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

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

To Study the Role of Telomerase in Cellular Replication

An introduction to telomeres:

A telomere is an important structure of the chromosome. Every chromosome consists of a special DNA-protein complex with a particular sequence. Thus telomeres are the caps at the end of the chromosomes. A human telomere is 3 to 20 kilobases in length. It consists of tandem repeats of TTAGGG-3’ sequence. The telomeric sequence is 3000 times repeated. When you go out on a sunny day, would you not prefer to wear a cap? Sun’s scorching heat would be unbearable and wearing a cap would protect you from the damaging heat. Similarly, telomeres are caps on the chromosomes.

 There are three important functions of telomeres:

1.     Telomeres provide structural stability to the chromosomes by sealing their ends.

2.     They protect the ends of the chromosomes from damage or formation of rings.

3.     Telomeres protect the chromosomes from fusing with other DNA.

A telomerase enzyme is a ribonucleoprotein complex. It acts like a reverse transcriptase. It plays a crucial role in terminal chromosomal maintenance. Telomerase adds telomeric repeats to the chromosomal ends or DNA termini. A telomerase enzyme compensates for incomplete replication. Telomerase can elongate the telomeres. They enable the cells to distinguish between natural chromosomal ends and double-stranded breaks. They maintain chromosome stability.

Species	Telomere repeat sequence	Telomerase RNA template sequence
Human	5’-TTAGGG-3’	5’-CUAACCCUAAC-3’
Oxytricha	5’-TTTTGGGG-3’	5’-CAAAACCCCAAAACC-3’
Tetrahymena	5’-TTGGGG-3’	5’-CAACCCCAA-3’

Table: The telomeric repeat sequences and telomerase RNA template sequences in humans, oxytricha, and tetrahymena respectively.

Structure of a telomere:

A telomere consists of the following components:

1.     DNA sequence:

A telomere consists of 5’TTAGGG 3’ sequence. A hexamer unit is present in 2000 copies.

2.     Loops

Several studies indicate the occurrence of telomeres in the form of loops. Most of the telomeres end in a loop known as T-loop, where the double-stranded telomeric tract curves around. The T-loop formation protects from exonucleases. There is a displacement loop of TTAGGG repeats known as a D loop. Telomeres can also form specialized structures known as G-quadruplex DNA. These structures are composed of guanine tetrads or G-quartets. Guanine tetrads are square planar arrays of four, hydrogen-bonded guanines. Hoogsteen base pairing is common among them. This G-quartets stack upon each other and provide telomere protection.

3.     Protein components

·        TRF1: It is known as a telomere repeat binding factor 1. It binds to the telomere at T-loop. TRF-1 inhibits telomerase-dependent elongation.

·        TRF 2: It is known as telomeric repeat binding factor 2. It is involved in the formation of T-loop. The overexpression of TRF2 in somatic cells leads to telomere shortening.

·    hRAP 1: It is a human homolog of yeast protein. It is involved in determining the length of the telomere.

·    TIN 2: It is known as a TRF-1 interacting nuclear factor. It promotes pairing of telomere repeats.

·        TANK1: It promotes telomere elongation.

Image 1: Chromosomes showing their telomeres

What is telomere shortening?

DNA replication is an important process of the cell. In this process, DNA is duplicated using replication enzymes. However, during DNA replication, the enzymes may skip replicating the ends of the DNA. As a result, few telomeric sequences get skipped. Thus, the replicated DNA is slightly shorter than the original. After many replications, the telomere sequences become too short. As a result, the cell division stops and the cell undergo apoptosis. This phenomenon leads to aging in human. The process of telomere shortening is observed in somatic cells and not in the germ cells, antibody-producing cells, and cells which constantly replace the gut epithelium. The telomerase enzyme is prominently present in these cells. That is why these cells do not undergo telomere shortening. Somatic cells are deficient in the telomerase enzyme.

