Genetics of hemoglobinopathies

Hemoglobin protein possesses an oxygen-carrying capacity. It occurs in the red blood cells. The functions of hemoglobin include PH maintenance, buffering action, and transportation of carbon dioxide. Hemoglobin is a complex protein. It consists of two main components such as the haem and the globin. Haem transports oxygen. The globin consists of four polypeptide chains. Each polypeptide chain consists of iron-containing haem group. The globin chain protects haem from oxidation. The structure of hemoglobin gets designed in a sophisticated way. Thus, it creates an internal environment of hydrophobicity and protects the iron from oxidation. However, the external structure of hemoglobin helps to get solubilized in water.

Alterations in the structure of hemoglobin lead to abnormalities in hemoglobin structures. These structures possess an affected oxygen-carrying capacity. The term Haemoglobinopathy depicts a condition resulting in disorders related to altered hemoglobin. There are two main categories of Haemoglobinopathies. The first type of hemoglobinopathy arises due to a change in the structure of the hemoglobin chain. The second type of hemoglobinopathy arises due to the synthesis of a faulty globin chain.

Image: Genetics of hemoglobinopathies

What is anemia?

Anemia is a condition leading to a decrease in red blood cells. Thus, the levels of hemoglobin also tend to decrease. Symptoms of anemia include weakness, fatigue, and shortness of breath. Sometimes, the individual may lose consciousness. There are three main types of anemia. Number one type of anemia includes anemia due to blood loss. Second, it occurs due to decreased production of RBCs. The third type of anemia occurs due to an increased breakdown of RBCs. A decreased production of RBCs occurs due to factors such as iron deficiency, vitamin B12 deficiency, and thalassemia. The reason behind the increased breakdown of RBCs is sickling of RBCs due to genetic changes.

The following table shows the types of hemoglobin and the mutations associated with the same.

Types of hemoglobins	Changes or alterations	Hemoglobinopathy
HbS	A point mutation or an alteration in the beta chain of globin leads to the replacement of glutamic acid to valine amino acid (at the 6th position).	Sickle cell anemia
HbC	A point mutation in the beta-globin genes leads to the replacement of glutamic acid to lysine (at the 6th position).	Hemolytic anemia, RBC sickling.
HbE	A point mutation in the beta-globin genes leads to the replacement of glutamic acid to lysine (at the 26th position).	Hemolytic anemia, RBC sickling.
Hb Lepore Hb Anti-Lepore	Fusion of the delta-beta chains (in Lepore) Fusion of beta-delta chains (in anti-Lepore)	Hemoglobin Lepore syndrome Hemoglobin anti-Lepore syndrome
Hb constant spring	Beta-point mutation in the termination codon.	Hemolytic anemia
Hb Grady	Alpha (116-118) duplication mutation.	Unstable Hemoglobin

Table: Types of hemoglobins and alterations leading to different kinds of hemoglobinopathies.

The genetics of Haemoglobinopathies:

Synthesis of hemoglobin takes place either in the mitochondria or cytosol. It occurs through a piece of special genetic machinery. There are two main types of globins such as alpha and beta globins. They get synthesized by alpha and beta globin genes respectively. Specific chromosomes consist of genes responsible for the production of globin chains. Chromosome 16 consists of the genetic code responsible for synthesizing alpha subunit of hemoglobin. Thus it helps in synthesizing alpha chain. Chromosome 11 consists of the genetic code responsible for synthesizing the beta subunit of hemoglobin. Thus, it helps in synthesizing beta chain.

Changes in the globin chain structure affect the formation of hemoglobin. They get associated with diseases such as sickle cell anemia. The changes in the structure of the globin chain occur due to mutations in the globin genes. A mutation either arises spontaneously or gets induced through certain factors. It also occurs due to mutagen exposure. Many types of mutations get observed in globin genes such as point mutations, deletions, insertions or frameshift mutations. Disorders also occur due to alterations in the globin chain synthesis. An example includes thalassemia.

