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13
CHROMOSOME REPLICATION
2.
These insults may cause depurination of DNA, deamination of cytosine to uracil,
or pyrimidine dimerization, which must be repaired by DNA repair enzymes.
3.
The normal response to DNA damage is to stall the cell in the G
1
phase of the cell
cycle until the damage is repaired.
4.
The system that detects and signals DNA damage is a multiprotein complex called
BASC (BRCA1-associated genome surveillance complex). Some the components
of BASC include ATM (ataxia telangiectasia mutated) protein, BRCA1 protein,
and BRCA2 protein.
B. TYPES OF DNA DAMAGE
1.
Depurination.
About 5000 purines (As or Gs) per day are lost from DNA of each
human cell when the N-glycosyl bond between the purine and the deoxyribose
sugar-phosphate is broken. This is the most frequent type of lesion and leaves the
deoxyribose sugar-phosphate with a missing purine base, that is, an apurinic (AP)
site.
2.
Deamination of cytosine to uracil.
About 100 cytosines (C) per day are sponta-
neously deaminated to uracil (U). If the U is not corrected back to a C, then upon
replication instead of the occurrence of a correct C-G base pairing, a U-A base pair-
ing will occur.
3.
Pyrimidine dimerization.
Sunlight (UV radiation) can cause covalent linkage of
adjacent pyrimidines forming, for example, thymine dimers.
A. GENERAL FEATURES
1.
DNA repair involves DNA excision of the damaged site, DNA synthesis of the cor-
rect sequence, and DNA ligation.
2.
DNA repair involves the following enzymes:
a.
DNA glycosylase removes damaged bases while leaving the phosphodiester
backbone intact which creates an apurinic/apyrimidinic (AP) site.
b.
AP endonuclease creates a nick in the phosphodiester backbone at the AP site.
c.
DNA polymerase
d.
DNA ligase
B. TYPES OF DNA REPAIR
1.
Base excision repair.
This type of repair removes a single damaged base from a
DNA strand.
2.
Nucleotide excision repair.
This type of repair removes a group of damaged bases
from a DNA strand.
3.
Mismatch repair.
This type of repair removes a segment of the newly synthesized
DNA strand that contains mismatch base pairs. The newly synthesized DNA strand is
recognized by its lack of methylation. DNA mismatch repair enzymes are needed
for the following reason: If DNA polymerase does not recognize a base pair mis-
match during DNA replication and does not correct the base pair mismatch using
its 3
S 5 proofing reading exonuclease activity during replication, then a base
pair mismatch occurs that cannot be corrected by DNA polymerase. Therefore,
DNA mismatch repair enzymes are necessary to correct base pair mismatches
missed by DNA polymerase.
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14
CHAPTER 2
A. XERODERMA PIGMENTOSUM (XP; Figure
2-3)
1.
XP is an autosomal recessive genetic dis-
order caused by mutations in nucleotide
excision repair enzymes, which results in
the inability to remove pyrimidine dimers
in individuals who are hypersensitive to
sunlight (UV radiation).
2.
The XPA gene and the XPC gene are two
of the genes involved in the cause of XP.
XPA gene located on chromosome 9q22.3
encodes for a DNA repair enzyme. The
XPC gene located on chromosome 3p25
also encodes for a DNA repair enzyme.
3.
Clinical features include:
sunlight (UV
radiation) hypersensitivity with sunburn-
like reaction, severe skin lesions around
the eyes and eyelids, and malignant skin
cancers (basal and squamous cell carcino-
mas and melanomas) whereby most indi-
viduals die by 30 years of age. Figure 2-2
(top) shows a young girl in the early
stages of XP with a sunburn-like reaction
on the cheeks. Figure 2-2 (bottom) shows
a boy in the late stages of XP with malig-
nant skin cancers in the facial area.
● Figure 2-3 Xeroderma Pigmentosum.
B. COCKAYNE SYNDROME (CS)
1.
CS is an autosomal recessive genetic disorder caused by mutations in excision
repair enzymes involved in transcription-coupled nucleotide excision repair.
2.
The ERCC8 gene and the ERCC6 gene are two of the genes involved in the cause
of CS. The ERCC8 gene located on chromosome 5 encodes for a excision repair
cross-complementing group 8 enzyme. The ERCC6 gene located on chromosome
10q11 encodes for excision repair cross-complementing group 6 enzyme.
3.
Clinical features include:
sunlight (UV radiation) hypersensitivity, short stature,
premature aging, impaired development of nervous system, premature aging, hear-
ing loss, and eye abnormalities (pigmentary retinopathy).
● Figure 2-4 Ataxia-Teleangiectasia.
C. ATAXIA-TELANGIECTASIA (AT; Figure 2-4)
1.
AT is an autosomal recessive genetic disor-
der caused by mutations in DNA recombi-
nation repair enzymes on chromosome
11q22-q23 which results in individuals who
are hypersensitive to ionizing radiation.
2.
The ATM gene (AT mutated) is one of the
genes involved in the cause of AT. The ATM
gene located on chromosome 11q22 en-
codes for a protein where one region resem-
bles a PI-3 kinase (phosphatidylinositol-3
kinase) and another region resembles a
DNA repair enzyme/cell cycle check-
point protein.
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15
CHROMOSOME REPLICATION
3.
Clinical features include:
ionizing radiation hypersensitivity; cerebellar ataxia
with depletion of Purkinje cells; progressive nystagmus; slurred speech; oculocu-
taneous telangiectasia (permanent dilation of preexisting small blood vessels cre-
ating focal red lesions) initially in the bulbar conjunctiva followed by ear, eyelid,
cheeks, and neck; immunodeficiency; and death in the second decade of life. A
high frequency of structural rearrangements of chromosomes 7 and 14 is the cyto-
genetic observation with this disease. Figure 2-4 (top) shows the appearance of
telangiectasia of the bulbar conjunctiva. Figure 2-4 (bottom) shows widespread
telangiectasia of the cheeks and nose.
