Download Biology 10.1 How Proteins are Made:

Biology 10.1 How Proteins are Made:
How
Proteins
are
Made
10/31/2009
Decoding the Information of DNA:
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Traits, such as eye color, are
determined by proteins that
are built according to
instructions coded in DNA.
Proteins are not built directly
from DNA however. Ribonucleic
acid is also involved.
Like DNA, ribonucleic acid is a
molecule made of three
nucleotides linked together.
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RNA
How Proteins are Made:
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RNA a Blueprint Copy
When an organism needs to use
the data stored in the genome,
e.g. to build components of a new
cell, a copy of the required DNA
part is made.
This copy is called RNA and is
almost identical to DNA. Just
like DNA, RNA is an abbreviated
form of a chemical name which in
the case of RNA is ribonucleic
acid.
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How Proteins are Made:
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RNA : 3 differences
between DNA and RNA
Unlike the double stranded
DNA, RNA is only made up of a
single strand.
Furthermore, the base T,
thymine, is replaced by U, uracil
in RNA.
RNA nucleotides also contain the
five-carbon sugar ribose rather
than the sugar deoxyribose,
which is found in DNA
nucleotides.
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How Proteins are Made:
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RNA
 This RNA string is used by the
organism as a template when it
builds protein molecules,
sometimes called the building
blocks of the body.
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For example, your muscles and
hair are mostly made up of
proteins.
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How Proteins are Made:
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RNA: Transcription
A gene’s instructions for making
a protein are coded in the
sequence of nucleotides in the
gene.
The instructions for making a
protein are transferred from a
gene to an RNA molecule in a
process called transcription.
Cells than use two different
types of RNA to read the
instructions on the RNA molecule
and put together the amino acids
in a process called translation.
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How Proteins are Made:
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RNA : Transcription
The entire process by which
proteins are made based on the
information encoded in DNA is
called gene expression or protein
synthesis.
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Transfer of Information from DNA to RNA
RNA: Transcription
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The first step in the making of a
protein, transcription, takes the
information found in a gene in
the DNA and transfers it to a
molecule of RNA.
RNA polymerase , an enzyme
that adds and links
complementary RNA nucleotides
during transcription, is required
for this process.
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Transfer of Information from DNA to RNA
RNA: Transcription Steps
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Step 1:
 Transcription begins when RNA
polymerase binds to the gene’s
promoter; a specific sequence of
DNA that acts as a “start” signal
for transcription.
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Step 2:
 RNA polymerase than unwinds and
separates the two strands of the
double helix, exposing the DNA
nucleotides on each strand.
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Transfer of Information from DNA to RNA
RNA: Transcription Steps
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Step 3:
 RNA polymerase adds and than
links complementary RNA
nucleotides as it “reads” the gene.
 RNA polymerase moves along the
nucleotides of the DNA strand
that has the gene, like a train
moves along a track.
 Transcription follows the base-
pairing rules for DNA replication
except that in RNA, uracil, rather
than thymine, pairs with adenine.
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Transfer of Information from DNA to RNA
RNA: Transcription Steps
 As transcription proceeds, the
RNA polymerase eventually
reaches a “stop signal” in the DNA.
 The stop signal is a sequence of
bases that marks the end of each
gene in eukaryotes, or the end of a
set of genes in prokaryotes.
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Transfer of Information from DNA to RNA
RNA: Transcription
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When the RNA nucleotides are
added during transcription, they
are linked together with covalent
bonds.
As RNA polymerase moves down
the strand, a single strand of
RNA grows.
Behind RNA polymerase, the two
strands of DNA close up forming
hydrogen bonds between
complementary bases, reforming
the DNA double-helix.
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Transfer of Information from DNA to RNA
RNA: Transcription
 Like DNA replication,
transcription uses DNA
nucleotides as a template for
making a new molecule.
 In DNA replication, the new
molecule made is DNA.
 In RNA transcription, the
new molecule made is RNA
instead.
 In DNA replication, both
strands of DNA serve as
templates.
 In transcription, only part of
one of the strands of DNA
(gene) serves as a template.
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Transfer of Information from DNA to RNA
RNA: Transcription:
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Transcription in prokaryote
cells occurs in the cytoplasm.
