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Vivek Mannava
Connie Allen
Honors Biology
24 May 2021
Cellular Processes & Functions
The building blocks of life, otherwise known as cells, are an essential component for
something to be considered as living. Many types of cells exist, each with its own molecular
makeup, niche, and function. Every cell has its own way of maintaining homeostasis. This can be
done through an array of different cellular processes utilizing many distinct organelles. Without
exception, cells possess an assortment of varying organelles and cellular processes in order to
attain a state of homeostasis.
Cell Function
Cells are imbued with many organelles, and they use these to perform various processes
in themselves. I will be using the following paragraphs in this section to explain more about the
main organelles in cells and their functions.
Cell Wall
The purpose of a cell wall is to help support the shape of the cell and protect the cell.
Plants, bacteria, and some fungi have cell walls. Cell walls are mainly composed of
polysaccharides such as cellulose, chitin, glucans, and peptidoglycan. Plant cell walls contain
cellulose, fungi contain chitin and glucans, and bacteria contain peptidoglycan.
Cell Membrane
The cell membrane is an essential component of the cell as it is responsible for enclosing
the cell’s organelles while acting as a semi-permeable barrier for cellular transport. It is primarily
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made up of a phospholipid bilayer, which consists of two layers of a hydrophilic head and a
hydrophobic tail. The arrangement of a phospholipid bilayer is as follows: head, tail, tail, and
head. The head of the phospholipid is made up of phosphates and glycerol. Because H2O is a
polar molecule, it is attracted to the phospholipid head. For the cell to maintain a balance of
water inside and outside of the cell, one head is placed on the outermost side of the bilayer, and
the other head is placed on the innermost layer side. The tail of the phospholipid is made up of a
saturated fatty acid and an unsaturated fatty acid. Fats interact poorly with water, therefore the
tail is hydrophobic. Due to the combination of hydrophobic and hydrophilic particles, the cell
membrane can maintain a semi-permeable boundary that not only protects the cell but also is
involved in cellular transport.
Cellular Transport
The cellular membrane has two main forms of moving particles across it: passive and
active transport. Passive transport requires no energy or adenosine triphosphate (ATP) from the
cell, while active transport does. The reason for the requirement of ATP in active transport is
because active transport moves particles from a low concentration to a high concentration.
Particles naturally move from a region of high concentration to one of low concentration without
requiring energy. This is the basis of passive transport in the cell. Meanwhile, it is also the reason
why active transport requires energy.
There are three forms of passive transport: simple diffusion, facilitated diffusion, and
osmosis. Simple diffusion deals with small, nonpolar molecules such as water, gases, and fatty
acids. Only these types of particles can pass through the phospholipid bilayer without help from
transport proteins. Facilitated diffusion, on the other hand, requires the help of transport proteins.
Protein channels that lie throughout the cell membrane act as a pipeline through which large,
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polar molecules can pass through. Osmosis is when water molecules diffuse across a
semi-permeable membrane due to solute concentration. When a solution has a lower
concentration of solute than another solution, osmotic pressure causes another solution to rush
towards it. This solution is said to be hypotonic. If a solution has a higher concentration of solute
than another solution, osmotic pressure acts upon it, causing it to rush towards that other
solution. This solution is called a hypertonic solution. In the case that both solutions have the
same amount of solute, movement would occur to maintain a balance between the two solutions.
These solutions would be called isotonic solutions.
Active transport implicates the usage of energy, but where does this energy get used?
ATP is expended in protein pumps which carry particles from low concentrations to higher
concentrations. These protein pumps are exceedingly specific to the type of molecules that they
carry in. Using the sodium-potassium pump as an example, we can deduce how definitive each
protein pump is to the type of molecule that it lets in. With each ATP, this pump expels three
sodium ions and takes in two potassium ions.
Cytoplasm
Cytoplasm exists for the main purpose of supporting the cell. Although it is involved in
many other cellular processes, its main function is to give the cell shape. Without the cytoplasm,
the cell would be a shriveled blob. The cytoplasm is mostly composed of a fluid made of salt and
water. The cytoplasm also contains enzymes that break down waste. Additionally, anaerobic
respiration occurs in the cytoplasm. Anaerobic respiration consists of glycolysis and
fermentation, which both occur respectively. Glycolysis is where glucose gets broken down into
pyruvates that then get broken down in fermentation. Fermentation can break down the products
from glycolysis into lactic acid or ethanol. Lactic acid fermentation occurs in animals and some
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types of bacteria. Ethanol fermentation can occur in fungi, plants, and some bacteria. If
fermentation does not occur and oxygen is present, then this goes into the mitochondria instead
to perform aerobic respiration.
