1. DNA Replication/Protein Synthesis Topic 2.7

2. DNA replication DNA molecule is unzipped down the middle by an enzyme Helicase Helicase: straightens out the helix and breaks the hydrogen bonds between nitrogenous bases DNA Ploymerase inserts new nucleotides thus formimg new complementary strands of DNA DNA replication is semi-conservative (one side of the strand is “conserved”)

3. DNA Replication

6. Meselson and Stahl’s experiment showing semi-conservative replication. Used radioactive isotopes (N 15 instead of N 14 ) N 15 is heavier than N 14 Used density gradient centrifugation to separate generations of DNA Results support semi-conservative replication

7. Meselson and Stahl

9. Protein Synthesis DNA provides the instructions for making specific proteins. The bridge between DNA and protein synthesis is RNA. RNA is chemically similar to DNA, except that it contains ribose as its sugar and substitutes the nitrogenous base uracil for thymine. – An RNA molecule consists of a single strand.

10. RNA – What does it do? RNA has a different function than DNA. Whereas DNA provides the instructions for protein synthesis, RNA does the actual work of protein synthesis. RNA takes from DNA the instructions on how the protein should be assembled, then—amino acid by amino acid—RNA assembles the protein.

11. Types of RNA 1.Messenger RNA – mRNA 2.Ribosomal RNA – rRNA 3.Transfer RNA - tRNA

12. mRNA Messenger RNA (mRNA) Reads the DNA code inside the nucleus. Carries the instructions to the rest of the cell.

13. rRna Ribosomal RNA (rRNA) Contained in the ribosomes. Helps with the assembly of proteins. Binds to the mRNA and uses the instructions to assemble the amino acids in the correct order.

14. Transfer RNA (continued) A tRNA molecule consists of a strand of about 80 nucleotides that folds back on itself to form a three-dimensional structure. – It includes a loop containing the anticodon and an attachment site at the 3’ end for an amino acid.

15. tRNA Transfer RNA (tRNA) Transfers amino acids to ribosomes for protein assembly. Single, folded strand that delivers the proper amino acid to the site at the right time.

16. Transfer RNA Like other types of RNA, tRNA molecules are transcribed from DNA templates in the nucleus. Once it reaches the cytoplasm, each tRNA is used repeatedly to pick up its designated amino acid in the cytosol (cytoplasm), to deposit the amino acid at the ribosome, to return to the cytosol and pick up another copy of that amino acid.

17. Protein Synthesis (continued) There are two parts to protein synthesis. – Transcription Copying DNA in the form of RNA Occurs in the Nucleus – Translation Read the RNA and construct the protein. Occurs in the Cytoplasm – In prokaryotes this process happens entirely in the cytoplasm as there is no nucleus.

18. Transcription During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand. – This process is used to synthesize any type of RNA from a DNA template. Transcription of a gene produces a messenger RNA (mRNA) molecule.

19. During transcription, one DNA strand, the template strand, provides a template for ordering the sequence of nucleotides in an RNA transcript – The complementary RNA molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 17.3

20. Building Amino Acids The genetic instructions for a polypeptide chain are written in DNA as a series of three-nucleotide words. Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids. Each triplet is called a codon In the triplet code, three consecutive bases specify an amino acid, creating 4 3 (64) possible code words. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

21. Each codon specifies which one of the 20 amino acids will be incorporated at the corresponding position along a polypeptide. Because codons are base triplets, the number of nucleotides making up a genetic message must be three times the number of amino acids making up the protein product. – It would take at least 300 nucleotides to code for a polypeptide that is 100 amino acids long. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

22. Codons / Amino Acides – 61 of 64 triplets code for amino acids. – The codon AUG not only codes for the amino acid methionine but also indicates the start of translation. – Three codons do not indicate amino acids but signal the termination of translation.

23. Translation Translations is the RNA-directed synthesis of a polypeptide In the process of translation, a cell interprets a series of codons along a mRNA molecule. Transfer RNA (tRNA) transfers amino acids from the cytoplasm’s pool to a ribosome. The ribosome adds each amino acid carried by tRNA to the growing end of the polypeptide chain.