#15 Biochemistry Lecture (DNA Structure and Replication) from Kevin Ahern's BB 350





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Published on Apr 27, 2013

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Lecture Highlights

(Note the first part of the video repeats material from another video from a previous year. There was no video #14 in this year.

10. Chromatin is a complex of DNA and proteins called histones.

11. Histones are proteins that associate with DNA and allow it to coil up. DNA-histone complexes are called nucleosomes. When you put many nucleosomes together, you get chromatin. Chromosomes contain chromatin.

12. DNA strands can easily be separated by heat, acid, or base.

13. Single strands of nucleic acid (DNA or RNA) absorb light at 260 nm more strongly than double strands of nucleic acid in the same quantity. Thus, one can monitor the denaturation of nucleic acid by monitoring its absorbance at 260 nm as the denaturation proceeds.

14. The transition temperature (Tm) for a nucleic acid denaturation is the mid-point of the Tm transition

15. Renaturation is the opposite of denaturation. Renaturation puts single strands together. Both denaturation and renaturation are important for anyone running the polymerase chain reaction (PCR).

16. RNA differs from DNA in having ribose (instead of deoxyribose) and uracil (instead of thymine).

17. There are three main types of RNA found in all cells. They are transfer RNA (tRNA), ribosomal RNA (rRNA), and messenger RNA (mRNA). We will talk about these later. For now, you should know that mRNAs carry the genetic code necessary for making proteins, tRNAs carry amino acids for translation and rRNAs are components of ribosomes that make proteins.

18. tRNAs carry an amino acid at one end and a three base sequence called an anti-codon loop at the other. The anticodon loop is complementary to the three base codon found in mRNA. The amino acid is specific for each anti-codon and thus for each codon, as well. The genetic code requires matching the right amino acid with the right anti-codon on a tRNA. Please note that the genetic code is written as the pairing of the codon with the amino acid that the tRNA brings to it.

19. Ribosomal RNA is found in ribosomes and helps the ribosome by catalyzing the formation of peptide bonds during protein synthesis. We'll see at least one other function soon.

20. Information in cells flows from DNA to RNA (called transcription) and from RNA to Protein (called translation). This is known as the Central Dogma. Some retroviruses have an enzyme called reverse transcriptase that allows them to make DNA from RNA. This last process is called reverse transcription.

21. DNA replication is catalyzed by an enzyme known as DNA polymerase. All DNA polymerases catalyze the synthesis of DNA in the 5' to 3' direction only.

22. Replication of each DNA strand occurs by a different scheme. The leading strand is made in a single continuous piece. The lagging strand is made in short segments called Okazaki fragments. The two different replication strategies arise from the fact that both must occur in the 5' to 3' direction. The lagging strand segments can only be started after the leading strand synthesis opens up the duplex sufficiently. This occurs repetitively during synthesis, generating multiple lagging strand fragments.

23. Note that leading AND lagging strand synthesis are both occurring at the same replication fork AND that both leading and lagging strand synthesis are occurring exclusively in the 5' to 3' direction.

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