Download Transcription and genetic code

17
FROM GENE TO
PROTEIN
Genes specify proteins via transcription
and translation
The products of Gene Expression: A Developing
Story
Basic Principles of Transcription and Translation
The genetic code
The genetic code must have evolved very early in the history of
life
The products of Gene Expression:
It was hypothesized earlier that “one gene–one
enzyme”. As researchers learned more about
proteins, they slightly modified this hypothesis:
• While most enzymes are proteins, many proteins
are not enzymes (e.g. keratin, insulin, ect…).
Proteins that are not enzymes are still, nevertheless,
gene products. Scientists began to think in terms of
one gene–one protein.
• Also, many proteins are comprised of two or more
polypeptide chains, each chain specified by its own
gene (e.g. globulin chains of hemoglobin).
• As a result of this new information, the previous
hypothesis has been restated as one gene-one
polypeptide.
Basic Principles of Transcription and
Translation
• Genes provide the instructions for making
specific proteins.
• The bridge between DNA and protein synthesis is
RNA.
• RNA is chemically similar to DNA, except that it
contains ribose as its sugar and substitutes the
nitrogenous base uracil for thymine.
– An RNA molecules almost always consists of a single
strand.
• In DNA or RNA, the four nucleotide monomers
act like the letters of the alphabet to
communicate information.
• The specific sequence of hundreds or thousands
of nucleotides in each gene carries the
information for the primary structure of a
protein, the linear order of the 20 possible
amino acids.
• To get from DNA, written in one chemical
language, to protein, written in another, requires
two major stages, transcription and
translation.
• Transcription is the synthesis of RNA under
the direction of DNA.
• During transcription, a DNA strand provides a
template for the synthesis of a complementary
RNA strand.
• This process is used to synthesize any type of RNA
from a DNA template.
• Transcription of a gene produces a messenger
RNA (mRNA) molecule.
• Translation is the actual synthesis of
polypeptide under the direction of mRNA.
• During translation, the cell must translate
the base sequence of an mRNA molecule
into the amino acid sequence of a
polypeptide.
• Translation occurs at ribosomes.
• The basic mechanics of transcription and
translation are similar in eukaryotes and
prokaryotes.
• Because bacteria lack nuclei, transcription and
translation are coupled.
• Ribosomes attach to the leading end of a mRNA
molecule while transcription is still in progress.
Fig. 17.3a
• In a eukaryotic cell, almost all transcription
occurs in the nucleus and translation occurs
mainly at ribosomes in the cytoplasm.
• In addition, before the
primary transcript
(pre-mRNA) can leave the
nucleus, it is modified in
various ways during
RNA processing
before the finished
mRNA is exported
to the cytoplasm.
Fig. 17.3b
• To summarize, genes program protein synthesis
via genetic messenger RNA.
• The molecular chain of command in a cell is :
DNA
RNA
•Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
protein.
The Genetic Code
Codons: Triplets of Bases
•
If the genetic code consisted of a single nucleotide or
even pairs of nucleotides per amino acid, there
would not be enough combinations (4 and 16
respectively) to code for all 20 amino acids.
•
Triplets of nucleotide bases are the smallest units of
uniform length that can code for all the amino acids.
•
In the triplet code, three consecutive bases specify
an amino acid, creating 43 (64) possible code words.
•
The genetic instructions for a polypeptide chain are
written in DNA as a series of three-nucleotide words.
• During transcription, one DNA strand, the
template strand, provides a template for
ordering the sequence of nucleotides in an RNA
transcript.
• The complementary RNA
molecule is synthesized
according to base-pairing
rules, except that uracil is
the complementary base
to adenine.
• During translation, blocks
of three nucleotides,
codons, are decoded into
a sequence of amino acids.
Fig. 17.4
• During translation, the codons are read in the 5’
3’ direction along the mRNA.
• Each codon specifies which one of the 20 amino
acids will be incorporated at the corresponding
position along a polypeptide.
• Because codons are base triplets, the number of
nucleotides making up a genetic message must
be three times the number of amino acids making
up the protein product.
• It would take at least 300 nucleotides to code for a
polypeptide that is 100 amino acids long.
Cracking the Genetic Code
• The first codon was deciphered in 1961 by
Marshall Nirenberg of the National Institutes of
Health.
• Marshall Nirenberg determined the first match,
that UUU coded for the amino acid
phenylalanine.
• He created an artificial mRNA molecule entirely of
uracil and added it to a test tube mixture of amino
acids, ribosomes, and other components for protein
synthesis.
• This “poly(U)” translated into a polypeptide
containing a single amino acid, phenyalanine, in a
long chain.
• Other more elaborate techniques were required to
decode mixed triplets such a AUA and CGA.
