Why is it difficult to observe individual chromosomes with a light microscope during interphase?

A) The DNA has not been replicated yet.

B) The spindle must move them to the metaphase plate before they become visible.

C) They leave the nucleus and are dispersed to other parts of the cell.

D) Sister chromatids do not pair up until division starts.

E) They have uncoiled to form long, thin strands.

Photo of a normal human karyotype
Photo of a normal human karyotype

The correct answer is E) They have uncoiled to form long, thin strands.

Eukaryotic genetic material is present on strands of DNA which are found arranged around histone proteins forming chromosomes. Humans have 46 chromosomes, but other organisms have a different number.

The chromosomes in humans are not visible during interphase of the cell cycle because the DNA is still tightly packed with the histones. It is only during cell division that separate chromosomes can be visualized because they have changed form to attach to spindle fibers in preparation for division.

Chromosomes have a centromere which is a region where kinetochore proteins assemble for the attachment of microtubule proteins of the spindle fiber during mitosis.

The chromosomes also have telomeres which are parts of DNA forming the far ends of the structure. These telomeres are believed to provide protection for the DNA.

One of the best techniques to view chromosomes is by completing a karyotype. This is a process that is often used when genetic testing is done on a fetus.

The cells of the fetus are grown in culture and then the process of cell division is halted when the cells enter metaphase or prometaphase. Fixing and staining is done to make the banding patterns on chromosomes obvious.

A karyogram is made based on a photo of these chromosomes. Homologs are arranged and a picture is made that will show if there are any problems.

DNA

DNA is the molecule deoxyribonucleic acid, which consists of several nucleotide units that are bonded together. Each nucleotide consists of a phosphate group, deoxyribose sugar, and a nitrogen base. The nitrogen can be one of four possible bases, adenine, thymine, cytosine and guanine.

The molecule is made of two strands that are linked together at complementary nitrogen bases. Base pairing rules are such that adenine can only bond with thymine and guanine with cytosine and vice versa.

It is the sequence of these nitrogen bases that make up the genetic code. This code is transcribed and translated into proteins, which is how our genes are expressed. The entire DNA molecule is also coiled to form a double helical structure.

Chromosomes

The number and relative size of chromosomes vary in different organisms. Primitive organisms such as bacteria often have only one circular shaped chromosome, while more complex life forms have many more that are often in the shape of threads.

Humans, for instance, have 46 chromosomes, that occur in the body cells (autosomes). Chromosomes are made of several strands of DNA, and in the case of eukaryotes, additional proteins that are called histones.

The DNA strands wind tightly around the histones at specific locations, which in humans are approximately every 200 base pairs along the nucleic acid.

Structure

There are four histone proteins that occur in pairs to form a structure known as an octamer. An additional histone attaches and helps further bind the DNA to the octamer.

Processes of methylation and acetylation or deacetylation function to change how strongly these chromosomes are attached to the histone proteins. It is necessary for them to become less tightly bound and packed for cell division and protein synthesis to occur.

Each chromosome also contains a region known as a centromere that is narrower than the rest of the chromosome and is often at the center of the structure. This is where kinetochore proteins form for the attachment of spindle fibers during cell division.

Telomeres are pieces of DNA that are found at the ends of the chromosomes. They function as a protective structure and it is believed that they become shorter with each division of the cell.

DNA synthesis and mitotic chromosomes

The DNA is replicated during the S stage (synthesis) of interphase. This is necessary because the genetic material has to be doubled in order for two new cells with the correct number of genes to be formed at the end of mitosis.

After the DNA has copied itself each chromosome has two sections known as chromatids, which are joined at the centromere. The kinetochore proteins also become organized in preparation for attachment of microtubule proteins of the spindle.

A separate spindle fiber attaches to each chromatid during prophase of mitosis. The chromosomes line up at the middle of the cell in metaphase and then the spindle fibers contract during anaphase.

When these microtubular fibers contract they pull the chromatids apart to opposite ends of the cell. The next stage of mitosis is telophase, which is when chromosomes form chromatin again and a nuclear envelope reforms.

Finally, the cytoplasm divides in cytokinesis to form two new cells each with the same amount and type of genetic material present. Each cell has the same number of chromosomes present as well.

Techniques to view chromosomes

Chromosomes can sometimes be seen even through a light microscope. However, it is only during certain stages of the cell cycle in some stages of cell division that they can be seen.

The chromosomes are not visible through a microscope during the interphase stage of the cell cycle, because they are tightly packed as chromatin.

The best time for examining chromosomes is during the prometaphase or metaphase stage of mitosis. This is because the chromosomes are visible as homologous pairs that are found at the center of the cell.

A karyotype is a pictorial profile of a person’s chromosomes that can be prepared and used as part of genetic testing. For instance, fetal cells can be obtained from chorionic villus sampling and the chromosomes examined for abnormalities.

The procedure involves growing the cells and then halting the mitosis of cells during the metaphase or prometaphase stage.

This is done by using colchicine that stops the spindle from contracting.  Various fixatives and stains are used so that the banding patterns on the chromosomes become visible so that they can be distinguishable.

A photo is taken and then the individual chromosomes are cut out and organized into a karyogram, which is a picture in which the chromosomes are arranged into homologous pairs.

References

  1. C O’Connor (2008) Karyotyping for chromosomal abnormalities. Nature Education.
  2. National Human Genome Research Institute (2015). Chromosomes. Retrieved from genome.gov.
  3. US National Library of Medicine (2019). What is a Chromosome? Retrieved from nih.gov.
  4. Editors of Encyclopedia Britannica (2018). Chromosome. Retrieved from Encyclopedia Britannica.
  5. Editors of Encyclopedia Britannica (2018). Genetic testing. Retrieved from Encyclopedia Britannica.

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