There are two ways in which the telomere shortening occurs:

The extreme 3’ end of the DNA is difficult to copy. The natural position of the priming site may be beyond the end of the template. Thus, the lagging strand copy is incomplete, because the last Okazaki fragment is not complete. The resulting daughter molecule has a 3’ overhang and gives rise to a grand-daughter molecule which is shorter than the original one.

Another reason for a shortened telomere is the position of the primer. It is at an extreme 3’ end of the lagging strand.

The length of the telomere is a useful parameter in the process of aging.

What is telomere extension?

The telomerase contains a unique RNA-protein complex. The 5’ end of the telomerase consists of 5’-CUAACCCUAAC-3’ sequence. The central region of this sequence is a reverse complement of the telomere repeat. This repeat has a 5’- TTAGGG-3’ sequence.

The extension of telomeric DNA follows five steps. First, the telomerase RNA pairs with the ends of the molecule. Next, the telomeric DNA gets extended at a short distance. A stem-loop structure determines the length of the telomeric DNA. After extension of the telomeric DNA, the telomerase moves further through the translocation process. It starts base pairing the next fragment. In this way, the telomere gets extended.

The completion of telomere extension is unique. A new Okazaki fragment is primed and synthesized, which converts 3’ extensions into the complementary double-stranded ends. The t-loop is formed when a free 3’ end of telomere loops back and invades the double helix.

A process of replicative senescence takes place in the cells. The changes in the structure of the telomeres cause replicative senescence. Cellular senescence leads to changes in cell morphology and gene expression. It is triggered when cells acquire few critically short telomeres.

Eventually, the telomeres become very short. The telomere-protein complex gets disrupted and leads to DNA damage. P53 involved apoptosis is a similar kind of damage. Thus, for a normal senescent, further cell division is blocked.

The finite ability of telomeric DNA replication is known as the Hayflick limit.

Image 2: Telomere extension

Telomeres and cancer:

Most of the cancer cells have active telomerase enzyme. As a result, these cells keep on multiplying. The remaining cells employ an important mechanism known as alternative lengthening of telomeres (ALT) for telomere maintenance.

Most of the cancer cells contain telomerase, a crucial factor for the immortality of the cells.

Suppose there is a mutation in a gene controlling normal cell cycle arrest. Such a cell will divide in spite of having a very short telomere. This cell has the capability of becoming immortal. It may also carry plenty of mutations triggering to cancer.

“In sum, the primary cause of the cancer is a cellular mutation. Telomerase activity and telomere extension are secondary to cancer.”

What are anti-telomerase drugs?

Anti-telomerase drugs work against the telomerase. These drugs target the action of telomerase and block this enzyme. As a result, the cells do not undergo division. However, some side effects may occur due to the complete blocking of telomerase. Cells such as antibody-producing cells, the immune cells, germ cells, and other cells are dependent on telomerase. Thus there could arise some serious effects of using anti-telomerase drugs.

Conclusion:

More research has to be carried out before launching anti-telomerase drugs. Also, we must note that telomerase activity is not the only cause of cancer. Smoking, consumption of alcohol, pollution, chemical mutagens, and other factors may also contribute to the risk of cancer. Mainly these factors induce mutations in the cells. Thus, for designing a cancer drug, the focus should be on the underlying mutations.

References:

[1] Telomeres and Telomerase in Aging, Disease, and Cancer: Molecular Mechanisms, K. Lenhard Rudolph

[2] Genome Instability in Cancer Development, Erich A. Nigg

[3] Topics in Anti-Cancer Research, Atta-ur Rahman, Khurshid Zaman

[4] Concepts Of Genetics, 7/E (With Cd), Klug
[5] Molecular Biology of the Gene, 5th Ed, Pearson Education, 2004: Gene, Pearson Education, Inc

© Copyright, 2018  All Rights Reserved

A review on human chromosomes

An Introduction to chromosomes:

The ability of the chromosomes in getting stained reveals the true nature of the chromosome (derived from the Greek work word. The word chroma indicates color). These stainable bodies appear like threads under a microscope. They contain the genetic material, mainly the DNA coiled around the proteins. Human chromosomes are visible with when the cell is undergoing mitotic or meiotic cell division. There are total 46 chromosomes in each human cell.

