MICROBIOLOGY

A)  GENES: STRUCTURE, REPLICATION & MUTATION: Structure of Chromosomes : The term “chromosome” is used to refer to a nucleic acid molecule that is the repository of genetic information in a virus, a bacterium, a eukaryotic cell, or an organelle. It also refers to the densely colored bodies seen in the nuclei of dye-stained eukaryotic cells, as visualized  using the light microscope. Chromatin Consists of DNA and Proteins : The eukaryotic cell cycle produces remarkable changes in the structure in the chromosome.  In nondividing eukaryotic cells (in G0) and those in interphase (G1, S, and G2), the chromosomal material, chromatin, is amorphous and seems to be randomly dispersed in certain parts of the nucleus. In the S phase of interphase the DNA in this amorphous state replicates, each chromosome producing two sister chromosomes (called sister chromatids) that remain associated with each other after replication is complete. The chromosomes become mu...

A) GENES: STRUCTURE, REPLICATION & MUTATION: DNA Super coiling:  Cellular DNA, as we have seen, is extremely compacted, implying a high degree of structural organization. The folding mechanism not only must pack the DNA but also must permit access to the information in the DNA. Before considering how this is accomplished in processes such as replication and transcription, we need to examine an important property of DNA structure known as Supercoiling.          “Supercoiling” means the coiling of a coil. A telephone cord, for example, is typically a coiled wire. The path taken by the wire between the base of the phone and the receiver often includes one or more supercoils. DNA is coiled in the form of a double helix, with both strands of the DNA coiling around an axis. The further coiling of that axis upon itself  produces DNA supercoiling. As detailed below, DNA supercoiling is generally a manifestation of structural st...

A)  Genes: Structure, Replication & Mutation GENES & CHROMOSOMES :  The size of DNA molecules presents an interesting biological puzzle. Given that these molecules are generally much longer than the cells or viral particles that contain them,how do they fit into their cellular or viral packages?   CHROMOSOMAL ELEMENTS :           Cellular DNA contains genes and intergenic regions, both of which may serve functions vital to the cell. The more complex genomes, such as those of eukaryotic cells, demand increased levels of chromosomal organization, and this is reflected in the chromosomes’ structural features. We begin by considering the different types of DNA sequences and structural elements within chromosomes. Genes Are Segments of DNA That Code For Polypeptide Chains & RNAs : Our understanding of genes has evolved tremendously over the last century. Classically, a gene was defined as a portion of a...

A)  Genes: Structure, Replication & Mutation STEPS IN PROKARYOTIC DNA SYNTHESIS   When the two strands of the DNA double helix are separated, each can serve as a template for the replication of a new complementary strand. This produces two daughter molecules, each of which contains two DNA strands with an anti parallel orientation . This process is called semi conservative replication because, although the parental duplex is separated into two halves (and, therefore, is not “conserved” as an entity), each of the individual parental strands remains intact in one of the two new duplexes . The enzymes involved in the DNA replication process are template-directed polymerases that can synthesize the complementary sequence of each strand with extraordinary fidelity. The reactions described in this section were first known from studies of the bacterium Escherichia coli (E. coli ), and the description given below refers to the process in pro...

A) Genes: Structure, Replication & Mutation    Nucleic Acid Structure:   The structure and synthesis of purine and pyrimidine nucleotides as you all know.  These nucleotides can be combined to form two types of nucleic acids; Deoxyribonucleic acid(DNA) & Ribonucleic acid( RNA). Ribonucleic acid    is composed of the ribonucleosides of adenine, guanine,   cytosine, and uracil (instead of thymine).  In both DNA and RNA, nucleosides are joined by phosphate groups to form long polynucleotides. Nucleic acids are required for the storage and expression of genetic information. There are two chemically distinct types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid . DNA, the repository of genetic information, is present not only in chromosomes in the nucleus of eukaryotic organisms, but also in mitochondria and the chloroplasts of plants. Prokaryotic cells, which lack nuclei, have a single chromosome, ...

  A) Genes: Structure, Replication & Mutation Concepts 1. The two kinds of nucleic acid, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), differ from one another in chemical composition and structure. In procaryotic and eucaryotic cells, DNA serves as the repository for genetic information. 2. DNA is associated with basic proteins in the cell. In eucaryotes these are special histone proteins, whereas in procaryotes nonhistone proteins are complexed with DNA. 3. The flow of genetic information usually proceeds from DNA through RNA to protein. A protein’s amino acid sequence reflects the nucleotide sequence of its mRNA. This messenger is a complementary copy of a portion of the DNA genome. 4. DNA replication is a very complex process involving a variety of proteins and a number of steps. It is designed to operate rapidly while minimizing errors and correcting those that arise when the DNA sequence is copied. 5. Genetic information is contained in the nuc...