Animal Genetics

Mutation and genetic polymorphism

Mutation is the process by which the sequence of base pairs in a DNA molecule is altered. A mutation, then, is a change in either a DNA base pair or a chromosome.

A cell with a mutation is a mutant cell. If a mutation happens to occur in a somatic cell (in multicellular organisms), the mutant characteristic affects only the individual in which the mutation occurs and is not passed on to the succeeding generation. This type of mutation is called a somatic mutation. In contrast, mutations in the germ line of sexually reproducing organisms may be transmitted by the gametes to the next generation, producing an individual
with the mutation in both its somatic and its germ-line cells. Such mutations are called germ-line mutations.

The mutation rate is the probability of a particular kind of mutation as a function of time, such as the number of mutations per nucleotide pair per generation or the number per gene per generation.

The mutation frequency is the number of occurrences of a particular kind of mutation, expressed as the proportion of cells or individuals in a population,
such as the number of mutations per 100,000 organisms or the number per 1 million gametes.

Effects of Mutation:

Mutations can affect individuals in a variety of ways. Among the consequences of mutation are the following:

Types of point mutations

 There are two basic types of mutations:

 

In an inverse mutation, a DNA sequence of nucleotides is reversed. Inversions can occur among a few bases within a gene or among longer DNA sequences that contain several genes.

Translocations are the transfer of a piece of one chromosome to a non-homologous chromosome. They are often reciprocal, with the two chromosomes swapping segments with each other.

The effects of small DNA changes depend on the region of the gene involved.

A. A nucleotide substitution in a coding region may be any of the following:

  1. Silent or synonymous mutations consist in changing one codon for another that codes for the same amino acid; eg TTT (Leu) —> TTC (Leu).
  2. Missense mutations involve a change of codons that cause substitution of one amino acid for another. E.g. Sickle cell hemoglobin: beta codon 6 GAG (Glu) —> GTG (Val).
  3. Nonsense mutations involve change from one of the 61 codons that specify amino acids to one of the three termination codons. E.g. In the form of thalassemia common in the Mediterranean region, Hb beta thal-1, codon 17 AAG (Tyr) —> TAG (Ter). This causes premature termination of the protein chain.
  4. Sense mutations involve change from a termination codon to one that codes for amino acids. E.g. Hemoglobin Constant Spring alpha 142 TAA (Ter) —> CAA (Gln). Translation continues beyond the normal termination until another termination codon is encountered.

 

B. Nucleotide substitutions can also affect the rate of transcription.

  1. A change in the regulatory regions can abolish transcription.
  2. In some instances, such as a mutation that causes persistence of fetal hemoglobin beyond the fetal period, nucleotide substitution in the 5' regulatory region causes the gene to be transcribed when it should be shut off.

C. Nucleotide substitutions can affect RNA processing.

  1. Substitutions in the exon/intron boundaries can prevent normal splicing.
  2. Substitutions within an intron or exon can create new splice sites.
  3. Changes in the 3' direction (downstream) from the coding regions can interfere with addition of poly-A tail.

D. Insertion or deletion of one or two nucleotides destroys the reading frame. Such frameshift mutations cause absence of any functional gene product. The most common mutation that causes cystic fibrosis involves loss of three nucleotides, which leaves the reading frame intact but the protein is nonfunctional.

 

E. Many mutations that cause loss of gene function are deletions of larger numbers of nucleotides. E.g. Duchenne muscular dystrophy.

F. The mutations that occur at a particular locus may be quite heterogeneous. Some may knock out gene function completely, others may reduce the function, and still others may have no effect on the phenotype, i.e., they are normal variants. As a result of this heterogeneity, many patients described as "homozygous" for a recessive allele may actually be heterozygous for two different defective alleles. Such a person is often called a compound heterozygote.

Genetic polymorfism

Genetic Polymorphism A difference in DNA sequence among individuals, groups, or populations. Sources include SNPs, sequence repeats, insertions, deletions and recombination. (e.g. a genetic polymorphism might give rise to blue eyes versus brown eyes, or straight hair versus curly hair). Genetic polymorphisms may be the result of chance processes, or may have been induced by external agents (such as viruses or radiation). If a difference in DNA sequence among individuals has been shown to be associated with disease, it will usually be called a genetic mutation. Changes in DNA sequence which have been confirmed to be caused by external agents are also generally called "mutations" rather than "polymorphisms." [Source: PHRMA Genomics Lexicon]

Genetic Mutation A change in the nucleotide sequence of a DNA molecule. Genetic mutations are a kind of genetic polymorphism. The term "mutation," as opposed to "polymorphism," is generally used to refer to changes in DNA sequence which are not present in most individuals of a species and either have been associated with disease (or risk of disease) or have resulted from damage in°icted by external agents (such as viruses or radiation). [Source: PHRMA Genomics Lexicon]

Polymorphism at the protein level

Variations in the DNA sequences can be studied by protein polymorphism. But much genetic variation in the genome, in each case, remained undetected. Why?

Polymorphisms at the DNA level

The document was created: 03. 08. 2020 23:16:23
Source: http://web2.mendelu.cz/af_291_projekty2/vseo/