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.



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.



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 %.



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 sa 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.



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 m in size.



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 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.