Sickle cell anemia arises due to the changes in the structure of hemoglobin. It leads to hemolytic anemia. Hemolytic anemia is a condition resulting in the elimination of RBC’s from the bloodstream before completing their lifespan. Thus, the survival of RBCs gets reduced. Abnormal hemoglobin leads to sickle-shaped RBC’s with less lifespan. Sickle cell anemia is an example of an autosomal recessive type of inheritance. The RBCs do not look like normal ones. They show crescent-shaped structures. Mutation in the beta polypeptide gene leads to sickle cell anemia. The sickle-shaped cells do not pass through the blood vessels easily. Instead, they get stuck up in the blood vessels. As a result, the oxygen-carrying capacity gets reduced. Sickle cell trait describes the carriers of sickle cell disease. Individuals with the Sickle cell trait show heterozygosity for the mutant gene. The carrier himself will be normal with no symptoms. However, the next generation develops a complete disease.

Thalassaemia also shows the characteristic of hemolytic anemia. In such cases, the globin genes possess normal structures. They synthesize normal globin chains. However, they get produced in fewer amounts. 

References:

[1] Hemoglobinopathy-Wikipedia

[2] NCBI

[3] PubMed

Copyright, 2019, Study Genetics Online. All Rights Reserved.

Chromosome jumping

It helps in isolating the non-contiguous clones from a genomic library. These clones skip a region between the known points on a chromosome. Chromosome jumping acts as a tool for bypassing the regions that do not allow the cloning process. It plays a crucial role in physical mapping techniques for generating genomic markers. Chromosome jumping helps in searching for a specific gene. The regions difficult to clone include certain repetitive sequences. It helps in analyzing the sequences separated by more than 100-kilobases. Chromosome jumping involves two alternative procedures. The first method uses rare cutting enzymes. The second method uses frequent cutters. Chromosome jumping is also known as chromosome hopping. Consider the phage DNA.

A method using the rare cutters:

These cutters belong to the class of endonucleases. The first step involves the isolation of the genomic DNA. The isolated DNA gets a treatment with the restriction enzyme (rare cutter). The process is known as restriction digestion. It allows the isolation of fragments with 100 kb sizes. These fragments get subjected to a specialized electrophoretic technique known as the pulse field gel electrophoresis. The separated fragments get easily circularized later on. This method gives rise to three types of fragments. The first type includes the fragments having sequences with 100-kilobase sizes. The second type of fragments includes sequences of the original augments with lighted sequences such as the junction fragments. The third type of fragments includes other types of genomic sequences.

A method using frequent cutters:

The frequently cutting endonucleases lead to partial digestion of the fragments. First, the genomic DNA gets isolated from the sample. Next, the restriction enzymes carry out partial digestion of the genomic DNA. The fragments obtained from the partial digestion get selected based on the size. The next step involves the pulse field gel electrophoresis. The fragments obtained from the pulse field gel electrophoresis get cloned into a suitable vector.

Cloning:

The fragments obtained from both the procedures get cloned into a suitable vector with the presence of a selectable marker. The vector gets introduced into the host such as E. coli bacteria. After this, the cells get plated on a medium to check the growth of the plaques. The bacteriophages genome having the selectable marker gets replicated to form the plaques. They are known as the jumping clones. Next, the jumping fragments get identified through the nucleic acid hybridization. It involves using a specific probe. Thus, it is possible to identify the fragments undergoing repetitive hopping process.

Image: Chromosome jumping library depicting jumping clones

Chromosome jumping library:

It includes a collection of recombinant molecules obtained through chromosome jumping. The two types of jumping libraries include the general jumping and the specific jumping libraries. The general jumping libraries include sequences starting at any genomic locations. They travel to specific distances. These libraries involve sequences obtained from chromosome jumping involving frequent cutters. The specific jumping libraries show a specific jumping of the clones. They jump from the rare restriction site to the adjacent restriction sites. These libraries involve sequences obtained from the chromosome jumping methods using the rare cutters. The examples include libraries constructed using Not-I restriction endonucleases. Thus, with the help of chromosome jumping libraries, the clones jump from one restriction site to the other.

