1. The Molecular Basis of Inheritance
Chapter 16
The Molecular Basis of Inheritance

2. Overview: Life’s Operating Instructions
Hereditary information is encoded in DNA and reproduced in all cells of the body
This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits
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3. Figure 16.1
Figure 16.1 How was the structure of DNA determined?

4. Sugar–phosphate backbone
Figure 16.5
Sugar–phosphate backbone
Nitrogenous bases
5 end
Thymine (T)
Adenine (A)
Cytosine (C)
Figure 16.5 The structure of a DNA strand.
Phosphate
Guanine (G)
Sugar (deoxyribose)
DNA nucleotide
Nitrogenous base
3 end

5. Key features of DNA structure (b) Partial chemical structure
Figure 16.7a
5 end C G
Hydrogen bond C G
3 end G C G C T A
3.4 nm T A G C G C C G A T
1 nm C G T A C G G C C
Figure 16.7 The double helix. G A T A T
3 end A T
0.34 nm T A
5 end
(a)
Key features of DNA structure
(b) Partial chemical structure

6. (c) Space-filling model
Figure 16.7b
Figure 16.7 The double helix.
(c) Space-filling model

7. Replication: Basics
When?
What?
Key players and roles?

9. Replication

10. Meselson-Stahl experiment

11. Which one can we eliminate?

14. How does DNA’s structure effect the replication process?
Enzymes can only add to the 3’ end
Leading and lagging strands

15. Origin of replication 3 5 RNA primer 5 3 Sliding clamp 3
Figure 16.15b
Origin of replication 3 5 5
RNA primer 3
Sliding clamp 3
DNA pol III
Parental DNA 5 3 5
Figure Synthesis of the leading strand during DNA replication. 5 3 3 5

16. Overall directions of replication
Figure 16.15a
Overview
Leading strand
Lagging strand
Origin of replication
Primer
Leading strand
Lagging strand
Figure Synthesis of the leading strand during DNA replication.
Overall directions of replication

18. Replicating the Ends of DNA Molecules
Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes
The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends
This is not a problem for prokaryotes, most of which have circular chromosomes
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19. Ends of parental DNA strands Leading strand Lagging strand 3
Figure 16.20 5
Ends of parental DNA strands
Leading strand
Lagging strand 3
Last fragment
Next-to-last fragment
Lagging strand
RNA primer 5 3
Parental strand
Removal of primers and replacement with DNA where a 3 end is available 5 3
Second round of replication
Figure Shortening of the ends of linear DNA molecules. 5
New leading strand 3
New lagging strand 5 3
Further rounds of replication
Shorter and shorter daughter molecules

20. Ends of parental DNA strands Lagging strand 3
Figure 16.20a 5
Leading strand
Ends of parental DNA strands
Lagging strand 3
Last fragment
Next-to-last fragment
RNA primer
Lagging strand 5 3
Parental strand
Figure Shortening of the ends of linear DNA molecules.
Removal of primers and replacement with DNA where a 3 end is available 5 3

21. Second round of replication
Figure 16.20b 5 3
Second round of replication 5
New leading strand 3
New lagging strand 5 3
Figure Shortening of the ends of linear DNA molecules.
Further rounds of replication
Shorter and shorter daughter molecules

22. Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres
Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules
It has been proposed that the shortening of telomeres is connected to aging
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23. Figure 16.21
Figure Telomeres.
1 m

24. If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce
An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells
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25. The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions
There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist
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26. Concept 16.3 A chromosome consists of a DNA molecule packed together with proteins
The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein
In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid
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27. Chromatin, a complex of DNA and protein, is found in the nucleus of eukaryotic cells
Chromosomes fit into the nucleus through an elaborate, multilevel system of packing
For the Cell Biology Video Cartoon and Stick Model of a Nucleosomal Particle, go to Animation and Video Files.
Animation: DNA Packing
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28. Nucleosome (10 nm in diameter)
Figure 16.22a
Nucleosome (10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)
DNA, the double helix
Histones

29. Replicated chromosome (1,400 nm)
Figure 16.22b
Chromatid (700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
30-nm fiber
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome
Replicated chromosome (1,400 nm)
Looped domains (300-nm fiber)
Metaphase chromosome

30. DNA double helix (2 nm in diameter)
Figure 16.22c
DNA double helix (2 nm in diameter)
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

31. Nucleosome (10 nm in diameter)
Figure 16.22d
Nucleosome (10 nm in diameter)
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

32. Chromatin undergoes changes in packing during the cell cycle
At interphase, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping
Though interphase chromosomes are not highly condensed, they still occupy specific restricted regions in the nucleus
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33. Figure 16.23
Figure Impact: Painting Chromosomes
5 m

34. Loosely packed chromatin is called euchromatin
Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
Loosely packed chromatin is called euchromatin
During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin
Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions
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35. Histones can undergo chemical modifications that result in changes in chromatin organization
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36. Evolution of the Genetic Code
The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after being transplanted from one species to another
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37. (a) Tobacco plant expressing (b) Pig expressing a jellyfish
Figure 17.6
Figure 17.6 Expression of genes from different species.
(a) Tobacco plant expressing
(b) Pig expressing a jellyfish
a firefly gene
gene