The chemistry of life (3) 28/05/2012 08:31:00 3.5.1 Compare the structure of RNA and DNA. DNA and RNA both consist of nucleotides, which contain a sugar, a base and a phosphate group. However there are a few differences. Firstly, DNA is composed of a double strand forming a helix whereas RNA is only composed of one strand. Also the sugar in DNA is deoxyribose whereas in RNA it is ribose. Finally, both DNA and RNA have the bases adenine, guanine and cytosine. However DNA also contains thymine, which is replaced by uracil in RNA. 3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase. DNA transcription is the formation of an RNA strand, which is complementary to the DNA strand. The first stage of transcription is the splitting up of the DNA double helix. Then, the free RNA nucleotides start to form an RNA strand by using one of the DNA strands as a template. This is done through complementary base pairing, however in the RNA chain, the base thymine is replaced by uracil. RNA polymerase is the enzyme used in the formation of the RNA strand and the splitting up of the double helix. The RNA strand then becomes longer and then separates from the DNA template. The DNA strands then reform a double helix. The strand of RNA formed is called messenger RNA. 3.5.3 Describe the genetic code in terms of codons composed of triplets of bases. A triplet of bases (3 bases) forms a codon. Each codon codes for a particular amino acid. Amino acids in turn link to form proteins. Therefore DNA and RNA regulate protein synthesis. The genetic code is the codons within DNA and RNA, composed of triplets of bases that eventually lead to protein synthesis. 3.5.4 Explain the process of translation, leading to polypeptide formation. Translation is the process through which proteins are synthesized. It uses ribosomes, messenger RNA which is made up of codons and transfer RNA that has a triplet of bases called the anticodon. The first stage of translation is the joining of messenger RNA to the small subunit of the ribosome. The transfer RNA’s have a specific amino acid attached to them that links to their anticodons. A transfer RNA molecule will join to the ribosome however its anticodon must match the codon on the messenger RNA. This is done through complementary base pairing. These two form a hydrogen bond together. Another transfer RNA molecule then bonds. Two transfer RNA molecules can bind at once. Then the two amino acids on the two transfer RNA molecules form a peptide bond. The first transfer RNA then separates from the ribosome and the second one takes it’s place. The ribosome moves along the messenger RNA to the next codon so that another transfer RNA can bind. Again, a peptide bond is formed between the amino acids and this process continues. This forms a polypeptide chain and is the basis of protein synthesis. 3.5.5 Discuss the relationship between one gene and one polypeptide. A polypeptide is formed by amino acids liking together through peptide bonds. There are 20 different amino acids so wide ranges of polypeptides are possible. Genes store the information required for making polypeptides. The information is stored in a coded form by the use of triplets of bases that form codons. The sequence of bases in a gene codes for the sequence of amino acids in a polypeptide. The information in the genes is decoded during transcription and translation leading to protein synthesis. 5/28/2012 8:31:00 AM 3.6.1 Define enzyme and active site. Enzymes: Globular proteins which act as catalysts of chemical reactions. Active site: Region on the surface of an enzyme to which substrates bind and which catalyses a chemical reaction involving the substrates. 3.6.2 Explain enzyme–substrate specificity. The active site of an enzyme is very specific to its substrates as it has a very precise shape. This results in enzymes being able to catalyze only certain reactions as only a small number of substrates fit in the active site. This is called enzyme-substrate specificity. For a substrate to bind to the active site of an enzyme it must fit in the active site and be chemically attracted to it. This makes the enzyme very specific to it’s substrate. The enzyme-substrate complex can be compared to a lock and key, where the enzyme is the lock and the substrate is the key. 3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity. Temperature as both positive and negative sides to enzyme activity. When the temperature rises, the speed of reaction increases, bcause heat make shte particles move faster, so there is more collision between the walls therefore the speed of reaction increases, however when the temperatures rise too high, the enzymes may denature and then they will not do the job anymore. The enzymre denature because the high temperatures make the molecule vibrate and split up, so they stop functioning. Enzymes usually have an optimum pH level where the best work at, if the pH goes beyond that, then the enzymes aren’t going to work at their best and most efficient. An increase in substrate concentration causes the rate of reactiong to increase because the enzymes can work on more substrate. However if all active sites re taken, the enzymes will not be enough, so as long as there are active sites, more substrate will increase the rate of reaction. 5/28/2012 8:31:00 AM