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Year : 2013 | Volume : 1 | Issue : 1 | Page : 1 - 3  


CME
Genetics Made Simple

Sampth kumar V1, Vani N2, Rajasree T. K.3, Anantha Kumari4, Hassan Q5

1Associate Professor of Biochemistry, Malla Reddy Institute of Medical Sciences (MRIMS), Hyderabad, 2Professor of Biochemistry, Osmania Medical College, Hyderabad, 3Professor & HOD of Anatomy, MRIMS, 4Professor of Anatomy, Deccan College of Medical Sciences, Hyderabad, 5Professor of Genetics, Kamineni Academy of Medical Sciences, Hyderabad

Introduction :

Genetics has been made into parts of curriculum for both undergraduates and Postgraduate students. As we all know genetics is an emerging field of medicine. Now-a-days lots of research is going in the field of genetics, for both therapeutic and investigation purpose. This C.M.E was conducted to update the basics of genetics to both medical teachers and students. There are many faculties in medicine, who have not been exposed to any of procedures on genetics. Keeping this in view we have also conducted a workshop on Karyotyping which is a basic procedure and essence of genetics.

Replication, Transcription, Translation and Gene expression

Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together by hydrogen bonds between G-C and A-T base pairs. Duplication of the genetic information occurs by the polymerization of a new complementary strand onto each of the old strands of the double helix during DNA replication. The expression of the genetic information stored in DNA involves the translation of a linear sequence of nucleotides into a co-linear sequence of amino acids in proteins. A limited segment of DNA is first copied into a complementary strand of RNA. This primary RNA transcript is spliced to remove intron sequences, producing mRNA molecule. Finally, the mRNA is translated into protein in a complex set of reactions that occur on a ribosome. The amino acids used for protein synthesis are first attached to a family of t-RNA molecules, each of which recognizes, by complementary base-pairing interactions, particular sets of three nucleotides in the mRNA (codons).

The sequence of nucleotides in the mRNA is then read from one end to the other insets of three, according to a universal genetic code. The large r-RNA of the ribosome which catalyzes the formation of peptide bonds during protein synthesis. [1] Alterations in gene expression allow a cell to adapt to environmental changes, developmental cues, and physiological signals. Gene expression can be controlled at multiple levels by changes in transcription, RNA processing, localization, and stability or utilization. Gene amplification and rearrangements also influence gene expression. [2]

Recombinant DNA technology

The recombinant DNA revolution in biology is rooted in the repertoire of enzymes that act on nucleic acids. It depends, first, on having enzymes that can cut, join, replicate DNA and reverse transcribe RNA. Restriction enzymes cut very long DNA molecules into specific fragments that can be manipulated. These endo – nucleases recognize specific base sequences in double-helical DNA and cleave both strands of the duplex, forming specific fragments of DNA. These restriction fragments can be separated and displayed by gel electrophoresis. The pattern that they form on the gel is a fingerprint of a DNA molecule. A DNA fragment containing a particular sequence can be identified by hybridizing it with a labeled single-stranded DNA probe (Southern blotting).DNA probes for hybridization reactions, as well as new genes, can be synthesized by the automated solid - phase method. DNA ligases join the fragments together. The availability of many kinds of restriction enzymes and DNA ligases makes it feasible to treat DNA sequences as modules that can be moved at will from one DNA molecule to another. Thus, recombinant DNA technology is based on nucleic acid enzymology.

The exquisite sensitivity of PCR makes it a choice technique in detecting pathogens and cancer markers, in genotyping, and in reading DNA from fossils that are many thousands of years old. Recombinant DNA technology is beginning to significantly alter the practice of medicine by providing new diagnostic and therapeutic agents and revealing molecular mechanisms of disease. Entire genomes, including the human genome, are being deciphered. New insights are emerging, for example, into the regulation of gene expression in cancer and development and the evolutionary history of proteins as well as organisms. New proteins can be created by altering genes in specific ways to provide detailed views into protein function. Clinically useful proteins, such as hormones, are now synthesized by recombinant DNA techniques. Crops are being generated to resist pests and harsh conditions. The new opportunities opened by recombinant DNA technology promise to have broad effects. [3]

Inborn errors of metabolism

Genetically determined inborn errors of metabolism are individually rare, but collectively they affect more than 1 in 500 individuals in the population. An abnormality in deoxyribonucleic acid (DNA) is expressed by the production of an enzyme that is deficient in its function of catalyzing and modulating the conversion of substrate to product. The resulting clinical picture ranges from normal to severely affected. Inborn errors of metabolism are generally inherited in an autosomal recessive or, less frequently, in a sex-linked recessive manner. The major categories are Organic acidemias, Fatty acid oxidation defects, Primary Lactic acidosis, Amino acidurias, Urea cycle defects, Disorders of carbohydrate metabolism, Lysosomal storage disorders, Peroxisomal disorders, disordered steroidogenesis and disorders of metal metabolism. The explosion in the application of recombinant DNA methods to inborn errors of metabolism has altered our approach to these disorders in a number of diagnostic and therapeutic ways by utilizing DNA probes for prenatal diagnosis. [4]

