Types Of Eukaryotic Chromosomes: 3 Facts You Should Know


In this post you will find the information about types of eukaryotic chromosomes and the main components of eukaryotic chromosomes.

Eukaryotic chromosomes are complex nuclear component of individual capable of heredity, mutation and  wrapped with a specific histone proteins.

Do eukaryotes have multiple chromosomes?

Eukaryotes have multiple chromosomes containing 46 chromosomes including male and female gender chromosomes. Long stretch of DNA gets coiled to a great extent so that it can fit inside the nucleus, so it does by forming the chromatin structures by super coiling of the DNA. Chromatin are thread like mesh which gives the typical structure called chromosomes. These structures can be seen during cell division.

  • Chromosomes are present in pairs, in total there  are 23 pairs of chromosomes including one pair of gender chromosomes or allosomes .
  • The rest of 22 pairs are called autosomes. Eukaryotes have multiple chromosomes, on the basis of shape there are four types of chromosomes, metacentric, submetacentric, acrocentric and telocentric, while telocentric type of chromosome is absent in humans.
  • These three types of chromosomes make different arrangements to form the karyotype of the individual.  The chromosomes have two arms P and Q.
  • We can easily identify the P and Q arm, P arm is smaller while Q arm  is larger.
  • Each chromosome contains a structure called centromere which connects the sister chromatids or arms but its actual function is to make spindle fibre formation during the cell division.

Types of eukaryotic chromosomes

Chromosomes are made up of chromatids joined with the help of centromere (a site where both chromatids are attached). Eukaryotic chromosomes are divided into different types on the basis of size, staining properties, location of centromere and size of chromatids.

The different arrangement of chromosomes in an individual is known as karyotype of that individual.

Types of chromosomes on the basis of  structure and location of centromere.

Chromosomes are divided into four types on the basis of centromere position.

  • Metacentric
  • Sub-metacentric
  • Acrocentric
  • Telocentric

In metacentric  chromosome the centromere is present in the centre of the chromatids. The two arms are of exactly the same length. In sub-metacentric, centromere is present slightly away from the centre, or we can say it has smaller P arm and larger Q arm. In the acrocentric, the centromere is almost present towards the end region of the chromatids. In telocentric the centromere is present at the extreme end, however the human karyotype does not contain the telomeric type of chromosomes.

Types of chromosomes from Web2

The chromosomes have two arms P and Q. We can easily identify the P and Q arm, P arm is smaller while Q arm  is larger. Each chromosome contains a structure called centromere which connects the sister chromatids or arms but its actual function is to make spindle fibre formation during the cell division.

Types of eukaryotic chromosomes on the basis of Sex chromosomes

Humans have 46 chromosomes, having 23 pairs of chromosomes. The 22 pairs are called autosomes and the 23rd pair of chromosomes is sex chromosome which is also called allosomes.

To be precise, the autosomes have no function to deal with the gender of the individual. The allosome or gender chromosome deals with the gender of the individual and determines whether it will be a boy or a girl. If the chromosome are identical, i.e., X and X, then the individual will be the female. If both the chromosome are different, i.e., X and Y, then individual will be male. Both contains genes that code for proteins which make up our body.

Two main components of eukaryotic chromosomes

The two main components of eukaryotic chromosomes are DNA and histone protein.

  • DNA
  • Histone protein

In chromosomes, the chemical composition of DNA is about 40%. DNA found in eukaryotes is long, unbroken and double stranded, whereas protein composition is 50-60%.  DNA is negatively charged and this charge is due to phosphorus . DNA is composed of 140 million of nucleotide. Histones are positively charged proteins which are strongly attached to the negatively charged DNA to form  the complex called nucleosomes.

types of eukaryotic chromosomes
The major structures in DNA compaction: DNA, the nucleosome, the 10 nm “beads-on-a-string” fibre, the 30 nm fibre and the metaphase chromosome from Wikipedia

Nucleosome is the structural and functional repeating unit of chromatin. There are four types of different  histone proteins subunits  : H2A, H2B, H3 and H4. They all are twice in number. Each subunit is repeated twice by forming it to eight subunit structure. This structure is called histone octamer.  This histone octamer wraps approximately 146 base pairs of DNA.   There is addition of one more H1 protein which wraps another 20 base pairs forming a structure called chromatosome. This H1 protein act as a linker between two nucleosomes.

DNA is wrapped around histones. It forms chromatin which is somehow gives the structure of beads attached on the strings. In eukaryotes, there is lot of information compared to prokaryotes. In eukaryotes the some genes codes for proteins while some don’t. mRNA formed in eukaryotes contains introns and exons, exons code for proteins while introns don’t, so there is lot of junk material in the DNA. So it become necessary for the DNA to be condensed  so that it can be placed in small nucleus.

But the chromatin is not fully condensed form, it is semi-condensed. During the time of cell division, this semi-condensed chromatin further condensed to form chromosome. And chromosome can be easily stained and visualize under a compound microscope.

Number of chromosomes varies in different species

SpecieNo. Of Chromosome
Penicillium1 chromosome
Frog26 chromosomes
Honey bee32 chromosomes
Human beings46 chromosomes
Monkey 48 chromosomes
Rat40 chromosomes
Corn20 chromosomes
Ferns500 chromosomes
Drosophila8 chromosomes

Structure of Eukaryotic Chromosome

A typical eukaryotic chromosome structure consists of chromatid, secondary constriction (also known as satellite) and primary constriction (also called centromere).

Heterochromatin and Euchromatin

HeterochromatinEuchromatin
Heterochromatin is very tightly packedEuchromatin is lightly packed inside the nucleus.
They are less functional.They are active and functional
The DNA present in heterochromatin region do not involve in replication, transcription and translation.They are involved in replication, transcription and translation.
Heterochromatin vs. Euchromatin from Wikipedia

Features of histone proteins

  • They are positively charged.
  • They are rich in arginine and lysine.
  • They are positively charged at physiological pH ,i,e 7., howver charge may vary according to the pH given.
  • They are present only in eukaryotes, prokaryotes lacks histone proteins.
  • They are conservative in nature.
  • They are water soluble.

Histone proteins are of two types: Replication dependent and Replication Independent

Replication dependent Replication independent
These type of proteins occur only in S phase of the cell cycleThese proteins can occur in any phase of the cell cycle.
Their mRNA lacks poly A tail, but there is an exception in case of budding in yeastThey have poly A tail in their mRNA
E.g; H1, H2A, H3 and H4E.g; H3.3, CNPA

Conclusion

To wrap up this post, we conclude that Chromosomes are made up of chromatids joined with the help of centromere .Nucleosome is the structural and functional repeating unit of chromatin. There are four types of different  histone proteins subunits  : H2A, H2B, H3 and H4. Eukaryotes have multiple chromosomes depending upon the species.

Saif Ali

Hi, I am Saif Ali. I obtained my Master's degree in Microbiology and have one year of research experience in water microbiology from  National Institute of Hydrology, Roorkee. Antibiotic resistant microorganisms and soil bacteria, particularly PGPR, are my areas of interest and expertise. Currently, I'm focused on developing antibiotic alternatives. I'm always trying to discover new things from my surroundings.  My goal is to provide readers with easy-to-understand microbiology articles. If you have a bug, treat it with caution and avoid using antibiotics to combat SUPERBUGS. Let's connect via LinkedIn:http://linkedin.com/in/saif-ali-80b5098b

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