Blueprint of Life Topic 18: Protein Synthesis Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

DOT POINT(s) explain the relationship between proteins and polypeptides outline, using a simple model, the process by which DNA controls the production of polypeptides

Introduction Proteins are large, complex macromolecules made up of one or more long chains called polypeptides. www.answers.com

Introduction Each polypeptide chain consists of a linear sequence of many amino acids joined by peptide bonds. (There are about 20 different amino acids and these can be linked together in chains of up to 300.) One or more polypeptides can be twisted together into a particular shape, resulting in the overall structure of a protein. www.answers.com

Introduction The sequence and arrangement of amino acids determines the configuration of the protein. Any change in the amino acid sequence that results in a change in the shape of the protein molecule could affect the ability of the protein to carry out its function in the cell. hanguyenbiologyhlblog.blogspot.com

Protein Synthesis Multicellular organisms are made up of a variety of different cells. For example, humans have over 200 different types of cells, including skin cells, muscle cells, blood cells and many others. Despite differing in structure, every cell that has a nucleus has a full copy of the same coded genetic information in its DNA. globalmedicaldiscovery.com

Protein Synthesis This encoded information directs the production of cell products such as polypeptides which form proteins, the key to cell specialisation and differentiation. In specialised cells, coded instructions for the production of a particular protein (or group of proteins) are ‘switched on’. www.dailytelegraph.com.au

Protein Synthesis This ensures that the cell develops a particular structure, in keeping with the type of tissue to which it belongs. For example in skin tissue, genes for the pigment protein melanin and for the protein keratin will be switched on in each cell, ensuring that the cells become skin cells. www.sciencedaily.com

The Process DNA never leaves the nucleus—it holds the original copy of all instructions. In order for a cell to make proteins, only the relevant instructions for those proteins are accessed in the DNA nucleotide sequence. www.doeaccimphal.org.in

The Process Since the DNA instructions must remain in the nucleus, an intermediate molecule—messenger RNA (mRNA)—is created; this carries a transcribed copy of the relevant instructions from the nucleus to the ribosomes in the cytoplasm. The ribosomes can be considered as the ‘machinery’ that translates the message carried by the mRNA into a cell product such as protein. www.biologycorner.com

The Process The sequence of information transfer necessary for DNA to direct the production of proteins is summarised below, in a framework known in genetics as the central dogma. DNA → RNA → protein www.nvo.com

The Chemicals Involved in Protein Synthesis DNA DNA consists of long chains of nucleotides wound into a double helix. The sequence of nucleotide bases determines the meaning of the message— because it codes for the sequence of RNA nucleotides and ultimately the sequence of amino acids that form the polypeptide chain. academic.pgcc.edu

The Chemicals Involved in Protein Synthesis RNA Like DNA, RNA is a nucleic acid made from a chain of nucleotides, but it differs from DNA in the following ways: ■ Most RNA is single-stranded. ■ The sugar in RNA is ribose sugar (not deoxyribose sugar as in DNA). ■ RNA has the nitrogenous base uracil (U ) instead of thymine (T). www.makingthemodernworld.org.uk

The Chemicals Involved in Protein Synthesis There are three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA): ■ mRNA is single-stranded and is not twisted into a helix. mRNA molecules are a few thousand bases long, much shorter than DNA. They are found in both the nucleus and cytoplasm. They function as an intermediate molecule, carrying information from DNA in the nucleus to the ribosomes in the cytoplasm. notesforpakistan.blogspot.com

The Chemicals Involved in Protein Synthesis ■ tRNA molecules occur in the cytoplasm that carry amino acids which build the new proteins. Each one is 75 nucleotides long and twisted into the shape of a clover leaf. On one end of the tRNA there are three unpaired bases called an anticodon, which attach the tRNA to its complementary bases on the mRNA strand. The other end of the tRNA is able to bind with an amino acid temporarily. Each tRNA molecule will only attach to one particular amino acid. The specific sequence of three bases at the anticodon end determines which amino acid will be carried by that tRNA. pinterest.com

The Chemicals Involved in Protein Synthesis ■ rRNA forms a structural part of the ribosomes. library.thinkquest.org

Protein Synthesis Protein synthesis can be thought of using a ‘computer’ analogy. The DNA could be considered as equivalent to the software, the mRNA is an intermediate made using a software program (e.g. a graph or Word document or diagram) and the protein would be the hard copy, printed out. Handout: Figure 4.7 The Biological Process of Protein Synthesis in a Cell www.astropix.com

The Steps Involved in Protein Synthesis 1. An enzyme, RNA polymerase, binds to a part of the DNA called the promoter and the DNA ‘unzips’— that is, the DNA unspirals, hydrogen bonds between the two strands break, and the strands separate over a short length, just in that part of the DNA that holds the gene to be used. Only one strand of DNA contains the genetic information to make a protein; rather confusingly, it is called the non-coding strand or sense strand; the other strand is called the coding strand or anti-sense strand.

The Steps Involved in Protein Synthesis 2. Transcription of the gene occurs, controlled by the enzyme RNA polymerase: the sense strand of the DNA acts as a template and RNA nucleotides are assembled, forming a complementary single stranded mRNA molecule (that is, DNA is transcribed into mRNA). The sequence of nucleotide bases on the mRNA molecule is the same as the DNA coding strand, except that it has U instead of T.

The Steps Involved in Protein Synthesis 3. The mRNA moves out of the nucleus and into the cytoplasm, where it encounters some of the millions of ribosomes in the cell.

The Steps Involved in Protein Synthesis 4. Translation: the ribosomes move along the mRNA molecule and, as they do so, they attach tRNA molecules by temporarily pairing the bases of the tRNA anticodons with their complementary triplets of bases (codons) on the mRNA.

The Steps Involved in Protein Synthesis 5. The amino acids are linked together by another enzyme to form a polypeptide chain. The amino acids are then spliced off their tRNA carriers.

The Steps Involved in Protein Synthesis 6. The tRNAs move away from the mRNA, leaving the growing chain of amino acids and move back into the cytoplasm where they can pick up another amino acid and be reused.

The Steps Involved in Protein Synthesis 7. The polypeptide chain may be joined by one or more other polypeptides; they are further processed and folded into their correct shape, forming a protein.

The Steps Involved in Protein Synthesis 8. The mRNA is broken down into its individual nucleotides which can be reused.

Advances in Understanding A gene was at first defined as a sequence of nucleotides that codes for one protein. Advances in understanding of the biochemical functioning of cells led to the definition changing to a sequence of nucleotides that codes for one polypeptide chain. en.wikipedia.org

Advances in Understanding More recent research has shown that some genes code mRNA and tRNA, which are not proteins at all, and that in other instances one gene may code for more than one polypeptide sequence (due to the splicing and rearrangement of blocks of mRNA before translation). Therefore the definition of a gene may need to change to a more functional concept: a sequence of nucleotides that codes for any molecular cell product. wp.stockton.edu

Activity -Students to watch video: http://youtu.be/983lhh20rGY -Students watch video: http://highered.mcgraw- hill.com/olc/dl/120077/micro06.swf a

Homework -Students to complete The Process of Protein Synthesis worksheet a