Mistakes in Meiosis
Mutations that occur in a somatic cell may do two of the following:
Kill the cell
Make the cell cancerous
However, whatever the effect is, the fate is the same in both cases; it will disappear when the cell in which it occurred, or its owner, dies
In short, somatic mutations are not heredity and are therefore, advantageous mutations because they are not inherited
Heritable mutation; it moves from one generation to the next. Found in every cell descended from the zygote (has the mutant gamete).
Meiotic errors are accidental, with no genetic predisposition (influence)
Accidental = the dance of chromosomes
The failure of homologous chromosomes to separate is called nondisjunction
The homologous chromosomes fail to separate properly leading to two of their gametes having an extra copy of a chromosome (trisomy after fertilization, n+1) and the other two gametes lack a chromosome completely (monosomy after fertilization, n-1)
The sister chromatids fail to separate leading to one gamete having an extra copy of a chromosome and the other gamete lacking a chromosome and the last two
Mistakes in meiosis may also occur when a stage in meiosis is induced before the previous stage is completed. For example, if anaphase is initiated before both kinetochores of a replicated chromosome become attached to microtubules from opposite spindle poles, daughter cells are produced that have a missing or extra chromosome; nondisjunction. The result of this is the production of a human egg with an extra chromosome; trisomy in its chromosome 21. An individual with trisomy in chromosome 21 has Down’s syndrome.
Its frequency increases with the age of the mother
Individuals with the syndrome are prone to developing leukemia and Alzheimer’s disease
Individuals are sexually underdeveloped and sterile
Short life span
Checkpoint controls, however, help reduce mistakes in cell-cycle events. They help reduce the development of Down’s syndrome
Checkpoint controls inhibit the passage of abnormal activities…through the cell cycle
The formation of a zygote with a gamete that is either trisomic (2n+1) or monosomic
(2n-1) with a normal gamete has a chromosomal alteraltion known as aneuploidy
The chromosome alteration that the zygote will have is that it will have an abnormal number of a particular chromosome; it will be either trisomic or monosomic for that chromosome
Aneuploid zygotes do not survive to produce successful offspring.
Sex chromosomes and chromosome 21 are more prone than others to aneuploidy
Patterns in the occurrence of meiotic errors suggested this
Aneuploidy in autosomal chromosomes are rare because they are not compatible (are unable to exist, work…) with life because being aneuploid means that there is a higher concentration of genes than normal, creating an imbalance that will be too high leading to the death of the cell. But this is not the case for chromosome 21,
Chromosome 21: Smallest autosomal chromosome; its genes can survive some imbalance (aneuploidy)
Chromosome Y: is a small chromosome that carries few genes
Chromosome X: The X chromosome can survive without the correct number of chromosomes, i.e. it can survive with aneuploidy
The unability of the sex chromosomes to separate properly into their gametes (nondisjunction) produces aneuploid conditions.
These aneuploid conditions affect genetic balance less than the autosomal aneuploid conditions and the reason lies in the sex chromosomes, X and Y:
X chromosomes: extra copies of the X become inactivated as Barr bodies in somatic cells
Y chromosome: carries few genes
The inactivation of One X chromosome happens in the palcenta of mammals; at the embryonic stage, after fertilization. Therefore, only one X is active in most mammalian female somatic cells.
Inactivated as Barr bodies, it is contracted into a Barr body that is located inside the nuclear membrane. Barr bodies are distinctive condensed chromatin structures. It is a dense object that lies along the inside of the nuclear envelope in female mammalian cells. Represents an inactivated X chromosome. The inactivation of an X chromosome is a random event occurring independently in embryonic cells; the event does not depend on other embryonic cells.
Now both males and females have equal amount of genes on the X chromosome.
The X chromosome that forms the Barr body has a gene called XIST that is active on it . XIST produces RNA that has the ability to trigger DNA methylation and X-inactivation.
Happens in males. Males have an extra X chromosome, hence they are XXY.
One of the X is inactivated but it still has a little affect in the phenotype of the males; some breast enlargement and other female body characteristics are common. Normal intelligence. They have male sex organs but their testes are abnormally small and the man is sterile (unable to produce sperm and therefore infertility is common). They have cognitive (acquiring knowledge) and behavioural problems. Mental retardation.
Disproportionate growth of the legs.
Males with an extra Y chromosome:
XYY
Do not exhibit any defined syndrome.
Individuals with XYY are taller than average
Females with trisomy X (XXX)
Are normal, they cannot be distinguished from XX females however, only by karyotype.
45, X0 individuals are phenotypically female; they are sterile because their sex organs do not mature. Estrogen replacement therapy allows girls with Turner syndrome to develop secondary sex characteristics. Most have normal intelligence. Short stature (height). Some distinctive physical features (“webbed neck”).
May be fertile but their wives may have miscarriages. Their offspring have the chance of prenatal death and chromosomal anomalies. Having another small chromosome Y can lead to aggressive behaviour. Having an extra Y doest not cause a big difference in balance, therefore, it does not have a large effect.
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2) Biology. Peter H. Raven, George B. Johnson, Susan Singer, Jonathan Losos. McGraw-Hill Science/Engineering/Math; 7 edition (January 8, 2004)
3) DNA Science: A First Course, Second Edition. David A. Micklos. Cold Springs Harbor Laboratory Press; 2 edition (January 8, 2003)