Chromosome jumping libraries have many applications. They include prenatal diagnosis, de novo assembly, and characterizing chromosomal rearrangements. Highly efficient bacterial genome assemblies were constructed using long jump chromosome libraries. The whole genome jumping library offers a gene-level resolution. Thus, it is beneficial for prenatal diagnosis and testing. A short jump library gets created using ligation of the DNA fragments with biotinylated, followed by circularization and affinity assays. However, it is less efficient due to its reduced genome coverage. Long jump library is efficient for longer DNA fragments. Another type of chromosome jumping library known as custom barcode jumping library distinguishes the junction fragments very efficiently. The E. coli vector transfection helps in amplifying larger DNA fragments in the Fosmid-jump library.

Enzymes used in chromosome jumping:

An endonuclease enzyme shows capability in cleaving the phosphodiester bond in the DNA strand. The technique of chromosome jumping involves restriction enzyme such as endonuclease type-II. These enzymes efficiently cleave longer DNA sequences. The following table depicts the examples of endonucleases used in chromosome jumping:

Endonuclease	Restriction site
Not I	GCGGCCGC
Sfi I	GGCCNNNNNGGCC
Pac I	TTAATTAA
BssHII	GCGCGC

Table: Endonucleases used in chromosome jumping and their restriction sites

Certain regions in the mammalian genome show the presence of rare nucleotide repeats. Such regions also get considered through enzyme treatment.

Genetic disorders arise due to defective genes or mutations. However, certain genetic disorders, the mutant genes or the gene products remain unidentified. In such cases, it becomes difficult to know the exact details of the genes and the pathways involved. Thus, identifying these genes and cloning them becomes the most difficult task. Also, the molecular markers prove to be inefficient in identifying such genes. The reason involves very large molecular distances. Reverse genetics is a subfield of genetics involving investigation of a gene or a protein function. The first step involves directed mutagenesis using the knowledge of a DNA or a protein sequence. Next, the programmed mutations get introduced back to the genome. However, it is difficult to identify the unknown sequences responsible for causing a genetic disorder. The problem could be solved using a chromosome jumping library. A review mentioned the chromosome jumping library constructed for the cloning of DNA sequences. These sequences lie a hundred kilobases away from the gDNA start point.

References:

[1] Encyclopedia of Genetics, Genomics, Proteomics, and Informatics, by George P. Rédei

[2] The Dictionary of Genomics, Transcriptomics, and Proteomics, By Guenter Kahl

[3] Plant Chromosomes, by Archana Sharma

[4] Introduction to Plant Biotechnology, by H. S. Chawla

[5] Molecular Biology and Genetic Engineering, by P. K. Gupta

Copyright, 2019, Study Genetics Online

Meiosis

Unlike mitosis, meiosis does not occur in the somatic cells. Instead, it occurs in the gametes. The doubling of the gametic chromosomes occurs due to meiosis. It occurs in two stages. The first meiotic stage leads to the reduction of the chromosome number from the diploid to haploid. The second meiotic division is the same as the mitotic division. A single chromosomal duplication precedes the two divisions. Meiosis also consists of stages such as prophase-I, metaphase-I, anaphase-I, and telophase-I in its meiosis-I stage. The meiosis-II stage also consists of prophase-II, metaphase-II, anaphase-II, and telophase-II. Let us discuss each of the phases in details.

Meiosis-I		Meiosis-II
Prophase-I	Leptonema or leptotene	Prophase-II
	Zygonema or zygotene
	Pachynema or pachytene
	Diplonema or diplotene
	Diakinesis
Metaphase-I		Metaphase-II
Anaphase-I		Anaphase-II
Telophase-I		Telophase-II

Table: Different stages involved in Meiosis-I and Meiosis-II

Meiosis-I:

Following are the subphases of meiosis-I:

Image: Different stages of Meiosis-I and Meiosis-II

Prophase-I:

It shows a similarity with the mitotic prophase. However, a slight difference in the substages makes the prophase-I different from the mitotic prophase. The meiotic prophase-I consists of five substages such as leptotene, zygotene, pachytene, diplotene, and diakinesis. The leptotene stage involves the coiling of the chromosomes. It helps in committing the cell to enter the meiosis. The chromosomes look like threads in leptotene. They get an orientation of the bouquet. It is known as the bouquet configuration. Each chromosome in the leptotene looks like a single chromosome. The pachytene stage reveals the two chromosomes. The DNA replication occurs well before the leptotene stage.