Mitochondria
As mentioned, aerobic respiration takes place in the mitochondria. Aerobic respiration
consists of three steps: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis
converts glucose into pyruvates, which produces 2 ATP. The Krebs cycle takes these pyruvates
and converts them into six molecules of nicotinamide adenine dinucleotide (NADH), two
molecules of flavin adenine dinucleotide (FADH2), two molecules of ATP, and four molecules of
CO2. Although the Krebs cycle does not consume oxygen, it cannot occur without the presence
of oxygen. The equation for the Krebs cycle is as follows:
2 acetyl groups + 6NAD+ + 2FAD + 2ADP + 2P → 4CO 2+ 6NADH + 6H+ + 2FADH2 +
2ATP
After the Krebs cycle is over, we move onto the electron transport chain. In the electron transport
chain, oxidative phosphorylation takes place. The electron transport chain gets its name from this
process because protons and electrons are transferred from NADH and FADH2 into oxygen. The
energy from oxidative phosphorylation powers enzymes named ATP synthases, which specialize
in catalyzing the production of ATP from adenosine diphosphate (ADP) and phosphate. At the
end of this process, water and 34 ATP are produced. The equation for aerobic respiration is:
C6H12O6 + 6O2 → 6CO2 + 6H2O + 38 ATP
This equation shows glucose being oxidized by the Krebs cycle and producing CO2. The electron
transport chain creates H2O as a byproduct as well. Finally, the 38 ATP is from glycolysis (2
ATP), the Krebs cycle (2 ATP), and the electron transport chain (34 ATP).
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Anaerobic respiration has only two steps, which include glycolysis and fermentation.
Unlike aerobic respiration, it does not require oxygen. However, without oxygen, only a scant
amount of energy can be produced as the mitochondria cannot be utilized. Glycolysis is the first
step of anaerobic respiration, and it produces two ATP by splitting glucose into pyruvates.
Because these pyruvates cannot be used due to the mitochondria’s requirement of oxygen, they
have to be broken down in fermentation. Lactic acid fermentation occurs in animals and some
bacteria, and ethanol fermentation occurs in plants, yeast, and some bacteria. The equation for
lactic acid fermentation is:
C6H12O6 → 2C3H6O3
This equation describes a glucose molecule being broken half into two lactic acid molecules. The
equation for ethanol fermentation is:
C6H12O6 → 2C2H5OH + 2CO2
This equation depicts a glucose molecule being converted into two ethanol molecules and two
carbon dioxide molecules.
Chloroplast
The chloroplast is an essential organelle where photosynthesis occurs. This organelle is
where solar energy from the sun gets exploited and turned into chemical energy in the form of
glucose. Only plants, particular protists, and particular bacteria can have chloroplasts. Two types
of reactions take place in photosynthesis. These reactions are light-dependent reactions and
light-independent reactions. Light-dependent reactions take place in photosystems inside the
chloroplast. Photosystems are composed of chlorophyll, and they use this chlorophyll to harvest
solar energy by using it to control electrons. Then light-independent reactions come into play,
where electrons are carried using energy carrier molecules such as ATP and nicotinamide
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adenine dinucleotide phosphate (NADPH). The energy from light-dependent reactions is used in
light-independent reactions in this way in order to combine water and carbon dioxide to
synthesize glucose. The formula of photosynthesis is as described:
6CO2 + 6H2O → C6H12O6
This equation shows six carbon dioxide molecules and six water molecules combining using
solar energy in the chloroplast’s photosystems during light-dependent and light-independent
reactions.
Vacuole
A vacuole is a membrane-bound organelle that mainly stores a cell’s wastes. The vacuole
is responsible for making sure these wastes are properly disposed of and contained. Vacuoles are
created and destroyed during endocytosis and exocytosis. Endocytosis creates vacuoles because
material enters the cell, and exocytosis expels vacuoles out of the cell, causing the vacuole to be
destroyed. Only plant vacuoles have a variation compared to regular vacuoles; they are larger
because they are mostly used for water and glucose storage.
