Chapter 9
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Part 3: The Continuity of Life
How Genes Work
Review
DNA is made of
molecules which are composed
of
. Each nucleotide has a central
, a
group and a
base.
The physical
structure
of DNA is a double
: two
strands of
twisted
around
each other.
DNA
involves these steps:
The
enzyme
unwinds the double helix, and
lays down an RNA
.
DNA
adds
nucleotides
to each
strand.
DNA
joins the fragments together.
A simplistic model of cellular communication called the
central
views the genetic information flow from
to
in two steps: transcription and translation, resulting in the
of a gene.
Transcription takes place in the
.
RNA
binds to a region on the DNA called the
to assemble a
RNA transcript which contains coding regions called
and noncoding regions called
.
The
exons
are
together to form mature
RNA (mRNA) which exits the nucleus.
Sequences of 3 nucleotides on the mRNA serve as
to translate the genetic code into sequences of
acids, using a
genetic
.
Translation takes place in the
on
.
The ribosome binds mRNA; in turn a
RNA
(tRNA), whose
is complementary to the
on the mRNA, binds to the complex.
The tRNA moves from the
site to the
site and exits at the
site,
adding
a specific amino acid to the growing
chain, until a
codon on the mRNA is reached.
Gene expression can be turned on or off by regulating access to the
, which controls a cluster of genes that are transcribed together as a unit called an
, as shown in prokaryotes.
The inducible
lac
operon is turned off in the
absence
of lactose when a
binds to the
. When lactose is present, the operon is turned
on
when
acts as an inducer by inactivating the
.
A
mutation
is a change in the
, due to errors in DNA
or to damage from physical or chemical agents called
.
Mutations
that change a single base pair are called
mutations caused by base
.
Insertions and deletions can affect the reading
and lead to
mutations that affect large sections of the protein.
Some genes can move from one place on the chromosome to another through
and result in either a
mutated
gene or abnormal
of the gene.
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Oct 3, 2008
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