Download 03 Bases of genetic

Bases of genetic
Nucleus & Related Structures
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A nucleus is present in all eukaryotic cells that divide. If
a cell is cut in half, the anucleate portion eventually dies
without dividing. The nucleus is made up in large part of
the chromosomes, the structures in the nucleus that
carry a complete blueprint for all the heritable species
and individual characteristics of the animal.
Except in germ cells, the chromosomes occur in pairs,
one originally from each parent. Each chromosome is
made up of a giant molecule of deoxyribonucleic acid
(DNA).
Endoplasmic Reticulum
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The endoplasmic reticulum is a complex series of tubules
in the cytoplasm of the cell. The outer limb of its
membrane is continuous with a segment of the nuclear
membrane, so in effect this part of the nuclear
membrane is a cistern of the endoplasmic reticulum.
The tubule walls are made up of membrane. In rough or
granular endoplasmic reticulum, granules called
ribosomes are attached to the cytoplasmic side of the
membrane, whereas in smooth or agranular endoplasmic
reticulum, the granules are absent.
RNA
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The nucleus of most cells contains a nucleolus, a
patchwork of granules rich in ribonucleic acid
(RNA). In some cells, the nucleus contains
several of these structures.
Nucleoli are most prominent and numerous in
growing cells. They are the site of synthesis of
ribosomes, the structures in the cytoplasm in
which proteins are synthesized
DNA3
ribosomes
DNA4
DNA5
DNA6
Ribosomes
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The ribosomes that become attached to the endoplasmic
reticulum synthesize all transmembrane proteins, most
secreted proteins, and most proteins that are stored in
the Golgi apparatus, lysosomes, and endosomes. All
these proteins have a hydrophobic signal peptide at one
end.
The Golgi apparatus, which is involved in processing
proteins formed in the ribosomes, and secretory
granules, vesicles, and endosomes are discussed below
in the context of protein synthesis and secretion.
The Genome
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DNA is found in bacteria, in the nuclei of eukaryotic cells,
and in mitochondria. It is made up of two extremely long
nucleotide chains containing the bases adenine (A),
guanine (G), thymine (T), and cytosine (C).
The chains are bound together by hydrogen bonding
between the bases, with adenine bonding to thymine
and guanine to cytosine. An indication of the complexity
of the molecule is the fact that the DNA in the human
haploid genome (the total genetic message) is made up
of 3 × 109 base pairs.
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Formation of mRNA by
Transcription of DNA
A segment of the DNA
molecule is opened, and
RNA polymerase (an
enzyme that is not shown)
assembles nucleotides into
mRNA according to the
base-pair combinations
shown in the inset.
Thus the sequence of
nucleotides in DNA
determines the sequence of
nucleotides in mRNA. As
nucleotides are added, an
mRNA molecule is formed.
The Human Genome
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When the human genome was finally mapped several
years ago, there was considerable surprise that it
contained only about 30,000 genes and not the 50,000
or more that had been expected. Yet humans differ quite
markedly from their nearest simian relatives.
The explanation appears to be that rather than a greater
number of genes in humans, there is a greater number
of mRNAs—perhaps as many as 85,000. The implications
of this increase are discussed below.
Transcription & Translation
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The strands of the DNA double helix not only replicate
themselves, but also serve as templates by lining up
complementary bases for the formation in the nucleus of
messenger RNA (mRNA), transfer RNA (tRNA), the RNA
in the ribosomes (rRNA), and various other RNAs.
The formation of mRNA is called transcription and is
catalyzed by various forms of RNA polymerase. Usually
after some posttranscriptional processing (see below),
mRNA moves to the cytoplasm and dictates the
formation of the polypeptide chain of a protein
(translation). This process occurs in the ribosomes.
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Posttranscriptional
Change in mRNA
An intron is cleaved
from between two
exons and is
discarded. The exons
are spliced together
by spliceosomes to
make the functional
mRNA.
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Translation of mRNA to
Produce a Protein
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To start protein
synthesis a ribosome
binds to mRNA. The
ribosome also has
two binding sites for
tRNA, one of which
is occupied by a
tRNA with its amino
acid. Note that the
codon of mRNA and
the anticodon of
tRNA are aligned and
joined. The
other tRNA binding
site is open.
