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Where are genes located?
GENE AND CHROMOSOME
2141591 Cell and Molecular Biology
Kanokporn Boonsirichai
Molecular Biology of the Cell. 2002.
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts,
and Peter Walter.
Chromosomes can be stained and
visualized
What is the genetic
material?
Frederick Griffith
Oswald Avery
Griffith’s experiment 1928; Avery’s 1944
DNA Building Blocks
DNA Base Pairing
• Purines: ‘A’ & ‘G’
• Pyrimidines: ‘C’ & ‘T’
Chargaff rules
• ‘A’ H-bonds (2) with ‘T’
• ‘C’ H-bonds (3) with ‘G’
DNA Double Helix
Average = 10.4 nt per turn
Flow of Information
Eucaryotic DNA is packaged in a set of
chromosomes
Genes are strung together on a
chromosome
Chromosomal aberration can be
determined from the banding pattern
Gene Structure
How is DNA packaged in a nucleus?
Sequences required for chromosomal
replication
Human Chromosome 22
Nucleosomes are the Basic Unit
• 48 x 106 nt pairs
Interphase chromatin
30 nm thick
• Stretched out end-to-end = 1.5 cm
• Interphase chromosome = 1,000-fold compaction
Experimentally
unpacked chromatin
• Metaphase = 2 microns (10,000-fold compaction)
Chromatin = DNA + histones/non-histone proteins
• DNA wraps ~1.65 turns on the
nucleosome core (146 bp)
• H-bonds
• Hydrophobic interaction
• Ionic interaction
Histone structures
Zigzag model for
the 30-nm
chromatin fiber
How does chromatin form a loop?
Condensins complex
• Histones help to
compact the
nucleosomes
Interphase Chromosome Structure
• Euchromatin
– Less condensed
– 30-nm structure
• Heterochromatin
–
–
–
–
Highly condensed
Less well understood structurally
Additional level of folding
Requires additional sets of proteins
Chromatin structure can be
interrupted
Chromatin Remodeling Complex
Chromatin Remodeling Complex
Figure 5-27 Essential Cell Biology (© Garland Science 2010)
Covalent Modification of
Histone Tail