Nucleus
The nucleus is the cell’s regulator organelle. Its main function is to store DNA and be the
cell’s command center. Due to this, it covers about 10% of the cell’s volume. Its makeup consists
of a nuclear envelope surrounding it, chromatins that contain DNA, the nucleolus, and
nucleoplasm. The nucleoplasm is similar to the cytoplasm; it envelops all of the extra space
inside the nucleus for structure. On the nucleus’s surface, the nuclear envelope exists as a
phospholipid bilayer that separates the nucleus from the cytoplasm of the cell. The nuclear
envelope is layered with nuclear pores that only allow certain materials to enter and leave the
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nucleus. The nucleolus is a ribosome-making factory within the nucleus. Finally, chromatins in
the nucleus exist to package DNA because it would be too lengthy otherwise.
We have gone through the composition of the nucleus, but we still have not covered why
the nucleus needs to store DNA, and how it controls the cell itself. DNA, or deoxyribonucleic
acid, is the primary proponent of protein synthesis. DNA has three main ingredients that inhabit
its makeup. These ingredients are phosphates, deoxyribose sugar, and nitrogenous bases. These
are arranged into subunits named nucleotides. The backbone of DNA is composed of
deoxyribose sugar and phosphates. These ingredients hold up the nitrogenous bases, which are
the main part of DNA because they are the ones that hold information for protein synthesis.
These nitrogenous bases are adenine, guanine, cytosine, and thymine. Adenine pairs with
thymine, and guanine pairs with cytosine. The way that these nitrogenous bases are arranged on
DNA is how proteins can be synthesized. These base pairs complement each other on the two
sides of the backbone, consequently resulting in a double helix form factor.
Because cells have to reproduce, DNA has to be replicated. DNA replication occurs in
the nucleus and consists of three phases. The DNA’s double helix unwinds and causes the DNA
to flatten in the first step, initiation. Enzymes, such as helicase, unzip the complementary stands
causing them to separate into two individual strands. Then, the second step, elongation,
continues with building the new DNA. Due to the complementary nature of the nitrogenous
bases in DNA, each split strand can be used as a template by adding the complementary base pair
to it. An enzyme, DNA polymerase, completes this process by adding nucleotides along the
strands. Termination, the final step, has all of the remaining gaps in the DNA filled, and then the
DNA is sealed.
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Protein synthesis was aforementioned, but what are its specifics? Protein synthesis begins
with transcription in the nucleus. Transcription has the same three phases of DNA replication
initiation, elongation, and termination. This is because another nucleic acid is being
manufactured. Ribonucleic acid, or RNA, is DNA except it is a single strand of base pairs
instead of two strands put together. Ribosomal RNA (rRNA), Messenger RNA (mRNA), and
Transfer RNA (tRNA) are all types of RNA in most cells. Because DNA cannot move out of the
nucleus and because its only purpose is to store information, RNA exists as a messenger that tells
the cell how to produce proteins. Initiation begins with the enzyme RNA polymerase signaling
the DNA to unwind. Then, elongation adds nucleotides to an mRNA strand. Finally, termination
is where the mRNA strand ejects from the DNA. Transcription takes the complement of the
nitrogenous base in the DNA. For example, this means that if adenine is present, then thymine
will be taken. Thymine is also replaced with uracil in mRNA. The purpose of this messenger
RNA is to communicate with ribosomes outside of the nucleus on how to assemble proteins.
Transcription ends when mRNA leaves the nucleus out of the nuclear pores.
Ribosomes
The next and concluding stage of protein synthesis is translation. The mRNA that was
previously transcribed in the nucleus arrives at a ribosome. Ribosomes are special organelles in
the body that are composed of ribosomal RNA and proteins. The ribosome takes in the mRNA,
reading it, and assigning transfer RNA molecules to complement the mRNA strand. The mRNA
is read three base pairs at a time, otherwise known as codons. These codons are read by
anticodons in the ribosome using tRNA. Transfer RNA strands carry amino acids, which means
that the mRNA can be translated into a chain of amino acids. After the amino acids are
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assembled employing peptide bonds, a polypeptide chain is synthesized. The protein has only
finished synthesis if it has been folded. This final process happens in the endoplasmic reticulum.
Types of Cells & Organisms
Like all things on Earth, cells are divided into prokaryotic and eukaryotic cells.
