What is Down syndrome?
Down syndrome, also known as trisomy 21, is a genetic disorder caused when abnormal cell division results in an extra copy of chromosome 21.
What are chromosomes?
Chromosomes are threadlike structures made of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell. In plants and animals (including humans), chromosomes reside in the nucleus of cells.
How does trisomy occur?
Trisomy occurs when an error during the formation of the egg or the sperm results in either one having an extra chromosome. So after the egg and sperm unite, the resulting cells will also have three copies of chromosome 21.
Regular cell division: Meiosis
There are two possible types of cell division in the human body: Mitosis and Meiosis.
Mitosis is the division of a cell into two cells that are identical to the initial cell
Meiosis is the division of a cell into four gametes: It leads to the creation of four gametes with 23 chromosomes.
Meiosis can be divided into two cell divisions, each of which are divided into 4 steps: Prophase, Metaphase, Anaphase and telophase
Here are the steps of a normal meiosis:
Prophase I: chromosomes pair up
Metaphase I: The pairs of chromosomes line up at equator
Anaphase I: The chromosomes of each pair are seperated
Telophase I-Prophase II: Cell pinches in the middle and divides into two daughter cells
Metaphase II: Chromosomes line up at equator
Anaphase II: Each chromosome is composed of two chromatids. During this phase, the chromatids of each chromosomes are pulled apart
Telophase II: Both cells pinch in the middle and divide; leading to the formation of 4 cells
What causes trisomy 21 during meiosis?
Trisomy can be generally caused by two types of abnormalities during cell division:
An error can occur during the separation of chromosomes during anaphase I
An error can occur during the separation of chromatids during anaphase II
According to the center for disease control and prevention, 95% of people with down syndrome have trisomy 21. However, This syndrome can also be caused by Translocation, which means that an extra part or a whole extra chromosome 21 is present, but it is “trans-located” to a different chromosome rather than being a separate chromosome 21. Such cases represent 3% of the people with down syndrome
What are the symptoms of down syndrome?
According to the center for disease control and prevention, common physical attributes for people with down syndrome can be:
A flattened face, especially the bridge of the nose
Almond-shaped eyes that slant up
A short neck
Small ears
A tongue that tends to stick out of the mouth
Tiny white spots on the iris (colored part) of the eye
Small hands and feet
A single line across the palm of the hand (palmar crease)
Small pinky fingers that sometimes curve toward the thumb
Poor muscle tone or loose joints
Shorter in height as children and adults
Mosaic down syndrome
It should however be noted that not all of the people with down syndrome have these attributes. In fact, some people who have mosaic down syndrome may not present any of these features because some of their cells have 3 copies of chromosome 21, while other cells have the typical two copies of chromosome 21.
Assessed and Endorsed by the MedReport Medical Review Board
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