Protein Synthesis and Mutations 1.5 Understand that a gene is a length of DNA containing a sequence of bases that code for a specific protein. What is a gene? • A gene is a small section of DNA that determines a particular feature. Genes determine a person's characteristics by instructing cells to produce certain proteins. • Genes code for a sequence of amino acids, which when combined, form a specific protein. • Amino acids are molecules that attach by peptide bonds to form proteins. • A protein is a polymer made up of a long chain of amino acids. A protein can also be called a polypeptide chain. • This chain folds up into a specific shape, determined by the order of the amino acids. This is what determines the structure and function of each specific protein. • A codon (also known as a triplet) is a sequence of three nucleotides in a DNA or RNA molecule that codes for a specific amino acid. A codon of DNA bases codes for one amino acid. A protein is a chain of amino acids, folded into the correct shape. Why do we need proteins? • They are critical to most of the work done by cells and are required for the structure, function and regulation of the body’s tissues and organs. • Proteins are needed to repair body cells and tissues including recovery after illness or injury. • They produce enzymes that are needed for digestion. • Produce hormones that control body functions. • Also, can be a secondary source of energy. 1.6 Know that RNA is a second type of Nucleic acid that has the following features; Single stranded, Contains ribose, Contains uracil That it is used to take information from DNA in the nucleus to the ribosomes for the synthesis of proteins. RNA – Ribonucleic Acid • Similar to DNA as they are both made up of Nucleotides. However the sugar in RNA is different to the sugar in DNA. The sugar in RNA is Ribose. • RNA is single stranded and therefore does not form the double helix structure. • RNA does not have the base Thymine, instead this is replaced with the base Uracil. Uracil will still bond with Adenine as its complimentary base pair. • There are two types of RNA that are involved in the process of protein synthesis which means making proteins. • Messenger RNA = mRNA • Transfer RNA = tRNA Protein Synthesis 1.8 Describe Protein Synthesis as: Transcription – The formation of mRNA in the nucleus - The transfer of mRNA to ribosomes in the cytoplasm. - Translation of the genetic code by tRNA from mRNA codons. - The formation of a polypeptide chain using amino acids. • Protein synthesis is the process of creating more proteins that the body needs to function. • There are two key steps in protein synthesis which are called; Transcription and Translation. Transcription • The first step of transcription is the unzipping of the double helix structure of the DNA by breaking the weak hydrogen bonds between the base pairs. This exposes both strands so that a copy of one can be made. • The enzyme RNA polymerase attaches to the DNA in a non-coding region just before the gene. • RNA polymerase moves along the DNA strand. Free RNA nucleotides form hydrogen bonds with the exposed DNA strand nucleotides by complementary base pairing to form a strand of Messenger RNA (mRNA): • REMEMBER - RNA nucleotides contain the same bases as DNA, except that Thymine (T) is replaced by Uracil (U). So Uracil (U) will bond with Adenine (A). • Because the opposite base bonds with the exposed DNA bases, the strand of mRNA is an opposite copy of the DNA strand (except that U replaces T). We call this a complementary copy. • The newly formed strand of mRNA is now ready to leave the nucleus and travel to the ribosome. Translation • After transcription, the mRNA leaves the nucleus, moves into the cytoplasm and binds to the ribosome. The strand passes through the ribosome. • Then Transfer RNA (tRNA) molecules that are complementary to the mRNA arrive at the ribosome. • The bases on the mRNA are read in three's (codon) and code for specific amino acids. • tRNA molecules transport specific amino acids to the ribosome which they leave behind shortly after lining up opposite the mRNA. Because there are three mRNA bases for each tRNA molecule, we call these the Anti codons. • Used tRNA molecules exit the ribosome and collect another specific amino acid. • The amino acids then bond with each other and polypeptides are formed. After translation, the polypeptide is finally folded into the correct shape and becomes a protein. Peptide bonds form between the adjacent amino acids to finalise the structure. Transcription Translation Mutations 1.7 Understand that a DNA mutation involves a change in the sequence of bases that could lead to a change in the amino acid sequence and phenotype of an individual. Mutations • Mutations are rare, random changes in the base sequences of DNA that can be inherited. • They can lead to alterations in the polypeptide chain which can lead to non functional proteins or can lead to a change in the phenotype. • This could also be problematic in DNA that codes for structural proteins. If a sequence for a key structural protein is altered, it may lead to big problems in the body. • Mutations in non-coding DNA can also cause problems as they can interfere with and stop the transcription of DNA. Hence, they can alter how genes are expressed. Phenotype: The observable characteristics in an individual resulting from the expression of genes Types of Mutations • Sometimes, when DNA is replicating, mistakes are made and the wrong nucleotide is used. • The result is a gene mutation, which can change the sequence of the bases in a gene. • This can lead to the gene coding for the wrong amino acid in a protein. • There are four ways in which gene mutations can occur; Duplication, deletion, substitution and inversion. Duplication In duplication, the nucleotide is inserted twice rather than once. This will alter the entire base sequence because each triplet after the point where the mutation occurred is changed. This leads to the whole gene being different and will code for a different protein. Deletion When a deletion mutation occurs, a whole nucleotide is missed out. This will change the entire base sequence. Each triplet after the mutation is altered and the whole gene is different. This means the gene will now code for a different protien. Substitution In a substitution mutation, a different nucleotide is used. The triplet of bases where the mutation occurred is changed and this might code for a different amino acid. If it does the structure of the protein molecule will be different. This may produce a significant change in how the protein functions, or it may stop the protein from functioning at all. However, most amino acids have more than one triplet code, so the new triplet may not code for a different amino acid and therefore could mean that the protein will have its normal structure and function. Inversion In an inversion mutation, the sequences of the bases in a triplet are reversed. The effects of this mutation are similar to those of substitution because only one triplet is affected which means it may or may not result in a different amino acid and altered protein structure. Mutations • Mutations that occur in body cells, such as the heart, intestines or skin, will only affect the particular cell in which they occur. • If the mutation is very harmful, the cell will die, and the mutation is lost. • If the mutation doesn't significantly affect the functioning of the cell, the cell may not die and then when the cell divides, a group of cells containing the mutant gene will form. • When the person dies, the mutation will be lost. • Mutations can only be passed onto the next generation if they occur in the gametes (sex cells) or in the cells that divide to form gametes. • This is how genetic diseases begin.
