Are Chromosomes Eukaryotic: 5 Facts You Should Know

Chromosomes are present in both prokaryotic and eukaryotic cells, the difference being in their size, shape, number and location.

To answer, “are chromosomes eukaryotic”, yes, they can be. Eukaryotes have different types of chromosomes. Their nuclear DNA which acts as the genetic material is very different from the organellar DNA that are inherited from the cytoplasm of the mother’s gamete.

Can chromosomes be eukaryotic?

The nuclear chromosomes found in most eukaryotic cells represent a typical eukaryotic chromosome. They usually have discrete arms but their structure can vary throughout the cell cycle depending on the phase of the cell cycle. In the interphase of the cell cycle, chromosomes can be in two forms, heterochromatin and euchromatin.

Euchromatin

Loosely packed chromosomes or chromatin networks, represent the euchromatin. They are transcriptionally active and undergo crossing over during cell division.

Heterochromatin

These are highly condensed DNA segments that do not undergo transcription and are usually found in eukaryotic cells only. Due to the high compaction, they do not undergo crossing over during cell division. Inactivated X chromosome in mammalian females and polytene chromosome present in insect larvae are examples of heterochromatin.

Do eukaryotes have chromosomes?

Every living cell has chromosomes which may have different shape, size, number and arrangement but they all act as the genetic material of the cell. Eukaryotic cells generally have linear and multiple chromosomes, whereas a prokaryotic cell possesses small, circular and single chromosome.

Different eukaryotic organisms have different number of chromosomes which are always diploid in number.

How many chromosomes do eukaryotes have?

The number of chromosomes or the genome size or the karyotype of any species is different from that of the other. These differences have come into existence because of the constant evolution of an organism that undergoes the pressure of natural selection.

are chromosomes eukaryotic
Image credit: Wikimedia Commons. Human male karyotype- 46,XY .

C-value

C-value is the amount of DNA present in the nucleus of an unreplicated gamete, regardless of the ploidy level of a species.

C-value paradox

It is the absence of correlation between the amount of DNA in a haploid cell and the chromosome number and the complexity of a eukaryotic organism. It is expected for a more complex organism to have a greater number of chromosomes with a larger size of genome. But findings have suggested otherwise, which brought into existence, the C-value paradox.

For example, some species of Sunflowers and Helianthus have four thirty times the amount of DNA than that in others. DNA content in the cells of some species of salamanders is nearly 30 times than that of the cells in humans.

Species Genome size (in bp) No. of chromosomes
Drosophila melanogaster
(fruit fly)
180 x 106 8
Arabidopsis thaliana
(Thale cress)
125 x 106 10
Drosophila virilis
(Fruit fly)
364 x 106 12
Zea maize
(maize)
2400 x 106 20
Saccharomyces cerevisiae
(yeast)
12 x 106 32
Necturus maculosus
(Common mudpuppy)
81174 x 106 38
Triticum aestivum (wheat) 16000 x 106 42
Homo sapiens
(Human beings)
3200 x 106 46
List of genome sizes and numbers of chromosomes of some organisms.

Extragenic DNA

Genome of all eukaryotic organism possesses huge and variable amount of repetitive extragenic DNA, that neither code for any protein, nor have any known regulatory functions. Two well studied examples of extragenic repeats of DNA are tandem repeats, such as minisatellite and microsatellite, and interspersed repeats such are LINEs and SINEs.

Nearly a million copies of 300 base pairs long Alu element, a SINE transposon, accounts for more than ten percent of the total human genome. The extragenic DNA represents nearly around 98 percent of the human genome while it accounts for more than 99.9 percent of the genome in Necturus maculosus (mudpuppy salamander).

Another great example of C-value paradox within a genus is Drosophila virilis having a genome size nearly two times than that of Drosophila melanogaster, where non- coding, multiple repeats of just four short sequences account for more than 40 percent of the genome of Drosophila virilis.

What type of chromosome do eukaryotes have?

A eukaryotic cell may possess several types of DNA which are encased in different organelles of the cell. Not only that, a single eukaryotic cell can contain both prokaryotic like and eukaryotic like DNA based on the types of organelles it is present it.

DNA Circle Medium
Image credit: Free SVG. Prokaryotic chromosome structure
Nuclear Chromosomes Organellar Chromosomes
Large in size. Small in size.
Linear chromosomes. Circular chromosome.
Highly condensed DNA. DNA is not condensed.
Histones are present are help in chromosome condensation. Histones are absent as the chromosomes are not condensed.
Undergoes replication during cell division process. Undergoes replication, independently of the cell division.
Telomerase is present and protects the ends of the chromosomes from damage. Telomerase is absent as the circular chromosomes do not have an open end.
Introns or the non- coding regions of DNA are present. Introns are absent and only exons or coding regions are found in the chromosomes.
Inherited from both the parents. Inherited from mothers only.
Multiple number of chromosomes are present in each cell except in gametes. Single chromosome is present in each cell. An extra, circular chromosomal structure called plasmid may exist.
Two copies of each chromosome is present (one from each parent). Multiple copies of the same chromosomes may exists in a cell.
Difference between Eukaryotic and Prokaryotic chromosomes.
Condensed Eukaryotic Chromosome
Image credit: Wikimedia Commons. Condensed eukaryotic chromosome.

Where are eukaryotic chromosomes found?

Eukaryotic cells possess a true nucleus which safe keeps the genome of the cell within a two layered nuclear membrane. This is one of the key points to differentiate between prokaryotic and eukaryotic cells. The prokaryotic cells do not possess a true nucleus, so the genetic material stay suspended within the cell matrix without any membrane protecting it.

Apart from the nuclear genetic material, some of the eukaryotic organelles possess DNA as well which undergo independent replication. Mitochondria in eukaryotic cells and chloroplast in the plant cells possess self- replicating DNA.

Organellar DNA are small and circular in nature that are passed on to the progeny from the mother cell. This is because during fertilisation of the eggs, only the genetic material or the nuclear DNA of the male gamete is allowed to enter the egg. Hence, the mitochondria contained in the egg is passed on to the next generation.

It is believed that these organelles, such as mitochondria and chloroplasts, became a part of the present- day eukaryotic cell due to endosymbiosis of self- replicating primitive cells and there are several evidences to support that.

1055px CtDNA.svg
Image credit: Wikimedia Commons. Circular chloroplast DNA.
  • Mitochondria and chloroplasts are highly specified, double membranous organelle.
  • They have a single circular DNA and separate set of ribosomes, similar to that of a prokaryotic cell.
  • Their lack of histone proteins and absence of introns is similar to that of a prokaryotic cell.
  • They can undergo DNA replication, transcription, translation and organelle division, independently from the cell division.
  • Their DNA encodes only for proteins, that are necessary for their specialised functions.

Conclusion

Eukaryotic chromosomes are of different types depending on their location within a cell and they do not share their evolutionary history with each other.

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