Owen Lyke
Period 1
10/19/10
DNA Essay
INTRODUCTION
Deoxyribonucleic acid is one of the most important biological components
for life on Earth. First isolated by Friedrich Miescher, DNA is the genetic material at
the center of all living organism’s cells. Without DNA life as we know it couldn’t exist
(no really.) DNA is the mechanism that allows life to change and adapt, it allows for
incredible amounts of different species of organisms and the diversity of life on this
planet. DNA is made of nucleotides in the form of a double helix that is connected by
four bases. DNA is reproduced in reproduction and gives individual cells their jobs
to function as part of a group. DNA also defines the individual characteristics of
every organism, such as whether it has gills or lungs, fins or feet. Mutations in DNA
caused by damage or natural process, make new individuals and continue the
process of evolution.
Structure
DNA is found in three forms, A DNA, B DNA, and Z DNA. B DNA, the most
common form of DNA found in functional living organisms is a right-handed double
helix discovered by James D. Watson and Francis Crick in 1953. IN B DNA the helix
makes a complete revolution every 3.4 nanometers and the distance between base
pairs is .34 nanometers. In every type of DNA there are two grooves made by the
backbones, these are called the major and minor grooves according to their size. Z
DNA takes the form of a left-handed helix that makes a complete turn every 4.6
nanometers with base pairs .383 nm apart. A DNA is rarely found in living
organisms, and is typically found in high solutions of salt or alcohol. A DNA makes a
complete revolution every 2.3 nm and has base pairs spaced .209 nm apart.
Nucleotides
Nucleotides can be used by the cell as energy, or to signal other cells through
cellular signaling, but their most important use is as the structure of DNA and RNA.
Nucleotides are made up of a nitrogenous base, a five carbon sugar, and one to three
phosphate groups. The phosphate attaches to either the 2nd, 3rd, or 5th carbon on the
sugar molecule, with the 5th carbon being the most common attachment point. The
make-up of the nucleotide also defines whether the helix is ribonucleic or
deoxyribonucleic (RNA of DNA). If the nucleotide is composed with the sugar ribose,
it will form the support of RNA, and if it uses a deoxyribose sugar it will build DNA.
Double Helix
The double helix structure of DNA allows the information stored to be
compacted into a smaller unit than if it was a flat “ladder.” This allows cells to
devote more space to other critical structures like the mitochondrion or chloroplast
and less to the nucleus. The backbones of DNA and RNA are long chains of either
deoxyribonucleotides or ribonucleotides. In A and B DNA structures the spiral
moves in a right-handed direction. In Z DNA, the backbones spiral in a left-handed
direction. The two backbones of DNA are connected by the four base molecules,
which attach one end to the backbone and the other side to a corresponding base
pair. These two sides split up when they are copied for reproduction.
Bases
The bases of DNA and RNA are the molecules that actually make up the
information that is stored by DNA and RNA. The bases are located between the two
structural supports of DNA. The bases are also what connect the two backbones. In
A DNA, the bases are a distance of .209 nm apart on the helix, in B DNA, they are .34
nm apart, and in Z DNA, the bases are spaced .383 nm from each other. There are
only four molecules that are used to store information in the genome. These
chemicals are adenine, thymine, guanine, and cytosine. Despite the fact that DNA
and RNA only use four molecules to communicate their information, there is almost
no limit to the amount of information it can represent. Take computers for example,
they only use two “molecules,” 1 and 0, and they can still run vastly complex
computer programs, similar to a cell. One of the four bases will only fit together with
one of the other three bases. For example, cytosine only pairs with guanine and
adenine only pairs with thymine. What actually stores the genetic information is the
order that these bases are paired along the DNA. A combination of AT, AT, CG, AT,
will be a different code than AT, CG, CG, AT.
Reproduction
For a living organism to grow and adapt to its surroundings, its cells need to
be able to multiply. The cell cycle represents the entire life of the cell, which is
described by PMATI. PMATI stands for prophase, metaphase, anaphase, telophase,
and interphase. Each of these phases are defined by different processes that occur
during them. The interphase dominates the majority of the cell cycle. During the
interphase cycle, a cell duplicates its’ DNA, synthesizes proteins, and copies
centrioles, the processes of the interphase are essential to mitosis and meiosis.
During the reproduction of a cell, the cell’s DNA is copied and half of each
chromosome is left in the two resulting daughter cells.
Mitosis
Mitosis is the reproduction of a cell that results in the production of two
daughter cells. In the cell cycle, mitosis is represented by the prophase, metaphase,
anaphase, and the telophase. Each phase has a different role in the reproduction of a
cell. In the prophase, the nucleus begins to fade and the chromatin (which is
replicated DNA and other proteins) condense to become chromosomes. Each
chromosome is made up of two chromatids which both contain the same genetic
information. The cytoskeleton is broken down to form the mitotic spindle that
grows from the centrosomes. The next step, the prometaphase, is in between the
prophase and the metaphase and not include in the PMATI acronym. In the
prometaphase, the nucleus breaks down until it is unrecognizable. During this step,
mitotic spindle fibers elongate and attach to the kinetochores, protein bundles at the
center of chromosomes where chromatids attach, other spindles overlap each other
at the cell’s center. The metaphase is the time when spindle fibers apply tension that
lines up the chromosomes at the center of the cell. The mitotic spindle fibers shorten
and pull apart the chromosomes at the kinetochores during the anaphase. In the
telophase, the chromatids arrive at the cell poles and the mitotic fibers that pulled
them disappear. Finally, at the end of mitosis, a cell undergoes cytokinesis. During
cytokinesis, the microtubules that made up the overlapping spindles break down
and a contractile ring splits the cell into to identical copies. Now the cytoskeleton
begins to reconstruct for the interphase.
Meiosis
Meiosis is another form of cellular division than mitosis. Mitosis is asexual
while meiosis is sexual. The steps for meiosis are prophase I, metaphase I, anaphase
I, telophase I, prophase II, metaphase II, anaphase II, and telophase II because
meiosis performs two divisions. Each phase does the same steps as in mitosis, but
they are repeated twice. Because meiosis has two divisions instead of one, the
number of chromosomes in each daughter is halved. This is okay because meiosis is
sexual reproduction and the number of chromosomes that start in the parent cell
are twice as much as need to be in the organism’s cells for it to function properly.
Also, it is due to meiosis that evolution is possible. If all cells reproduced by mitosis,
every orgamism on the planet would be the same. When meiosis occurs, two
animal’s chromosomes are divided and mixed around like a deck of cards. This
produces four daughter cells that all have traits from both the mother and father
cell, allowing for evolution to act.
DNA Reproduction/RNA
Cells Jobs
Organs
Organ Systems
Defines Organism
Characteristics
Alleles
Chromosomes
Damage
Chemical
Radiation
Evolution
CONCLUSION