Protein Synthesis Function of DNA • The bases in the DNA molecule act as a code • Each triplet of bases code for one amino acid • Definitions: codon – a triplet of bases Amino acids – basic building blocks of proteins • DNA cannot leave the nucleus of the cell – how does the DNA code get from the nucleus to the ribosomes in the cytoplasm where proteins are made? • The code of DNA is transferred to RNA (ribonucleic acid) • RNA “carries” the message outside the nucleus to the ribosomes where the proteins are made • Differences between DNA and RNA ! RNA is single stranded ! Ribose sugar ! Uracil replaces thymine ! Travels out of nucleus ! Three types of RNA Three types of RNA • Ribosomal RNA (r-RNA) Combines with protein to make ribosomes • Messenger RNA (m-RNA) Carries instructions for protein synthesis from DNA to ribosomes • Transfer RNA (t-RNA) - A short molecule with three exposed nitrogen bases which are complementary to the codons on the mRNA. These three exposed bases are called anticodons. - The function of t-RNA is to carry the amino acids from the cytoplasm to the mRNA for assembly in the proper sequence. Protein Synthesis – the process Each triplet or codon on the DNA and then on the RNA codes for a particular amino acid There are twenty different amino acids which make up all the different proteins in our body For most amino acids, there are several codons - see table page 637 Once the amino acids have been brought to the ribosome in the correct sequence according to the mRNA, each one is bonded to the next with a peptide bond. This bond is formed by removing a molecule of water from two amino acids in a process called dehydration. Many amino acids bonded together are often called a polypeptide molecule. Along with the codons for the amino acids, there are also codons to start the protein (initiator codons) and to stop the protein (terminator codons) Any errors in the original DNA, may cause errors in the mRNA and ultimately in the sequence of amino acids which are formed. The change of one amino acid in a protein can cause the protein to be faulty and not work properly. These mistakes are called gene mutations and will be discussed later in the course in more detail. Steps in Protein Synthesis 1. Transcription of m-RNA from DNA 2. Translation of m-RNA into amino acid sequence 1. Transcription of RNA: a. An enzyme called RNA polymerase cause the DNA molecule to be opened up and read. b. RNA nucleotides match up with one side of the DNA only and form a single stranded molecule complementary to the DNA c. Once the RNA molecule is formed it is then released from the DNA molecule which then returns to its original structure of a double helix d. All three types of RNA are made using DNA as pattern e. The m-RNA is a relatively long molecule (1 000 to 10 000 nucleotides) f. The t-RNA molecule is a relatively short molecule (75-85 nucleotides) and is folded like a clover leaf). One end of the t- RNA molecule attaches to a particular amino acid and the other end has a group of unbonded bases called the anticodon. Eg. Amino acid asparagine DNA code – TTG m- RNA codon – AAC t-RNA anticodon – UUG 2. Translation of mRNA into proteins a. The mRNA attaches to the ribosome b. The initiator site is read (AUG) c. The t-RNA molecules transport amino acids to the ribosome according to the sequence of bases on the mRNA d. The protein molecule is built with one amino acid after another being added to the polypeptide chain. This process is called elongation. e. At the end of the m-RNA strand is a terminator codon which ends the process f. The protein molecule is now released from the ribosome and is transported wherever it is needed.