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Chromosomes

     The DNA of an organism are packed in their cells to protected, they also regulate the accessibility of the DNA. Packaging of DNA helps conserve space in the cells. Approximately, two meters of the human DNA can fit into a cell that is only a few micrometers wide. Chromosomes are made up of DNA segments. Chromosomes carry all the information that help a cell grows, survive and reproduce. DNA segments with specific patterns are called genes. The chromosomes are found in the nucleus of the cell. In prokaryotic organisms, the DNA is not present in the nucleus; the DNA floats in the cytoplasm in area called the nucleoid.
     The chromosomes vary widely between different organisms. Eukaryotic cells have large number of linear chromosomes and cells of prokaryotes have smaller and circular DNA. Cells may contain more than one type of chromosome, like in most eukaryotic cells, the mitochondria and the chloroplasts in plant cells possess their own set of chromosomes.
     In nucleus of eukaryotic organism, the chromosomes are packed by proteins to form a compact structure called chromatin. This condensation allows long molecules of DNA to fit into the cell nucleus. Chromosomes are more condensed then the chromatin and they are essential for cell division. The chromosomes are replicated, divided and passed on to the daughter cells, to ensure genetic diversity and survival of the progeny.

     Duplicated chromosomes contain tow identical copies known as chromtids or sister chromatids, they are joined by a centromere. Compaction of the chromosomes during the cell division process results in the four-arm structure.
   Recombination of chromosome plays a vital role in genetic diversity. Incorrect multiplication of the chromosomes may lead to mitotic failure or death of the cell, it may lead to apoptosis and sometimes may be cancerous.
     Chromosomes are organized structure of DNA and proteins found in cells. They are thread-like structures located inside the nucleus of animal and plant cells. Chromosomes are made up of proteins and a molecule of deoxyribonucleic acid (DNA). Chromosomes are passed on from parents to offspring. The term chromosome is derived from a Greek word 'chroma' which means 'color' and 'soma' which means 'body'. The chromosomes are named so because they are cellular structures or cellular bodies and they are strongly stained by some dyes used in research. Chromosomes play an important role that ensures DNA is copied and destructed accurately in the process of cell division. In most of the organisms chromosomes are arranged in pairs in the nucleus of the cell. We have 23 pairs of chromosomes.

 

Chromosomes structure: 
* In eukaryotic cells, chromosomes are composed of single molecule of DNA with many copies of five types of histones. 
* Histones are proteins molecules and are rich in lysine and arginine residues, they are positively charged. Hence they bind tightly to the negatively-charged phosphates in the DNA sequence. 
* A small number of non-histone proteins are also present, these are mostly transcription factors. Transcription factors regulate which parts of DNA to be transcribed into RNA. 
* During most of the cell's life cycle, chromosomes are elongated and cannot be observed under the microscope. 
* During the S phase of the mitotic cell cycle the chromosomes are duplicated. 
* At the beginning of mitosis the chromosomes are duplicated and they begin to condense into short structures which can be stained and observed easily under the light microscope.

                                        
* These duplicated condensed chromosomes are known as dyads. 
* The duplicated chromosomes are held together at the region of centromeres. 
* The centromeres in humans are made of about 1 - 10 million base pairs of DNA. 

* The DNA of the centromere are mostly repetitive short sequences of DNA, the sequences are repeated over and over in tandem arrays. 
* The attached, duplicated chromosomes are commonly called sister chromatids. 
* Kinetochores are the attachment point for spindle fibers which helps to pull apart the sister chromatids as the mitosis process proceeds to anaphase stage. The kinetochores are a complex of about 80 different proteins. 
* The shorter arm of the two arms of the chromosome extending from the centromere is called the p arm and the longer arm is known the q arm.
Bacterial chromosomes: Bacterial chromosomes contain circular DNA molecule unlike the linear DNA of vertebrates. Most of chromosomes are circular DNA molecules and there are no free ends to the DNA. The bacterial DNA is packaged into a single chromosome into a continuous loop. The DNA is folded or coiled to fit into the cell. The compaction of the DNA involves the binding of proteins to the DNA that help form initial loops which is then coiled.

 

Prokaryotic chromosome: Prokaryotes like the bacteria and archaea typically have a single circular chromosome.

