Genes are the building blocks of heredity. All living things inherit the genetic information specifying their structure and function from their parents. Likewise, all cells exist from pre-existing cells and therefore the genetic information has to be replicated and transmitted from one generation to the other at each cell division CITATION Coo00 l 7177 (Cooper, 2000).
The first evidence leading to the identification of DNA as the genetic material came from studies in bacteria. It dates back to 1928 while Fredrick Griffith was trying to find a vaccine against Pneumonia. He had two strains of the bacterium, one pathogenic and one non-pathogenic. He found that when he killed the pathogenic bacteria with heat and then mixed the remains with living bacteria of the non-pathogenic strain, some of the living cells became pathogenic. In addition, the living cells inherited the new trait of pathogenicity by all the descendants of the transformed bacteria. It had become clear that some chemical component of the dead pathogenic cells caused this heritable change even though the identity of the substance was not known. Griffiths work went on to inspire the work of Avery, Maclyn McCarty, and MacLeod. They later went on to identify DNA as the transforming substance, however some scientists were still sceptical (Urry et al.,).
Further evidence that DNA is the genetic material came from Erwin Chargaff. It was known that DNA was a polynucleotide that has three components: a nitrogenous base, a phosphate group and a pentose sugar named deoxyribose. The nitrogenous base could be Adenine, Thymine, Cytosine and Guanine. He then analysed the base composition in a number of organisms and found that it varies from one species to another. He also found that the percentage of Adenine and Thymine bases were roughly equal and that of Cytosine and Guanine bases were as well. This was unexplained until the discovery of the double helix in 1953 by James Watson and Francis Crick. They used an X-ray image that was produced by Rosalind Franklin to come to the conclusion that DNA was helical and made up of two strands. Chargaff’s rule of base equivalence was explained by this (Urry et al.,).
A DNA molecule is made up of two strands of nucleotides coiled together into a double-helix shape. Between the nitrogenous bases are hydrogen bonds which stabilize the strand. Another factor that adds to the stability of the DNA molecule is that a purine base always bonds to a pyrimidine base. This means that Adenine always binds with Thymine and Guanine binds with Cytosine. This type of bonding contributes to keeping the diameter of the molecule constant CITATION JOS60 l 7177 (LEDERBERG, 1960) .
DNA is composed of genes and inherited traits are determined by genes. Genes provide the code for making specific proteins but do not make them directly. They do so using RNA, a molecule much like DNA but has Uracil as nitrogenous base instead of Thymine and has a ribose sugar. In order for DNA to direct the synthesis of proteins gene expression needs to happen. Gene expression is divided into two stages: transcription and translation (Urry et al.,).
Transcription has three stages: initiation, elongation and termination. In initiation RNA polymerase binds to the promoter of the DNA strands that begins to unwind. The RNA polymerase also initiates RNA synthesis at the starting point on the template strand. As the RNA polymerase moves down the length of the DNA molecule the RNA strand elongates, this part is called elongation. The final stage in transcription, termination, is where the RNA transcript is released and the RNA polymerase is detached. The RNA strand reads only one strand of DNA and will be complimentary to it. The RNA is a “transcript” of the gene’s instructions. The genes determine the sequence of the nucleotide bases along the RNA molecule that is being made. The RNA molecule is called mRNA (messenger RNA) because it contains the genetic message from the DNA, which serves as a template. The mRNA leaves the nucleus and enters the cytoplasm. The next stage, translation, is where the mRNA molecule has to be decoded and translated into a sequence of amino acids that makes up a polynucleotide chain. The first step in translation is when a small ribosomal sub-unit binds to mRNA and an initiator tRNA. The start codon, AUG, indicates the start of translation and codes for the amino acid methionine. The first tRNA enters the P-site of the ribosome and the rest that follows enters the A-site. Each tRNA will be carrying an amino acid and has an anticodon that is complimentary to the codon on the mRNA strand. The amino acids bind in a specific order, according to mRNA and that forms whichever protein is needed. When the polypeptide chain is complete a release factor that is shaped like a tRNA will enter the A-site and the polypeptide chain will be released. Transcription and translation happen in all organisms (Urry et al.,).
Living organism are distinguished by their ability to produce their own kind. In order for this to happen DNA must first be replicated. DNA replication occurs in the S-Phase of interphase during the cell cycle. It is important for reproduction, growth and repair. Before DNA is replicated it has to “unzip” by breaking the hydrogen bonds between bases. This creates two single strands with the help of the enzyme Helicase. Helicase makes the DNA into a “Y” shape, creating a replication fork- this region is the template for replication. The two DNA strands are anti-parallel and run in opposite directions. The one strand runs in the 5′ to 3′ direction (lagging strand) while the second one runs in the 3′ to ‘5 direction (leading strand). DNA can only be synthesized in the 5′ to 3’direction. The two strands are therefor replicated with different processes to accommodate for the different direction. The leading strand is synthesized continuously and is the easiest to replicate. An RNA Primase enzyme lays down an RNA primer which binds to the 3’ end of the strand at the origin of replication. This allows DNA polymerase to create a new strand by process of elongation. The polymerase binds to the primer and starts adding base pairs, in the direction of the replication fork, that are complimentary to the strand during replication. The other strand, the lagging strand, begins replication with multiple primers each of which is only several bases apart. The DNA fragments that are added by DNA polymerase are called Okazaki fragments. This strand is synthesized discontinuously since these fragments are not connected. Once both strands are synthesized all the primers are removed and DNA ligase joins the Okazaki fragments together. In the end two DNA molecules are produced. The cell now has double the amount of DNA and can be divided into two daughter cells (Urry et al.,).
DNA replication is extremely important for the preservation of heredity. During the process of DNA replication there are many proof reading mechanisms that detect and corrects mistakes that were made during replication. DNA polymerase is one of these mechanisms. Sometimes errors are not detected and can lead to a mutation. These DNA molecules that have the mutations can undergo replication once again and the mutation will be carried on in the replicated DNA molecule and the offspring of the organism. The progeny of an organism will contain 50% maternal DNA and the other 50% will be paternal. This happens in meiosis, a process in which the chromosome number in an organism is halved due to DNA replicating once and dividing twice. Meiosis forms gametes, reproductive cells with half as much DNA as somatic cells, and progeny are formed when male and female gametes fuse. The individual resulting from the syngamy will have inherited their traits from their parents (Urry et al.,).
In conclusion, genes program protein synthesis with the help from genetic material in the form of RNA. Since all cells exist from pre-existing cells this means that the genetic material has to be transmitted from generation to generation. Meiosis and fertilization plays an important role in the transmission in multicellular, diploid organisms. Single celled organisms often undergo mitosis to produce an entirely new, identical organism to the parent. All of this would not be possible if it weren’t for the double helical structure of DNA and the process of replication (Urry et al.,).
BIBLIOGRAPHY Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Heredity, Genes, and DNA.
LEDERBERG, J., 1960. A View of Genetics. 131(3396), pp. 269-276.
Lisa A. Urry, Micheal L. Chain, Steven A. Wasserman, Peter V. Minorsky, Jane B. Reece, Neil A. Campbell. Campbell Biology. 11th ed.