INTRODUCTION OF GENETIC VARIABILITY

The amount by which individuals ill a population differ from one another due to their genes, rather than their environment is called genetic variability. the study of genetic variability is part of population genetics. Genetic variability is caused by two factors: mutations and genetic recombination. first of Bosselman’s Hod iy ersitv described “To what extent does ‘rick of genetic variability itself threaten a species or populations existance How much should ‘genetic variability be protected?”

Importance of genetic variability

Genetic variability has great importance for the survival of an species. Low genetic variability can put the survival of specie in danger.For example the Iris potato famine occurred due to low genetic  variability for potatoes. thus the single fungus destroyed all the same crops of potato

GENE MUTATION

The changes to the genetic material (either DN.% ir P NA) are called mutation. Mutations ran he caused by

L. Copying errors in the genetic material during cell division

  1. By exposure to radiation. chemicals (mutagens).
  2. – Viruses may causes mutations
  3.  Mutations can occur under cellular control during processes such as meiosis or hypermutation.In multicellular organisms, mutations can be subdivided into two types:
    • Germline Mutations: These mutations can be passed on to descendants.
    • Somatic Mutations: The somatic mutations cannot be transmitted to descendants in animals. Plants sometimes can transmit somatic mutations to their descendants asexually or sexually (in case when Bower buds develop in somatically mutated part of plant).

    Effects of mutations

    Mutations create variation in the gene pool. Therefore, mutations can be:

    (a)   Deleterious: These mutations can be less favorable (or

    (deleterious.) mutations. These are removed from the gene pool by natural selection.

    (b)     Beneficial: But more favorable (beneficial or advantageous) tend to accumulate through evolution.

    (c)     Neutral mutations: Neutral mutations are defined as mutations whose effects do not influence the fitness of either the species or the individuals of the species. These can accumulate over time due to genetic drift. DNA repair is able to revert most changes before they become permanent mutations. Therefore, majority of mutations have no significant effect. Many organisms have mechanisms for eliminating permanently mutated somatic cells.

    Classification of Mutations Classification by effect on structure

    The DNA sequence of a gene can be altered in a number of ways. Gene mutations have varying effects on health. It depends on where they occur and whether they alter the function of essential proteins. Structurally, mutations can be classified as:

    (a)        Smallscale mutations: These mutations affecting one or a few nucleotides. These are

    I. Point mutations: The mutations caused by exchange of single nucleotide are called point mutations. These are often caused by chemicals or malfunction of DNA replication. Most common is the transition that exchanges a purine for a purine (A   G) or a pyrimidine for a pyrimidine, (C •—T). A transition can be caused by nitrous acid, base mispairing, or mutagenic base analogs such as 5-bromo-2-deoxyuridine (B’rdU). Less common is a transversion, which exchanges a purine for a pyrimidine or a

    pyrimidine for a purine (C/TA/G). A point mutation can be
    reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion. Point mutations that occur within the protein coding region of a gene are classified into three kinds:

    • Silent mutations: which code for the same amino acid.
    • Missense mutations: which code for a different amino acid.
    • Nonsense mutations: which code for a stop codons.

    2. Insertions: The mutations in which one or more extra nucleotides are added into the DNA is called insertion. They are usually caused by transposable elements, or errors during replication of repeating elements ,(e.g. AT repeats). It has two types:

    • Splice site mutation: In this case, the coding region of a gene may alter splicing of the mRNA (splice site mutation).
    • Frameshift mutations: These mutations cause a shift in the reading frame (frameshift). It significantly alter the gene product.

    3. Deletions: The removal one or more nucleotides from the DNA is called deletion. Like insertions, these mutations can alter the reading frame of the gene. They are irreversible.

    (b)  Largescale Mutations: These mutations causes

    change in chromosomal structure. including:

    I. Amplifications (or gene duplications) leading to multiple copies of chromosomal regions. It increases the dosage of the genes located within them.

    1. Deletions of large chromosomal regions, leading to loss of the genes within those regions.
    2. Mutations whose effect is to juxtapose previously separate pieces of DNA. It brings together separate genes to form functionally distinct fusion genes. These include:
    • Chromosomal translocations: interchange of genetic parts from non homologous chromosomes.
    • Interstitial deletions: removing .regions of DNA from a single chromosome. thereby apposing previously distant genes.
      • Chromosomal inversions: reversing the orientation of a chromosomal segment.

      4. Loss of heterozygosity: loss of one allele, either by a deletion or recombination event, in organisms which previously had two.

      (c) Classification of mutation by effect on function

      I. Amormphic mutations or Loss-of-function mutations are the result of gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive.

      1. Neomorphic mutations or Gain-of-function mutations change the gene product such that it gains a new and abnormal function. These mutations usually have dominant phenotypes. These are also called a neomorphic mutation.
      2. Actinomorphic mutations or Dominant negative mutations have an altered gene product that acts antagonistically to the wild-type allele. These mutations usually result in an altered molecular function (often inactive). They are characterized by a dominant or semi-dominant phenotype.
      3. Lethal mutations are mutations that lead to a phenotype incapable of effective reproduction.

