Abnormal chromosomes most often happen as a result of an error during cell division. Chromosome abnormalities often happen due to one or more of these:. Meiosis my-OH-sis is the process in which sex cells divide and create new sex cells with half the number of chromosomes.
Sperm and eggs are sex cells. Meiosis is the start of the process of how a baby grows. Normally, meiosis causes each parent to give 23 chromosomes to a pregnancy. When a sperm fertilizes an egg, the union leads to a baby with 46 chromosomes. These problems can cause pregnancy loss. Or they can cause health problems in a child. A woman age 35 years or older is at higher risk of having a baby with a chromosomal abnormality. Many chromosomes have two segments, called "arms," separated by a pinched region known as the centromere.
The shorter arm is called the "p" arm. The longer arm is called the "q" arm. The body is made up of individual units called cells. Your body has many different kinds of cells, such as skin cells, liver cells and blood cells. In the center of most cells is a structure called the nucleus. This is where chromosomes are located. The typical number of chromosomes in a human cell is 23 pairs, holding an estimated total of 20, to 25, genes.
One set of 23 chromosomes is inherited from the biological mother from the egg , and the other set is inherited from the biological father from the sperm.
Of the 23 pairs of chromosomes, the first 22 pairs are called "autosomes. The mother and father each contribute one set of 22 autosomes and one sex chromosome. For a century, scientists studied chromosomes by looking at them under a microscope.
In order for chromosomes to be seen this way, they need to be stained. Once stained, the chromosomes look like strings with light and dark "bands," and their picture can be taken. A picture, or chromosome map, of all 46 chromosomes is called a karyotype.
The karyotype can help identify abnormalities in the structure or the number of chromosomes. To help identify chromosomes, the pairs have been numbered from 1 to 22, with the 23rd pair labeled "X" and "Y. In the past decade, newer techniques have been developed that allow scientists and doctors to screen for chromosomal abnormalities without using a microscope.
The comparison can be used to find chromosomal abnormalities where the two samples differ. One such method is called noninvasive prenatal testing. This is a test to screen a pregnancy to determine whether a baby has an increased chance of having specific chromosome disorders. The test examines the baby's DNA in the mother's blood. There are many types of chromosome abnormalities. However, they can be organized into two basic groups: numerical abnormalities and structural abnormalities.
Numerical Abnormalities: When an individual is missing one of the chromosomes from a pair, the condition is called monosomy. When an individual has more than two chromosomes instead of a pair, the condition is called trisomy. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes.
It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people.
Chromosomes vary in number and shape among living things. Most bacteria have one or two circular chromosomes. Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome.
When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells. Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria.
This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy.
When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. The constricted region of linear chromosomes is known as the centromere.
Although this constriction is called the centromere, it usually is not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome's end. The regions on either side of the centromere are referred to as the chromosome's arms. Centromeres help to keep chromosomes properly aligned during the complex process of cell division. As chromosomes are copied in preparation for production of a new cell, the centromere serves as an attachment site for the two halves of each replicated chromosome, known as sister chromatids.
Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling.
In many types of cells, telomeres lose a bit of their DNA every time a cell divides. Eventually, when all of the telomere DNA is gone, the cell cannot replicate and dies. White blood cells and other cell types with the capacity to divide very frequently have a special enzyme that prevents their chromosomes from losing their telomeres.
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