Translation of mRNA to Produce a Protein
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By occupying the
open tRNA binding
site the next tRNA is
properly aligned
with mRNA and with
the other tRNA.
Translation of mRNA to Produce a Protein
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An enzyme within the
ribosome catalyzes a
synthesis reaction to
form a peptide bond
between the amino
acids. Note that the
amino acids are now
associated with only
one of the tRNAs.
Translation of mRNA
to Produce a Protein
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The ribosome shifts position
by three nucleotides.
The tRNA without the amino
acid is released from the
ribosome, and the tRNA with
the amino acids takes its
position. A tRNA binding site
is left open by the shift.
Additional amino acids can be
added by repeating steps 2
through 4. Eventually a stop
codon in the mRNA ends the
production of the protein,
which is released from the
ribosome.
Translation of mRNA to
Produce a Protein
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Multiple
ribosomes attach
to a single mRNA.
As the ribosomes
move down the
mRNA, proteins
attached to the
ribosomes
lengthen and
eventually detach
from the mRNA.
Cell Cycle
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The cell cycle is divided into
interphase and mitosis.
Interphase is divided into
G1, S, and G2 subphases.
During G1 and G2, the cell
carries out routine metabolic
activities. During the S
phase DNA is replicated. (a)
Following mitosis, two cells
are formed by the process
of cytokinesis.
Each new cell begins a new
cell cycle. (b) Many cells exit
the cell cycle and enter the
G0 phase, where they
remain until stimulated to
divide, at which point they
reenter the cell cycle.
Cell Cycle
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Obviously, the initiation of mitosis and normal cell
division depends on the orderly occurrence of events
during what has come to be called the cell cycle. A
diagram of these events is shown in.
There is intense interest in the biochemical machinery
that produces mitosis, in part because of the obvious
possibility of its relation to cancer. When DNA is
damaged, entry into mitosis is inhibited, giving the cell
time to repair the DNA; failure to repair damaged DNA
leads to cancer.
Mitosis
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Interphase. DNA, which
is dispersed as
chromatin, replicates.
The two strands of each
DNA molecule separate,
and a copy of each
strand is made.
Consequently,
two identical DNA
molecules are
produced. The pair of
centrioles replicates to
produce two pairs of
centrioles
Mitosis
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Prophase. Chromatin
strands condense to
form chromosomes.
Each chromosome is
composed of two
identical strands of
chromatin called
chromatids, which are
joined together at one
point by a specialized
region called the
centromere. Each
chromatid contains one
of the DNA molecules
replicated during
interphase.
Mitosis
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Metaphase.
The chromosomes
align along the
equator with
spindle fibers from
each pair of
centrioles, located
at opposite poles
of the cell,
attached to their
centromeres.
Mitosis
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Anaphase. The
centromeres separate,
and each chromatid is
then referred to as a
chromosome. Thus, when
the centromeres divide,
the chromosome number
doubles, and there are
two identical sets of
chromosomes.
The two sets of
chromosomes are pulled
by the spindle fibers
toward the poles of the
cell.
Mitosis
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Telophase. The migration
of each set of
chromosomes is
complete. A new nuclear
envelope develops from
the endoplasmic
reticulum, and the
nucleoli reappear.
During the latter portion
of telophase the spindle
fibers disappear, and the
chromosomes unravel to
become less distinct
chromatin threads.
Mitosis
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Interphase. Cytokinesis,
which continued from
anaphase through
telophase, becomes
complete when the plasma
membranes move close
enough together at the
equator of the cell to fuse,
completely separating the
two new daughter cells,
each of which now has a
complete set of
chromosomes (a diploid
number of chromosomes)
identical to the parent cell.
Meiosis
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In germ cells, reduction division (meiosis) takes place
during maturation. The net result is that one of each pair
of chromosomes ends up in each mature germ cell;
consequently, each mature germ cell contains half the
amount of chromosomal material found in somatic cells.
Therefore, when a sperm unites with an ovum, the
resulting zygote has the full complement of DNA, half of
which came from the father and half from the mother.
The term "ploidy" is sometimes used to refer to the
number of chromosomes in cells. Normal resting diploid
cells are euploid and become tetraploid just before
division. Aneuploidy is the condition in which a cell
contains other than the haploid number of chromosomes
or an exact multiple of it, and this condition is common
in cancerous cells.