• By the mid-1960s the entire code was
determined.
• 61 of 64 triplets code
for amino acids.
• The codon AUG not
only codes for the
amino acid methionine
but also indicates the
start of translation.
• Three codons do
not indicate amino
acids but signal
the termination
of translation.
Fig. 17.5
• The genetic code is redundant but not
ambiguous.
• There are typically several different codons that
would indicate a specific amino acid.
• However, any one codon indicates only one amino
acid.
• [If you have a specific codon, you can be sure of the
corresponding amino acid, but if you know only the amino
acid, there may be several possible codons.]
• Both GAA and GAG specify glutamate, but no other amino
acid.
• Codons synonymous for the same amino acid often
differ only in the third codon position.
• In summary, genetic information is encoded as a
sequence of nonoverlapping base triplets, or
codons, each of which is translated into a
specific amino acid during protein synthesis.
• To extract the message from the genetic code
requires specifying the correct starting point.
• This establishes the reading frame and subsequent
codons are read in groups of three nucleotides.
• For example, the sequence of amino acids - Trp - Phe
- Gly - Arg - Phe - can be assembled in the correct
order
only
if
the
mRNA
codons
UGGUUUGGCCGUUUU are read in the correct
sequence and groups.
• The cell reads the message in the correct frame as a
series of non-overlapping three-letter words: UGG UUU - GGC - CGU - UUU.
CHAPTER 17
FROM GENE TO
PROTEIN
Section B: The Synthesis and Processing of
RNA
1. Transcription is the DNA-directed synthesis of RNA: a closer
look
2. Eukaryotic cells modify RNA after transcription
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
17.2 Transcription is the DNA-directed
synthesis of RNA: a closer look
• Messenger RNA is transcribed from the template
•
•
strand of a gene.
RNA polymerase separates the DNA strands at the
appropriate point and links the RNA nucleotides as
they base-pair along the DNA template.
Like DNA polymerases, RNA polymerases can add
nucleotides only to the 3’ end of the growing
polymer.
• Genes are read 3’
5’, creating a 5’
3’
RNA molecule.
• Specific sequences of nucleotides along the DNA
mark where gene transcription begins and ends:
• RNA polymerase attaches and initiates transcription at
the promotor.
• The terminator signals the end of transcription.
• The promotor, terminator and the nucleotides in
between are called a transcription unit.
• Bacteria have a single type of RNA polymerase that
synthesizes all RNA molecules.
• In contrast, eukaryotes have three RNA polymerases
(I, II, and III) in their nuclei.
• RNA polymerase II is used for mRNA synthesis.
• Transcription
can be
separated
into three
stages:
initiation,
elongation,
and
termination.
Fig. 17.6a
A. RNA Polymerase Binding and Initiation of
Transcription
• The presence of a promotor sequence
determines which strand of the DNA helix is
the template.
• Within the promotor is the starting point for the
transcription of a gene.
• The promotor also includes a binding site for RNA
polymerase several dozen nucleotides upstream of
the start point.
• In prokaryotes, RNA polymerase can recognize and
bind directly to the promotor region.
• In eukaryotes, proteins called transcription
factors recognize the promotor region,
especially a TATA box, and bind to the
promotor.
• After they have bound
to the promotor,
RNA polymerase
binds to transcription
factors to create a
transcription
initiation complex.
• RNA polymerase
then starts
transcription. Fig. 17.7
B. Elongation of the RNA Strand
• As RNA polymerase moves along the DNA, it
untwists the double helix, 10 to 20 bases at time.
• The enzyme adds
nucleotides to the
3’ end of the
growing strand.
• Behind the point
of RNA synthesis,
the double helix
re-forms and the
RNA molecule
peels away. Fig. 17.6b
• A single gene can be transcribed simultaneously
by several RNA polymerases II, following each
other, at a time.
• A growing strand of mRNA trails off from each
polymerase.
• The length of each new strand reflects how far
along the template the enzyme has traveled from the
start point.
• The congregation of many polymerase
molecules simultaneously transcribing a single
gene increases the amount of mRNA
transcribed from it.
• This helps the cell make the encoded protein in
large amounts.
C. Termination of Transcription
• Transcription proceeds until after the RNA
polymerase transcribes a terminator sequence in
the DNA.
• In prokaryotes, RNA polymerase stops transcription
right at the end of the terminator.
• Both the RNA and DNA is then released.
• In eukaryotes, at a point 10-35 nucleotides
downstream from the terminator sequence
(AAUAAA signal), the pre-mRNA is released from
the.
• At a point about 10 to 35 nucleotides past this sequence,
the pre-mRNA is cut from the enzyme.