Prokaryotic cells are simple, small cells without membrane-bound organelles, while eukaryotic
cells are large, complex cells with membrane-bound organelles. The taxonomic system divides
organisms into Archaebacteria, Eubacteria, Protists, Fungi, Plants, and Animals. These divisions
are decided based on cell type, multicellular or unicellular, type of reproduction, where their food
is obtained from, way of reproduction, and other unique characteristics.
Prokaryotes
Prokaryotes are small cells with a simple structure. They do not have a nucleus or
membrane-bound organelles, and their genetic material floats openly in the cytoplasm. The
Archaebacteria and Eubacteria kingdoms both contain prokaryotic organisms. Also, all
prokaryotic organisms are single-celled organisms due to their simplicity. Due to DNA being the
factor that creates all proteins in the cell, it is consequently also responsible for the smaller cell
size in prokaryotes than eukaryotes because of the few chromosomes that prokaryotes contain.
All prokaryotes consist of a cell membrane, cell wall, ribosomes, a circular piece of DNA, and
cytoplasm. The size of prokaryotic cells ranges from .1 microns to 5 microns, while eukaryotes
range from 10 to 100 microns. All prokaryotes are haploid cells that reproduce asexually through
binary fission. They are able to divide extremely quickly due to their simplistic, small structure.
Prokaryotes also compose a large portion of the Earth’s biomass. Because of their simplicity,
they are considered to be the oldest forms of life on Earth.
Eukaryotes
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The endosymbiotic theory expresses how prokaryotes are the ancestors of eukaryotes, and how
eukaryotes evolved from prokaryotes. It states how some organelles in our cells were external
prokaryotes that were swallowed by an ancient prokaryote and used as organelles via forming a
symbiotic relationship. These ancient prokaryotes were presumably amoeba-like cells with a
nucleus that was mutated from its cellular membrane closing around its chromosomes. The four
pieces of evidence of this theory are the fact that chloroplasts and mitochondria have their own
circular DNA (not linear chromosomes), chloroplasts and mitochondria both divide by binary
fission, chloroplasts and mitochondria both have different types of ribosomes than eukaryotes,
and their relation to prokaryotes today. Unlike prokaryotic organisms, eukaryotes can reproduce
sexually, meaning that they are additionally diverse. Similar to prokaryotes, eukaryotes also
contain DNA, ribosomes, cell walls, cell membranes, and cytoplasm. Most eukaryotic organisms
are multicellular, unlike prokaryotes. Eukaryotic DNA is linear compared to the circular DNA in
prokaryotes. Unlike prokaryotes, eukaryotes contain many more organelles such as vacuoles,
nuclei, mitochondria, chloroplasts, vesicles, lysosomes, peroxisomes, the rough and smooth
endoplasmic reticulum, and the Golgi apparatus.
Sexual Reproduction
There are two contrasting varieties of cells in eukaryotic organisms that sexually
reproduce. These cells are somatic cells, normal body cells, and sex (or germ) cells. Somatic
cells are regular body cells that have a full double set of chromosomes from their parent
organism. Cells that meet this requirement are considered to be diploid cells. Somatic cells in the
body can include nerve cells, muscle cells, blood cells, liver cells, phloem cells, xylem cells, et
cetera. Germ cells are special body cells used for sexual reproduction. Some examples of germ
cells are sperm cells, egg cells, pollen cells, and spore cells. They are known to be haploid cells
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or cells with only one copy of their parents’ chromosomes. Haploid cells have half the number of
chromosomes that diploid cells have. To create genetic diversity, sexual reproduction exists, and
this is the primary reason why haploid germ cells combine during sexual reproduction to make a
zygote. The kingdoms that can sexually reproduce, and therefore contain these types of cells, are
certain types of animals, plants, and fungi.
