2) General features of human chromosomes. Chromosome mutations.
General features:
DEFINITION:
In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosomes are only visible during metaphase.
Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.”
Usually there are 23 pairs of chromosomes (46 in total) in all the body cells except the egg and sperm. This chromosome complement is diploid, meaning 2 sets of 23 chromosomes.
The number and appearance of chromosomes in the nucleus of an eukaryote cell is called a karyotype. The term is also used for the complete set of chromosomes in an individual organism.
During the formation of the egg or sperm, the chromosome pairs usually separate so that each egg or sperm cell contains only one copy of each of the 23 pairs of chromosomes.
TYPES OF CHROMOSOMES:
They are placed into broad categories depending on the position of the centromere.
- Metacentric: the centromere is in the middle, with arms of equal length (chromosomes 1, 2, 3,
- Sub-metacentric: the centromere is near the middle, with arms of slightly different length. 4-12, 16-20, X
- Acrocentric: the centromere is near one end, with arms of very different lengths (chromosomes 13-15 (big) and 21, 22, Y (small))
Chromosome mutations:
INCIDENCE:
- The earlier the abortion, the more likely to be a chromosomal abnormality.
- 50% of spontaneous abortions are chromosomal abnormalities, mostly triploidy, 45X0, trisomy 16.
- 98% of fetuses with Turner abort.
- Generally the incidence of chromosomal abnormalities is 6/1000.
WHEN TO SUSPECT IT:
- Unexplained infertility.
- Multiple abortions (>2).
- Prior case of defective baby.
- Problems with early growth and development.
- Stillbirth and neonatal death.
- Pregnancy in women of advanced age (>35 years).
- Presence of congenital anomalies: 45% have a single minor anomaly, 9% have 3 minor anomalies, 1,5% have a major anomaly. 2 or more major anomalies may represent a genetic syndrome or a chromosomal abnormality.
A major anomaly is never normal, it has a functional significance.
A minor anomaly is sometimes normal, it has no functional significance.