 The chromosome size of most bacteria is from only 160,000 base pairs to 12,200,00 base pairs. Some bacteria in exceptions contain a single linear chromosome. The base sequences in prokaryotic chromosomes are less than in eukaryotic cells. Bacterial chromosomes have a single origin of replication from which the replication starts. In some archaea there are multiple replication origins. The prokaryotic genes are organized into operons and it usually it does not contain introns. Nucleus is absent in prokaryotes, the DNA is organized into a structure called the nucleoid. The DNA of the archaea are more organized, they are packaged within structures similar to eukaryotic nucleosomes. The chromosomes in the prokaryotes and plasmids are generally supercoiled like that of the eukaryotes. The DNA are released into the relaxed state for the process of transcription, replication and regulation.
 

Eukaryotic chromosome: In eukaryotes the chromosomes are multiple large, linear and are present in the nucleus of the cell. Each chromosome typically has one centromere, one or two arms that project from the centromere, the arms are usually not visible during most of the time. Most of the eukaryotes have a small circular genome in the mitochondria. Some of the eukaryotes have small linear or circular chromosomes in the cytoplasm.

To fit into the compartment in which it is contained the DNA has to be condensed and the degree to which it is condensed is expressed as its packaging ratio. Packaging ratio is the length of the DNA divided by the length into which it is packaged. The chromosome of eukaryotic organisms consists of complexes made of DNA and protein, and it is organized in a condensed manner. This condensation permits the large amount of DNA to be stored in the nucleus of the cell.
 

Human chromosomes: Humans chromosomes are of two types autosomes and sex chromosomes. Genetic traits that are linked to the sex of the person are passed on through the sex chromosomes. The rest of the genetic information is present in the autosomes. Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell. Many copies of mitochondrial genome are present in human cells.

Sex chromosomes:
    Sex chromosomes differ in form of size, behavior from the ordinary chromosome. The sex chromosomes determine the sex of an individual during reproduction. These sex chromosomes differ between the male and the females. Females have two copies of X chromosome, males have one X chromosome and one Y chromosome. In the process of sexual reproduction in humans, two different gametes fuse to form a zygote.


                                     

Homologous chromosomes: Homologous chromosomes are also known as homologs or homologues. The homologous chromosomes are pairs of chromosomes that are approximately of the same length, position of centromere, and pattern of staining, genes for the same characteristic are at a corresponding loci. In an organism one of the homologous chromosome is inherited from the mother and the other from the father. These chromosomes are usually not identical, but they carry the same type of genes. During the process of mitosis the daughter chromosomes carry the same sequence of nucleotide, assuming there are no errors during the replication process. The genome in diploid organisms is composed of homologous chromosomes one of homologous pair is the maternal chromosome and the other is the paternal chromosome. During the process of meiosis the homologous chromosomes cross over. Homologous chromosomes are not identical but they are similar. The genes are carried in the same order, but the alleles for the trait may not be similar.


                                 

Functions of Chromosomes:
Genetic Code Storage:
Chromosome contains the genetic material that is required by the organism to develop and grow. DNA molecules are made of chain of units called genes. Genes are those sections of the DNA which code for specific proteins required by the cell for its proper functioning. 

 

Sex Determination: Humans have 23 pairs of chromosomes out of which one pair is the sex chromosome. Females have two X chromosomes and males have one X and one Y chromosome. The sex of the child is determined by the chromosome passed down by the male. If X chromosome is passed out of XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops. 
 

Control of Cell Division: Chromosomes check successful division of cells during the process of mitosis. The chromosomes of the parent cells insure that the correct information is passed on to the daughter cells required by the cell to grow and develop correctly. 
 

Formation of Proteins and Storage: Proteins are essential for the activity of a cell. The chromosomes direct the sequences of proteins formed in our body and also maintain the order of DNA. The proteins are also stored in the coiled structure of the chromosomes. These proteins bound to the DNA help in proper packaging of the DNA.

 

Chromosomal Defects:
1. Klinefelter syndrome:
Klinefelter syndrome or Klinefelter's syndrome (KS) also known as 47, XXY or XXY, is the set of symptoms that result from two or more X chromosome in males. The primary feature is sterility. Often symptoms may be subtle and many people do not realize they are affected. Sometimes symptoms are more prominent and may include weaker muscles, greater height, poor coordination, less body hair, smaller genitals, breast growth, and less interest in sex. Often it is only at puberty that these symptoms are noticed. Intelligence is usually normal, however, reading difficulties and problems with speech are more common. Symptoms are typically more severe if three or more X chromosomes are present.
     Klinefelter syndrome usually occurs randomly. An older mother might increase the risk slightly. The condition is not inherited from one's parents. The underlying mechanism involves at least one extra X chromosome in addition to a Y chromosome such that there is a total of 47 or more chromosomes rather than usual 46. KS is diagnosed by the genetic test known as a karyotype.