The zygonema stage or the zygotene is the early mid-phase of the prophase-I. The chromosomes get shortened in this stage. The chromosomes in the stage are known as homologous chromosomes. The pairing of these chromosomes occurs only in the meiosis. It does not occur during mitosis. Mitotic recombination occurs very rarely.  The homologous chromosomes are a pair of essentially identical chromosomes. They, later on, involve synapsis. The genes in the homologous chromosomes belong to the common ancestor. Hence, they get retained. Synapsis involves a point-by-point pairing of the homologous chromosomes. It occurs mainly during the zygonema stage. Mainly the dipteran tissues such as the Drosophila salivary glands undergo synapsis. Parallel, dense elements showing lateral position surround the medial complex. These elements form a ribbon-like tripartite structure. This structure is known as a synaptonemal complex. It occurs in the central axes of the paired homologous chromosomes present in a pachytene bivalent. Synaptonemal complex helps in maintaining the parallel configuration of the lateral elements.

Synapses also play a crucial role in forming the zipper-like structures along the length of the chromatids. It also reduces the chromosomal threads into the half. Their appearance becomes like bivalents instead of a single chromosomal look. The bouquet-like arrangement also involves the role of telomeres (chromosomal ends). The pachytene stage, each paired chromosome separates into two sister chromatids. Exceptions include the centromeric regions. The division occurs longitudinally. It forms chromatin tetrads. Meaning, four chromatids occur due to the longitudinal division. Next step involves a localized breakage. It exposes the non-sister chromatids. These non-sister chromatids exchange the genetic material with each other.

The process of exchange is known as the crossing over. It plays a crucial role in giving a different set of alleles to the progeny, slightly differing from the parental alleles. Hence, it gives rise to the new genetic material (recombinants). The disassembly of the synaptonemal complex commits the cell to the diplotene stage. Each pair of sister-chromatids in a tetrad start separating in this phase (except the places of exchange). The cross-shaped structure arising due to the overlapping chromatids is known as chiasmata. The process of terminalisation brings the chiasma towards the end of the tetrads. Diakinesis involves tight coiling of the chromosomes. The nucleolus and the nuclear envelope disappear. The first meiotic division produces two secondary gametocytes containing the dyads.

 Metaphase-I:

It involves a complete breakdown of the nuclear envelope. It shows an alignment of the bivalents on the equatorial plane. It gives rise to spindle formation followed by the microtubule attachment.

Anaphase-I, Telophase-I, and cytokinesis:

In the anaphase, the chromosomal disjoining occurs. The chromosomes migrate to the opposite poles, thereby moving the centromeres. However, the centromeres do not get separated from their sister chromatids. They remain attached. It leads to the formation of a new nuclear envelope. Next, the cell divides into two through a process known as cytokinesis.

Meiosis-II:

The second meiotic division mimics the mitotic division. The chromosomes start getting condensed in the prophase-II. The chromosomes get aligned on the equator in the metaphase-II. The centromeres split, moving the chromatids apart on the opposite poles. The anaphase-II plays a crucial role in proper centromere splitting. After the telophase-II, the microscopic observations reveal well-defined chromosomes.

Gene segregation:

Half the numbers of chromosomes occur in a haploid cell. Thus, it is a result of meiosis. A diploid cell enters the meiotic phase and results in haploid cells. Two meiotic divisions occur after the DNA replication which occurs only once (in the S phase). However, the haploid nuclei fuse together if the diploid nuclei are required. Thus, meiosis helps in maintaining the chromosome number. The chromosomes aligning at the equatorial plane may either be paternally derived ones or maternally derived ones. There involves no restriction in the alignment of the chromosomes. Thus, nuclei derived out of meiosis contain a combination of both the paternal and the maternal chromosomes. The process of variation leads to variation. It gives rise to recombinants.

