OUTCOMES PROTEIN SYNTHESIS IB Biology Core Topic 3.5 Transcription and Translation 3.5.1 Compare the structure of RNA and DNA. 3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase. 3.5.3 Describe the genetic code in terms of codons composed of triplets of bases. 3.5.4 Explain the process of translation, leading to polypeptide formation. 3.5.5 Discuss the relationship between one gene and one polypeptide. OVERVIEW ANIMATION http://learn.genetics.utah.edu/content/begin/dna/f irefly/ CONTEXT Genes are instructions on how to make proteins. Proteins are made in two steps: 1. Transcription The gene information is made into an mrna messenger that goes to a ribosome 2. Translation The ribosome translates the messenger into a sequence of amino acids that will join together to make a functioning protein RIBONUCLEIC ACID (RNA) RNA is single stranded RNA uses the sugar Ribose RNA uses the base Uracil instead of Thymine
STRUCTURES OF DNA AND RNA DNA Contains the 5 carbon sugar deoxyribose Contains thymine Is in the shape of a double helix There is only 1 type of DNA DNA can t leave the nucleus RNA Contains the 5 carbon sugar ribose Contains uracil instead of thymine Is a single strand There are 3 types of RNA: m-rna, t-rna, r-rna RNA leaves the nucleus DNA TRANSCRIPTION Transcription is the process by which a strand of RNA is produced from a DNA template. As seen above, DNA cannot leave the nucleus; it is too valuable. Instead, it makes a copy of its code in the form of mrna which can leave the nucleus. The mrna can carry the information needed (genetic code) from the nucleus out into the cytoplasm and to the ribosome where the polypeptide or protein will be formed. DNA is much longer RNA is shorter strands TRANSCRIPTION Transcription is the process by which the base sequence of DNA is copied into an mrna molecule. Transcription involves the following steps which are carried out by the enzyme RNA polymerase. 1. The double helix structure of the DNA molecule unwinds and the two strands separate. This means the hydrogen bonds between the nitrogen bases must be broken. 2. One of the sides of the DNA strand is used as a template to build the mrna strand using free floating nucleotides. Transcription occurs in one direction (5 to 3 ).
3. The nucleotides of the new mrna strand are bonded together and then the mrna separates from the strand of DNA. The hydrogen bonds will form again between the complementary base pairs in the DNA strand and it will return to its double helix form. 4. The newly formed mrna strand is identical to the complementary side of the DNA template strand except uracil replaces thymine. THE GENETIC CODE The genetic code is located on strands of DNA and it is read in triplets known as codons. Three nitrogen bases code for a single amino acid and this triplet of bases is known as a codon. There are 64 codons in total.
TRANSLATION The process of translation involves reading the code on the mrna and using that code to build a polypeptide. Translation takes place inside the ribosomes of eukaryotic cells. Translation also involves another type of RNA known as trna or transfer RNA. This molecule is located in the cytoplasm of the cells and it contains a special triplet of bases known as the anticodon. The anticodon allows the trna to carry a specific amino acid to the ribosome where it is needed in the synthesis of the polypeptide. The mrna that has been produced during transcription binds with the small subunit of the ribosome. The mrna is carrying the genetic code in a sequence of codons which each code for a particular amino acid. The attachment of the mrna to the small subunit brings in a trna molecule that binds with the complementary codon on the mrna. Then the larger subunit of the ribosome attaches forming a functional ribosome. The ribosome begins reading (translating) the genetic code of the mrna and the corresponding trna molecules will bind to the ribosome in one of two sites located inside the ribosome. The trna molecule will only bind if it carries the anticodon that is complementary to the codon being read on the mrna. The trna molecules are carrying the amino acids needed in the formation of the polypeptide. The two amino acids inside the ribosome form a peptide bond. The first trna molecule (in the p site) will detach and leave the ribosome. The second trna molecule (in the a site) will shift over making room for the next trna molecule and its corresponding amino acid.
The ribosome continues moving down the strand of mrna and each time it reads a new codon a new amino acid is attached to the polypeptide until the entire polypeptide is formed. INITIATION Once the polypeptide is complete (a stop codon has been read) the ribosome breaks into its subunits and releases the mrna and the newly formed polypeptide. ELONGATION TERMINATION LET S TRY IT TOGETHER... http://learn.genetics.utah.edu/content/begin/dna/t ranscribe/
GENES AND POLYPEPTIDES Polypeptides, also known as proteins, are long chains of amino acids that can be differentiated by their particular sequence. Each polypeptide must be linked in a precise sequence to form the correct protein and the code for this sequence is located in a sequence of bases on genes. Genes store the code for each polypeptide and the information stored in that code is copied and decoded during the processes of transcription and translation. ONE GENE ONE POLYPEPTIDE? The one gene, one polypeptide hypothesis that has been taught for years suggests that only one gene could produce a specific protein. However recent research, particularly that of the human genome project, has shown us that some genes do not work that way. Some genes code for a single strand of mrna which can then be modified in several different ways resulting in several different possible proteins.