A karyotype is a representation of the chromosomes in a cell. Karyotypes show what parts of the chromosome are protein-coding genes and what parts are not.
They also show if the chromosome is normal or abnormal. Abnormalities can be due to mutations in gene proteins that coat the chromosome (called telomeres) or that are part of the chromosome itself (called centromeres).
The way to prepare a karyotype varies depending on what stage of mitosis the chromosomes are photographed in. The main stages of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase.
Chromosome preparation
Once the cell has divided and the chromosomes have been distributed to both daughter cells, the chromosomes can be seen in a process called karyotyping.
Karyotyping is the process of labeling and organizing all of the chromosomes in a cell. This is done by photographing the chromosomes and grouping them by size and shape.
Chromosomes are photographed at interphase, which is the period between cell divisions. Interphase occurs right before mitosis begins, so this is a very precise time to take pictures of the chromosomes.
There are two types of interphase: leptotene and zygotene. Leptotene is when the chromosome fibers are thin and easily visible, while zygotene is when the chromosomes are tightly coiled. Both of these stages make it easier to see the chromosome fibers.
Stage 1: Pre-prep chromatin stage
Before the chromosomes can be seen, they must be extracted from the nucleus. This is done by using a chemical solution called a cytogenetic buffer.
This solution contains chemicals that disentangle the chromosomes and reduce their susceptibility to damage. It also contains substances that facilitate their observation and classification.
The solution used to prepare the karyotype depends on the stage of mitosis in which the chromosomes are photographed. For this reason, the cytogenetic buffer has different components depending on what stage of mitosis is being photographed.
For example, in prophase cells are photographed before chromosome compaction, so a less dense solution is needed to extract the chromosomes for preparation of the karyotype.
Similarly, in metaphase cells, all of the chromosomes need to be visible, so a more dense solution is needed to extract them for karyotyping.
Stage 2: Diplotene stage
Once the chromosomes are in the correct order, the karyotype can be completed. This is typically when the cells are in the diploid stage of mitosis.
Diploid refers to the fact that the cells contain two sets of chromosomes, one from your mother and one from your father.
This is the stage at which chromosomes can be photographed to create a karyotype due to the distinct appearance of the chromosomes. They are tightly coiled and easily spotted.
The length of time it takes to prepare a karyotype depends on how many cell types you are looking at and how many stages of mitosis you need to observe. It can take a few hours or several days depending on the urgency and detail required.
Preventing contamination is very important so that all cells appear as normal as possible.
Stage 3: Tetraploid stage
A karyotype is a picture of all the chromosomes in a cell. The word comes from the Greek words karyon, which means nucleus, and genre, which means type.
Most cells in your body have two sets of chromosomes: one set from your mother and one from your father. These are called gonosomes. When a cell undergoes the process of mitosis to divide into two new cells, the chromosomes line up and sort into pairs.
Because human cells only have two sets of chromosomes (with the exception of sperm and ovum cells), when you look at a karyotype you are actually looking at two copies of each chromosome. For this reason, a karyotype shows what color bands each chromosome is, not what genes it contains.
There are twenty-three pairs of chromosomes in total, plus one pair of sex chromosomes that determine gender.
Stage 4: Late anaphase/early telophase stage
As mitosis progresses, the chromosomes that are present in the cell become visible. This is because the DNA within the chromosome becomes exposed as the cell membrane breaks down.
The length of time it takes for this to occur is what differentiates the stages of mitosis. The stage that identifies which phase of mitosis a chromosome is photographed in to create a karyotype depends on when this occurs.
The late anaphase stage is when the chromosomes are photographed after they have separated from each other. At this point, all of the chromosomes are extended and visible, which makes it easy to identify which ones are abnormal or mutated.
The early telophase stage is when the cell begins to return to its normal shape. At this point, part of the nuclear membrane re-forms around the now-divided cells, covering up some of the chromosomes.
Early prometaphase stage
Now that you know the difference between mitosis and meiosis, how chromosomes are photographed during the preparation of a karyotype, and at which stage of mitosis chromosomes are usually photographed in the preparation of a karyotype, you will be ready to learn about the last part of this series!
chromosomes are usually photographed in the preparation of a karyotype during early prometaphase. This is when the kinetochores are visibly attached to microtubules.
Kinetochores are protein complexes that attach the chromosome to microtubules. They become visible at this stage due to increased RNA within the cell nucleus.
When looking at prometaphase cells under a microscope, you will notice two sets of chromosomes lined up next to each other before moving apart and then duplicating. This is why there are two sets of chromosomes in prometaphase cells- one set is what was previously hidden inside of the cell nucleus and one set is what is emerging.
Mid-prometaphase stage
As mentioned before, the chromosomes are most visible during the mid-prometaphase stage. This is because the chromosomes are in line with each other, and they are aligned with the poles of the cell.
The chromosomes can be seen as a band across the cell as well. This makes it easy to photograph or examine them. The rest of the stages of mitosis are more difficult to see the chromosome structure due to variability in shape and alignment.
Another reason that karyotypes use this stage is because it is one stage before cell division is complete. By photographing them at this point, researchers can easily see what sort of chromosome number or structure they have, and what sort of cells they are due to having completed this stage.
Early metaphase stage
Karyotypes are most often prepared from cells in the metaphase stage of mitosis. This is because chromosomes are longest and clearest in this stage.
Interphase occurs between cell divisions, and during this period the cell grows and duplicates its DNA so it can divide into two identical daughter cells.
DNA replication starts at one point on the chromosome, called the origin of replication, and extends to the other end of the chromosome, called the terminus.
During interphase, chromosomes exist in two forms: diffuse and compact. In the diffuse form, the DNA is loosely strung together; in the compact form, it is tightly bundled together. Both forms are distinguishable by electron microscopy.
