MUTATIONS

MUTATIONS

1.    Mutation is an alteration or change in the genetic information of a living and, therefore, it will produce a change in the features of the person or animal which suffer from that.

2.    The genetic unit that is able to mutate is the gene, which is the hereditary information that take place in the DNA.

3.    A consequence of mutations may be a genetic disease.

4.    According to the mechanism that caused the change in the genetic material, there are three types of mutations: MOLECULAR or GENETIC mutation, GENOMIC mutation and CHROMOSOMAL mutation.

MOLECULAR or GENETIC MUTATION: affects the chemical formation of genes.  

1.    It occurs by altering the nucleotide sequence of DNA.

2.    It leads a nucleotide change producing a protein with an amino acid sequence that it’s incorrect.

3.    In this type of mutations we found that instead of a nucleotide there is another. This change of one nucleotide for another may involve, or not, an amino acid change or could generate a stop codon, that is, not determined amino acid in the genetic code.

4.    If the nucleotide change creates a stop codon, the protein is interrupted and is not functional.

5.    The molecular mutations is caused by:

o   Replacement. Where there should be a nucleotide, is inserted another. For example, instead of cytosine there is a thymine. The replacements may be:

-  Transitions: substitution of one purine by another purine or a pyrimidine by another pyrimidine.

-  Transversions: replacement of a purine for pyrimidine or vice versa.

o   Phase shift. When inserted or deleted one or more nucleotides, produce a fails leading the formation of non-functional proteins.

6.    EXAMPLES:

 ALBINISM. Albinism is a genetic condition in which there is an absence of melanin in the eyes, skin and hair in humans or animals and it’s caused by a mutation in the genes. It is hereditary and appears with the combination of both parents that carry the recessive gene.

 SICKLE CELL ANEMIA. It’s a disease that affects hemoglobin, a protein that is part of red blood cells and is responsible of oxygen transport. It’s genetic and provokes lack of movement of red blood cells and some of their symptoms are pain in the extremities.

GENOMIC MUTATION: Alterations in the number of chromosomes. They can be of two types:

1.    Aneuploidy: an increase or decrease in the number of chromosomes. They can be:

a.         Nulisomías: when instead of two chromosomes in each pair there is none.

b.         Monosomies: when instead of two chromosomes of each pair there is one. For example: Turner Syndrome (XO)

c.         Trisomies: when instead of two chromosomes of each pair there are three. For example: Down Syndrome (chromosome 21), Edwards Syndrome (chromosome 18), Patau syndrome (chromosome 13).

d.         Tetrasomías: when instead of two chromosomes of each pair there are four.

                                                                                                                

2.    Euploidía: affects complete sets of chromosomes.

-       The monoploidía or haploidy is the existence of a single chromosome, instead of to be diploid.

-       Polyploidy: presence of additional sets of chromosomes. People are often sterile.

CHROMOSOMAL MUTATION: affects the number of chromosome, or the number or arrangement of genes on a chromosome. It is produce by mistakes during gametogenesis (formation of gametes by meiosis) or of the first divisions of the zygote. It can be divided into 3 parts:

1.    Chromosomal arrangements, which involve changes in chromosome structure:

o    Deletion and duplication, which can modify chromosome segments. Example: Chi du Chat Syndrome (or Cat Cry), presented growth retardation and low birth weight and their crying is like a cat's meow.

o    Inversion and translocation, which lead a loss of genetic information. For example: Philadelphia Syndrome that caused the formation of a deadly cancer that is called leukemia. Some of the symptoms are: tiredness, fever, lack of appetite, sweats, etc.

2.    Aneuploidies, an increase or decrease in the number of chromosomes. Examples:

a.    Down's syndrome, trisomy carriers such as the 21. Gametes carry the translocated chromosome plus one normal, so when fertilized with the opposite gamete, the product will have three chromosomes 21.

b.    Edward’s syndrome, known as trisomy 18. It is characterized by the presence of an extra chromosome in the 18.

c.    Patau syndrome, known as trisomy 13. It is a genetic disease from the presence of one additional chromosome 13. The victims die shortly after birth, most at 3 months, and at most reach the year.

d.    Klinefelter syndrome is a chromosomal abnormality that affects only men and provokes hypogonadism, that is, a disorder in the testicles. It is based on a genetic disorder that develops incorrect separation of chromosomes during meiosis. As we know, men have sex chromosomes 46, XY and women have sex chromosomes 46, XX. In Klinefelter syndrome, the man has at least an extra X chromosome.

3.    Polyploidy. Presence of additional sets of chromosomes. (As we have seen before)