Cell Protein Production Chapter 3 © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia To learn about the stages of protein synthesis click on the stage names in blue. Transcription Protein Formation Summary Translation © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Transcription Transcription is the process by which the mRNA is formed using the code in a DNA molecule The genetic instructions are copied (or transcribed) from the DNA to the mRNA molecule The enzyme RNA polymerase begins the process of making mRNA © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Transcription (cont.) RNA polymerase makes the double-stranded DNA molecule come apart, usually about 17 base pairs at a time RNA polymerase then transcribes (copies) the bases on one strand of the DNA to make a complementary molecule of mRNA © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Transcription (cont.) RNA molecules have the base uracil (U) instead of thymine At the end of the gene there is a sequence of bases that tells the RNA poly-merase to stop copying and as a consequence the mRNA molecule is released © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Transcription (cont.) The strand that is copied is called the template strand because it is the template from which the mRNA is made. The other strand is known as the coding strand Not all of the bases in the DNA molecule are code for an amino acid. The non- coding sequences are called introns © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Transcription (cont.) The base sequences that code for amino acids are called exons The functional mRNA will leave the nucleus and direct protein synthesis in the cytoplasm Return © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Translation Translation is the production of a protein using the information that is coded in the mRNA molecule In the cytoplasm a ribosome attaches to one end of the mRNA molecule. using a sequence that is the binding site for the ribosome © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Translation (cont.) The ribosome then moves along the mRNA three bases at a time, reading the bases as it goes When the ribosome reaches the start codon (AUG) it starts making the protein Return © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Protein Formation Transfer RNA (tRNA) bring the individual amino acids to the ribosome to be joined together as proteins Each tRNA molecule has a section that binds to its corresponding amino acid © 2010 McGraw-Hill Australia

Protein Formation (cont.) The loop of the tRNA has three nitrogen bases that form an anticodon. These three bases bind with the complementary bases of a codon on the mRNA molecule The anticodon determines the type of amino acid carried by the tRNA © 2010 McGraw-Hill Australia

Protein Formation (cont.) As the ribosome reads the codons on the mRNA, tRNA molecules with the matching anticodon are brought in The amino acids carried by the tRNA are joined together so the protein is assembled with the amino acids in the correct sequence © 2010 McGraw-Hill Australia

Protein Formation (cont.) Once the tRNA has delivered its amino acid it detaches from the ribosome and can pick up another amino acid from the cytoplasm One mRNA often has 10 or 20 ribosomes reading its code at the same time This means that a cell could produce over 150 000 protein molecules per second Return © 2010 McGraw-Hill Australia

© 2010 McGraw-Hill Australia Summary Each gene in the DNA in the cell nucleus carries the code for making a protein from amino acids Transcription The two strands of the DNA in a gene separate mRNA forms with bases that are complementary to those on the template strand of the DNA The mRNA molecule that is formed undergoes modification – the introns are removed leaving just the exons mRNA travels from the nucleus to the cytoplasm Translation mRNA attaches to a ribosome The ribosome moves along the mRNA reading the code. At the start codon it begins making the protein For each codon on the mRNA, a tRNA with a matching anticodon brings the correct amino acid The amino acids are bonded together to form the protein © 2010 McGraw-Hill Australia