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Medical Genetics
02 人类基因
human gene
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Genes are entities that parents
pass to offspring during
reproduction.
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These entities encode
information essential for the
construction and regulation of
proteins (such as enzymes) and
other molecules that determine
the growth and functioning of
the organism.
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1. Gene Chemistry
Composed as a "double
helix" - two linked chemical
strands.
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Linked together by base pairs that can
assume one of four variant forms:
thymine (T)
guanine (G)
adenine (A)
cytosine (C)
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James Watson and Francis Crick display their newly
discovered model of DNA, the molecular basis of genetics.
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Base sequence form the "genetic
code" that initiates the two major
functions of genes: protein systhesis
and replication.
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2. Gene Structure
Richard J. Roberts and Phillip
A. Sharp (1993 Nobel Prize winner)
made their amazing discovery of the
existence of split genes in the
adenovirus, when they examined a
hybridized nucleic acid molecule
made between a adenoviral mRNA
and its template DNA in the electron
microscope.
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They observed that the mRNA was
much shorter in length and thus was not
encoded as an equal colinear segment of
the DNA molecule . Instead, large loops of
unhybridized DNA (A, B and C in figure)
were seen. Their interpretation was that
the mature messenger RNA was derived
from four discountinuous segments on the
viral DNA.
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The segments retained in the mRNA
they called exons and the intervening
sequences (A, B and C), which are excised
during mRNA processing and maturation,
are called introns.
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GT-AG rule
The predominance of this 'GTAG' rule supports the hypothesis that
spliceosomal introns are spliced in
yeasts, as in most eukaryotes, by a
conserved mechanism.
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2. Human Genome
A genome is all the DNA in an
organism, including its genes. Genes
carry information for making all the
proteins required by all organisms.
These proteins determine, among
other things, how the organism looks,
how well its body metabolizes food
or fights infection, and sometimes
even how it behaves.
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Simple Sequence DNA
All eukaryotes have more than one
type of simple sequence DNA. The repeat
unit length is usually 5-10 bp but may be
as short as 2 or as long as 200. These
repeat units are arranged in head to tail
tandem arrays which vary in length. The
longest tandem array known so far is that
of human alphoid DNA which has a total
array length of approximately 5 million
base pairs but certain simple sequences
have an array length of only a few
hundred base pairs.
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Most simple sequence DNA is
found in heterochromatin particularly at the centromere and at
the ends or telomeres of
chromosomes - though some simple
sequences are scattered at different
positions on different chromosomes.
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Some simple sequence families form
the basis for DNA fingerprinting. These
families are called minisatellites.
Minisatellites have a repeat unit length of
15-100bp and a total array length of 0.5
to 30 kb. The short array length is the
reason they are called minisatellites to
distinguish them from normal satellite
DNA which has array lengths in the range
of a few hundred kb.
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Microsatellite DNA
Microsatellite DNA differs from
minisatellite DNA in that the repeat
units are less than 5bp in length for
example (TG)n or (AAT)n. These
microsatellites are highly
polymorphic in the length of their
repeat arrays and so they can also
be used in forensic analysis, as well
as to identify related groups of
individuals within populations.
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The advantage of microsatellites over
minisatellites is that the size of each allele
is much smaller and they are therefore
more amenable to PCR analysis. Their
small size also means that they are more
likely to remain intact in highly degraded
DNA such as that from forensic samples or
from ancient DNA - for example from
Egyptian mummies.
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Microsatellites are also involved in
a number of human genetic diseases
which have been shown to be due to
variations in the numbers of
trinucleotide repeats.
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Human genome project
Collective name (Human Genome
Initiative ) for several projects begun in
1986 by DOE to create an ordered set of
DNA segments from known chromosomal
locations, develop new computational
methods for analyzing genetic map and
DNA sequence data, and develop new
techniques and instruments for detecting
and analyzing DNA. This DOE initiative is
now known as the Human Genome
Program. The joint national effort, led by
DOE and NIH, is known as the Human
Genome Project.
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Begun formally in 1990, the U.S.
Human Genome Project was a 13year effort coordinated by the U.S.
Department of Energy and the
National Institutes of Health. The
project originally was planned to last
15 years, but rapid technological
advances accelerated the completion
date to 2003.
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• identify all the approximately 20,000-25,000
genes in human DNA,
• determine the sequences of the 3 billion chemical
base pairs that make up human DNA,
• store this information in databases,
• improve tools for data analysis,
• transfer related technologies to the private sector,
and
• address the ethical, legal, and social issues (ELSI)
that may arise from the project.
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3. DNA replication
Before a cell can divide, it must
duplicate all its DNA. In eukaryotes,
this occurs during S phase of the cell
cycle.
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The Steps:
1. A portion of the double helix is
unwound by a helicase.
2. A molecule of a DNA polymerase
binds to one strand of the DNA and begins
moving along it in the 3' to 5' direction,
using it as a template for assembling a
leading strand of nucleotides and
reforming a double helix. In eukaryotes,
this molecule is called DNA polymerase
delta (δ).
