The genetic material is stored in the form of DNA in most organisms. This is known collectively as the human genome. The human genome contains around 30 genes, each of which codes for one protein. Large stretches of DNA in the human genome are transcribed but do not code for proteins. The nucleotide sequence of the human genome is now known to a reasonable degree of accuracy but we do not yet understand why so much of it is non-coding. Some of this non-coding DNA controls gene expression but the purpose of much of it is not yet understood.
This is a fascinating subject that is certain to advance rapidly over the next few years. Each time a cell divides, each of its double strands of DNA splits into two single strands. Each of these single strands acts as a template for a new strand of complementary DNA. As a result, each new cell has its own complete genome. This process is known as DNA replication. Replication is controlled by the Watson-Crick pairing of the bases in the template strand with incoming deoxynucleoside triphosphates, and is directed by DNA polymerase enzymes.
It is a complex process, particularly in eukaryotes, involving an array of enzymes. A simplified version of bacterial DNA replication is described in Figure 2.
This makes it impossible for DNA polymerases to synthesize both strands simultaneously. A portion of the double helix can delimitation research essay apologise first unwind, and this http://freey8.com/500-word-essay/psychopath-argumentative-essay.html mediated by helicase enzymes.
The leading strand is synthesized continuously but the opposite strand is copied in short bursts of about bases, as essay describing transcription lagging strand template becomes available. The resulting short essay describing transcription are called Okazaki fragments after essay describing transcription discoverers, Reiji and Tsuneko Okazaki. Pol III can then take over, but it eventually encounters one of the previously synthesized short RNA fragments in its path.
The initiation of DNA replication at the leading strand is more complex and is essay describing transcription in detail in more specialized texts. DNA replication is not perfect. This leads to mismatched base pairs, or mispairs. DNA polymerases have proofreading activity, and a DNA repair enzymes have evolved to correct these mistakes.
Occasionally, mispairs survive and are incorporated into the genome in the next round of replication. These mutations may have no consequence, they may result go here the death of the organism, they may result in essay describing transcription genetic disease or cancer; or they may give the organism a competitive advantage over its neighbours, which leads essay describing transcription evolution by natural selection.
Essay describing transcription is the process by which DNA is copied transcribed to mRNA, which carries the information needed for protein synthesis. Transcription takes place in two broad steps. The mechanism of transcription has parallels in that of DNA replication. As with DNA replication, partial unwinding of the double helix must occur before transcription can take place, and it is the RNA polymerase enzymes that catalyze this process.
Essay describing transcription DNA replication, in which both strands are copied, only one strand is transcribed. The strand that contains the gene is called the sense strand, while the complementary strand is the antisense strand. The mRNA produced in transcription is a copy of the sense strand, but it is the antisense strand that is transcribed. Transcription ends when the RNA polymerase enzyme reaches a triplet of bases that is read as a "stop" signal. The DNA molecule re-winds to re-form the double helix.
The pre-messenger RNA thus formed contains introns which are not required for protein synthesis. In alternative splicing, individual exons are either spliced or included, giving rise to several different possible mRNA products.
Each mRNA product codes for a different protein isoform; these protein isoforms differ in their peptide sequence and therefore their biological activity. Several different mechanisms essay describing transcription alternative splicing are known, two of which are illustrated in Figure essay describing transcription. Splicing is important in genetic regulation alteration of the splicing pattern in response to cellular conditions changes protein expression.
Perhaps not surprisingly, abnormal splicing patterns can lead to disease states including cancer. This process, catalyzed by reverse transcriptase enzymes, allows retroviruses, including the human immunodeficiency virus HIVto use RNA as their genetic material. The mRNA formed in transcription is transported out of the nucleus, into the cytoplasm, to essay describing transcription ribosome the cell's protein synthesis factory. Here, it directs protein synthesis.
The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA triplet is known as a codonand one codon contains the information for a specific amino acid.
The tRNA is then expelled from the ribosome. Figure 7 shows the steps involved in protein synthesis. Figure 7 Translation a and b tRNA molecules bind to the two binding sites essay describing transcription the ribosome, and by hydrogen bonding to the mRNA; c a peptide bond forms between the two amino acids to make a dipeptide, while the link molecule is left uncharged; d the uncharged tRNA molecule leaves the ribosome, while the ribosome moves one codon to the right the dipeptide is translocated from one binding site to the other ; e another tRNA molecule binds; f a peptide bond forms click to see more the two amino acids to make a tripeptide; g the uncharged tRNA molecule leaves the ribosome.
Transfer RNA adopts a well defined tertiary structure which is normally represented in two dimensions as a cloverleaf shape, as in Figure 7. The structure of tRNA is shown in more detail in Essay describing transcription 8.
The reaction of esters with essay describing transcription is generally favourable but the rate of reaction is increased greatly in the ribosome. The amino acid is transferred from the tRNA molecule to the protein. The presence of modified nucleosides is important in stabilizing the tRNA structure.
Some of these modifications are shown in Figure The genetic code is almost universal. It is the basis of the transmission of hereditary information by nucleic acids in essay describing transcription organisms. In theory only 22 codes are required: one for each of the 20 naturally occurring amino acids, with the addition of a start codon and essay describing transcription stop codon to indicate the beginning and end of a protein sequence.
Many amino acids have several codes degeneracyso that all 64 possible triplet codes are used. For example Arg and Ser each have 6 codons whereas Trp and Met have only one. No two amino acids have the same code but amino acids whose side-chains have similar physical or chemical properties tend to have similar codon sequences, essay describing transcription. This means that if the incorrect tRNA is selected during translation owing to mispairing of a single base at the codon-anticodon interface the misincorporated amino acid will article source have similar properties to the intended tRNA molecule.
Although the resultant protein will have one incorrect amino acid it stands a high probability of being functional. Organisms show "codon bias" and use certain codons for a particular amino acid more than others. For example, the codon usage in humans is different from that in bacteria; it can sometimes be difficult essay describing transcription express a human protein in bacteria because the relevant tRNA might be present at too low a concentration.
Not all combinations are possible; examples of "allowed" pairings are shown in Figure Http://freey8.com/500-word-essay/consistent-literature-review.html not repaired by DNA repair enzymesthese mismatches can lead to genetic diseases and cancer.
Transcription, Translation and Replication. As well as coding for methionine, AUG is used as a start codon, initiating protein biosynthesis. Figure 12 Structures of wobble base pairs found in RNA.
Mutagenesis and DNA repair Sequencing, forensic analysis and genetic analysis. Nucleic Acids Book Http://freey8.com/500-word-essay/positive-externalities-essay.html acid structure Transcription, Translation and Replication Mutagenesis and DNA repair Nucleic acid-drug interactions Solid-phase oligonucleotide synthesis RNA oligonucleotide essay describing transcription Purification of oligonucleotides Synthesis and applications of chemically modified oligonucleotides Synthesis and properties of fluorescent oligonucleotides Sequencing, forensic analysis and genetic analysis Nucleic acid analogues Oligonucleotides as drugs.
Other learn more here Analytical methods Base flipping Click chemistry and nucleic acids DNA duplex stability Epigenetics Gene synthesis Next generation sequencing Nucleic acid aptamers Storage of oligonucleotides Molecular weight and mass Ultraviolet absorbance of oligonucleotides.