·    Autosomes: There are total 44 autosomes or 22 pairs. Each pair consists of homologous chromosomes. In a pair, one chromosome comes from the father and the other chromosome comes from the mother.

·        Sex Chromosomes: There are two different types of sex chromosomes, X and Y respectively.

The average size of the human metaphase chromosome is 5 millimeters. Chromosomes tightly coil and get condensed during metaphase. Chromosomes appear in different shapes during each phase of the cell cycle. They appear thread-like during interphase. During metaphase, chromosomes look like rod-shaped. They look like V, J or rod-shaped during anaphase. Von Hartz coined the term chromosome. The scientists who first discovered the structure of chromosomes were Schleiden, Virchow, and Bütschli. Walter Sutton and Theodor Boveri independently developed the chromosome theory of inheritance in 1902.

An interphase nucleus contains strands of a material called chromatin. There are two regions in the chromatin, mainly coiled and extended regions.

·    Heterochromatin: It is the dark staining area of the chromatin. There are two types of heterochromatin. Constitutive heterochromatin contains repetitive sequences. It is present near the centromere. Constitutive heterochromatin never expresses itself. There is one more type of heterochromatin, known as facultative heterochromatin, which expresses itself.

·        Euchromatin: It is the light staining area of the chromatin.

The chief constituent of chromatin is DNA, the blueprint of life. At the time of cell division, chromatin strands coil into compact structures, so that they easily fit into the cell nucleus. Chromosomes appear as thick rods only during the cell division. They uncoil and form chromatin at the end of cell division.

Image: Human chromosomes revealed through karyotyping

Structure of the chromosome:

Metaphase chromosome appears clearly under the microscope. Following are the principal point to be discussed:

·        Chromatids: Each metaphase chromosome consists of two symmetrical halves parallel to each other. They are called chromatids. These chromatids are present in the form of chromonema during prophase. There are two types of chromatids mainly, sister and non-sister chromatids. Two chromatids are present on a single chromosome. Thus they are called sister chromatids. The concept of dyad describes a pair of sister chromatids. These structures join with the help of a centromere.  Non-sister chromatids are either of the two chromatids of a chromosome pair.

·        Centromere: A centromere is a light staining constricted area to which both the chromatids are attached. A centromere divides the chromatids into short and long arms respectively. The short arm is known as “p” arm. The long arm is known as “q” arm. Centromere produces a primary constriction. It is the position of the centromere. It is different for different chromosomes. Secondary constriction is also known as nucleolar organizer region. It is involved in the formation of the nucleolus. Human centromere consists of several hundred kilobases of repetitive DNA. Centromeres are the sites where spindle fibers are attached. Thus, centromere helps in the movement of the chromosome.

·     Satellite: A satellite is a region that is attached to the chromosome by a thread of chromatin. It is present at the distal region of the arm of the chromosome.

·        Telomere: A special DNA-protein complex is present at the ends of the chromosomes. This complex is known as a telomere, with tandem repeats of TTAGGG-3’ sequences between 3-20 kilobases in length. Telomeres are not genes since they do not code for any functional molecule. Telomeres provide structural stability to the chromosomes by sealing their ends. Telomeres protect the chromosomes from damage. They also protect the chromosome from fusing into a ring or binding to other DNA.

Classification of chromosomes:

1.     Classification based on the position of the centromere:  

·     Metacentric: The two arms are almost equal in their lengths. The location of the centromere is at the center of the chromosome.

·   Submetacentric: The two arms of the submetacentric chromosomes are unequal in length. The location of the centromere is slightly away from the center.

·   Acrocentric: One arm of an acrocentric chromosome is short. Whereas, the other arm is long.