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3. Because DNA synthesis can only occur
5' to 3', a molecule of a second type of
DNA polymerase (epsilon, ε, in eukaryotes)
binds to the other template strand as the
double helix opens. This molecule must
synthesize discontinuous segments of
polynucleotides (Okazaki fragments).
Another enzyme, DNA ligase I then
stitches these together into the lagging
strand.
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When the replication process is complete, two DNA
molecules — identical to each other and identical
to the original — have been produced. Each
strand of the original molecule has
• remained intact as it served as the template for
the synthesis of
• a complementary strand.
This mode of replication is described as semiconservative: one-half of each new molecule of
DNA is old; one-half new.
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4. Gene Expression
The majority of genes are expressed as
the proteins they encode. The process
occurs in two steps:
Transcription = DNA → RNA
Translation = RNA → protein
Taken together, they make up the "central
dogma" of biology: DNA → RNA → protein.
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Here is an overview
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Gene Transcription: DNA → RNA
DNA serves as the template for
the synthesis of RNA much as it does
for its own replication.
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Some 50 different protein transcription factors
bind to promoter sites, usually on the 5′ side of
the gene to be transcribed.
An enzyme, an RNA polymerase, binds to the
complex of transcription factors.
Working together, they open the DNA double helix.
The RNA polymerase proceeds down one strand
moving in the 3′ → 5′ direction.
In eukaryotes, this requires — at least for proteinencoding genes — that the nucleosomes in front
of the advancing RNA polymerase (RNAP II) be
removed. A complex of proteins is responsible for
this. The same complex replaces the
nucleosomes after the DNA has been transcribed
and RNAP II has moved on.
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As the RNA polymerase travels along the DNA strand,
it assembles ribonucleotides into a strand of RNA.
Each ribonucleotide is inserted into the growing RNA
strand following the rules of base pairing. Thus for
each C encountered on the DNA strand, a G is
inserted in the RNA; for each G, a C; and for each
T, an A. However, each A on the DNA guides the
insertion of the pyrimidine uracil . There is no T in
RNA.
Synthesis of the RNA proceeds in the 5′ → 3′.
As each nucleoside triphosphate is brought in to add
to the 3′ end of the growing strand, the two
terminal phosphates are removed.
When transcription is complete, the transcript is
released from the polymerase and, shortly
thereafter, the polymerase is released from the
DNA.
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Types of RNA
mRNA. This will later be translated into a polypeptide.
rRNA. This will be used in the building of ribosomes: machinery
for synthesizing proteins by translating mRNA.
tRNA. RNA molecules that carry amino acids to the growing
polypeptide.
snRNA (small nuclear RNA ). DNA transcription of the genes
for mRNA, rRNA, and tRNA produces large precursor
molecules ("primary transcripts") that must be processed
within the nucleus to produce the functional molecules for
export to the cytosol. Some of these processing steps are
mediated by snRNAs.
snoRNA (small nucleolar RNA ). These RNAs within the
nucleolus have several functions (described below).
miRNA (microRNA). These are tiny (~22 nts) RNA molecules
that appear to regulate the expression of messenger RNA
(mRNA) molecules. [Discussion]
XIST RNA. This inactivates one of the two X chromosomes in
female vertebrates.
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Small Nuclear RNA (snRNA)
Approximately a dozen different genes
for snRNAs, each present in multiple
copies, have been identified. The snRNAs
have various roles in the processing of the
other classes of RNA. For example, several
snRNAs are part of the spliceosome that
participates in converting pre-mRNA into
mRNA by excising the introns and splicing
the exons.
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Small Nucleolar RNA (snoRNA)
As the name suggests, these small (60–300
nts) RNAs are found in the nucleolus where they
are responsible for several functions:
• Some participate in making ribosomes by helping
to cut up the large RNA precursor of the 28S,
18S, and 5.8S molecules.
• Others chemically modify many of the
nucleotides in rRNA, tRNA, and snRNA molecules,
e.g., by adding methyl groups to ribose.
• Some have been implicated in the alternative
splicing of pre-mRNA to different forms of mature
mRNA.
• One snoRNA serves as the template for the
synthesis of telomeres
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RNA Processing: pre-mRNA → mRNA
All the primary transcripts produced in
the nucleus must undergo processing
steps to produce functional RNA molecules
for export to the cytosol. We shall confine
ourselves to a view of the steps as they
occur in the processing of pre-mRNA to
mRNA.
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Gene Translation: RNA -> Protein
By means of transfer RNA
molecules, each specific for one
amino acid and for a particular
triplet of nucleotides in mRNA called
a codon. The family of tRNA
molecules enables the codons in a
mRNA molecule to be translated
into the sequence of amino acids in
the protein.
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The Steps of Translation
1. Initiation
2. Elongation
3. Termination
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RNA Interference
Here small RNA molecules bind
to the complementary portion of a
mRNA and prevent it from being
translated by ribosomes or trigger its
destruction.
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