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Chemistry 501 Handout 24 Genes and Chromosomes Chapter 24. Dep. of Chemistry & Biochemistry Prof. Indig. Lehninger. Principles of Biochemistry. by Nelson and Cox, 5 th Edition; W.H. Freeman and Company. Colinearity of the coding nucleotide sequences of DNA and mRNA
Dep. of Chemistry & Biochemistry
Lehninger. Principles of Biochemistry.
by Nelson and Cox, 5th Edition; W.H. Freeman and Company
and the amino acid sequence of a polypeptide chain.
DNA molecules are much longer than the cellular or viral packages that contain them
The mtDNA is replicated each time the mitochondrion divides
Short interspersed elements
100 to 300 bp
Long interspersed elements
6 to 8 kbp
(encode enzymes for transposition)
1.5 to 11 kbp
Supercoiling induced by separating the strands
of a helical structure
DNA underwinding is defined
by topological linking number
s = DLk/Lk0 = -2/200 = -0.01
1% of the helical turns present in the
DNA (in its B form) has been removed
Linking number applied to
closed-circular DNA molecules
Negative and positive
Twist (Tw) and Writhe (Wr)
Measure of the coiling of the
Lk = Tw + Wr
In addition of causing supercoiling and making strand separation somewhat easier,
the underwinding of DNA facilitates structural changes in the molecule
facilitates the partial strand
separation needed to promote
cruciform formation at
Type 1: single strand breaks;
changes Lk in increments of 1
Type 2: double strand breaks;
changes Lk in increments of 2
Generally relax DNA by removing negative supercoils
eukaryotic type IIA topoisomerase
Two ATPs are bound and hydrolyzed during this cycle
their DNA, or express their genes, and they die.
They are important drug targets for bacterial infections and cancer
Topoisomerase inhibitors are
important pharmaceutical agents
The supercoils are right-handed in a negatively supercoiled DNA molecule, and they
tend to be extended and narrow rather than compacted, often with multiple branches.
provides a much greater
degree of compaction
(tight left-handed turns)
Same DNA molecule, drawn in scale
rather, they tend to position themselves at certain locations
In some cases seems to
depend on a local abundance
of A=T base pairs in the DNA
helix where it is in contact with
The chromosomal material, chromatin, consists of DNA and proteins.
Nucleosomes are the
fundamental organizational unit
Beginning with nucleosomes,
eukaryotic chromosomal DNA
is packed in a sucession of
higher-order structures that
ultimately yield the compact
Eight histone proteins
Two of each: H2A, H2B, H3, and H4
Nucleosome with 146 bp of bound DNA
The DNA binds in a left-handed solenoidal supercoil that
circumnavigates the histone complex 1.8 times
Loops of chromosomal DNA
attached to a nuclear scaffold.
Next level of organization
(after 30 nm fiber)
DNA compaction in eukaryotes
is likely to involve coils upon coils
(Structural Maintenance of Chromosomes)
also bound to additional
regulatory proteins (not shown)
Model for the roles of cohesins and condensins during the eukaryotic cell cycle.
Looped domains of the E. coli chromosome.
E. coli nucleoids.
attached to a scaffoldlike structure.
Information that is passed from
one generation to another but is
not encoded in DNA is referred
to as epigenetic information.
Much of it is in the form of covalent modification of histones and/or placement of histone variants in chromosome
(common to all core histones)
Sites of Lys/Arg residue methylation and Ser phosphorylation are indicated
to which a particular histone variant binds.
bound by the nucleosomes with the histone variant.