POLYTENE CHROMOSOMES

• These are over-sized chromosomes.
• They have developed from standard chromosomes.
• They are commonly found in the salivary glands of Drosophila melanogaster.

Salivary glands in Drosophila (people.hsc.edu)

• Polytene chromosomes are formed when specialized cells undergo repeated rounds of DNA replication without any cell division occurring in the cell.
• This in turn increases the cell volume forming a giant chromosome such as the polytene chromosome.
• Multiple rounds of replication produce many sister chromatids that remain synapsed together forming polytene chromosomes.
• Other than simply increasing the nuclei's volume and causing cell expansion, there is also a certain metabolic advantage in the formation of polytene chromosomes.
• The advantage is the result of a high level of gene expression in the presence of multiple copies of genes.

  

  A polytene chromosome in Drosophila melanogaster. (www.mun.ca)

• Example: In Drosophila melanogaster the chromosomes of the larval salivary glands undergo many rounds of endoreduplication in order to produce large amounts of glue before pupation.
• Example: Again in Drosophila melanogaster, within the organism there is tandem duplication of various polytene bands located near the centromere of the X chromosome. This results in the Bar phenotype of kidney-shaped eyes.

Structure

• These chromosomes have characteristic light and dark banding patterns. The banding patterns can be used to identify chromosomal rearrangements and deletions.
• Dark banding frequently corresponds to inactive chromatin, whereas the light banding is usually found at areas with higher transcriptional activity.
• The banding patterns of D. melanogaster was sketched by Calvin B Bridges in 1935.
• The polytene chromosomes in Drosophila have been used to support the “Theory of genomic equivalence”. This theory suggests that all the cells in the body maintain the same genome.
• Chromosomal Puffs are diffused uncoiled regions of the polytene chromosomes that are sites of RNA transcription.

Chromosomal puff (www.tutorsglobe.com)

• A Balbiani ring is a large chromosome puff.
• Polytene chromosomes are usually about 200 um in length.
• The chromonema of these chromosomes divide but do not separate. This makes them stay together in a large size.
• Another such type of chromosomes is the Lampbrush chromosome.
• Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881.

Chironomus Larvae (mmp.vfu.cz)

• Their hereditary nature was however confirmed only in Drosophila melanogaster I the early 1930s by Emil Heitz and Hans Bauer.
• They are known to occur in the secretory tissues of other dipteran insects such as Malpighian tubules of Sciara and also in protists, plants, mammals or in the cells from other insects.
• The largest polytene chromosomes described thus far occur in larval salivary gland cells of the Chironomid genus Axarus.
• Polytene chromosomes are also used to identify the species of Chironomid larvae that are notoriously difficult to identify.

IMMUNOLOGY- SET 01

Introduction to infections

• Based on relationship to hosts, microorganisms are classified as; Saprophytes and Parasites.
• SAPROPHYTES: The word is from Greek “sapros” meaning decayed and “phyton” meaning plants.
• Free living microbes that subsist on dead and decaying organic matter.
• They are found in soil and water and are important for degradation of organic materials.
• They are incapable of multiplying in living tissues which means they have less relevance for infectious diseases.
• But, saprophytes like Bacillus subtilis may infect devitalized host whose natural resistance is lowered. Such infections are termed Opportunistic infections.
• PARASITES: can establish themselves and multiply in hosts.
• Parasites are either pathogens or commensals.
• PATHOGENS: From Greek words, “pathos” meaning suffering and “gen” meaning produce which roughly translates to “disease-producing”.
• COMMENSALS: From Latin words, “com” meaning with and “mensa” meaning table. These parasites live together with the host without causing any disease causing effects.
• However, many commensals behave as “facultative pathogens” and can produce disease when resistance is lowered. 

Classification of Infections

• Primary Infections: initial infections with a parasite.
• Re – Infections: subsequent infections by same parasite.
• Secondary Infections: new parasite infects when resistance is lowered by presence of a previous one.
• Focal Infections: or focal sepsis, infection or sepsis at a localized site like appendix or tonsils which produces generalized effects.
• Cross infections: When a patient is already suffering from a disease, a new infection is set up from another host or another external source.
• Nosocomial infections: Cross- infections occurring in hospitals are called as nosocomial infections. The term is derived from the Greek work “nosocomion” meaning hospital.
• Iatrogenic infections: refers to physician-induced infections resulting from investigative, therapeutic or other procedures.
• Depending on whether the source of infection is within or outside the host’s own body, infections are classified as Endogenous or Exogenous.
• Endogenous- source is outside host’s body; Exogenous- source is inside host’s own body.
• Based on clinical effects, they may be classified as;
• In apparent infection – clinical effects are not apparent. Also called as “Sub-clinical infection”.
• Atypical infection – The typical or characteristic clinical manifestations of the particular infectious disease are not present.
• Latent infection – Some parasites, following infections may remain in the tissues in a latent or hidden form, proliferating and producing disease when the host resistance is lowered.