Transcription in eukaryote
cells occurs in the nucleus,
where the DNA is located.
During transcription, many identical
RNA molecules are made
simultaneously from a single gene.
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The Genetic Code
RNA:
The Genetic Code: ThreeNucleotide “Words”
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Different types of RNA are made
during transcription, depending on the
gene being expressed. When a cell
needs a particular protein, it is
messenger RNA that is made.
Messenger RNA (mRNA) is a form of
RNA that carries the instructions for
making a protein from a gene and
delivers it to the site of translation.
The information is translated from the
language of RNA, nucleotides, to the
language of proteins, amino acids.
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The Genetic Code
RNA:
The Genetic Code: ThreeNucleotide “Words”
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The RNA instructions are
written as a series of threenucleotide sequences on the
mRNA called codons.
Each codon along the mRNA
strand corresponds to an amino
acid or signifies a start or stop
signal for translation.
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The Genetic Code
RNA:
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The Genetic Code: Three-Nucleotide “Words”
In 1961, an American biochemist Marshall Nirenberg, deciphered the
first codon by making artificial mRNA that contained only the base
uracil.
The mRNA was translated into a protein made up entirely of
phenylalanine amino-acids subunits.
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The Genetic Code
RNA: The Genetic Code: Three-Nucleotide “Words”
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Nirenberg concluded that the codon UUU is the instruction for the
amino acid phenylalanine. Later, scientists deciphered the other
codons.
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RNAs Role in Translation:
RNA: The Genetic Code: ThreeNucleotide “Words”:
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The complete list of codons, in
their groups of threes, makes
up the genetic code
deciphered by scientists that
provides the instructions for
all the amino acids and the
“start” and “stop” signals that
are coded by each of the 64
mRNA possible combinations.
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RNAs Role in Translation:
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Translation takes place in the
cytoplasm. Here transfer RNA
molecules and ribosomes help in
the synthesis of proteins.
Transfer RNA (tRNA) molecules
are single strands of RNA that
temporarily carry a specific amino
acid on one end.
Each tRNA is folded into a
compact shape and has an
anticodon . An anticodon is a
three-nucleotide sequence on a
tRNA that is complementary to
an mRNA codon.
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RNAs Role in Translation:
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Ribosomes are composed of both proteins and ribosomal RNA
(rRNA). Ribosomal RNA molecules are RNA molecules that are part
of the structure of ribosomes.
A cell’s cytoplasm contains thousands of ribosomes. Each ribosome
temporarily holds one mRNA and two tRNA molecules.
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RNAs Role in Translation:
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Translation is the process of synthesis of a protein by ribosomes, using
mRNA as a template.
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The genetic message in mRNA is 'read' by organelles called ribosomes in
order to make a particular protein. tRNA is also required for this process.
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tRNAs are specific for one particular amino acid and each tRNA carries
required amino acids to the ribosome in order to synthesize the
polypeptide chain.
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RNAs Role in Translation:
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The ribosome 'reads' the mRNA language in the 5' to 3' direction.
Each codon (sets of three nucleotide bases) specifies one amino acid
from which proteins are made.
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RNAs Role in Translation:
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So, the mRNA language indicates the sequence of amino acids for the
synthesis of a protein.
The mRNA language begins with the codon AUG (initiation codon,
which starts making a protein chain) and ends with UAA, UAG or UGA
(stop codons also called terminators of a protein chain).
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As each codon is 'read', the amino acids are carried to the site of
formation of the polypeptide chain by the particular tRNA.
Each tRNA has an anticodon that are opposite to the particular codon
on the mRNA e.g. if the mRNA codon is AGG then the matching tRNA
anticodon is UCC.
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Once a amino acid is bound to the forming polypeptide chain the next
codon is read by the ribosome.
The sequence of reading the mRNA and adding an amino acid continues
until the 'stop' sequence (codon) is recognized.
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As the mRNA moves across the
ribosome, another ribosome can
find the AUG codon on the same
mRNA and begin making a
second copy of the same
protein.
In this way many copies of the
same protein are made from a
single mRNA molecule.
With few exceptions, the
genetic code is the same in all
organisms. For this reason, the
genetic code is often described
as being nearly universal.
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