The aforementioned were zygotes, the preliminary stage of sexually reproducing
eukaryotes. To gain a more centralized understanding of how zygotes turn into their respective
fully formed organism, I will describe these processes in humans. Reproduction in humans
occurs when a male inserts haploid gametes, in the form of sperm, into a female. This sperm cell
converges with a haploid egg cell and merges. Once the sperm cell is assimilated into the egg
cell, a zygote forms. Zygotes are diploid because their chromosome number doubles when the
sperm and egg fuse. This is because two haploid cells equal a diploid cell. Human haploid cells
contain 23 chromosomes, and human diploid cells contain 46 chromosomes. The zygote matures
as cells divide inside of it. As this happens, it moves towards the uterus, where it will turn into a
blastocyst. The zygote takes about five days to turn into a blastocyst. Implantation then occurs,
where the blastocyst affixes itself into the uterine lining. 15 days into the process, an embryonic
disc forms. This embryonic disc is attached to a maternal artery that provides nourishment. All
while this is happening, cells are constantly differentiating inside the organism. The embryonic
disc has three layers the endoderm, the mesoderm, and the ectoderm. These layers will
differentiate to form every single organ in the human body. The fourth week is when the
embryonic disc will turn into an embryo with a slight human shape. During the next six weeks,
arms, legs, eyes, and the nervous system will form. At this time, all body systems will have
appeared in undeveloped form. When week ten arrives, the embryo will turn into a fetus, and an
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umbilical cord will have fully developed. During the next twenty to thirty weeks until childbirth,
the fetus will fully develop all of its bones, muscles, skin, brain, and many other parts.
Now that I have provided a physical overview of how sexually reproducing eukaryotes
can reproduce, I can get into the genetic reasoning of how cells differentiate. Cells in the body
differentiate due to factors such as external signaling proteins, temperature, and oxygen. When
cells differentiate, the structure of DNA and its surrounding histone proteins change. Histone
proteins help DNA condense into chromatins so that it may fit inside of the nucleus. Due to cell
differentiation, histone proteins bind tightly to DNA so that only the genetic material needed for
the certain type of cell being specified is revealed. Transcription proteins, which designate how
much mRNA needs to be transcribed, also come into action. Both of these processes cause the
cell to differentiate from a stem cell into a differentiated somatic cell.
Eukaryotic Taxonomic Kingdoms
Although a few animals reproduce via asexual reproduction, most animals are sexually
reproducing eukaryotes. Also, they are multicellular heterotrophic organisms, meaning that they
need to take in their energy from the environment instead of producing it themselves. A unique
characteristic they possess is the ability to move. Unlike plant cells and some fungi cells, animal
cells do not have cell walls. Animals are the consumers in the ecosystem.
Plants are also multicellular eukaryotic organisms like animals. However, they do not
possess the ability to move, nor are they heterotrophic. Plants are autotrophic, meaning that they
produce their food by the way of photosynthesis. Plant cells contain cell walls with cellulose,
unlike animal cells which do not have cell walls. Plants are the decomposers in the ecosystem.
Plants can reproduce through sexual reproduction.
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Unlike plants and animals, fungi can be multicellular or unicellular organisms. Like
animals, fungi are also heterotrophic. They are the decomposers in the ecosystem. Like animals
and plants, fungi can also reproduce sexually. Fungi also have a cell wall like plants, but their
cell wall contains chitin instead of cellulose.
Protists are unicellular organisms, unlike most fungi, plants, and animals. They can feed
heterotrophically or autotrophically, and they can reproduce asexually or sexually. The protist
kingdom is often classified as a junk drawer because it contains many organisms that do not
belong in the remaining three eukaryotic kingdoms.
Cellular Cycle
Eukaryotic cells tend to spend their lifespans in a never-ending cycle of growth and cell
division. This cycle consists of a growth phase known as interphase and a varying replication
phase which can be either mitosis or meiosis.
Interphase
The stages of interphase in a cell varies based on cell type. Normally, interphase is split
into G1, S, and G2 respectively, but some cells have a special G0 phase. G phase stands for gap
phase, and it is where growth will primarily occur in the cell. G0 occurs when cells no longer
have to divide. They may re-enter the cell cycle again later on, or perpetually remain in G0. In
G1, organelles are replicated, and the cell grows. The next phase in interphase is the S phase or
the synthesis phase. During this phase, DNA is replicated in the nucleus of the cell. Interphase
finishes off with G2, the second gap phase. The cell produces proteins, organelles, grows and
prepares for mitosis/meiosis. After G 2, the cell has finished interphase.
Cell Cycle Regulation Genes & Cancer
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Genes are a prerequisite for the cell cycle to transpire. Two types of genes control the
pace of the cell cycle. These genes two are classified as proto-oncogenes and tumor-suppressor
genes. Proto-oncogenes help the cell progress through the cell cycle. They code for proteins that
enhance cell growth. Tumor suppressor genes impede cell growth, rectify errors in DNA, and
determine when apoptosis occurs. Apoptosis occurs when the cell is at the end of its life cycle. It
is known as programmed cell death.
