1. Nucleotides, Nucleic Acids and Heredity
Bettelheim, Brown, Campbell and Farrell
Chapter 25—part 2

2. DNA and RNA The three differences in structure between DNA and RNA are
DNA bases are A, G, C, and T; the RNA bases are A, G, C, and U
the sugar in DNA is 2-deoxy-D-ribose; in RNA it is D-ribose
DNA is always double stranded; there are several kinds of RNA, all of which are single-stranded

3. RNA
RNA molecules are classified according to their structure and function

4. Other RNAs
RNA molecules are classified according to their structure and function
snRNA –small nuclear RNA ( b)
Combine to make snRNPs to help processing of mRNA for export from nucleus
miRNA—microRNA
Bind to mRNA in development
siRNA—small interfering RNA
Knock out mRNA for genes that are undesirable

5. t-RNA Structure
Contains some modified nucleotides, such as 1-methylguanosine
Actual 3-D shape is an L
Often shown as a 2-D “clover leaf” shape with three “loops”
Some H-bonding between bases at base of loops
Anticodon bases contained on middle loop
3’ end has CCA as terminal bases
3’ end carries the amino acid

6. “Clover Leaf” Structure of tRNA

7. Fig

8. Nucleic Acids and Heredity
Chromosomes exist in pairs (23 pairs in humans)
Inherit one DNA copy from each parent.
Most cells in our body contain copies of both
Genetic information is carried in the sequence of bases along the DNA strands.
Information is passed to daughter cells when cell divides.

9. Genes, Exons, and Introns
Gene: a segment of DNA that carries a base sequence that directs the synthesis of a particular protein, tRNA, or mRNA
there are many genes in one DNA molecule
in bacteria the gene is continuous
in higher organisms the gene is discontinuous
Exon: a section of DNA that, when transcribed, codes for a protein or RNA
Intron: a section of DNA that does not code for anything functional

10. Genes, Exons, and Introns
introns are cut out of mRNA by ribozymes before the protein is synthesized

11. Central Dogma of Molecular Biology
DNA → RNA → Protein

12. Processes involved in transfer of hereditary information
Replication: DNA → DNA (identical copy)
Transcription: DNA → RNA (m-RNA)
Translation: RNA → protein

13. DNA Replication
Replication involves separation of the two original strands and synthesis of two new daughter strands using the original strands as templates
DNA double helix unwinds at a specific point called an origin of replication
DNA replication is bidirectional: chains are synthesized in both directions from the origin of replication
At each origin of replication, there are two replication forks where new polynucleotide strands are formed

14. Two NEW Strands Formed

15. Steps in Replication 1.Weaken DNA-Histone interactions
Histone Acetylase interferes with +/- interaction by adding acetyl group to lysine amino groups
2.Relax higher DNA superstructure
Topoisomerases (gyrases) eliminate supercoiling of DNA by binding to one strand (via tyrosine and phosphate bond), nicking DNA, uncoiling DNA and rejoining DNA segments
3.Unwind Double Helix
Helicases attach to one strand and separate the two strands (uses ATP for energy)

16. DNA Replication

17. Steps in Replication 4.Primer/Primases
Short RNA sequences needed to start DNA synthesis
Catalyzed by Primase
5.Polymerization (actual synthesis of new DNA strands)
DNA Polymerase
Catalyzes attachment base to new strand
New base is complementary to template base
Short Okasaki fragments formed (ca 200 bases)
6.Ligation
DNA ligase joins Okasaki fragments

18. DNA Replication
DNA is synthesized from its 5’ -> 3’ end (from the 3’ -> 5’ direction of the template)
the leading strand is synthesized continuously in the 5’ -> 3’ direction toward the replication fork
the lagging strand is synthesized discontinuously as a series of Okazaki fragments, also in the 5’ -> 3’ direction, but away from the replication fork
Okazaki fragments of the lagging strand are joined by the enzyme DNA ligase
replication is semiconservative: each daughter strand contains one original template strand and one newly synthesized strand

19. DNA Replication
Leading strand
Lagging strand

20. DNA Replication Leading strand (new strand synthesized from 5’ to 3’)
Lagging strand (new strand runs from 3’ to 5’ but synthesis of individual Okasaki fragments is 5’ to 3’)

22. Fig. 24.9

23. DNA Replication

24. Bond Formation in Replication

25. DNA Replication
Old strand:
New strand:
ATTCGTAAAGGTC
TAAGCATT

26. DNA Repair
Cells have DNA repair enzymes that can detect, recognize, and repair mutations in DNA
Base excision repair (BER): one of the most common repair mechanisms
DNA glycosylase recognizes the “damaged” base and cuts out the base, leaving the sugar-phosphate backbone
AP (apurinic or apyrimidinic) site created

27. DNA Repair Endonuclease catalyzes the hydrolysis of the backbone
an exonuclease liberates the sugar-phosphate unit of the damaged site
DNA polymerase inserts the correct nucleotide
DNA ligase seals the backbone to complete the repair

28. DNA Repair
NER (nucleotide excision repair) removes and repairs up to units by a similar mechanism involving a number of repair enzymes