Download RNA and Protein Synthesis

RNA and Protein
Synthesis
Section 12-3
Objectives for this section
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Compare and contrast DNA and RNA
Name the 3 main types of RNA
Describe transcription and the editing of RNA
Identify the genetic code
Summarize translation
Explain the relationship between genes and
proteins
Introduction
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The structure of DNA explains how it can be
copied, but it does not tell how a gene works.
Genes—coded DNA instructions that control
the production of proteins within the cell
Introduction
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We now know that the central dogma of
biology is DNA to RNA to protein
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The first step in decoding the genetic message is
to copy part of the DNA nucleotide sequence into
RNA (ribonucleic acid)
These RNA molecules contain the coded
instructions for making proteins
The Central Dogma of Bilogy
The Structure of RNA
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RNA, like DNA, is a long chain of
nucleotides
RNA consists of the same components as
DNA
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A 5-carbon sugar
A phosphate group
A nitrogenous base
The Structure of RNA
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There are 3 main differences between DNA
and RNA
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The sugar in RNA is ribose instead of the
deoxyribose in DNA
RNA is generally single-stranded
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RNA contains uracil instead of thymine
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The Structure of RNA
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RNA is like a disposable copy of a segment
of DNA
In many cases, RNA is a copy of a single
gene—the ability to copy a single DNA
sequence into RNA makes it possible for a
single gene to produce hundreds or even
thousands of RNA molecules
Types of RNA
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RNA molecules have many functions, but in
the majority of cells most RNA molecules are
involved in just one job—protein synthesis
There are 3 main types of RNA
Types of RNA
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Messenger RNA (mRNA)—carry copies of
instructions for assembling amino acids into
proteins; serve as “messengers” from DNA to
the rest of the cell
Types of RNA
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Ribosomal RNA (rRNA)—a component of
ribosomes
Types of RNA
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Transfer RNA (tRNA)—works during the
construction of a protein; transfers each amino
acid to the ribosome as it is specified by the
coded messages in the mRNA
Types of RNA
Transcription
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Transcription—when RNA molecules are
produced by copying part of the nucleotide
sequence of DNA into a complementary
sequence in RNA
Transcription requires the enzyme RNA
polymerase—binds to DNA and separates the
DNA strands. Then uses one strand of DNA
as a template from which nucleotides are
assembled into a strand of RNA
Transcription
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How does the RNA polymerase know where
to start and stop making the RNA copy?
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The enzyme will only bind to regions of DNA
known as promoters, which have specific base
sequences
Similar signals tell the RNA polymerase
where to stop
Transcription
RNA Editing
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The first molecule of mRNA (known as the
pre-mRNA) produced by copying the DNA
sequence is like a rough draft and it requires
editing
DNA contains sequences of nucleotides called
introns, which are not involved in coding for
proteins
RNA Editing
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The DNA sequences that code for proteins are
called exons, because they are expressed in
the synthesis of proteins
When an RNA molecule is formed, it contains
both introns and exons
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The introns are cut out of the RNA molecule
while it is still in the nucleus
The remaining exons are spliced back together
and form the final pre-mRNA molecule
RNA Editing
The Genetic Code
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Remember that proteins are made by joining
amino acids into long chains called
polypeptides
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Each polypeptide contains a combination of any
or all of the 20 different amino acids
The properties of proteins are determined by
the order in which different amino acids are
joined together to produce polypeptides
The Genetic Code
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The “language” of mRNA instructions is
called the genetic code
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RNA contains 4 different nitrogenous bases (U,
C, G, A)
How can a code with only 4 letters translate into
20 different amino acids?
The genetic code is read 3 letters at a time, so
each “word” of the coded message is 3 bases long
The Genetic Code
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Each 3-letter “word” in mRNA is known as a
codon
A codon consists of 3 consecutive nucleotides
that specify a single amino acid that is to be
added to the polypeptide
Example: UCGCACGGU would be read as
UCG-CAC-GGU. These codons represent 3
amino acids: serine-histidine-glycine
The Genetic Code
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Because there are 4 different bases, there are
64 possible 3-base codons.
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Note that some amino acids can be specified by
more than one codon
There are also “start” and “stop” codons
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Start codons (AUG) tell where protein synthesis is to
begin
Stop condons (3 different ones) tell where the
end of the polypeptide is
The Genetic Code
Translation
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The mRNA molecule has been transcribed
and serves as instructions, but we need
something to read the instructions and put
them to use
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In the cell, the ribosome takes care of this
Translation—the decoding of an mRNA
message into a polypeptide chain (protein)
Translation
Steps in translation
 Begins when an mRNA molecule in the
cytoplasm attaches to a ribosome
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Each codon of the mRNA moves through the
ribosome and the proper amino acid is brought to
the ribosome by the tRNA
Translation
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Each tRNA carries only one kind of amino
acid and picks it up based on the anitcodon it
is carrying
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Example: if the anticodon is UUU, the tRNA
would pick up the amino acid with the codon
AAA (they are opposites)
What codon would these anticodons
pick up??
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ACG
UGG
CAG
GGG
GCC
CAA
Translation
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The ribosome forms a peptide bond between
the first and second amino acids
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At the same time, it breaks the bond with the
tRNA molecule and releases it
The ribosome moves on the third amino codon,
where a tRNA molecule brings it the amino acid
specified by the third codon
Translation
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The polypeptide chain continues to grow until
the ribosome reaches a stop codon on the
mRNA molecule
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When the stop codon is reached, it releases the
newly formed polypepetide and mRNA molecule,
completing the process of translation
Genes and Proteins
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What does protein synthesis have to do with the
color of a flower, eye color, or height?
Remember that many proteins are enzymes, which
catalyze and regulate chemical reactions.
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A gene that codes for an enzyme to produces pigment
controls flower color.
Proteins are specific tools that build or operate
components of living cells.