Introduction to protein synthesis Some key aspect to learn and understand A Protein is a polymer which consists of monomers (repeating building blocks), called amino acids. A protein consists of 50 or more amino acids. Every protein is synthesised in accordance with instructions contained in DNA The new proteins will have structural and functional roles in cells. Other substances which are also examples of proteins are amongst others enzymes and hormones. Life cannot exist without proteins, enzymes, hormones and other proteins.
Introduction to protein synthesis Each cell’s DNA can be seen as a book of protein-building instructions. The alphabet used in this book is simple A, T, G and C. The DNA molecules for the formation of proteins occurs in genes on chromosomes. Memorise the matching of these bases and between A and U (RNA). It takes two steps, transcription and translation, to carry out a gene’s protein-building instructions.
Overview of protein synthesis Transcription (takes place in nucleus) DNA serves as template to assemble a single strand of mRNA from the cell’s pool of free nucleotides. Afterward, RNA moves into cytoplasm Translation (cytoplasm and ribosome) mRNA & tRNA control the assembly of amino acids into proteins Summary: DNA RNA Protein transcription translation
Process of transcription The enzyme polymerase controls the process. The DNA unwinds and unzips by breaking the weak hydrogen bonds between the complementary bases. One of the strands of the unwound DNA acts as a template for the formation of the complementary strand of mRNA by using the free nucleotides in the cytoplasm. A nucleotide with base C on the template will link the nucleotide with G on the mRNA. Adenine on DNA template links nucleotide with Uracil in mRNA.
Diagram to show transcription DNA unwinds and unzips. Complementary mRNA strand (Blue strand) forms.
DNA molecule illustrates two genes (each of them with their own codons on two different mRNAs)
Transcription (Cont.) If the DNA base triplet sequence is GGT, the mRNA codon (sequence of three bases) will be CCA. The sugar-phosphate bonds form to link the mRNA nucleotides together. This forms a single strand with the nucleotides in the correct sequence. Once the mRNA has formed,it moves away from the DNA template and leaves the nucleus via the nucleus membrane pore. The mRNA carries the ‘message’ /codons to the ribosome.
Forming and movement of mRNA From the DNA template a mRNA is formed in the nucleus Single strand mRNA moves through the pores in the nucleus membrane into the cytoplasm of a cell
Process of translation Translation means to change something into another form. The code is translated into a protein. The mRNA strand becomes attached to a ribosome with the codons exposed. Each tRNA molecule carries one particular amino acid from the pool of amino acids in the cytoplasm to the ribosome. Here, the tRNA’santicodon matches up with the codon on the mRNA so that the amino acids are placed in the correct sequence.
Process of translation (cont.) The amino acids were added to the growing chain of amino acids. Peptides bonds are form to link adjacent amino acids in the correct sequence (according to the codons on the mRNA). Proteins consist of 50 or more amino acids.
Process of translation (Cont.) • Once a tRNA has placed its amino acid into the correct sequence, the tRNA molecule is released from the ribosome. • The tRNA molecule will then pick upanother amino acid of the same type to continue the process until the complete protein has been formed. • The chain of amino acids elongates one amino acid at a step until the last particular codon, specifying ‘stop’ codon which indicate that the chain of amino acids is completed. • The protein is released from the ribosome.
Amino acid Growing part of protein Peptide bond Incoming tRNA with a particular amino acid attached Outgoing tRNA without an amino acid Anticodon mRNA 4 Codons Ribosome Diagram illustrating the translation process
Drawings to illustrate the complete protein synthesis process Write the anticodons for the four codons on the mRNA. Translation (3 & 4) Transcription (1 & 2) Use these drawings to describe the whole process. Underline the key words in your four paragraphs. Memorise the key words.
You don’t have to memorise the combinations of triplets. Use the matching bases of DNA and RNA to determine the triplets. Complete the following table to show the 3 bases in each case TGC UGC AUA ATA GCA GCA TGT UGU Memorise the following: Pairing bases of DNA: A with T or T with A and C with G or G with C Pairing bases of RNA: A with U or U with A and C with G or G with C
‘Stop’-codons (interesting information) Codons UAA, UAG en UGA are ‘stop’ kodons. These codons don’t have tRNS molecules. They terminate the formation of a specific protein and the protein is released from the ribosome.