·      Telocentric: A telocentric chromosome has only one arm.

2.     Standard classification: It is also known as Denver classification. It classifies chromosomes into seven groups, depending on the length of the chromosomes.

·  Group A: This group consists of pairs of chromosomes 1, 2 and 3. Chromosome 1 is the largest human chromosome. It represents 8% of the total DNA content.  Chromosome 2 is the second one. The third chromosome represents 6.5 % of the total DNA content.

·       Group B: This group consists of pairs of chromosome 4 and 5.

·    Group C: This group consists of pairs of chromosome 6, 7, 8, 9, 10, 11 and 12.

·        Group D: This group consists of pairs of chromosomes 13, 14 and 15.

·        Group E: This group consists of pairs of chromosomes 16, 17 and 18.

·        Group F: This group consists of pairs of chromosomes 19 and 20.

·        Group G: This group consists of pairs of chromosomes 21 and 22.

·  Sex Chromosomes: There are two types of sex chromosomes. X chromosome and Y chromosome are called sex chromosomes.

3.     Paris nomenclature: According to this method, the long and short arms of the chromosomes have specific regions that get stained. These regions are further stained using banding techniques. Such techniques may not only help to identify specific chromosomes, but also find out the location within the chromosome. Banding techniques may help to detect minor structural abnormalities.

What is sex chromatin?

The nucleus in the interphase is in the resting phase. An interphase nucleus shows a dark stain chromatin mass attached on one side of the nuclear membrane. Sex chromatin is also known as the Barr body. It is observed only in females. However, the chromatin determination using the Barr body is not as accurate as the Karyotyping technique.

What are chromosomal aberrations?

Chromosomal mutations or aberrations are variation in the normal chromosome structure or chromosome number.

A deletion is a chromosomal mutation in which a part of a chromosome is missing. Chromosomal breaks result in deletions. Sometimes an entire chromosome may get deleted. Duplication may lead to doubling of a chromosomal segment. Excision of a chromosomal segment follows reinsertion leading to an inversion. A translocation is a chromosomal mutation in which a chromosome segment gets positioned in a different location in the genome.

Chromosome Analysis:

Chromosome analysis indicates a proper diagnosis of many clinical conditions. It is a microscopic analysis of chromosomes in the dividing cells. Chromosomal analysis can detect chromosome number and structure.

Uses of chromosome analysis are as follows:

·        Detection of congenital malformations, mental retardation, and repeated abortions.

·        Prenatal diagnosis

·        Diagnosis of malignancies

Karyotyping:

Karyotyping is a test to evaluate the number and the structure of the chromosomes. In this procedure, the metaphase chromosomes are obtained and photographed. The procedure of karyotyping is specialized. Peripheral blood lymphocytes, bone marrow cells or amniotic fluid samples are collected and analyzed for chromosomes. A photo-micrograph reveals chromosomes scattered randomly. These chromosomes are arranged into groups, using the software. A karyotype can be sued to detect chromosomal abnormalities.

Chromosome Banding:

Analysis of chromosome becomes precise with the help of banding techniques. There are four types of banding techniques such as G-banding, Q-banding, R-banding, and C-banding. A unique pattern of light and dark bands are obtained using the G-banding technique.

Fluorescence in-situ hybridization (FISH):

FISH is a new diagnostic technique that involves a single-stranded probe annealing to its complementary sequence. FISH can be used to detect minute chromosomal aberrations, malignancies, and study of chromosomes.

References:
[1] Medical genetics, G.P. Pal
[2] Human Genetics, 3/e, Gangane
[3] Vogel and Motulsky's Human Genetics: Problems and Approaches, Friedrich Vogel, Gunter Vogel, Arno G. Motulsky
[4] Biology for the IB Diploma: Standard and Higher Level, Andrew Allott
[5] Principles of Medical Genetics, Thomas D. Gelehrter

© Copyright, 2018  All Rights Reserved