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Mon 2:30-3:30
Wed 1:30-2:30
359 Boyer Hall
The Molecular Structure of
Genes and Chromosomes
Fig. 21-1
Chromatid
Fig. 21-35
Centromere
Fig. 21-1
Chromatid
Centromere
Fig. 10-3
~ 1/2 of human genes are solitary genes, like the SUR2 gene.
There is only one gene of similar sequence in the haploid
genome. Comparing the sequence of the human and mouse
SUR2 genes, the exons are >90% identical, but the introns
are <10% identical.
SUR2 gene:
exon
intron
40 kb
In contrast, the globin genes are members of a gene family.
The -globin genes on chromosome 11 have exons that are
>90% identical. They are also >80% identical to the -globin
genes on another chromosome.
~1/2 of all human protein coding genes are duplicated, or
members of a gene family (>2 closely related genes).
Fig. 10-3
How did the duplicated genes arise?
Fig. 10-3
Gene duplication by unequal crossing-over between
homologous repeats:
Paternal
chromosome
Fig. 10-4
Maternal
chromosome
Fig. 21-34
Fig. 10-34
Gene duplication by unequal crossing-over between
homologous repeats:
Paternal
chromosome
Fig. 10-4
Maternal
chromosome
How did the duplicated genes arise?
Through gene duplication as a result of unequal crossingover between homologous chromosomes during meiosis.
Fig. 10-3
Repititious DNA
Simple sequence DNA ~3% of human DNA
(total protein coding DNA <2%)
Tandem repeats of sequences 1 to ~500 bp in length.
Commonly called “satellite DNA”
Micro-satellites: most 1-4 bp in
length tandemly repeated in regions
up to 150 bp in length. Thought to
have accumulated during rare
mistakes in DNA synthesis when the
nascent daughter strand “slipped”
along the parental template strand
so that additional bases were
inserted into the daughter strand.
Nascent daughter strand
5’ NNNNNNNNCAGCAGCAGCAGCAG
3’ NNNNNNNNGACGACGACGACGACGACGACGACNNNNN 5’
Template strand
5’ NNNNNNNNCAGCAGCAGCAGCAG
3’ NNNNNNNNGACGACGACGACGACGACGACGACNNNNN 5’
Daughter strand
“slips” backward
GC
A A
C
G
5’ NNNNNNNNCAGCAG
CAG
3’ NNNNNNNNGACGACGACGACGACGACGACGACNNNNN 5’
DNA replication
GC
continues A A
C
G
5’ NNNNNNNNCAGCAG
CAGCAGCAGCAGCAGNN
3’ NNNNNNNNGACGACGACGACGACGACGACGACNNNNN 5’
Differences in the number of
repeats of a microsatellite sequence
in the DNA of different individuals
arise through unequal crossing over
between chromosomes during
meiosis.
Fig. 10-6
“DNA fingerprinting” depends on
differences between every individual in the
number of tandem repeats of specific types
of simple sequence DNA that occur in
stretches of 500 bp--20 kb, called
“minisatellite” sequences.
These differences in the number of tandem
repeats of simple sequence DNA result
from “unequal crossing-over during
recombination between homologous
chromosomes during meiosis.
Fig. 10-7 a minisatellite l33.6; b l33.15; c l33.5
DNA fingerprinting by PCR using primers that bind
to the single copy sequences that flank
“minisatellite” repeats.
Most simple sequence DNA is
composed of repeats of 14-500 bp
tandemly repeated in stretches of 20100 kb.
Most of these long simple sequence
DNAs are found at centromeres of
chromosomes and telomeres where
they are thought to participate in the
structure of these specialized regions
of chromosomes.
Fig. 10-5
The most common type of repititious DNA in multicellular
organisms (accounting for ~ half of human DNA) is called
“interspersed repeats”
(also known as ‘moderately repeated DNA’ and
‘intermediate-repeat DNA’)
These do not occur in tandem arrays, but rather individual
copies of the same, or nearly the same sequence of ~100 bp
to ~10 kb found at tens of thousands to millions of different
positions on each of the chromosomes.
These resulted from repeated insertions of transposons into
new sites during the evolution of modern organisms. (Or to
the action of enzymes encoded by transposons on other
genomic sequences.)
Fig. 10-3
Fig. 10-3
Fig. 4-15
Exon duplication by unequal crossing over between
homologous repeated sequences on either side of an exon.
exon n
Multimerization of exons by unequal crossing over between
homologous exons:
exon n
exon n+1
+
Recombinant
chromosomes
Multimerization of exons by unequal crossing over between
homologous repeats:
L1
exon n
exon n+1
+
Recombinant
chromosomes
Fig. 4-15