Cytogenetics

History of Cytogenetics has been classified into various stages. The dark era this is before 1956, hypotonic era from 1956 to 1960, Trisomy era from 1960 to 1980. The golden era which is 1980 onwards, this is the decade where genetics has progressed enormously. Whatever recent advanced we are seeing today has taken place in golden era. The earliest developments taken place in genetics during the period of Gregor Mendel- Austrian Monk in 18th Century. [5]He has given the Mendelian laws which include law of dominance, law of segregation, law of independent assortment. Even after 100 years of Gregor Mendel’s discovery genetics was unknown subject. Then again in 1956 Watson, Crick and Wilkins revised genetics and given molecular structure of DNA in the same year. After the discovery of DNA, then research in genetics shifted to chromosomes. Each chromosome is made up of DNA coiling with some proteins in it. This coiling model of chromosome is called as solenoid model of chromosome. A chromosome will have centromere, long arm, and short arm. Depending upon the location of centromere chromosomes are classified into meta-centric, submeta-centric, acro-centric, telo-centric.

Then comes the most basic procedure done to study chromosomes, in which cells are cultured, stained with some dyes, and then dropped from a height to show the metaphase spread. Now the photographs of the metaphase spread will be taken and individual chromosomes will be studied in detail. Depending upon the size and shape, chromosomes are classified into seven groups. In Karyotyping if chromosomes are stained with Giemsa-stain it is called as G-banding. If it is stained by Quinacrine it is called as Q-banding. By Karyotyping we can identify the structural and numerical abnormalities of chromosomes.

Chromosomal abnormalities are divided into structural abnormalities and numerical abnormalities. Structural abnormalities include translocation, inversion, ring chromosomes, deletion, and duplications. [6] Some of structural abnormalities diseases are Cri du chat syndrome, Angel man syndrome, Prader willi syndrome. Numerical abnormalities occur due to errors in mitotic or meiotic division, with gain or loss of chromosomes. Polyploidy is change in complete set of chromosomes or Aneuploidy is change in number of chromosomes. Down syndrome, Tunrer syndrome, Patau syndrome, Kleinfelters syndromes are result of numerical abnormalities of chromosomes. [6] The etiological factors for genetic diseases are uni -factorial and multi factorial. Environmental factors also have role in genetic diseases.

Patterns of Inheritance

Pattern of inheritance describe different ways in which genes handed down from parents to their off springs through several generations, may express themselves. Genes carried on chromosomes are responsible for the development of inherited characters or traits. [7] When single gene pair determined a trait is known as single gene trait and its mode of inheritance from one generation to next generation follows Mendel’s law of unit inheritance and segregation. When many genes determine a trait, the trait is known as polygenic and it follows the pattern of polygenic inheritance for transmission from generation to generation. Following are the various types of inheritance. Autosomal dominant, Autosomal recessive, X linked dominant, X linked recessive, Y- linked inheritance and polygenic inheritance. An autosomal dominant trait is one which manifest in the heterozygous state. Any child born to a person affected by a dominant trait or disorder has a 1 in 2 chance of inherited it and being similarly affected. Some of the disorder which has got autosomal dominant inheritance is Achondraplasia, porphyria, congenital cataract, polycystic kidney disease. Autosomal recessive traits and disorders only manifest when the mutant allele is present in a double dose i.e. homozygous state. Autosomal recessive trait and disorders usually related to consanguineous marriages. In autosomal recessive disorder inherited gene has a chance 1 in 4(25%). Some of the autosomal recessive disorders are cystic fibrosis, alkaptonuria, and many other enzyme deficiency metabolic disorders. Sex linked inheritance refers to the pattern of inheritance shown by genes that are located on either of the sex chromosomes. Genes carried on X- Chromosomes are referred to being X-linked, while genes carried on the Y-Chromosomes are referred to as exhibiting Y-Linked inheritance. X-Linked recessive trait is one determine by gene carried on X- Chromosomes and usually manifest in males. Classical example for X- Linked recessive disorder is hemophilia. X-Linked dominant inheritance all though uncommon there are disorders that manifest in heterozygous female as well as males who has mutant allele on his single X-Chromosome. X-Linked dominant disorders are vitamin -D resistant rickets, Charcot Marie tooth disease. Y-linked inheritance implies only males are affected e.g., Hairy pinna. [8] Hardy -Weinberg principle denotes that dominant genes and traits in a population would tend to increase at expense of recessive one. Factors which influence Hardy – Weinberg principle are non- random mating, mutation, selection, small population size, gene flow migration.