Sources of infection

Humans:-

• Commonest source of infection
• Source may be patient or a carrier
• Carrier – A person who harbours the pathogenic microorganisms without suffering any ill effect because of it.
• There are several types of carriers;
• Healthy carriers – One who harbours the pathogen but has never suffered from the disease caused by the pathogen.
• Convalescent carriers – One who has recovered from the disease and continues to harbour the pathogen.
• Depending on duration of carriage, carriers are classified as Temporary or Chronic.
• Temporary carrier – Carriage lasts for less than six months.
• Chronic carrier – Carriage may last for several years and sometimes even the rest of one’s life.
• Contact carrier – A person who acquires the infection from a patient.
• Paradoxical carrier – A carrier who acquires it from another carrier.

Animals:-

• Act as sources of human infection.
• Reservoir hosts – The infection in animals may be symptomatic. These animals serve to maintain the parasite in nature and act as the reservoir of human infections.
• Zoonoses – Infectious diseases transmitted from animals to humans.
• Zoonotic diseases may be;
• Bacterial – Plague from rats, caused by Yersinia pestis which is an enterobacteriaceae.
• Viral – Rabies from dogs, Lyssani viruses, Rabies virus, Australian Bat Lyssani.
• Protozoal – Toxoplasmosis from cats, Toxoplasma gondii.
• Helminthic – Hydatid disease from dogs, Echino coccosis – tapeworms (Echino coccus)
• Fungal – Zoophilic dermatophytes from cats and dogs.

Insects:-

• Arthropod-borne disease – The diseases caused by transmission of pathogens by blood sucking insects.
• Vectors – Insects such as mosquitos, ticks, mites, flies, fleas and lice that transmit infections.
• Mechanical vectors – When the transmission of pathogens by vectors is mechanical. For e.g. Transmission of dysentery or typhoid bacilli by the domestic fly.
• Typhoid bacilli – Salmonella typhi
• Dysentery bacilli – Shigella sonnei, Shigella dysenteriae, Shigella flexneri
• Biological vectors – When the pathogen multiplies or undergoes part of its developmental cycle in the body of the vector.
• For e.g. Aedes aegypti mosquito – Yellow fever
• Anopheles mosquito – Malaria
• These vectors transmit the infection only when the pathogen has multiplied in them sufficiently or has undergone a developmental cycle.
• The interval between the time of entry of the pathogen into the vector and the vector becoming infective is called the extrinsic incubation period.
• Insects also act as reservoir hosts, e.g. Ticks in relapsing fever and spotted fever.

HUMAN MOLECULAR GENETICS - INTRODUCTION

MOLECULAR GENETICS 

•  Molecular genetics basically revolves around the interrelationship between 2 nucleic acids – DNA and RNA.
• The large amount of DNA in the nucleus is also composed of repetitive sequences that have been found to support chromosomal function. There is also a good amount of defective copies of functional genes. 
• The main story that is dealt with is the synthesis of polypeptides by these 2 nucleic acids.
• It is hypothesized that RNA may have been the hereditary material during early evolution.
• However, now most organisms contain their genetic information in DNA molecules that are more chemically stable.
• This genetic information is copied and transmitted to daughter cells.
• Some viruses still contain RNA as the genetic material. Some of them are Ebola virus, SARS coronavirus, Influenza virus, Hepatitis C virus (HCV), West Nile virus, Polio virus, Measles virus.
• DNA molecules are found mainly in the chromosome of the nucleus, and also in each mitochondrion. In plants, DNA molecules are also found in chloroplasts.
• A gene is a part of a DNA molecule that serves as a template for making a functional RNA molecule.
• In bacteria and other simple organisms, the DNA is packed within genes.
• In Eukaryotes, the nucleus contains tens of thousands of genes packed in chromosomes, whereas the mitochondrion or chloroplast contain only a few genes.

RNA Molecules

• There are many types of RNA molecules and can generally be divided into 2 broad classes.
• Coding RNA – contain coding sequence that can be decoded to generate a corresponding polypeptide sequence.
• Messenger RNA – carries genetic information from DNA to the protein synthesis machinery. (mRNA)
• The mRNA made in the nucleus has to be transported to the cytoplasm to interact with ribosomes and make proteins.
• The mRNA in the mitochondrion and chloroplasts make protein within the organelles.
• Functional end point of information stored in DNA – protein synthesis.

Central Dogma

• One-way direction of genetic information flow – DNA> RNA> Polypeptide
• Universality of this gene flow is why it is referred to as the central dogma.
• The process flow – Transcription> Translation. The 2 essential processes in cellular organisms.
• Transcription – DNA id used as a template by an RNA polymerase to synthesize a particular type of RNA.
• Translation – mRNA is decoded to make polypeptides at ribosomes.
• Ribosomes are large RNA-protein complexes that are found in the cytoplasm and also in mitochondria and chloroplasts.
• Genetic information is written as a linear sequence of nucleotides in DNA
• The sequence is decoded in group’s three nucleotides to give a linear sequence of nucleotides in RNA. Here the Thymine found in DNA is replaced by Uracil in RNA.
• This is again decoded in groups of three to give a linear sequence of amino acids in forming the polypeptide.
•  Eukaryotic cells, including mammalian cells contain nonviral chromosomal DNA sequences, such as members of the mammalian Line-1 repetitive DNA family that encode cellular reverse transcriptases.
•  Reverse transcriptases can produce DNA from an RNA template.
• Hence, the central dogma is not strictly valid.