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Historical Aspects
One of the most important questions which occupied the early molecular biologists was the mechanism by which the hereditary traits could be passed from one generation to another. In one experiment, Hammeling a Danish scientist could show in 1930 that the hereditary material is stored in the nucleus. He used a green alga called acetabularia mediterrania.
When the cap was cut off , a new cap was generated from the stalk and stem, however when the foot was removed no new foot was generated. In a second experiment he used a different strain - acetabularia crenulata - which has a flower like cap. When the stem of acetabularia crenulata, was crafted on the foot of acetabularia mediterrania the cap which developed was disc like. An obvious conclusion from these experiments was that the hereditary material was stored in the nucleus.

In the 1930ies by Griffith, who showed that streptococcus pneumonia occurs in two strains, strain S is the virulent strain, while strain R does not cause pneumonia. Mice injected with a mixture of live R strain and dead S strain developed pneumonia. Thus the DNA of the dead strain must have conjugated with the DNA of the live strain to make it virulent.

Another important experiment was done by Hershey and Martha Chase. T2-phage a bacterial virus, attaches itself to a bacterial cell and injects DNA into the cell. The viral DNA is replicated by the bacterial cell and expressed into proteins which are assembled into more viruses. Hershey and Chase could show that if a P-32 (beta rad., 1.71 MeV) labeled virus infected a cell, the label was found in the bacterial cell. However, if a S-35 (beta rad., 0.167 MeV) labeled virus infected the cell the label was found in the "empty" phage. Since the P-32 labels nucleic acids and S-35 labels protein, the researchers could establish that the hereditary material is DNA and not protein.

DNA

Every organism including microbes, plants and animals transmit their hereditary information in form of distinct units to the next generation. The material which contains this information is called nucleic acids. There are two types of nucleic acids: DNA and RNA. Most organisms store their hereditary information in the sequence of DNA bases with the exception of retro viruses, which use RNA instead of DNA. DNA or RNA is a polymer made from repeating units of nucleotides. Ribonucleotides, which make up RNA consist of ribose, a pyrimidine or purine base and phosphate. The repeating units of DNA are deoxyribonucleotides, which consist of deoxyribose, a pyrimidine or purine base and phosphate.

DNA Structure
Please review the structure of DNA and RNA as shown in your textbook.
DNA bases     DNA, RNA components     DNA strands (parallel)     DNA strands antiparallel     GC pair     ATpair     base pairing     double helix     F.C, J.W.    Nature Article    

Mechanism of DNA replication

Replication of DNA begins at a replication origin. Bacterial and viral DNA have only one replication origin, whereas eukaryotic DNA has many sites where DNA synthesis begins simultaneously. Replication origins are spaced between 30,000 to 300,000 nucleotides form each other. The first step in DNA synthesis is the unwinding of the double helix by a protein called helicase. DNA replication begins with the synthesis of a short segment of RNA molecules along the 3’ to 5’ and 5’ to 3’ strands of the DNA to be replicated. The newly synthesized DNA strands grows from the 5' to the 3' direction image. The enzyme which catalyzes this step is called primase and the short RNA segment is called primer. The DNA replication of the 3’ to 5’ strand starting from the RNA primer is catalyzed by a DNA polymerase . The newly synthesized strand ( called the leading strand) grows continuously from its 5’end to its 3’ end toward the replication fork
The replication of the 5' to 3’ strand is more complicated. Since DNA synthesis is always in the direction from the 5’ end of the newly synthesized strand (lagging strand) to its 3’ end, the replication proceeds in opposite direction from the replication fork, i.e. away from the replication fork. Thus, short segments (called Okazaki fragments) are synthesized in a discontinuous manner. Eventually the RNA primers are removed by an enzyme, replaced by DNA and the Okazaki fragments are joined together by a ligase.

Please review the practical applications of DNA replication such as PCR.

Genome: All DNA of a species is called the genome. Nuclear DNA is called nuclear genome. Genomes are separated into linear DNA segments which are called chromosomes. Each chromosome has a certain number of genes which have coding regions for m, r, t-RNA and non coding regions. Lower eukaryotic organisms have approximately 5000 coding genes, whereas complex species have about 60000 coding genes.

Chromosomes (electronmicrograph of chromosomes )

Human DNA is organized into long linear strands called chromatin (Structure of Chromatin ). During cellular division chromatin assumes the characteristic shape of chromosomes. Normal metabolizing cells or somatic cells in humans have 46 chromosomes, with 22 chromosomes having nearly identical copies of each other, called homologous chromosomes. In females chromosome 23 consists of two nearly identical copies called X-chromosome. In males chromosome 23 consists of a X-chromosome and a Y-chromosome. The shapes and size of the human chromosomes are best seen during the metaphase of the cell growth cycle ( karyo-typing ). The number of chromosomes can vary, depending on the species.

Cell Cycle

The process of mitosis is the basis for growth and repeated cellular division of the somatic cells. This asexual reproduction is also the mechanism by which bacteria reproduce.

Gap 1 phase (G1-phase) Most of the time the cell engages in metabolism and growth in a period called gap 1 (G1) which follows cellular division. During the G1 phase no DNA synthesis occurs, i.e. no cellular replication. A G1 phase can last from a few minutes to weeks or years, or a cell can be arrested in the G1 phase and never replicate again. For example most nerve cell of the human body are in a G1 arrest, whereas embryonic cells complete a cell cycle in less than 24 hours.
DNA synthesis phase (S-phase) During the S-phase all nuclear DNA is replicated. That means each of the 46 chromosomes will be replicated. The schematic diagram below shows the replication of one chromosomal pair (remember, each chromosome has a nearly identical copy, except chromosome 23 in males)

The central region of the chromosome is called centromer. This is also the region where the chromosome is joined with its newly synthesized copy. Before synthesis there were 46 chromosomes, after replication the number of chromosomes is still reported as 46, since only centromers are counted. The number of centromers has not changed. Pairs of joined chromosomes are called sister chromatids and they have identical genetic information since one has been copied from the other one. In a typical eukaryotic cell, the S-phase last 8 hours.

fertilization     haploid, diploid     fern life cycle