Pedigree charting

A family tree is a shorthand system of recording the pertinent information about a family. It usually begins with the person through whom the family came to the attention of the investigator. This person is referred to as the index case, proband or propositus, or if female, the proposita. The position of the proband in the family tree is indicated by an arrow. Information about the health of the rest of the family is obtained by asking direct questions about brothers, sisters, parents and maternal and paternal relatives, with the relevant information about the sex of the individual, affection status and relationship to other individuals being carefully recorded in the pedigree chart . Attention to detail can be crucial because patients do not always appreciate the important difference between siblings and half-siblings, or might overlook the fact, for example, that the child of a brother who is at risk of Huntington disease is actually a step-child and not a biological relative. [9]

Dermatoglyphics

The study of the patterns of the ridges on the palms, fingers soles, and toes is called Dermatoglyphics. Dermatoglyphics is important part of genetics because their correlation to genetic disorder has been suppressed by newer and more accurate methods of chromosomal analysis. [10] However such methods are still not evenly accessible at many places in various parts of the world and Dermatoglyphics are still used to diagnose many chromosomal disorders. Characteristic simian crease found in Down’s syndrome and many other chromosomal disorders. Dermatographic traits are genetically determined. They provide useful means of resemblance between twins in determination of twin zygosity.

Genetics in modern Medicine

Genetics has historically focused predominantly on chromosomal and metabolic disorders, reflecting the long-standing availability of techniques to diagnose these conditions. For example, conditions such as trisomy 21 (Down’s syndrome) or monosomy X (Turner’s syndrome) can be diagnosed using cytogenetics. Likewise, many metabolic disorders (e.g., phenylketonuria, familial hypercholesterolemia) are diagnosed using biochemical analyses. Recent advances in DNA diagnosis have extended the field of genetics to include virtually all medical specialties. In cardiology, for example, the molecular basis of inherited cardiomyopathies and ion channel defects that predispose to arrhythmias is being defined. In neurology, genetics has unmasked the pathophysiology of a startling number of neurodegenerative disorders. Hematology has evolved dramatically, from its incipient genetic descriptions of hemo-globinopathies to the current understanding of the molecular basis of red cell membrane defects, clotting disorders, and thrombotic disorders. DNA testing is most commonly performed by DNA sequence analysis for mutations, although genotype can also be deduced through the study of RNA or protein (e.g., apoprotein E, hemoglobin, immune histochemistry). For example, immune histochemical analysis of colorectal cancers for absence of expression of mismatch repair proteins has been proposed as a strategy for universal Lynch syndrome screening. The determination of DNA sequence alterations relies heavily on the use of polymerase chain reaction (PCR), which allows rapid amplification and analysis of the gene of interest. In addition, PCR enables genetic testing on minimal amounts of DNA extracted from a wide range of tissue sources including leukocytes, mucosal epithelial cells, and archival tissues. Amplified DNA can be analyzed directly by DNA sequencing or it can be hybridized to DNA chips or blots to detect the presence of normal and altered DNA sequences. Mutations in certain cancer susceptibility genes such as BRCA1 and BRCA2 may identify individuals with an increased risk for the development of malignancies and result in risk-reducing interventions. The detection of mutations is an important diagnostic and prognostic tool in leukemias and lymphomas.

Prenatal diagnosis of numerous genetic diseases in instances with a high risk for certain disorders is now possible by direct DNA analysis. Amniocentesis involves the removal of a small amount of amniotic fluid, usually at 16 weeks of gestation. Cells can be collected and submitted for karyotype analyses, FISH, and mutational analysis of selected genes. The main indications for amniocentesis include advanced maternal age (>35 years), an abnormal serum triple marker test (α-fetoprotein, β human chorionic gonadotropin, pregnancy-associated plasma protein A, or unconjugated estriol), a family history of chromosomal abnormalities, or a Mendelian disorder amenable to genetic testing. Prenatal diagnosis can also be performed by chorionic villus sampling (CVS), in which a small amount of the chorion is removed by a transcervical or transabdominal biopsy. Chromosomes and DNA obtained from these cells can be submitted for cytogenetic and mutational analyses. Direct DNA sequencing is increasingly used for prenatal diagnosis as well as for determination of hereditary disease susceptibility. Analysis of large alterations in the genome is possible using cytogenetics, fluorescent in situ hybridization (FISH), or Southern blotting. [5]

References

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