Module 2 from chapter 11.4 -- NonDisjunction PDF

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Summary

- Errors that occur in mitosis and meiosis processes can lead to cells having an incorrect amount of chromosomes (extra or missing chromosomes)...

Description

11.4 - NonDisjunction -

Errors that occur in mitosis and meiosis processes can lead to cells having an incorrect amount of chromosomes (extra or missing chromosomes) -

These are known as mutations

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Mutations can be lethal, have effects that cause complications but are able to be lived with, or have no major effect at all

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Nondisjunction: The failure of a pair of chromosomes to separate normally during anaphase of cell division -

One daughter cell receives an extra copy of chromosomes, while the other daughter cell fails to receive any chromosomes

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Takes place in mitosis -

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Can also take place in meiosis

Nondisjunction often observed in cancer cells

Nondisjunction in meiosis results in reproductive cells (Gametes) that have extra or missing chromosomes -

Failure to properly separate chromosomes can occur in both Meiosis I and Meiosis II -

Both result in either extra or missing chromosomes within the cells

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First-Division Nondisjunction: failure of chromosome separation in meiosis I -

Much more common

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Homologous chromosomes fail to separate properly during the first phase of meiosis (Figure to the left, step b.))

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One sister cell has an extra pair of chromosomes, while the other sister cell has

missing chromosomes -

Second-Division Nondisjunction: disjunction that occurs in the second Meiotic division -

Sister chromatids fail to separate

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Two gametes with normal chromosomes, one gamete with an extra chromosome, and one gamete with a missing chromosome

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Nondisjunction causes some human disorders that can have none to severe effects on human life expectancy and everyday living

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Down Syndrome: also known as Trisomy 21, is a condition resulting from the presence of an extra copy of chromosome 21 -

A more commonly known disorder that is a result of Nondisjunction

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Affected individuals have three copies of chromosome 21

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Side effects and traits of Down syndrome is led by the extra genes provided by the extra set of chromosome

Traits of Down Syndrome: -

Mental disability (to varying degrees)

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Short in height (due to delayed maturation of skeletal system)

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Low muscle tone -

Characteristic facial appearance

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Major heart defects

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Lower than average life expectancy (~55 years)

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Higher risk of dementia than the general population (15%-50%)

Maternal age is an identifier of potential abnormalities that can occur in their eggs -

Generally, the older that women are (45-50), the higher the risk of genetic diseases and disorders are (in comparison to mothers 15-20)

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Trisomy 13 and Trisomy 18 are much rarer Nondisjunction disorders that have more severe effects -

Babies born with these disorders rarely survive the first year of life

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Abnormalities of extra or missing chromosomes in sex chromosomes are much more common than autosomes

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Female Karyotype 47, XXX and Male Karyotype 47, XYY are found among healthy males and females -

The extra sex chromosomes are not found unless chromosomes are tested

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XYY does not have detectable effects because Y chromosomes contain genes that stimulate the embryo to take the male developmental pathway -

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Individuals do not have many extra genes

XXX does not have detectable effects because of the regulation of the X chromosome -

All X chromosomes (except one) are inactivated and gene expression is largely repressed

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Therefore, 47, XXX females have only one active X chromosome, same as 46, XX females

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Other sex chromosome abnormalities do have severe effects on human development -

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47, XXY (Male Karyotype) —> Klinefelter Syndrome -

Small testes, normal penis and scrotum

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Taller in height

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A certain degree of mental impairment

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Voice remains high-pitched

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Pubic and facial hair remains scarce

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A certain extent of enlargement of breasts

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Sterile

45, X (Female Karyotype) —> Turner’s Syndrome -

Short in height

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Webbing of skin between neck and shoulders

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No sexual maturation

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External sex organs are not developed

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Internal sex organs are small or fail to develop

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Menstruation does not occur

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Specific spatial disabilities and arithmetical skill challenges

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Chromosomal abnormalities showcased in living humans represent a minority of nondisjunction mutations -

Most mutations that occur will occur in fertilized eggs -

During pregnancy (late or early stages), spontaneous abortion will occur (miscarriage)...