Meiotic drive is a condition in which a meiotic division gives rise to an unequal recovery of the gametes produced by a heterozygote. It involves an intragenomic conflict.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

Copyrights, 2019 All Rights Reserved

Mitosis

Prokaryotes and eukaryotes differ in their cellular structures, the pathways, the machinery, and different processes. The cellular reproductive capacity of the eukaryotes is much higher than the prokaryotes. The cells follow a typical cycle for undergoing division and proliferation. The cell cycle studies help in knowing the pattern of growth and reproduction. The cellular compartmentalization bifurcates the functioning of every important cellular component. The nucleus and the cytoplasm have different roles to play. They involve great interactions and function in an efficient way. The study of cell cycle not only helps us in knowing about the cell division but also its way of differentiation.

Different types of cells exist in eukaryotic organisms. The cells either show haploidy or diploidy or any other condition. Each of them shows the presence of well-defined chromosomes. The G1, S, G2, and the M phases constitute the phases of a cell cycle. The M phase or the mitotic phase belongs to the somatic cell cycle. It is the final phase of the cell cycle and lasts for one hour. The interphase, lasting for 23 hours involves the G1, S, and the G2 phases. The division phase or mitosis plays a crucial role in cellular division. It consists of four subphases such as prophase, metaphase, anaphase, and telophase. Studying mitosis helps in finding out the mitotic index. It helps in determining the fraction of cells undergoing mitosis in a given sample. The mitotic index study helps in determining the division of cancer cells undergoing rapid division.

Image: Different stages of Mitosis

Mitotic apparatus:

Before knowing about the mitotic phase in detail, it is important to know the components involved in the mitosis phase. The mitotic apparatus consists of three main components such as the asters, the spindle, and the traction fibers. Each centrosome involves the formation of the asters. The spindle apparatus consists of a gelatinous structure. The connection of the centromeres to the centrosomes occurs with the help of the traction fibers.

Prophase:

The initial phase of the mitosis involving chromosome visibility is known as prophase. The word prophase indicates the initial stage involved in the division. The chromosomes occur in the highly condensed state. They show clear visibility under the microscope. The formation of the spindle apparatus starts during this phase. The spindle apparatus looks like a slender structure tapering towards the ends. The slenderness of the spindle arises due to the tubulin fibers. These tubulin fibers help in the chromosomal movements during the other phases of the mitotic phase. The chromosomes get attached to these fibers with the help of kinetochores. Each chromosome becomes longitudinally double. Exceptions include the regions near or at the centromere. Three types of microtubules include the polar microtubules, the kinetochore microtubules, and the astral microtubules. Hence, these structures help in organizing the spindle formation. The assembly occurs outside the nucleus during the prophase. The prophase further involves three stages. They include the early prophase, the middle prophase, and the late prophase.

The centrioles are the other organelles involved in the spindle formation. These structures show a composition of tubulin. They help in organizing the mitotic spindle. During the early prophase, these structures move apart, thereby indicating the beginning of the spindle formation. The early prophase chromosomes show a reduced structure. After some time, their visibility gets clarified. The nucleolus gets disappeared in this phase.

The middle prophase leads to the movement of centrioles further apart. It proceeds with the mitotic spindle formation. The late prophase depicts the movement of the centrioles towards the opposite sides. The spindle formation starts. It is the time for the chromosomes to coil and produce a series of compact structures known as gyres.

Metaphase:

The nuclear envelope starts getting disappeared in this phase. The kinetochores are an example of well-defined proteins playing a crucial role in chromosome-spindle attachment. They attach themselves very well to the centromere (structures of the chromosomes joining the sister chromatids). Hence, the chromosomes get aligned on the equatorial plane of the spindle. It occurs between the two spindle poles. It is also known as a metaphase plate. The chromosome gets aligned perfectly on the spindle. The cellular processing technique helps in arresting the cells. Hence, they get visualized under an electron microscope. The microscopic observations reveal the scaffolding patterns of the proteins surrounding the uncoiled DNA. Thus, it helps in studying the double-stranded DNA.

