Genetic
Genetics is the science of heredity and variation in living organisms.Knowledge of the
inheritance of characteristics has been implicitly used since prehistoric times for improving
crop plants and animals through selective breeding.However, the modern science of genetics,
which seeks to understand the mechanisms of inheritance, only began with the work of Gregor
Mendel in the mid-1800s.Although he did not know the physical basis for heredity, Mendel
observed that inheritance is fundamentally a discrete process with specific traits that are
inherited in an independent manner these basic units of inheritance are now called
genes.Following the rediscovery of Mendel's observations in the early 1900s, research in
1910s yielded the first physical understanding of inheritance that genes are arranged linearly
along large cellular structures called chromosomes. By the 1950s it was understood that the
core of a chromosome was a long molecule called DNA and genes existed as linear sections
within the molecule.
A single strand of DNA is a chain of four types of nucleotides hereditary information is
contained within the sequence of these nucleotides. Solved by Watson, Wilkins, and Crick in
1953, DNA's three-dimensional structure is a double-stranded helix, with the nucleotides on
each strand physically matched to each other. Each strand acts as a template for synthesis of
a new partner strand, providing the physical mechanism for the inheritance of information.The
sequence of nucleotides in DNA is used to produce specific sequences of amino acids, creating
proteins a correspondence known as the genetic code. This sequence of amino acids in a
protein determines how it folds into a three-dimensional structure, this structure is in turn
responsible for the protein's function. Proteins are responsible for almost all functional roles
in the cell. A change to DNA sequence can change a protein's structure and behavior, and this
can have dramatic consequences in the cell and on the organism as a whole.
Population Genetics
Population genetics is the study of the allele frequency distribution and change under the
influence of the four evolutionary forces natural selection, genetic drift, mutation and gene
flow. It also takes account of population subdivision and population structure in space. As
such, it attempts to explain such phenomena as adaptation and speciation. Population genetics
was a vital ingredient in the modern evolutionary synthesis, its primary founders were Sewall
Wright, J. B. S. Haldane and R. A. Fisher, who also laid the foundations for the related
discipline of quantitative genetics.Allele frequency.Allele frequency is a measure of the relative
frequency of an allele on a genetic locus in a population. Usually it is expressed as a
proportion or a percentage.
In population genetics, allele frequencies show the genetic diversity of a species population or
equivalently the richness of its gene pool. Allele frequency is defined as follows.A particular
chromosome locus and the gene occupying that locus.A population of individuals carrying n
loci in each of their somatic cells e.g. two loci in the cells of diploid species, which contain two
sets of chromosomes.A variant or allele of the gene,then the allele frequency is the fraction or
percentage of loci that the allele occupies within the population.For example, if the frequency
of an allele is 20% in a given population, then among population members, one in five
chromosomes will carry that allele. Four out of five will be occupied by other variants of the
gene.
Histogram
In statistics, a histogram is a graphical display of tabulated frequencies. A histogram is the
graphical version of a table that shows what proportion of cases fall into each of several or
many specified categories. The histogram differs from a bar chart in that it is the area of the
bar that denotes the value, not the height, a crucial distinction when the categories are not of
uniform width Lancaster, 1974. The categories are usually specified as non-overlapping
intervals of some variable. The categories bars must be adjacent.The word histogram is
derived from Greek histos 'anything set upright' as the masts of a ship, the bar of a loom, or
the vertical bars of a histogram gramma 'drawing, record, writing'. The histogram is one of
the seven basic tools of quality control, which also include the Pareto chart, check sheet,
control chart, cause-and-effect diagram, flowchart, and scatter diagram. A generalization of
the histogram is kernel smoothing techniques. This will construct a very smooth Probability
density function from the supplied data.
Note that for diploid genes the fraction of individuals that carry this allele may be nearly two
in five. If the allele distributes randomly, then the binomial theorem will apply 32% of the
population will be heterozygous for the allele i.e. carry one copy of that allele and one copy of
another in each somatic cell and 4% will be homozygous carrying two copies of the allele.
Together, this means that 36% of diploid individuals would be expected to carry an allele that
has a frequency of 20%. However, alleles distribute randomly only under certain assumptions,
including the absence of selection. When these conditions apply, a population is said to be in
Hardy-Weinberg equilibrium.The frequencies of all the alleles of a given gene often are
graphed together as an allele frequency distribution histogram. Population genetics studies the
different forces that might lead to changes in the distribution and frequencies of alleles in
other words, to evolution. Besides selection, these forces include genetic drift, mutation and
migration.
