Molecular Genetics

1. Molecular Genetics The Central Dogma of Biology!

2. First… a few more thoughts about replication

3. When DNA is “unzipped,” replication proceeds on both strands in opposite directions.

4. Replication Mistakes • DNA is copied many sections at a time, so replicationgoes quickly. • DNA is copied very accurately because of “proofreading” mechanism in DNApolymerase. • If a mistake is made, this enzyme can backtrack and replace the wrong nitrogen base.

5. Prokaryotic and Eukaryotic Replication • Because prokaryotic DNA is a single loop, replication begins at one place along the loop • Replication occurs in opposite directions until both replication forks meet • Eukaryotic DNA is linear and replication occurs at many sites along a chromosome • This allows faster replication than prokaryotic replication • Replication “bubbles” are formed along the chromosome • Replication of an entire human chromosome occurs in about eight hours

6. Section 8.4 Transcription • The Centraldogmaof molecular biology states that information goes from DNA to RNA to proteins • The central dogma involves three processes: • Replication—making copies of DNA from DNA • Transcription—making copies of DNA in the form of RNA • Translation—taking RNA to make a chain of amino acids or protein

7. The Central Dogma!

8. RNA • DNA stores and transmits the information needed to make proteins but it does not actually use that information to make proteins. • That is the function of RNA, ribonucleic acid.

9. RNA vs. DNA

10. Transcription • DNAis used a template. • One strand of DNA is the “sense” strand—this is the strand used for coding the mRNA in transcription. • The other is the “antisense” (or nonsense) strand and is not used for transcription.

11. Summary of Transcription • 1. RNA polymerasebinds to DNA and “unzips” it • 2. RNA polymerasewill add and link RNA nucleotides to the DNA template as it reads the gene • Hydrogen bonds form between bases of “unzipped” DNA and RNA nucleotides • 3.RNAis released from DNA strand

12. Three types of RNA • mRNA(messenger) • carries genetic information from DNA to the cytoplasm (site of translation) • serves as a pattern for assembly of amino acids • tRNA(transfer) • brings amino acids from the cytoplasm to a ribosome to make growing protein • each tRNA bonds to a specific amino acid • rRNA(ribosomal) • part of ribosomes • site of protein synthesis

13. Triplet Code • A codonis a three-nucleotidesequence of mRNA that codes for an amino acid • Each codon codes for an amino acid or a stop codon • **See page 244 in your textbook for the Genetic Code of mRNA codons.

14. Codon Wheel Remember, the codon on the mRNA corresponds to the amino acid!

15. Codon Chart on page 244 You must know how to use this chart!

16. Translation • After transcription, RNA moves from the nucleusto the cytoplasmto make proteins. • The translation occurs on ribosomes.

17. tRNA • One side contains a sequence of three bases called an anticodon, which bonds with the complementary mRNA codon • The other side carries a specific aminoacid

18. Summary of Translation • Ribosome attaches to the mRNA • Translation begins at the “start” codon, AUG • Codon AUG on the mRNA pairs with the anticodon UAC on tRNA • This tRNA carries methionine to the ribosome • Ribosome moves systematically along mRNA • mRNA codons pair with their tRNA anticodon • amino acid then add to the growing polypeptide chain

19. Summary of Translation This continues until ribosome reaches a “stop” codon (UGA, UAA, or UAG) Several ribosomes translating same mRNA at same time The ribosome complex breaks apart, releasing the mRNA molecule and the polypeptide