Anaphase:

The centromeres of the sister chromatids undergo a separation during the anaphase. They form two daughter chromosomes. The traction fibers help in the separation of the chromatids. They separate in such a way that their movement follows towards the spindle pole. The kinetochores also separate during this process. The process of disjunction leads to the conversion of the sister chromatids into the independent chromosomes. The microtubules start getting shorter and shorter. Thus, the two independent chromosomes move toward the poles. Due to the disjunction, the chromosomes appear in different shapes. They include V, J, T, X or rod-shaped structures. The shape, however, depends on the position of the centromere. The J-shaped chromosomes are known as the sub-metacentric chromosomes. The V-shaped chromosomes are known as the metacentric. Improper centromere split leads to chromosomal abnormalities. The factor defining the movement of the chromosomes towards the poles is known as the mitotic center. Example of the mitotic center includes the centriole.

Telophase:

Now the chromosomes align in two groups. They place themselves at the opposite ends of the cell. The uncoiling of the chromosomes starts in telophase. The spindle apparatus disappears. This phase helps in the completion of the nuclear division. The dissolution of the kinetochore microtubules occurs during this phase. The polar microtubules get elongated in this phase.

Cytokinesis:

One cell undergoes a division to give rise to two cells. Cytokinesis occurs after the telophase of the mitosis. The two cells have their own set of nuclei and organelles. First, the cell cytoplasm separates and cleaves at the middle of the telophase cell. Thus the cell divides into two. The mitotic spindle helps in determining the cleavage site. The thin ring of the actin filaments gets cleaved first. Hence, it plays a crucial role in the process of cleavage.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

Copyrights, 2019 All Rights Reserved

Prokaryotic and Eukaryotic Cells

Prokaryotic organisms first evolved on the earth. Many differences exist between the prokaryotic and eukaryotic organisms. Hence, their cellular structures and compartmentalization also vary. The functions and the biochemical pathways also vary from each other. The eukaryotic cells possess different types of organelles. The prokaryotic cells do not have all of them. Most of the prokaryotic cells possess microscopic and sub-microscopic structures. These cells do not get visualized with the naked eyes. The only way to study them involves culturing them and visualizing them under a microscope. The eukaryotes, on the other hand, show a wide range of organisms such as plants, insects, animals, birds, and human beings. The eukaryotic organisms possess different types of bodies, different cells, different proteins, and cellular processes. Thus, both prokaryotic and eukaryotic cells involve topics of interest for the researchers. Let us discuss the different points.

Image: A rough sketch of a eukaryotic cell and a prokaryotic cell

Cell size and shapes: The size of a eukaryotic cell varies from 10-100um. The size of the prokaryotic cell varies from 1-10um. The shapes of the cells also vary. Eukaryotic cells possess different types of cells with varied shapes in a particular organism. For example, the shape of the sperm cells differs from the oocytes or the smooth muscle cells. Similarly, the neuronal cells possess different shape. The prokaryotic organisms have rod-shaped cells, cocci, spiral-shaped cells or cork-screw shaped cells, and many other shapes.

Nucleus: The core region in the cell, known as the nucleus, plays a crucial role in giving space to the genetic machinery and other cellular activities. Eukaryotic cells possess a well-defined nucleus. The prokaryotes do not have a well-defined nucleus. The eukaryotic cells possess more than one chromosome in the nucleus. The prokaryotic cells have only one or very few chromosomes. However, they show the presence of extrachromosomal material known as the plasmids. The eukaryotic cells possess a membrane-bound nucleus.

Chromosomes: The prokaryotic chromosomes mostly possess a single-stranded or a double-stranded DNA. They possess a circular or a linear structure. The bacterial or the archeal chromosomes are known as a nucleoid. Eukaryotes possess a specific number of chromosomes. For example, humans possess forty-six chromosomes. The packaging proteins in the prokaryotic chromosomes are known as HU proteins. The packaging proteins in the eukaryotic chromosomes are known as histone proteins.

Cell wall: Present in plants and fungi. The prokaryotic cell wall consists of complex structures. The bacterial cell walls possess peptidoglycan. It consists of polysaccharide chains cross-linked by unusual peptides. The plant cell wall consists of cellulose. The fungal cell walls contain chitin.