Percentage
In mathematics, a percentage is a way of expressing a number as a fraction of 100 per cent
meaning per hundred. It is often denoted using the percent sign, %. For example, 45 % read
as forty-five percent is equal to 45 / 100, or 0.45.Percentages are used to express how large
one quantity is relative to another quantity. The first quantity usually represents a part of, or a
change in, the second quantity. For example, an increase of $ 0.15 on a price of $ 2.50 is an
increase by a fraction of 0.15 / 2.50 = 0.06. Expressed as a percentage, this is therefore a 6 %
increase.Although percentages are usually used to express numbers between zero and one, any
dimensionless proportionality can be expressed as a percentage.
For instance, 111 % is 1.11 and -0.35 % is -0.0035.Percentages are correctly used to express
fractions of the total. For example, 25 % means 25 / 100 or one quarter.Percentages larger
than 100 %, such as 101 % and 110 %, may be used as a literary paradox to express
motivation and exceeding of expectations. For example, We expect you to give 110 % of your
ability, however there are cases when percentages over 100 can be meant literally.The
fundamental concept to remember when performing calculations with percentages is that the
percent symbol can be treated as being equivalent to the pure number constant 1 / 100 = 0.01.
For example, 35 % of 300 can be written as 350.01300 = 105.To find the percentage of a
single unit in the whole, divide 100 by the whole. For instance, if you have 1250 apples, and
you want to find out what percentage of the 1250 apples a single apple represents, 100 / 1250
would provide the answer of 0.08 %.
Chromosome
A chromosome is a single large macromolecule of DNA, and constitutes a physically organized
form of DNA in a cell. It is a very long, continuous piece of DNA a single DNA molecule,
which contains many genes, regulatory elements and other intervening nucleotide sequences.
A broader definition of chromosome also includes the DNA-bound proteins which serve to
package and manage the DNA. The word chromosome comes from the Greek chroma, color
and sµa soma, body due to its capacity to be stained very strongly with vital and supravital
dyes.Chromosomes vary extensively between different organisms. The DNA molecule may be
circular or linear, and can contain anything from tens of kilobase pairs to hundreds of
megabase pairs. Typically eukaryotic cells cells with nuclei have large linear chromosomes
and prokaryotic cells cells without nuclei smaller circular chromosomes, although there are
many exceptions to this rule.
Furthermore, cells may contain more than one type of chromosome for example mitochondria
in most eukaryotes and chloroplasts in plants have their own small chromosome in addition to
the nuclear chromosomes.In eukaryotes nuclear chromosomes are packaged by proteins
particularly histones into chromatin to fit the massive molecules into the nucleus. The
structure of chromatin varies through the cell cycle, and is responsible for the compaction of
DNA into the classic four-arm structure during mitosis and meiosis. Prokaryotes do not form
chromatin, because the cells lack proteins required and the circular configuration of the
molecule prevents this.Chromosome is a rather loosely defined term. In prokaryotes, a small
circular DNA molecule may be called either a plasmid or a small chromosome. In viruses,
mitochondria, and chloroplasts their DNA molecules are commonly referred to as
chromosomes, despite being naked molecules, as they constitute the complete genome of the
organism or organelle.
Mitocondrian
Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes.In
cell biology, a mitochondrion plural mitochondria is a membrane-enclosed organelle that is
found in most eukaryotic cells.Mitochondria are sometimes described as cellular power plants,
because they generate most of the cell's supply of ATP, used as a source of chemical energy.
The number of mitochondria in a cell varies widely by organism and tissue type. Many cells
possess only a single mitochondrion, whereas others can contain several thousand
mitochondria.Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion
has its own independent genome. This DNA shows similarity to bacterial genomes, and,
according to the endosymbiotic theory, mitochondria are descended from free-living
prokaryotes.
The word mitochondrion comes from the Greek or mitos, thread or khondrion, granule.A
mitochondrion contains inner and outer membranes composed of phospholipid bilayers and
proteins. The two membranes, however, have different properties. Because of this
double-membraned organization, there are 5 distinct compartments within the mitochondrion.
There is the outer membrane, the intermembrane space the space between the outer and inner
membranes, the inner membrane, the cristae space formed by infoldings of the inner
membrane, and the matrix space within the inner membrane. Mitochondria range from 1 to 10
micrometers µm in size.
Eukariyote
Animals, plants, fungi, and protists are eukaryotes, organisms whose cells are organized into
complex structures by internal membranes and a cytoskeleton. The most characteristic
membrane bound structure is the nucleus. This feature gives them their name, also spelled
eucaryote, which comes from the Greek e, meaning good/true, and, meaning nut, referring to
the nucleus. In the nucleus the genetic material, DNA, is arranged in chromosomes. Many
eukaryotic cells also contain membrane-bound organelles such as mitochondria, chloroplasts
and Golgi bodies.
