Chromosomes are stick-shaped structures in the middle [nucleus] of each cell in the body. Each cell has 46 chromosomes grouped in 23 pairs. When a chromosome is abnormal, it can cause health problems in the body. Abnormal chromosomes most often happen as a result of an error during cell division. Chromosome abnormalities often happen due to 1 or more of these:
Errors during dividing of sex cells [meiosis]
Errors during dividing of other cells [mitosis]
Other causes of birth defects can include exposures of many types of substances [teratogens] to the developing baby.
Gendered terms are used here to talk about anatomy and health risk. Please use this information in a way that works best for you and your provider as you talk about your care.
Errors during dividing of sex cells [meiosis]
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.
But if meiosis doesn’t happen normally, a baby may have an extra chromosome [trisomy], or have a missing chromosome [monosomy]. These problems can cause pregnancy loss or health problems in a child.
A woman age 35 years or older is at higher risk of having a baby with a chromosomal abnormality. This is because errors in meiosis may be more likely to happen as a result of the aging process. Women are born with all of their eggs already in their ovaries. The eggs begin to mature during puberty. If a woman is 35 years old, the eggs in the ovaries are also 35 years old. You may be referred for genetic counseling or testing if you’re age 35 or older when you are pregnant. Men make new sperm ongoing. So age doesn’t increase the risk for chromosome abnormalities for older fathers. But some newer studies suggest that rare abnormalities do occur.
Errors during dividing of other cells [mitosis]
Mitosis [my-TOH-sis] is the dividing of all other cells in the body. It’s how a baby in the womb grows. Mitosis causes the number of chromosomes to double to 92, and then split in half back to 46. This process repeats constantly in the cells as the baby grows. Mitosis continues throughout your lifetime. It replaces skin cells, blood cells, and other types of cells that are damaged or naturally die.
During pregnancy, an error in mitosis can occur. If the chromosomes don’t split into equal halves, the new cells can have an extra chromosome [47 total] or have a missing chromosome [45 total].
Substances that cause birth defects [teratogens]
A teratogen [ter-AT-uh-jen] is something that can cause or raise the risk for a birth defect in a baby. They are things that a birth parent may be exposed to during pregnancy. Teratogens include:
Some medicines
Street drugs
Alcohol
Tobacco
Toxic chemicals
Some viruses and bacteria
Some kinds of radiation
Certain health conditions, such as uncontrolled diabetes
Researchers have 2 ways of finding out if a substance is a teratogen:
Animal studies. Animal studies are the main way to find out if a medicine or other substance is safe during human pregnancy.
A regular human cell has 46 chromosomes: 44 autosomes, which come in pairs, and 2 sex chromosomes, which specify whether someone is male [usually XY] or female [usually XX].
The pairs of autosomes are called "homologous chromosomes." Homologous chromosomes have all of the same genes arranged in the same order, but there are small differences in the DNA letters of the genes.
When cells divide to make more cells [mitosis] or reproductive cells [meiosis], and when reproductive cells join to make a new individual [fertilization], it is important that the new cells get the proper number of chromosomes. Read on to learn more about these processes.
This passing of genes from one generation to the next is called heredity. Simple organisms pass on genes by duplicating their genetic information and then splitting to form an identical organism. More complex organisms, including humans, produce specialised sex cells [gametes] that carry half of the genetic information, then combine these to form new organisms. The process that produces gametes is called meiosis.
Meiosis makes sperm and eggs
During meiosis in humans, 1 diploid cell [with 46 chromosomes or 23 pairs] undergoes 2 cycles of cell division but only 1 round of DNA replication. The result is 4 haploid daughter cells known as gametes or egg and sperm cells [each with 23 chromosomes – 1 from each pair in the diploid cell].
At conception, an egg cell and a sperm cell combine to form a zygote [46 chromosomes or 23 pairs]. This is the 1st cell of a new individual. The halving of the number of chromosomes in gametes ensures that zygotes have the same number of chromosomes from one generation to the next. This is critical for stable sexual reproduction through successive generations.
The phases of meiosis in humans
Interphase
Replication of DNA in preparation for meiosis. After replication, each chromosome becomes a structure comprising 2 identical chromatids.
Prophase I
The chromosomes condense into visible X shaped structures that can be easily seen under a microscope, and homologous chromosomes pair up. Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane dissolves.
Metaphase I
Paired chromosomes line up along the middle of the cell.
Anaphase I
The pairs of chromosomes separate and move to opposing poles. Either one of each pair can go to either pole.
Telophase I
Nuclear membranes reform. Cell divides and 2 daughter cells are formed, each with 23 chromosomes.
Prophase II
There are now 2 cells. DNA does not replicate again.
Metaphase II
Individual chromosomes line up along the middle of the cell.
Anaphase II
The chromosome copies [chromatids] separate and move to opposing poles.
Telophase II
Nuclear membranes reform. There are 4 new haploid daughter cells. In males, 4 sperm cells are produced. In females, 1 egg cell and 3 polar bodies are produced. Polar bodies do not function as sex cells.
Genetic variation is increased by meiosis
During fertilisation, 1 gamete from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote.
Recombination or crossing over occurs during prophase I. Homologous chromosomes – 1 inherited from each parent – pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination.
Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes.
This reshuffling of genes into unique combinations increases the genetic variation in a population and explains the variation we see between siblings with the same parents.
How does meiosis affect daughter cells?
Mitosis creates two identical daughter cells that each contain the same number of chromosomes as their parent cell. In contrast, meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell.How does meiosis 1 reduce the number of chromosomes in the daughter cells?
When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half.Does meiosis increase the chromosome number of the daughter cells?
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.What happens to the chromosome number per daughter cell in mitosis?
At the end of mitosis, the two daughter cells will be exact copies of the original cell. Each daughter cell will have 30 chromosomes. At the end of meiosis II, each cell [i.e., gamete] would have half the original number of chromosomes, that is, 15 chromosomes. 2.