Genome

•The genomes of most eukaryotes are larger and more complex than those of Prokaryotes

•However, the genome size of many eukaryotes does not appear to be related to genetic complexity.

•For example, the genomes of salamanders and lilies contain more than ten times the amount of DNA that is in the human genome, yet these organisms are clearly not ten times more complex than humans

•And although the genome of the small plant Arabidopsis thaliana is only about 5% the size of the human genome, it contains approximately 26,000 proteincoding genes, compared with 20,000 in the human genome.

•In molecular terms, a gene can be defined as a segment of DNA that is expressed to yield a functional product, which may be either an RNA (e.g., ribosomal and transfer RNAs) or a polypeptide.

•coding sequences (called exons) are separated by noncoding sequences (intervening sequences, or introns)

•many introns encode functional products, which may be proteins or noncoding RNAs. In these cases (called nested genes) one gene is contained within an intron of a larger gene

DNA

•The classical principles of genetics were deduced by Gregor Mendel in 1865

•a pair of inherited factors, which are now called genes. One gene copy (called an allele)

•Identification of DNA as the genetic material

•Chromosomes contain proteins as well as DNA, and it was initially thought that genes were proteins

•Griffith’s experiment: The experiments that defined the role of DNA were derived from studies of the bacterium that causes pneumonia (Pneumococcus).

•Thus a substance in the S extract (called the transforming principle) was responsible for inducing the genetic transformation of R to S bacteria.

•In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty established that the transforming principle was DNA

•both by purifying it from bacterial extracts and by demonstrating that the activity of the transforming principle is abolished by enzymatic digestion of DNA—but not by digestion of proteins

•But not accepted by all

•Harshy and Chase – DNA as genetic material= experiment on Bacteriophages

•The nucleic acids—DNA and RNA—are the principal informational molecules of the cell. Deoxyribonucleic acid (DNA) has a unique role as the genetic material, which in eukaryotic cells is located in the nucleus

•Different types of ribonucleic acid (RNA) participate in a number of cellular activities. Messenger RNA (mRNA) carries information from DNA to the ribosomes it serves as a template for protein synthesis.

•Two other types of RNA (ribosomal RNA and transfer RNA) are involved in protein synthesis

•DNA and RNA are polymers of nucleotides, which consist of purine and pyrimidine bases linked to phosphorylated sugars

•DNA contains two purines (adenine and guanine) and two pyrimidines (cytosine and thymine)

•Adenine, guanine, and cytosine are also present in RNA, but RNA contains uracil in place of thymine.

•The bases are linked to sugars (2′-deoxyribose in DNA, or ribose in RNA) to form nucleosides

•Nucleotides additionally contain one or more phosphate groups linked to the 5′ carbon of nucleoside sugars.

•The polymerization of nucleotides to form nucleic acids involves the formation of phosphodiester bonds between the 5′ phosphate of one nucleotide and the 3′ hydroxyl of another

•Oligonucleotides are small polymers containing only a few nucleotides; the large polynucleotides that make up cellular RNA and DNA may contain thousands or millions of nucleotides, respectively

Harshy and Chase experiment:

Our understanding of the three-dimensional structure of DNA, deduced in 1953 by James Watson and Francis Crick, has been the basis for present-day molecular biology.

Solenoid model :

•The genomes of prokaryotes are contained in single chromosomes, which are usually circular DNA molecules.

•In contrast, the genomes of eukaryotes are composed of multiple chromosomes, each containing a linear molecule of DNA.

•The DNA of eukaryotic cells is tightly bound to small basic proteins (histones) that package the DNA in an orderly way in the cell nucleus

•The complexes between eukaryotic DNA and proteins are called chromatin, which typically contains about twice as much protein as DNA. The major proteins of chromatin are the histones—small proteins containing a high proportion of basic amino acids (arginine and lysine) that facilitate binding to the negatively charged DNA molecule.

•There are five major types of histones— called H1, H2A, H2B, H3, and H4—which are very similar among different species of eukaryotes