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DNA and Protein Synthesis
California Standards 4 and 5
The Components and Structure of DNA
DNA is made up of
A nucleotide is a
made up of
There are four kinds of bases in in DNA:
The backbone of a DNA chain is formed by
of each nucleotide.
The nucleotides can be joined together in any order.
X-Ray Evidence
used X-ray
diffraction to get information about the structure of DNA.
She aimed an
at concentrated
DNA samples and recorded the scattering pattern of the X-rays
on film.
The Double Helix
Using clues from Franklin’s pattern,
built a model that explained how
DNA carried information and could be copied.
Watson and Crick's model of DNA was a
, in which
were wound around each
other, like a
The Components and Structure of DNA
Watson and Crick discovered that
can form only between certain
base pairs—
, and
This principle is called
of the DNA double helix has all the information needed to
by the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a single point and continues in two directions.
In eukaryotic chromosomes,
occurs at hundreds of places.
until each chromosome is completely copied.
The sites where separation and replication occur are called
Duplicating DNA
Before a cell divides, it
in a process called
Replication ensures that each resulting cell will have a complete set of DNA.
DNA Replication
During DNA replication, the
into two
strands, then produces two new complementary strands following the rules of base pairing.
Each strand of the double helix of
the new strand.
This form of replication is called
How Replication Occurs
DNA replication is carried out by
that “unzip” a molecule of DNA.
base pairs are broken and the two strands of DNA
The principal enzyme involved in DNA replication is
joins individual nucleotides to produce a DNA molecule
and then “
” each new DNA strand.
Replication can begin in the middle of the DNA chain.
The two chains unzip to form a
The two chains unzip until both chains have be separated and copied.
As the replication bubble expands, one of the new DNA chains is
replicated in
This is the “
” strand.
The opposite strand, being formed in the opposite direction is the “
” strand.
It is formed in
that are ultimately
by an enzyme called
DNA ligase attaches all DNA fragments together into a
RNA Function
are coded
that control the
Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into RNA.
The Structure of RNA
RNA consists of a
Each nucleotide is made up of 1
There are three main differences between RNA and DNA:
The sugar in
instead of deoxyribose.
RNA is generally
RNA contains
in place of
There are three main types of RNA:
carries copies of instructions for
assembling amino acids into proteins.
Ribosomes are made up of proteins and
During protein construction,
each amino acid to the ribosome.
What is transcription?
molecules are produced
part of a
into a complementary sequence in RNA.
This process is called
Transcription requires the
During transcription,
binds to DNA and
then uses one strand of
from which nucleotides are assembled into a strand of RNA.
RNA polymerase binds only to regions of DNA known as
Promoters are signals in DNA that indicate to the enzyme
make RNA.
RNA Editing
The DNA of eukaryotic genes contains sequences of nucleotides, called
, that are
in coding for proteins.
are called
When RNA molecules are formed, introns and exons are copied from
of RNA
The exons are the
to form
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (
consists of
on mRNA that specify a particular
amino acid.
specifies a particular
that is to be placed on the
polypeptide chain.
Some amino acids can be specified by more than one codon.
There is one codon
that can either specify the amino acid methionine or
serve as a
for protein synthesis.
There are
that do not code for any amino acid.
These “stop” codons signify the
What is translation?
Translation is the
message into
Translation takes place on
During translation, the cell uses information from
is transcribed in the nucleus, and then enters the
cytoplasm where it
when an mRNA molecule attaches to a ribosome.
As each codon of the mRNA molecule
moves through the ribosome, the
is brought into
the ribosome
In the ribosome, the amino acid is
transferred to the growing polypeptide
kind of
In addition to an amino acid, each
tRNA molecule has
These bases, called the
, are complementary to
one mRNA codon.
The ribosome binds new tRNA
molecules and amino acids as it moves
along the mRNA.
The process continues until the
ribosome reaches a stop codon.
The Roles of RNA and DNA
The cell
to prepare
The DNA stays in the nucleus.
The RNA molecules go to the protein building sites in the cytoplasm—the ribosomes.
Genes and Proteins
Many proteins are
, which catalyze and
Proteins are each specifically designed to build or operate a component of a living cell.
The sequence of bases in DNA is used as a template for mRNA.
of mRNA specify the sequence of
in a protein.
in the genetic material.
Kinds of Mutations
Mutations that produce changes in a
are known as
Mutations that produce changes in whole chromosomes are known as
chromosomal mutations.
Gene mutations involving a change in
are known as
because they occur at a single point in the DNA sequence.
Point mutations include
usually affect no more than a
The effects of insertions or deletions are more dramatic.
of a nucleotide
causes a shift in the grouping of codons.
Changes like these are called
Frameshift mutations may
that follows the point of the mutation.
Frameshift mutations can
so much that it is unable to perform its normal functions.
In an
, an
is inserted into a base sequence.
In a
, the
deleted and the reading frame is shifted.
Manipulating DNA
The Tools of Molecular Biology
How do scientists make changes to DNA?
Scientists use their knowledge of the structure of DNA and its chemical properties to study and
change DNA molecules.
Scientists use different techniques to:
from cells
into smaller pieces
the sequence of
in a DNA molecule
of DNA
, biologists make changes in the
DNA code of a living organism.
DNA Extraction
DNA can be extracted from most cells by a simple
chemical procedure.
The cells are opened and the DNA is separated from the
other cell parts.
Cutting DNA
Most DNA molecules are too large to be analyzed, so
cuts DNA at a
of nucleotides.
A restriction enzyme will cut a DNA sequence only if it
matches the sequence precisely.
The locations where the enzyme opens the DNA are called
Using the DNA Sequence
Cutting and Pasting
Short sequences of DNA can be assembled using
“Synthetic” sequences can be joined to “natural” sequences using enzymes that splice DNA together.
These enzymes also make it possible to take a
it to the
Such DNA molecules are sometimes called
Separating DNA
, DNA fragments travel through a
, a porous material.
than the longer lengths.
Gel electrophoresis can be used to
of different organisms or
different individuals.
Making Copies
is a technique that allows biologists to
A biologist adds short pieces of DNA that are
complementary to portions of the sequence.
DNA is heated to separate its two strands
starts making copies of the region between the primers
two separated strands
Transforming Bacteria
can be
so the bacteria
is common
Foreign DNA is first joined to a
found in
These rings are known as
Using restriction enzymes, human
the bacteria
, and
Transforming Plant Cells
To help improve the quality of many crops, plants receive
for a
variety of genes.
Plants receive genes that make them