AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

MODERN CONCEPT

• The classical concept of the gene started to break down as soon as it had been completely formulated.
• The nature of the genetic material became more accurate when Avery, Macleod and McCarty (1944) demonstrated that the substance causing transformation in bacteria was DNA.
• Support for the DNA theory of inheritance was gained when Alfred D. Hershey and Martha Chase demonstrated that DNA alone was responsible for the multiplication of bacteriophages.
• James D.Watson and Francis H. C. Crick (1953) demonstrated that deoxyribonucleic acid (DNA) is a double-stranded helix of nucleotides.
• Alexander Dounce and George Gamow independently presented the so-called colinearity hypothesis, according to which the linear structure of DNA determines the linear primary structure of a polypeptide.
• The concept of "one gene-one enzyme" hypothesis was proposed by George Beadle and Edward Tatum.
• Advances in biochemical genetics made the one gene-one enzyme hypothesis seem very unlikely and leading to a "one gene-one polypeptide" hypothesis instead.
• Proteins or enzymes are generally aggregates of more than one kind of polypeptides. This hypothesis is correct to a very great extent and applies to all organisms.
• However, it is now known that certain genes also direct the formation of ribosomal and t-RNA molecules and not the formation of proteins.

Structure of DNA

• DNA is a ncleic acid and nucleotides are the building blocks of all nucleic acid molecules. The structural unit consists of
  • Pentose sugar (2’ deoxy ribose)
  • phosphoric acid and
  • four nitrogenous bases: two of these nitrogenous bases are
    • Purines- a double ring structure
      • Adenine (A)
      • Guanine (G)
    • Pyrimidines- a single ring
      • Thymine (T)
      • Cytosine (C)
• In DNA, each base is chemically linked to a deoxyribose sugar, forming a nucleoside. A phosphate group is also attached to the sugar of each nucleoside yielding a nucleotide (a nucleotide is a nucleoside phosphate). It is common to name nucleotides as monophosphates i.e. deoxyadenosine 5’ –monophosphate and abbreviated as dAMP for a adenine containing nucleotides others are named as dGMP, dCMP, and dTMP. The double helix measures 20Aº (2.0 nm) in diameter.
• Two polynucleotide chains twisted around one another forming a rigt handed double-stranded helix in which Adenine pairs only with thymine (complementary base pairing) and Guanine with Cytosine. (The content of purine always equals the contents of pyrimidine, furthermore the amount of adenine and thymine are always equal and so are the amounts of guanine and cytosine-Chargaff’s rule). In the double helix, one polynucleotide strand is in 5’-> 3’ direction and the other is in 3’ -> 5’ direction and hence are said to be anti parallel.
• The nucleotides are joined together to form a polynucleotide chain in which 5’ carbon of one sugar is linked by its phosphate group to the 3’ carbon of the next sugar. (The chemical bonds by which the sugar components of the adjacent nucleotides are linked through the phosphate groups are called phosphor diester bonds). This 5’-> 3’-> 5’->3’ orientation of these linkages continues throughout the chain. In each polynucleotide chain there are a 5’ phosphonyl group (5’-p) at one end and 3’-hydroxyl group (3’-OH) at the other.
• Each chain makes one complete turn every 34 A⁰. The bases are spaced at 3.4 A⁰ such that there are ten base pairs per helical turn.
• Each base is paired to a base in other strand by hydrogen bonds. An AT pair has two hydrogen bonds and a GC pair has three hydrogen bonds.
• The double helix structure of DNA are closer together on one side of the helix than on the other. The major groove occurs where the backbones are far apart, the minor groove occurs where they are close together.

Gene

• A gene is a unit of information and corresponds to a discrete segment of DNA that encodes the amino acid sequence of a polypeptide.
• In higher organisms, the coding sequence of a gene is split into a series of segments called exons which are separated by non coding sequences called introns, which do not contain useful information.
• Introns are removed from RNA transcripts by a process called splicing during protein synthesis.
• Genes vary greatly in size from less than 100 bp to several million bp.

Central Dogma Concept

• The central dogma of genetics describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA -> RNA -> protein.
• This flow of information is unidirectional and irreversible.

Gene Expression

• Gene expression refers to the mechanism by which a gene expresses a phenotype.
• A gene contains the code or plan for a polypeptide chain in the form of a specific sequence of nucleotide bases.
• It transfers its codes to mRNA. It is called transcription.
• The code present in the mRNA is translated in the ribosome with the help to tRNA.
• The tRNA picks up the required amino acids and are linked in specific sequence as per the sequence of nucleotides present in the mRNA.
• The amino acids form a polypeptide chain. It functions as a structural protein or as an enzyme.
• In other words, the polypeptide contributes to the morphological or functional trait of the cell.

 

• The specificity of base pairing is such that each base along one polynucleotide strand of the DNA determines the base in opposite position on the other strand. Hence the sequence of bases along the two strands is complementary in nature.