesticides; the use of
chlorofluorocarbons (CFCs): and the destruction and fragmentation of natural habitats.
How do human activities influence and modify the environment?
There are many examples of how human activities influence and modify the environment.
Nonrenewable resources cannot be replaced. The burning of fossil fuels has many effects. It is
the main cause of air pollution, smog, and acid precipitation. It also causes the increase in
carbon dioxide in the atmosphere which causes the greenhouse effect and global warming. Acid
precipitation damages crops, kills organisms in aquatic ecosystems, and causes valuable nutrients
to leach from the soil. The use of pesticides has caused the evolution of resistant species and the
biological magnification of chemicals such as DDT in organisms in food chains. The use of
CFCs is responsible for the thinning of the ozone layer. The destruction and fragmentation of
natural habitats are responsible for the loss of biodiversity.
What is an adaptation?
Any structure, behavior, or internal process that enables an organism to respond to stimuli and
better survive in an environment is called an adaptation. Adaptations are inherited from previous
generations.
How do tropisms enable plants to survive in their environment?
A tropism is a plant’s directional growth response to a stimulus in their environment. Plants will
grow towards or away from a stimulus to better survive in an environment. Geotropism is the
response of plants to the force of gravity. Geotropism causes the roots of seedlings to grow
downward (positive geotropism) and the stems to grow upward (negative geotropism).
Phototropism is the growth response to light. If a plant is placed near a directional light source,
the plant will grow in the direction of the light. Thigmotropism is the response of a plant to
touch. Thigmotropism occurs when plants grow around a surface; vines contain tendrils that coil
around supporting objects.
How do hormones affect plant growth and survival?
A hormone is a chemical that is produced in one part of an organism that is transferred and
causes an effect in another part of the organism. Auxins cause the elongation of cells in stems
and are responsible for the growth patterns of tropisms. Gibberellins are growth hormones that
cause plants to grow taller by stimulating cell elongation. They also play a role in seed
germination and bud development. Cytokinins stimulate cell division. Ethylene gas promotes
ripening of fruit. Abscisic acid inhibits plant growth and cell division during times of stress.
How do structural adaptations help plants survive stressful environmental conditions?
There are many examples of plant adaptations that allow plants to survive in their environment.
Seeds of many plants will go dormant in unfavorable conditions. Some plants have modified
roots and stems that are used for storage in order to survive the winter. Deciduous trees drop
their leaves and go dormant for the winter. Conifers have many structural adaptations: thick
bark to protect from fire, a waxy coating on the leaves to reduce evaporation, and flexible
branches that allows them to bend under the weight of snow. There are also many adaptations
for reproduction: flowers can be pollinated in many ways, and there are various methods for
seed dispersal. There are also adaptations used for defense. Physical defenses include thorns
and spines. Chemical defenses include chemicals that taste bad, irritants, and poisons.
What is behavior?
Behavior is defined as anything an animal does in response to stimuli in its environment.
How do inherited behaviors increase an animal’s ability to survive stressful environmental
conditions?
Inherited behaviors are behavior patterns that are genetically programmed; they are often
referred to as innate behaviors. They include both automatic responses and instinctive behaviors.
A reflex is a simple automatic response that involves no conscious control. A fight or flight
response mobilizes the body for greater activity. An instinct is a complex pattern of innate
behavior. There are many examples of instinctive behavior. Courtship behavior is specific for
each species; animals will recognize the behaviors of members of their own species.
Territoriality reduces competition; a territory often contains the breeding grounds, feeding area,
shelter, and potential mates of an animal. Aggression is another behavior exhibited by animals to
fend off predators and competitors. A dominance hierarchy is a form of social ranking within a
group in which some members are more subordinate than others. Migration is the instinctive,
seasonal movement of a species. Hibernation is a state in which the body temperature drops,
oxygen consumption decreases, and breathing rates decline. Estivation is a state of reduced
metabolism that occurs in animals living in conditions of extreme heat.
How do learned behaviors increase an animal’s ability to survive stressful environmental
conditions?
Learned behavior is a result of previous experiences of an animal that modifies their current
behavior. Learned behavior has survival value because it allows animals to change their
behavior in a changing environment. There are many examples of learned behavior. Habituation
occurs when an animal is repeatedly given a stimulus that is not associated with any punishment
or reward. An animal has become habituated when it ceases to respond to a stimulus. Imprinting
is a form of learning in which an animal, at a specific critical time of its life, forms a social
attachment to another object. Other examples of learned behavior include trial-and-error
learning, conditioning, and insight.
What are adaptations for defense?
There are two categories of defense adaptations: mechanical defenses and chemical defenses.
Mechanical defenses are incorporated into the physical structure of the organism. They include
structures such as claws, tusks, singers, and shells. Another mechanical defense is camouflage;
examples include cryptic coloration, disruptive coloration, and countershading. Chemical
defenses occur when the animal produces stinging sensations, paralysis, poisoning, or just a bad
taste.
Essential Questions and Answers:
Solutions
What are the two parts of a solution?
A solution is composed of a solute placed into a solvent. The solvent will be the component with
the largest volume in the solution. Water is called the ‘Universal Solvent’ because it dissolves
many compounds, including salt.
What is the process of forming a solution?
Solvation is the process of surrounding solute particles with solvent particles. This causes the
solute to be dissolved and distributed throughout the solvent. Dissociation occurs when ionic
compounds separate into their component ions in a solution.
Compare and contrast soluble with insoluble and miscible with immiscible.
Solids which easily dissolve in a solvent are soluble. Liquids that mix together are miscible.
Solids that do not dissolve in a solvent are insoluble and liquids that do not mix together are
immiscible (Ex: oil in water). ‘Like dissolves like’; so polar solvents dissolve ionic solids and
polar liquids, while non-polar solvents dissolve non-polar compounds. Polar and ionic
compounds will not be soluble/miscible in non-polar solvents.
What are the factors that affect the rate at which a solution dissolves?
How quickly a solute dissolves into a solvent depends upon the characteristics of the solvent and
how easily the process of solvation can occur. Increasing the temperature (increasing molecular
motion), increasing the surface area (decreasing particle size), and agitation (stirring, shaking) all
increase the solvation process for solids and liquids. Increasing the pressure and decreasing the
temperature will increase the solvation of gases (ex: sodas).
What information can be gained from a solubility curve?
A Solubility curve tells the amount of a solute dissolved in 100 g of solvent at a given
temperature or pressure. A solution that is saturated (on the line) has the maximum amount of
solute dissolved at that specific temperature/pressure. An unsaturated solution (below the line)
can dissolve more solute. A supersaturated solution (above the line) has excess solute dissolved;
this excess can easily crystallize out of the solution.
How is concentration expressed?
Concentration is the amount of solute in a given amount of solvent or total solution. A
concentrated solution has a high concentration and a dilute solution has a low concentration. The
most common concentration is molarity which tells you the moles of solute per liters of solvent.
Molality is the moles of solute in kilograms of solvent and is used to avoid thermal expansion in
volume measurements.
How are colligative properties related to concentration?
Colligative properties are physical properties of solutions that depend on the concentration
(molality), not identity, of a solution. For example, increasing the concentration of an electrolyte
increases the ions dissociated in the solution, thus increasing the electric conductivity. Boiling
point elevation and freezing point depression occurs when you increase the concentration of salts
in water. Osmosis, the rate of diffusion across a membrane, increases due to the concentration
gradient.