Since proto-oncogenes and tumor suppressor genes control the behavior of the cell and
are therefore imperative to proper cell function, it is not illogical to assume that a major
disturbance would occur in the cell cycle if any malfunction occurred. These disturbances could
be the failure of tumor suppressor genes to activate and/or the over-activation of
proto-oncogenes. When mutations of these genes reach a climax, the cell is unable to control its
growth and division rate. This causes tumors in the body and can eventually lead to cancer.
Cancer is the uncontrollable propagation of an abnormal group of cells in the body. If cancer
occurs, then proto-oncogenes will be called oncogenes.
Cells can succumb to cancer in four general ways. An error may accidentally take place
during DNA replication affecting proto-oncogenes or tumor suppressor genes. Exposure to
certain chemicals could cause a mutation in DNA. Radiation exposure is also a cause of DNA
mutations. Finally, carcinogenic genes may be inherited from parents.
Mitosis & Meiosis
When cells finish interphase, they need to reproduce. They can do this in two ways:
mitosis and meiosis. However, somatic cells and sex cells need to reproduce in a different
process. This is because of their respective functions since somatic cells serve as regular body
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cells, while sex cells exist for the reproduction of their host organism. Due to this, somatic cells
undergo mitosis while sex cells go through meiosis.
Mitosis is composed of prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase starts with the condensation of DNA in the nucleus. Prometaphase continues with the
nuclear envelope dissolving. In metaphase, the chromosomes line up in the middle of the cell.
During anaphase, sister chromatids of each chromosome get pulled apart into each cell. Finally,
during telophase, the nuclear envelope forms on each cell, and the cells divide.
Meiosis is composed of the same phases as mitosis. The only difference is that they occur
twice instead of once. Some extra processes also take place during these same phases. These
processes include crossing over and independent assortment. Crossing over is when tetrads, two
groups of maternal and paternal sister chromatids, come together and exchange genetic
information. This means that similar DNA crosses over to each side. Random assortment occurs
when homologous chromosomes are being sorted. Homologous chromosomes are DNA that
carries the same genes but has different alleles. Although in mitosis this sorting is fixed, in
meiosis it is random. Crossing over and independent assortment take place in meiosis because of
extra variation in offspring. Variation in offspring is good considering that organisms can adapt
to their environment more adequately.
Now that we have covered the details of mitosis and meiosis, we can get into their
comparison. Mitosis results in two diploid daughter cells, unlike meiosis which results in four
haploid daughter cells. Diploid (2n) cells are cells that have the full set of chromosomes, and
haploid (n) cells are cells that have half the number of chromosomes that a regular cell has.
While mitosis occurs in somatic cells, meiosis occurs in sex cells. Mitosis produces regular body
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cells, unlike meiosis which produces gametes. Although meiosis includes processes such as
crossing over and independent assortment for variation, mitosis has no such processes for this.
Heredity
Heredity concerns the genetic inheritance from parents to their offspring. Cells receive all
of their genetic information from their parents. This genetic information is identical to the
information made from the convergence of the sperm and the egg, or a zygote. This zygote
divided through mitosis until the current cells existed. During this process, the stem cells
produced by the zygote differentiate into their respective niche cells.
Mendelian Inheritance
Gregor Johann Mendel was a famous scientist who discovered the laws of inheritance.
Mendelian genes and Non-Mendelian genes get their names from him. Genes are Mendelian
when one allele exhibits clear dominance over the other recessive allele. There also needs to be
only two types of alleles for the particular gene.
For example, one Mendelian gene commonly passed on in horses is curly or smooth hair.
The allele for smooth hair is dominant over curly hair, which is recessive. This information can
be showcased in the punnet square below:
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This punnet square shows the genotypes of organisms that have heterozygous parents with
smooth hair (Hh). Heterozygous refers to the possession of one dominant allele and one
recessive allele. Homozygous means that the respective organism has only one type of allele.
With Mendelian genes, an organism can either be heterozygous, homozygous dominant, or
homozygous recessive.
In this punnet square, the probability of the parents producing heterozygous (Hh),
homozygous dominant (HH), or homozygous recessive (hh) offspring is showcased. Each square
represents a 25% chance of that genotype. This translates to a 25% chance of a homozygous
dominant offspring, a 25% chance of a homozygous recessive offspring, and a 50% chance of a
heterozygous offspring. When the genotype gets expressed in the organism, it is referred to as a
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phenotype. The presence of dominant alleles determines the phenotype of the offspring.