The permeability of the nuclear membrane: The eukaryotic nuclear membranes show selective permeability. The prokaryotic cells do not have permeability mechanisms. The cell membrane helps manage the movement of water, carbon dioxide, and oxygen. It exhibits a selective permeability towards the ions and the organic molecules.

The cellular type: The eukaryotic organisms possess many cells. Thus, they are known as multicellular organisms. The prokaryotic organisms have a single cell. They are known as unicellular organisms. The eukaryotic cells also possess different kinds of cells such as immune cells, blood cells, somatic cells, stem cells, gametic cells, and many other types of cells. Most of the cells possess a nucleus (exceptions include red blood cells). The chromosomal segregation of the somatic and the gametic cell also varies. The prokaryotic cells lack all the above points.

Mitochondria and Chloroplast: These organelles play a crucial role in producing energy in the form of ATP. They possess a separate DNA known as mitochondrial DNA or the mtDNA. The chloroplasts possess pigment chlorophyll. It plays a crucial role in photosynthesis. The eukaryotic cells possess a mitochondrion and chloroplasts (in plants and algae). The prokaryotic cells lack mitochondrion. They do not possess chloroplast. However, they show the presence of scattered chlorophyll. The origin of the mitochondria and chloroplast involved a free-living prokaryotic origin. These organisms, later on, invaded eukaryotic cells and got established there. The theory explaining the above concept is known as endosymbiont theory. Eukaryotic cells originated as anaerobic organisms. They lacked mitochondria and chloroplasts. After many years, a eukaryotic cell established a relationship with a purple non-sulfur bacteria. The purple non-sulfur bacteria involved a key process known as oxidative phosphorylation, which proved to be beneficial for the eukaryotic cell. Thus, it started getting atmospheric oxygen thereby depending on the prokaryotic cells. Thus, the mitochondrion came into existence. The chlorophyll producing plants ingested the oxygen-producing photosynthetic bacteria. Thus, the chloroplast came into existence.

Lysosomes and peroxisomes: The eukaryotic cells possess lysosomes and peroxisomes. The prokaryotes lack them. Lysosomes possess hydrolytic enzymes capable of breaking down many different biomolecules. They help in disposing of the unwanted things from the cell. The peroxisome is also known as a microbody. It plays an important role in catabolizing the long chain fatty acids.

Endoplasmic reticulum: Eukaryotes possess endoplasmic reticulum. The prokaryotes do not have an endoplasmic reticulum.

Ribosomes: Prokaryotes have 70S ribosome. Eukaryotes have 80S ribosomes. The eukaryotic ribosomes have a larger size as compared to the prokaryotic ribosomes. Ribosomes play a crucial role in translation (RNA to protein). The process of translation occurs slightly differently in the prokaryotic and eukaryotic cells. The initiator methionine gets modified to N-formyl-methionine in the case of the prokaryotic translation process. The eukaryotic translation process does not involve the modification of the methionine. Unlike prokaryotic sequences, the eukaryotic mRNAs do not possess Shine-Dalgarno sequences. Instead, they employ a short sequence known as the Kozak sequence.

Vesicles and the Golgi apparatus: The eukaryotic cells have the vesicles and Golgi apparatus. The prokaryotic cells lack Golgi complex and vesicles.

Genetic recombination: Meiosis and fusion of gametes occur in the eukaryotes. The eukaryotic DNA undergoes recombination. The prokaryotic DNA also undergoes recombination. However, it involves partial genetic recombination. It mostly chooses unidirectional DNA transfer or a vector mediated DNA transfer.

Microtubules: The eukaryotic cells possess microtubules. The prokaryotes do not have microtubules. However, rarely they occur in very few prokaryotic organisms. The rare bacterial microtubules possess a smaller diameter as compared to the eukaryotic microtubules.

References:

[1] How Eukaryotic and Prokaryotic Cells Differ, Raina G. Merchant and Lesli J. Favor

[2] Eukaryotic and Prokaryotic Cell Structures: Understanding Cells With and Without a Nucleus, By Leslie Favor, Ph.D.

[3] Website Diffen: Eukaryotic Cell vs. Prokaryotic Cell

Copyright, 2019 All Rights Reserved