D. HEREDITARY NONPOLYPOSIS COLORECTAL CANCER (HNPCC; or Warthin-
Lynch Syndrome)
1.
HNPCC is an autosomal dominant genetic disorder caused by mutations in DNA
mismatch repair enzymes which results in the inability to remove single nu-
cleotide mismatches or loops that occur in microsatellite repeat areas.
2.
The four genes involved in the cause of HNPCC include:
a.
MLH1 gene located on chromosome 3p21.3, which encodes for DNA mis-
match repair protein Mlh1.
b.
MSH2 gene located on chromosome 2p22-p21, which encodes for DNA mis-
match repair protein Msh2.
c.
MSH6 gene located on chromosome 2p16, which encodes for DNA mismatch
repair protein Msh6.
d.
PMS2 gene located on chromosome 7p22, which encodes for PMS1 protein
homolog 2.
3.
These genes are the human homologues to the Escherichia coli mutS gene and mutL
gene that code for DNA mismatch repair enzymes.
4.
Clinical features include:
onset of colorectal cancer at a young age, high fre-
quency of carcinomas proximal to the splenic flexure, multiple synchronous or
metachronous colorectal cancers, and presence of extracolonic cancers (e.g., en-
dometrial and ovarian cancer; adenocarcinomas of the stomach, small intestine,
and hepatobiliary tract), and account for 3%–5% of all colorectal cancers.
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Summary of Chromosome Replication Machinery (Table 2-1)
16
CHAPTER 2
DNA REPLICATION MACHINERY
TABLE
2-1
Component
Function
DNA helicase
Recognizes the replication fork and opens up the double helix
DNA Topoisomerases (in general)
Alter the supercoiling of DNA (in general)
Type I topoisomerase
Introduces negative supercoiling (or relaxes positive supercoiling)
Type II topoisomerase
Introduces negative supercoiling (or relaxes positive supercoiling)
DNA gyrase
Introduces negative supercoiling (or relaxes positive supercoiling)
Revere gyrase
Introduces positive supercoiling
High-Fidelity DNA-Directed DNA
Polymerases
DNA polymerase
Synthesizes the lagging strand
DNA polymerase
Repairs DNA by base excision
DNA polymerase
Synthesizes mitochondrial DNA
DNA polymerase
Synthesizes the leading strand
DNA polymerase
Repairs DNA by nucleotide and base excision
Low-Fidelity DNA-Directed DNA
Polymerases
DNA polymerase
DNA polymerase
Involved in hypermutation in B and T lymphocytes
DNA polymerase
DNA polymerase
Telomerase*
Lengthens the end of the lagging strand to its full length
DNA primase
Synthesizes short RNA primers
DNA ligase
Catalyzes the formation of the 3
,5-phosphodiester bond; joins
DNA fragments
Single-stranded binding proteins
Maintain the stability of the replication fork
High fidelity
DNA sequence faithfully copied; Low Fidelity DNA sequence not faithfully copied (error prone).
*Telomerase is an RNA-directed DNA polymerase (or a reverse transcriptase).
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Meiosis and Genetic Recombination
17
Meiosis is a specialized process of cell division (contrasted with
mitosis which occurs in somatic cells; see Chapter 10: Cell Cycle) that occurs only in the
production of the gametes (i.e., occurs only in the testes and ovary). In general, meiosis
consists of two cell divisions (Meiosis I and Meiosis II), but only one round of DNA repli-
cation that results in the formation of four gametes, each containing half the number of
chromosomes (23 chromosomes) and half the amount of DNA (1 N) found in normal
somatic cells (46 chromosomes, 2 N). The various aspects of meiosis compared with mito-
sis are given in Table 3-1.
A. MEIOSIS I. Events that occur during Meiosis I include
1.
Meiosis S phase (DNA replication)
2.
Synapsis
a.
Synapsis refers to the pairing of 46 homologous duplicated chromosomes side
by side which occurs only in Meiosis I (not Meiosis II or mitosis).
b.
In female meiosis, each chromosome has a homologous partner whereby the
two X chromosomes synapse and crossover just like the other pairs of homol-
ogous chromosomes.
c.
In male meiosis, there is a problem because the X and Y chromosomes are very
different. However, the X and Y chromosomes do pair and crossover. The pair-
ing of the X and Y chromosomes is an end-to-end fashion (rather than along
the whole length as for all the other chromosomes) which is made possible by
a 2.6-Mb region of sequence homology between the X and Y chromosomes at
the tips of their p arms.
3.
Crossover
a.
Crossover refers to the equal exchange of large segments of DNA between the
maternal chromatid and paternal chromatid (i.e., nonsister chromatids) at the
chiasma which occurs during prophase (pachytene) of Meiosis I.
b.
Crossover introduces one level of genetic variability among the gametes and
occurs by a process called general recombination.
c.
During crossover, two other events (i.e., unequal crossover and unequal sis-
ter chromatid exchange) introduce variable number tandem repeat (VNTR)
polymorphisms into the human nuclear genome.
4.
Alignment.
Alignment refers to the condition whereby the 46 homologous dupli-
cated chromosomes align at the metaphase plate.
5.
Disjunction
a.
Disjunction refers to the separation of the 46 maternal and paternal homolo-
gous duplicated chromosomes from each other into separate secondary game-
tocytes (Note: the centromeres do not split).
b.
However, the choice of which maternal or paternal homologous duplicated
chromosomes enters the secondary gametocyte is a random distribution.
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