Heterozygous and homozygous dominant genotypes will all have the dominant allele expressed
in the phenotype. For example in the punnet square above, the chance for smooth hair is stated as
75% because three of the squares have dominant alleles present. Consequently, this causes the
chance for smooth hair to be 25%. It is crucial to observe that the chances for phenotypes are
different from genotypes because of dominant alleles.
Non-Mendelian Inheritance
However, not all genes are Mendelian genes. As implied by the name, Non-Mendelian
genes are the inverse of Mendelian genes. Non-Mendelian genes have two general differences
between them and Mendelian genes. First, they can have multiple alleles (2+), and second, one
of their alleles can not be dominant to the other. Since organisms are very complex, there are four
different types of Non-Mendelian genes.
Incomplete Dominance
The first of the four types is incomplete dominance. Incomplete dominance occurs when
two traits blend. For example, when a chicken with black feathers mates with a chicken with
white feathers, a blue chicken is produced. This is expressed in the punnet square below:
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This punnet shows a black chicken (F1) crossing with a white chicken (F2). The probability of
this outcome is 100% the same genotype (F1F2) and the same phenotype (blue).
Codominance
Codominance occurs when both alleles are dominant and are accordingly expressed
equally. An example of this would be a red-haired cattle mating with a white-haired cattle to
produce a cattle with red and white hair. This is indicated in the punnet square below:
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This punnet square shows a white-haired cow (HWHW) mating with a white and red haired cow
(HWHR). There is a 50% possibility that the children will be white-haired (HWHW) and a 50%
probability that they will be white and red haired (HWHR).
Multiple Alleles
Genes that have more than two alleles are multiple allelic genes. An example of this
would be a gene for fur in rabbits with four different possible alleles: the allele for black fur (C),
the allele for gray fur (cch), the allele for Himalayan fur (ch), and the allele for albino fur (c). This
means that a rabbit could possibly have black fur, gray fur, Himalayan fur, albino fur, and a mix
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of gray and Himalayan fur. This can be portrayed in the punnet square below:
This punnet square shows a parent with black fur mating with a parent with Himalayan fur. The
probability that their offspring would have black fur (Cch) is 50%, and the probability that their
offspring would have a mix of gray fur and Himalayan fur (chcch) is 50% as well.
Polygenic
A possibility exists for multiple genes to code for a single trait. When this occurs, the trait
is a Non-Mendelian polygenic trait. An example of this would be the length of pigs. If four XY
genes code for height and the amount of dominant alleles corresponds to a larger length, then
65,536 different phenotypes and genotypes would be possible.
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Errors in Gene Inheritance
Sporadically, mutations may appear in DNA during its replication. These mutations may
be substitutions, insertions, or deletions. A substitution occurs when a nitrogenous base in DNA
gets replaced accidentally with another base. Insertions occur when an extra nitrogenous base
gets implanted into the DNA sequence. Deletions occur when a nitrogenous base gets removed
from the DNA strand. Deletions and insertions cause frameshifts because nucleic acids are read
in groups of three nitrogenous bases known as codons. Deletions and insertions shift the frame of
the DNA causing information to be misaligned from these codons.
Another type of mutation is non-disjunction, which occurs during meiosis. The factor that
causes non-disjunction is the failed separation of homologous chromosomes in Anaphase I and
II. Extra chromosomes may be present in the daughter germ cells or some chromosomes may be
missing if non-disjunction has occurred. These same problems are passed on to the zygote,
blastocyst, embryonic disc, embryo, and offspring during its development. This causes a range of
different diseases in the organism that is all based on the specific chromosome that was
mishandled.
Conclusion
Cells, the elementary units of life, contain an assortment of varying structures and
mechanisms to maintain homeostasis in themselves and their respective organisms. DNA and
RNA, noteworthy ingredients in protein synthesis, are imperative to proper cell function. Cells
operate in a cell cycle, which regulates their growth and division rate. Cells pass on their DNA to
their offspring in a process called genetic inheritance. Eukaryotic cells and prokaryotic cells are
the two main types of cells. All of these characteristics are necessary for life to be sustained on
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Earth; they are a hallmark of the evolutionary amelioration that has swayed the planet for billions
of years.
Mannava 24
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