Eukaryotes often have unique flagella made of microtubules in a 9+2 arrangement.Cell
division in eukaryotes is also different from organisms without a nucleus. This process
involves separating the duplicated chromosomes, through movements directed by
microtubules. There are two types of these division prcesses. In mitosis one cell divides to
produce two genetically identical cells. In meiosis, which is required in sexual reproduction,
one diploid cell having two copies of each chromosome, one from each parent undergoes a
process of recombination between each pair of parental chromosomes, and then two stages of
cell division, resulting in four haploid cells gametes each of which has only a single
complement of chromosomes, each one being a unique mix and match of the corresponding
pair of parental chromosomes.Eukaryotes appear to be monophyletic and thus make up one of
the three domains of life. The two other domains bacteria and archaea prokaryotes without a
nucleus share none of the above features, though the eukaryotes do share some aspects of
their biochemistry with the archaea, and as such, are grouped with the archaea in the clade
Neomura.
Chloroplasts
Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct
photosynthesis. Chloroplasts absorb sunlight and use it in conjunction with water and carbon
dioxide to produce sugars, the raw material for energy and biomass production in all green
plants and the animals that depend on them, directly or indirectly, for food. Chloroplasts
capture light energy from the sun to conserve free energy in the form of ATP and reduce
NADP to NADPH through a complex set of processes called photosynthesis. It is derived from
the Greek words chloros which means green and plast which means form or entity.
Chloroplasts are members of a class of organelles known as plastids.Recently, chloroplasts
have caught attention by developers of genetically modified plants. In certain plant species,
such as tobacco, chloroplasts are not inherited from the male, and therefore, transgenes in
these plastids cannot be disseminated by pollen.
This makes plastid transformation a valuable tool for the creation and cultivation of
genetically modified plants that are biologically contained, thus posing significantly lower
environmental risks. This biological containment strategy is therefore suitable for establishing
the coexistence of conventional and organic agriculture. The reliability of this mechanism has
not yet been studied for all relevant crop species. However, the research programme Co-Extra
recently published results for tobacco plants, demonstrating that the contaiment of
transplastomic plants is highly reliable with a tiny failure rate of 3 in 1,000,000.
Evolutionary origin
Chloroplasts are one of the many unique organelles in the plant cell. They are generally
considered to have originated as endosymbiotic cyanobacteria i.e. blue-green algae. This was
first suggested by Mereschkowsky in 1905 after an observation by Schimper in 1883 that
chloroplasts closely resemble cyanobacteria. In that they derive from an endosymbiotic event,
chloroplasts are similar to mitochondria but chloroplasts are found only in plants and protista.
The chloroplast is surrounded by a double-layered composite membrane with an
intermembrane space it has its own DNA and is involved in energy metabolism. Further, it has
reticulations, or many infoldings, filling the inner spaces.In green plants, chloroplasts are
surrounded by two lipid-bilayer membranes.
The inner membrane is now believed to correspond to the outer membrane of the ancestral
cyanobacterium. Chloroplasts have their own genome, which is considerably reduced
compared to that of free-living cyanobacteria, but the parts that are still present show clear
similarities with the cyanobacterial genome. Plastids may contain 60-100 genes whereas
cyanobacteria often contain more than 1500 genes.Many of the missing genes are encoded in
the nuclear genome of the host. The transfer of nuclear information has been estimated in
tobacco plants at one gene for every 16000 pollen grains.In some algae such as the
heterokonts and other protists such as Euglenozoa and Cercozoa, chloroplasts seem to have
evolved through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a
second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four
membrane layers. In some cases, such secondary endosymbionts may have themselves been
engulfed by still other eukaryotes, thus forming tertiary endosymbionts.
Chloroplast structure
The internal structure of a chloroplast, with a granal stack of thylakoids circled.
Chloroplasts are observable morphologically as flat discs usually 2 to 10 micrometer in
diameter and 1 micrometer thick. The chloroplast is contained by an envelope that consists of
an inner and an outer phospholipid membrane. Between these two layers is the intermembrane
space.The material within the chloroplast is called the stroma, corresponding to the cytosol of
the original bacterium, and contains one or more molecules of small circular DNA. It also
contains ribosomes, although most of its proteins are encoded by genes contained in the host
cell nucleus, with the protein products transported to the chloroplast.Within the stroma are
stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids
are arranged in stacks called grana singular granum.
A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or
lumen. Photosynthesis takes place on the thylakoid membrane as in mitochondrial oxidative
phosphorylation, it involves the coupling of cross-membrane fluxes with biosynthesis via the
dissipation of a proton electrochemical gradient.Embedded in the thylakoid membrane is the
antenna complex, which consists of proteins, and light-absorbing pigments, including
chlorophyll and carotenoids. This complex both increases the surface area for light capture,
and allows capture of photons with a wider range of wavelengths. The energy of the incident
photons is absorbed by the pigments and funneled to the reaction centre of this complex
through resonance energy transfer. Two chlorophyll molecules are then ionised, producing an
excited electron which then passes onto the photochemical reaction centre.