      (d) Classification of mutations on the basis of causing agents

      There are two classes of mutations are spontaneous mutations (molecular dcay) and induced mutations caused by mutagens.

      I. Spontaneous mutations: The mutations caused due to

      molecular decay are called spontaneous mutations. These

      inc hide:


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      • Tautogmerisin – A based is changed by the repositioning of a hydrogen atom.
      • Depurination – Loss cf a purine base (A or G).
      • Deamination – Ch1 nges a normal base to an atypical base; C —› 11, (which can be corrected by DNA repair mechanisms), or      spontanei us    deamination    of    5-methycytosine (irreparable), or A      FIX (hypoxanthine).
      • Transition – A purine changes to another purine, or a pyrimidine to a pyrimidine.
      • • Transversion – A purine becomes a pyrimidine, or vice virsa.2. Induced mutations: The mutations caused by mutagens are called induced mutations: Induced mutations may be caused by chemicals like Nitrosoguanidine (NTG) or by radiation like ultraviolet radiations.Effects of mutations .

        I. Changes in DNA caused by mutation can cause errors in protein sequence, creating partially or completely non-functional proteins.

        1. If a mutation is present in a germ cell. this can give rise to offspring that carries the mutation in all of its cells. This is the case in hereditary diseases.
        2. Often, gene mutations that could cause a genetic disorder are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair mistakes in DNA.
        3. A very small percentage of all mutations actually have a positive effect. These mutations lead to new versions of proteins. These proteins help an organism and its future generations better adapt to changes in their environment.

        GENETIC RECOMBINATION

        Genetic recombination is the transmission-genetic process by which the combinations of alleles observed at different loci in two parental individuals become shuffled in offspring. Such shuffling can be the result of:

        (a)   Crossing over: The recombination via intra-chromosomal recombination is called crossing over).

        (b)   Independent assortment: The i nter-ch romososom a I recombination is called independent assortment.

        Recombination therefore only shuffles already existing genetic variation. It does not create new variation at the involved loci.

        In molecular biology. recombination is the molecular process by which genetic variation found associated at two different places in a continuous piece of DNA becomes disassociated (shuffled). In this process one or both of the genetic variants are replaced by different variants found at the same two places in a second DNA molecule. One mechanism leading to such molecular recombination is chromosomal crossing over. Such shuffling of variation is also possible between duplicated loci within the same DNA molecule.

        Sometimes, the number of loci in each of the recombinant molecules is changed by the shuffling process. It is called unbalanced recombination or unequal crossing over. Enzymes called recontbinases catalyze this reaction. RecA, the recombinase found in E. col i, is responsible for the repair of DNA double strand breaks (DSBs).

        (a) Crossing over

        Crossing over of one of the chromosomes inherited from each of one’s parents occurs during meiosis in that parent. After chromosomal replication during gameto genesis. the four available chromatids from charismata. During this time, homologous sites on two chromatids can mesh with one another and exchange genetic information. Immediately after replication. the tetrad formed by replication contains two, pairs of two identical chromatids. After crossing over. each of the four chromatids carries a unique set of genetic information.

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Mechanism of crossing over: Enzymes known as recombitutses catalyze the reactions that allow for crossover to occur. A recombinase creates a nick in one strand of a DNA double helix. It allows the nicked strand to pull apart from its complementary strand. It is attached to one strand of the double helix on the opposite chromatid. A second nick allows the unannealed strand in the second double helix to pull apart. It is anneal to the remaining strand. It forms a structure known as a cross-strand exchange or a Holliday junction. The Holliday junction is a tetrahedral structure. It can be pulled by other recombinases and move it along the four-stranded

(b) Independent assortment

In most eukaryotes. a cell carries two copies of each gene called allele. Each parent passes on one allele to each offspring. Even without recombination, each gamete contains a random assortment of chromatids. They choose randomly from each pair of chromatids available. With recombination, however, the gamete can receive a (mostly) random assortment of individual genes.

Recombination results in a new combination of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different. This process explains why offspring from the same parents can look so different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring. Hie fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination. This theory of independent assortment of alleles is fundamental to genetic inheritance.

Other types of recombinations

I. Conservative site-specific recombination: In this case, a mobile DNA element is inserted into a strand of DNA by means similar to that seen in crossover. A segment of DNA on the mobile element matches exactly with a segment of DNA on the target. It allows the enzymes called iMegrases to insert the rest of the mobile element into the target. Integrases are a special type of Recombinases.

  1. Transpositional recombination: Transpositional recombination does not require an identical strand of DNA in the mobile element to match with the target DNA. Instead, the integrases introduce nicks in both the mobile element and the target DNA. It allows the mobile DNA to enter the sequence. The nicks are then removed by ligases.
  2. Nonhomologous recombination: Recombination between DNA sequences that contain no sequence even though have similar homology is called nonhomologous recombination or recombination end joining.

 

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