Understanding DNA Structure and Replication

1. A 4-nucleotide-long segment of DNA. (arbitrary choice of bases) Only variation comes from nitrogenous base side chains

2. The general structure of a nucleic acid in terms of nucleotide subunits. DNA varies from RNA in using deoxyribose sugar not ribose, and having thymine instead of uracil. Otherwise it’s the same composition.

3. DNA GEOMETRY Rosalind Franklin in early 1950s used X-ray crystallography to study DNA structure. Watson and Crick used her info to develop a DNA double helix shape. In 3-D, two DNA strands twist into a double helix (spiral staircase).

4. DNA double helix like a spiral staircase antiparallel strands: each strand oriented 5‘3' in opposite directions. Strands are complementary, not identical. Central bases pair up to form “steps”

5. Base pairs composed of one purine paired with one pyrimidine. Width of the helix stays roughly equivalent with this arrangement

6. Phosphate–Sugar ester bonds are covalent and very strong. Base pairs are held together with weak Hydrogen bonds. This becomes important when we see how DNA works. Thymine to Adenine Cytosine to Guanine

7. (a) Thymine to adenine base pairing involves 2 H bonds. (b) Cytosine to guanine base pairing involves 3 H bonds.

8. DNA double helix emphasizing the hydrogen bonding between bases on the two chains. Keep in mind the weak H bonds in the interior, they are key!

9. For Heredity to Work: Biological info must be accurately copied and transmitted from each cell to all of its progeny. Most cells divide frequently, so DNA has to be easily copied (replicated) and separated before a cell splits in two. So how is DNA replicated?

10. DNA replication:2 strands of DNA double helix unwind (helicase enzymes) Separated strands = templates for new DNA strands. Free floating nucleotides pair with complementary bases on separated strands. Result is replication of DNA molecule.

11. The enzymeDNA polymerase catalyzes base pairing and acts only in 5’-to-3’ direction. One strand (top) grows continuously in the direction of the unwinding, the other strand grows in segments in the opposite direction. The segments (Okazaki fragments) in this chain are connected by a different enzyme, DNA ligase.

12. DNA replication usually occurs at multiple sites within a molecule, and replication is bidirectional from these sites.

13. Summary of DNA Replication Two identical daughter strands form. Then cell divides. Replication animation

14. Review: can you… • Compare and contrast the structure and • function of different types of nucleic acids • Draw the basic structure of nucleosides • and nucleotides • Explain the primary structure of nucleic acids and compare it to protein structure • Describe the structural properties of the • DNA double helix • Draw the steps involved in DNA replication • Compare & contrast RNA to DNA

15. Chromosomes are DNA strands wrapped aroundproteins. DNA is the template for protein production.

16. RNA is similar to DNA • but with these differences: • The sugar in RNA is ribose not • deoxyribose • The nitrogen base uracil replaces • thymine. • RNA molecules are single, not • double stranded. • RNA molecules are much smaller • than DNA molecules. • RNA enters & leaves the nucleus

17. Protein Synthesis • Proteins are synthesized in the cell through a series of steps involving • Transcription: DNA --> RNA • Translation: RNA --> Protein

18. Genes are a segment on a DNA strand that code for a hnRNA/mRNA molecule. The process of copying a complementary strand of mRNA off of a DNA gene is called transcription

19. The transcription of DNA to form RNA involves an unwinding of a portion of the DNA double helix. Only one strand of the DNA is copied during transcription.

20. RNA transcription

21. Initially heterogenous nuclear RNA (hnRNA) is formed Non-coding pieces (introns) are spliced out using small nuclear RNA and protein particles called “snurps” (snRNP).