 

2. Turner syndrome (TS): Turner syndrome also known as Ullrich–Turner syndrome, gonadal dysgenesis, and 45, X, is a condition in which a female is partly or completely missing an X chromosome.

Signs and symptoms vary among those affected. Often there is a short and webbed neck, low-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet at birth. Typically they are without menstrual periods, do not develop breasts, and are unable to have children. Heart defects, diabetes, and low thyroid hormone occur more frequently. Most people with TS have normal intelligence. Many, however, have troubles with spatial visualization such as that needed for mathematics. Vision and hearing problems occur more often.
      Turner syndrome is not usually inherited from a person's parents. There are no known environmental risks and the mother's age does not play a role. Turner syndrome is due to a chromosomal abnormality in which all or part of one of the X chromosomes is missing or altered. While most people have 46 chromosomes, people with TS usually only have 45. The chromosomal abnormality may be present in just some cells in which case it is known as TS with mosaicism. In these cases the symptoms are usually fewer and possibly there are none at all. Diagnosis is based on physical signs and genetic testing.
     There is no cure for Turner syndrome. Treatment, however, may help with symptoms. Human growth hormone injections during childhood may increase adult height. Estrogen replacement therapy can promote development of the breasts and hips. Medical care is often required to manage other health problems with which TS is associated.

   Turner syndrome occurs in between 1 in 2000 to 1 in 5000 females at birth. All regions of the world and cultures are affected about equally. People with TS have a shorter life expectancy, mostly due to heart problems and diabetes. Henry Turner first described the condition in 1938. In 1964 it was determined to be due to a chromosomal abnormality.
 

3. Down syndrome: Down syndrome is a set of physical and mental traits caused by a gene problem that happens before birth. Children who have Down syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of intellectual disability. This varies from person to person. But in most cases it is mild to moderate.
      Down syndrome is a lifelong condition. But with care and support, children who have Down syndrome can grow up to have healthy, happy, productive lives.
     Down syndrome is caused by a problem with a baby's chromosomes. Normally, a person has 46 chromosomes. But most people with Down syndrome have 47 chromosomes. In rare cases, other chromosome problems cause Down syndrome. Having extra or abnormal chromosomes changes the way the brain and body develop.
      Experts don't know the exact cause, but some things increase the chance that you'll have a baby with Down syndrome. These things are called risk factors.

 

Risk of having a baby with Down syndrome: 
* You are older when you get pregnant. Many doctors believe that the risk increases for women age 35 and older. 
* You have a brother or sister who has Down syndrome. 
* You had another baby with Down syndrome.
     Many children with Down syndrome are also born with heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. But most of these problems can be treated.
Doctor may suggest tests during pregnancy to find out if your baby has Down syndrome:
Screening tests: Screening tests, such as an ultrasound or a blood test during your first or second trimester. These can help show if the developing baby (fetus) is at risk for Down syndrome. But these tests sometimes give false-positive or false-negative results. 
Diagnostic tests: Diagnostic tests, such as chorionic villus sampling or amniocentesis. These can show if a baby has Down syndrome. You may want to have these tests if you have abnormal results from a screening test or if you are worried about Down syndrome.

 

Nucleic Acids:
    Nucleic acids allow organisms to transfer genetic information from one generation to the next. There are two types of nucleic acids.
     a. DNA (deoxyribonucleic acid) 
     b. RNA (ribonucleic acid)

 

Nucleic acids are composed of nucleotide monomers. Nucleotides contain three parts:
* A Nitrogenous Base 
* A Five-Carbon Sugar 
* A Phosphate Group

 

a. DNA (deoxyribonucleic acid): 
* DNA was discovered by Francis Harry Compton Crick, James Dewey Watson in 1953, Nobel Prize was awarded jointly to Francis Harry Compton Crick, James Dewey Watson in 1962. 
* DNA (deoxyribonucleic acid) is a type of macromolecule known as a nucleic acid. It is shaped like a twisted double helix and is composed of long strands of alternating sugars and phosphate groups, along with nitrogenous bases

      i. Adenine 
      ii. Thymine 
      iii. Guanine 
      iv. Cytosine.

                  

* It is organized into structures called chromosomes and housed within the nucleus of our cells. DNA contains the genetic information necessary for the production of other cell components, organelles, and for the reproduction of life.
 

Gene: A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes.
     Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

 

Genetic Disorders: 
* A genetic disorder is a genetic problem caused by one or more abnormalities in the genome, especially a condition that is present from birth (congenital). Most genetic disorders are quite rare and affect one person in every several thousands or millions. 

