2. PROTEIN SYNTHESIS The formation of new proteins using the code carried on DNA

3. Gene is Selected A gene is a section of DNA coding for a particular polypeptide or protein. When a protein is required by the cell or the organism as a whole, the gene for that protein is activated. Gene

4. Introns and Exons DNA is made up of coding sequences called exons and non coding sequences called introns. Exon Exon Intron Intron

5. Transcription An enzyme RNA Polymerase unwinds the section of DNA carrying the desired gene. Only one strand of the DNA (the template) is recognised. RNA Polymerase then binds to a promoter site and begins to form an RNA copy of the template strand (called messenger RNA (mRNA)).

6. Naming the Strands An mRNA copy of the top coding strand is made by base pairing with the bottom template strand. DNA Primary RNA Coding strand Template strand The DNA strand is unwound exposing the bases that carry the genetic code.

7. Messenger RNA Free nucleotides match up with the coded sequence of DNA. When bases pair, Uracil is present in RNA instead of Thymine. A single stranded primary transcript that is complementary to the template strand is formed. DNA template TAC GCC AAA TCC mRNA codons AUG CGG UUU AGG

8. Removal of Introns The primary transcript contains a copy of both introns and exons. Introns are removed and exons are spliced (joined) to form mature mRNA.

9. Movement of mRNA The completed single strand of mRNA migrates out of the nucleus through pores in the nuclear membrane. mRNA binds to ribosomes. The mRNA is divided into triplets of bases called codons. Different codons carry the code for a particular amino acid (AA) Other codons carry start and stop codes.

10. Important Codon Facts AUG is the start codon on all viable mRNA molecules. It also codes for the amino acid Methionine. UAA, UAG, UGA are stop codes. Many codons are degenerate – more than one codon codes for an AA. The code is universal. The same codes work in all organisms. 4 3 different combinations give 64 different codons.

11. Amino Acid Codons

12. Transfer RNA UAC Transfer RNA (tRNA) is a carrier molecule which picks up amino acids from the cytoplasm and carries them to the mRNA / ribosome complex. anticodon

13. Translation A triplet of bases on the tRNA called an anticodon matches with the mRNA codon. tRNA carrying its AA binds to the codon The ribosome moves along the mRNA activating new codons. AA’s are joined by peptide bonds to form long polypeptide chains.

14. Ribosome Action A U G C G G U U U A G G C C C U A G The ribosome acts as catalyst holding all the components together while bonds are formed. GC C AA 2 UA C AA 1

15. Peptide Bond Formation A U G C G G U U U A G G C C C U A G As the ribosome moves relative to the mRNA, amino acids are added until a stop signal is reached. AU C stop AA 1 AA 4 AA 3 AA 2 AA 5

16. Translation speed Rapid protein formation can be achieved when the mRNA is attached to more than one ribosome.

17. Electron Microscope View mRNA (purple) binds to a ribosome. Beads of AA’s (yellow) are joined to form a polypeptide.

18. Questions What name is given to a section of DNA coding for a polypeptide? The coding sequences of DNA are known as what? Name the enzyme responsible for unwinding the DNA helix. How does the enzyme recognize the start of a message? Which DNA strand is used in the base pairing process?

19. What name is given to the RNA strand first formed, that contains introns and exons? In RNA, which base binds with Adenine? What is the codon for the start of an mRNA? What are the three stop codons? What does the term “codons are degenerate” mean? Why is the codon a three base sequence?

20. What does the term, “the genetic code is universal” mean? What name is given to the process of forming mRNA? What is the full name for tRNA? What is the role of tRNA? mRNA has a codon, tRNA has an? What holds amino acids together in a polypeptide?

21. What is the role of the ribosomes? How do the ribosomes operate that allows long amino acid chains to form? What name is given to the process of converting an mRNA code into an amino acid sequence? Describe one way in which the rate of translation can be increased?