Transcript transfer RNA

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A B C D E F G H I J K L M
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N O P Q R S T U V
W X Y Z
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• The sole function of
DNA in each of our cells
is to synthesize protein.
• Today we’re going to
find out just how DNA
accomplishes this.
DNA
Transcription
RNA
Translation
Protein
• Muscle contraction
• Transporting O2 in the
bloodstream Hemoglobin
• Providing immunity
Antibodies
• Getting reactions going
Enzymes
•Determine structure and function
Cytoskeleton
The proteins that your DNA codes for help
determine:
• Where is most of the DNA in our cells?
Inside the nucleus
• Where in our cells does protein synthesis
occur? On the ribosomes
• Since DNA never leaves the nucleus to
perform protein synthesis, its code must find
a way to get out of the nucleus.
• Another nucleic acid known as RNA
helps out.
Think of DNA as the “keeper of the code”, or the “recipe” for the
organism that contains it. These banding patterns on chromosomes
represent “genes”. Genes are regions on chromosomes that code for
specific proteins.
While many parts of that code are important parts of the “recipe”, some
parts are simply “filler”, and are unnecessary, so far as we understand.
These unnecessary sections are called introns, and they are sequences
of DNA, within genes, which have no apparent purpose.
Within the cells of
multicellular
animals and plants
almost every gene
has introns, but
they’re fairly rare Exons are the working or
in prokaryotes, expressed segments of
and unicellular genes.
Both terms, "introns" and "exons," were coined by
eukaryotes.
Harvard biologist Walter Gilbert in 1978
• The transcription of most genes produces a messenger
RNA (mRNA)
• The translation of mRNA requires the work of the
transfer RNA (tRNA)
• Ribosomal RNA is the rRNA that the ribosomes are
composed of, and the actual site of protein synthesis
As we already learned, RNA is almost, but not exactly like
DNA.
 RNA contains the pyrmidine uracil, instead of thymine
 RNA is single stranded instead of a double helix
 RNA contains Ribose instead of deoxyribose
 RNA contains a hydroxyl group on the 2’ carbon, not a
hydrogen like DNA
In the nucleus, transcription occurs when a part of
the DNA molecule unzips, as if replication was
going to occur.
The protein that is needed by
the cell is coded for by a
specific sequence of
nucleotides. (a gene)
• a deoxyribose sugar
•A phosphate group
•A nitrogenous base
It is that specific section that unwinds when
transcription occurs. (instead of the whole strand)
Transcription is
initiated at a
promoter.
This promoter signals the start of a
gene. The section of DNA that gets
transcribed is called a transcription
unit.
An enzyme called RNA
polymerase catalyzes
nucleotide additions to
the growing RNA strand
complementing the DNA
5’
strand.
3’
This enzyme moves along
the DNA strand, reading
5’
it in a 3’ to 5’ direction,
3’
joining complementary
RNA nucleotides
When the protein coding section ends at the
terminator, the new RNA molecule is released as a
free transcript.
At this point the newly formed RNA is a “PremRNA”, and must be modified before its proteinbuilding instructions can be put to use.
A cap binding protein
complex (CBC) forms
at the 5’ end which is
needed when the
mRNA is exported
from the nucleus.
To the 3’ end, a poly-tail of about 100 to 300
nucleotides is found, which will help the newly
formed mRNA bind to a location on the ribosome.
This pre-mRNA still contains non-coding sections.
Some human genes can have 50 introns in a single gene! Typically there
are four introns within five exons, but the introns are generally much
larger than the exons. Introns must be spliced out from between the
exons before the mRNA can leave the nucleus.
Spliceosomes are composed of five small nuclear RNA proteins, called
snRNPs, (pronounced "snurps")
The protein building “words” encrypted within the
mRNA’s message are read three at a time. Each
base triplet is known as a codon.
Think of codons as being the
words that need to be
translated by the tRNA.
Things to notice about chart:
• there are 64 possible codon
combinations
• there are only 20 amino
acids
Methionine (AUG) is the start • most amino acids
codon which signals the
correspond to more than
beginning of translation, and
one codon code
there are several “stop”
codons.
•The mature mRNA (messenger RNA) finds its way to the
ribosome outside the nucleus, and continues to help in the
process of protein synthesis.
• The next step in the process involves an interpretation, or
“translation” of the mRNA’s message by a molecule called
tRNA (or transfer RNA)
• This message is translated as a three letter code, or triplet
code. Every three bases in sequence codes for a different
word in the protein puzzle.
• The three letter word (codon) brought to the ribosome by
the mRNA, is translated by the anticodon of the tRNA.
Activation: Occurs when an amino acid is covalently bonded to its
corresponding anticodon on the tRNA.
Initiation: Occurs when the mRNA is loaded onto the ribosome,
and the tRNA begins reading the code, looking for the start codon.
tRNA attaches to the 5’ end of the mRNA.
Methionine
Elongation: Refers to the time during translation when the
polypeptide or amino acid chain is being built or “elongated”. A series
of enzyme building and breaking occurs.
Termination: A stop
codon moves into the area
where the polypeptide chain
is being built. It is the signal
to release the mRNA
transcript from the ribosome.
At the same time, the
polypeptide chain is also
released from the ribosome
Just to make things a bit more difficult, there is a
“loosening” up of the base-pairing rule in translation.
For codon-anticodon interactions, the third base of
the codon isn’t read as “strictly” as the third base in
the DNA/mRNA sequence during transcription or
DNA replication.
This is called the wobble effect.
CUU; CUC; CUA
Look at your codon chart and find an example of different
codons that code for the same amino acid.
As long as the first two bases are the same, the last base
doesn’t matter all that much. Each of these mRNA codon
sequences will bond with a single type of tRNA that carries
leucine.
Mom fed the dog for Tad who was out all day.
Creating a protein is much like decoding a
sentence in a written language.
• Each three-base sequence codes for a different
amino acid. (each letter goes together to make a
different three letter word)
• The sequence of amino acids determines the type
of protein that is synthesized. (each word goes into
a sequence to make a certain sentence)
• Different sequences create different proteins.
(The sequence of words can make very different
sentences) Tad who was out for all day fed Mom the dog
Amino Acids
Using the codon chart, determine the
sequence of the amino acid chain that will
be created.
1. AUG = methionine 7. ACC = threonine
2. CUG = leucine
8. CAG = glutamine
9. GCA = alanine
3. AAA = lysine
4. CGC= arginine
10. GGU = glycine
5. CCU = proline
11. AGU = serine
6. GAA = Glutamic 12. UAA = Stop codon
acid