* Genetic disorders may or may not be heritable, i.e., passed down from the parents' genes. In non-heritable genetic disorders, defects may be caused by new mutations or changes to the DNA. In such cases, the defect will only be heritable if it occurs in the germ line. The same disease, such as some forms of cancer, may be caused by an inherited genetic condition in some people, by new mutations in other people, and mainly by environmental causes in still other people. Whether, when and to what extent a person with the genetic defect or abnormality will actually suffer from the disease is almost always affected by the environmental factors and events in the person's development. 
* Some types of recessive gene disorders confer an advantage in certain environments when only one copy of the gene is present. 
Apoptosis: Apoptosis is the process of programmed cell death (PCD) that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation, and global mRNA decay. 

Pleiotropy: Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits, an example being phenylketonuria, which is a human disease that affects multiple systems but is caused by one gene defect. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favours one specific version of the gene (allele), while selection on other traits favors a different allele. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways that contribute to different phenotypes. Genetic correlations and hence correlated responses to selection are most often caused by pleiotropy.
* Barbara McClintock discovered jumping genes in zeamays. 
The genetic code: The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells. Biological decoding is accomplished by the ribosome, which links amino acids in an order specified by mRNA, using transfer RNA (tRNA) molecules to carry amino acids and to read the mRNA three nucleotides at a time. The genetic code was discovered by naren berg and Hargovind Khurana and Nobel Prize was awarded to them in 1968.

DNA fingerprinting: DNA fingerprinting is a test to identify and evaluate the genetic information—called DNA (deoxyribonucleic acid) in a person's cells. It is called a "fingerprint" because it is very unlikely that any two people would have exactly the same DNA information, in the same way that it is very unlikely that any two people would have exactly the same physical fingerprint. The test is used to determine whether a family relationship exists between two people, to identify organisms causing a disease, and to solve crimes.
     Only a small sample of cells is needed for DNA fingerprinting. A drop of blood or the root of a hair contains enough DNA for testing. Semen, hair, or skin scrapings are often used in criminal investigations.
     A person who has DNA fingerprinting done voluntarily usually provides a sample of blood taken from a vein. DNA testing also can be done on cells obtained by a simple mouthwash or a swab of the cheeks inside the mouth, but these methods are not recommended.

 

c. RNA (ribonucleic acid):
    RNA stands for ribonucleic acid. It is an important molecule with long chains of nucleotides. A nucleotide contains a nitrogenous base, a ribose sugar, and a phosphate. Just like DNA, RNA is vital for living beings.

    The main job of RNA is to transfer the genetic code need for the creation of proteins from the nucleus to the ribosome. This process prevents the DNA from having to leave the nucleus.
      This keeps the DNA and genetic code protected from damage. Without RNA, proteins could never be made.
      Some RNAs are enzymes. It was widely believed for many years that only proteins could be enzymes. RNAs are now known to adopt complex tertiary structures and act as biological catalysts. Such RNA enzymes are known as ribozymes, and they exhibit many of the features of a classical enzyme, such as an active site, a binding site for a substrate and a binding site for a cofactor, such as a metal ion.
     One of the first ribozymes to be discovered was RNase P, a ribonuclease that is involved in generating tRNA molecules from larger, precursor RNAs. RNase P is composed of both RNA and protein; however, the RNA moiety alone is the catalyst.
    DNA is defined as a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. RNA molecules are involved in protein synthesis and sometimes in the transmission of genetic information.

      However unlike DNA, RNA comes in a variety of shapes and types. While DNA looks like a double helix and a twisted ladder, RNA may be of more than one type. RNA is usually single-stranded, while DNA is usually double-stranded. In addition, RNA contains ribose while DNA contains deoxyribose. Deoxyribose lacks one oxygen atom. RNA has the bases Adenine (A), Uracil (U) (instead of thymine in DNA), Cytosine (C) and Guanine (G).
     Deoxyribose sugar in DNA is less reactive because of C-H bonds. DNA is stable in alkaline conditions. DNA has a smaller groove where the damaging enzyme can attach which makes it harder for the enzyme to attack DNA.
    Ribose sugar however is more reactive because of C-OH (hydroxyl) bonds. RNA is not stable in alkaline conditions. RNA has larger grooves, which makes it easier to be attacked by enzymes.
     The helix geometry of DNA is of B Form. DNA can be damaged by exposure to Ultraviolet rays. The helix geometry of RNA is of A-Form. RNA strands are continually made, broken down and reused. RNA, however, is more resistant to damage by Ultra-violet rays.

Posted Date : 01-10-2022

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

 

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