Transcription & translation

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 uncoiling 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 involved in the formation of the RNA strand and the uncoiling of the double helix. The RNA strand then elongates 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 which 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 composed of codons and transfer RNA which has a triplet of bases called the anticodon. The first stage of translation is the binding of messenger RNA to the small subunit of the ribosome. The transfer RNA’s have a specific amino acid attached to them which corresponds to their anticodons. A transfer RNA molecule will bind to the ribosome however it’s 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 detaches 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 a wide range 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 which 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.