Human Chromosomes Flashcards
What are the light and dark bands in a chromosome?
Stained with Giemsa
Dark band = high A/T content (gene poor)
Light band = high G/C content (gene rich)
What are 4 important diseases that are caused by genes on the human X chromosome?
DMD - Duchenne muscular dystrophy
PHEX - Hypophosphatemia
FMR1 - Fragile X
F8 - Hemophilia A
What are the 3 shapes that a chromosome can take with regards to centromere location?
Metacentric = p and q arms are the same length
Sub-metacentric = p arm is shorter than q arm
Acrocentric = p arm is very very short
What genes tend to be located on the p arms of acrocentric chromosomes?
What implication does this have for damge or loss of these regions?
Which chromosomes are acrocentric in humans?
Genes for rRNA
Damge or loss off these genes does not result in phenotypic deficit b/c the organism has many other copies of the genes available
13, 14, 15, 21, 22
In general, extra copies of chromosomes or missing copies of chromosomes is lethal to the organism. What are 3 exceptions to this rule?
Down Syndrome - Trisomy 21
Turner Syndrome - X Monosomy
Triple X Syndrome
What is the name for the syndrome characterized by 69, XXX?
Triploidy syndrome - the individual has 3 copies of every chromosome. This condition usually leads to micarriages during gestation, or death shortly after birth if the pregnancy is carried to term.
Down Syndrome
- Craniofacial characteristics
- Extremeties
- Developmental signs
- Other related symptoms
- Shorter than normal head, flat facial profile, excess skin on back of neck
- Shorter than normal digits, palmar creases, widened gap between 1st and 2nd toes
- Mental retardation with IQ 50 - 60, equivalent to 8 year old child
- Hypotonia, hypothyroidism, congenital heard defects, hearing/vision problems
Why does trisomy 21 cause phenotypic changes (general)?
It is a problem with gene dosing - the individual is expressing too much genetic information
What is a robertsonian translocation?
Where a certain type of a chromosome becomes attached to another
What does 46, XY, t(7;9)(p21.1; q34.1) mean?
Male with translocation between short arm of chromosome 7 at band 21.1 and long arm of chromosome 9 at band 34.1
What is the most common cause of non-disjunctions during mitosis and meiosis?
Cohesin is a molecule that keeps the sister chromatids held together during mitosis/meiosis. It is supposed to release when at the point in division when the chromatids would be separated. When it does not release the sister chromatids, they can remain stuck together and lead to non-disjunction during division.
Why are non-disjunctions more common as women age?
Cohesin loses some of its performance abilities over time such that it loses ability to dissociate from chromosomes with age. Thus, risk of nondisjunction increases with increasing age due to failure of cohesin molecules.
What is a constitutional chromosome abnormality?
One that is present at conception (meiotic error), is present in all cells, interferes with normal growth and development, likely to result in severe consequences for growing fetus (miscarriage, birth defects, etc.)
What is an acquired chromosome abnormality?
Arises due to mitotic error, occurs during replication of cell, present in subet of cells and perpetuates with further division, risk factor for cancer
