Download DNA Functions

DNA Functions- Expression
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Expression of Information
!  A different subset of the genetic information is
read in each cell type.
!  Expression requires two processes.
-  Transcription - With DNA as a template,
mRNA, tRNA, and rRNA are made during
transcription.
-  Translation
"  Messenger RNA (mRNA) from transcription
is translated to produce proteins.
Transcription
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Three different types of RNA (ribonucleic acid)
produced:
!  Messenger RNA (mRNA)
!  Transfer RNA (tRNA)
!  Ribosomal RNA (rRNA)
RNA is made up of numerous nucleotides
assembled in exactly the same way as in DNA
except that…….
!  1. RNA is mostly single stranded and not a
helix.
!  2. the sugar is ribose instead of deoxyribose.
!  3. the base thymine is replaced by uracil.
Transcription
Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into proteins
from the nucleus to the ribosomes.
Transcription
Ribosomes are made up
of proteins and
ribosomal RNA (rRNA).
Transcription
During protein
construction, transfer
RNA (tRNA) transfers
each amino acid to the
ribosome. Each form of
tRNA has a specific
anticodon loop
Transcription
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In transcription, the DNA molecule unzips,
just as in replication.
RNA polymerase will attach to assemble the
new RNA molecule.
Only one side of the DNA molecule is
transcribed.
This is assured by the fact that RNA
polymerase is not attracted to just any
stretch of DNA, but only to certain DNA base
sequences, called “promoters.”
Transcription
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Promoters are sequences of bases that do
not determine protein structure but serve
only to send the message “RNA polymerase,
start here.”
Transcription is essentially identical to
replication. The differences are…
!  1. assembled with ribonucleotides instead
of nucleotides.
!  2. adenine pairs with uracil.
-  AND
Transcription
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3. daughter RNA strand does not stay
attached to the parent DNA strand. Instead it
separates from the DNA, and the DNA zips
back together. The RNA migrates out of the
nucleus of the cell and into the cytoplasm.
Transcription
Transcription
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RNA Editing
-  The DNA of eukaryotic genes contains
sequences of nucleotides, called
introns, that are not involved in coding
for proteins.
-  The DNA sequences that code for
proteins are called exons.
-  When RNA molecules are formed,
introns and exons are copied from DNA.
Transcription
The introns are
cut out of RNA
molecules.
The exons are
the spliced
together to form
mRNA.
Translation
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Translation is the part of the process where
the information from the gene is used to
create the amino acids and then the protein.
The ribosome has to take the nucleotide
bases and equate it with codons that specify
amino acids.
Each amino acid is represented by certain
codons.
The “language” that bridges the gap
between gene and amino acid is the genetic
code.
Translation
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The genetic code is based on codons.
A codon consists of three consecutive
nucleotides that specify a single amino acid
that is to be added to the polypeptide.
-  64 possible combinations
-  Code for 20 amino acids.
Translation
Translation
Translation
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Translation begins when an mRNA molecule
in the cytoplasm attaches to a ribosome.
As each codon of the mRNA moves through
the ribosome, the proper amino acid is
brought into the ribosome by tRNA.
In the ribosome, the amino acid is transferred
to the growing polypeptide chain [protein].
Each tRNA molecule carries only one kind of
amino acid.
In addition to an amino acid, each tRNA
molecule has three upaired bases
Translation
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These bases, called the anticodon, are
complementary to one mRNA codon.
In the case of the tRNA molecule for
methionine, the anticodon bases are UAC,
which pair with the methionine codon, AUG.
The ribosome has a second binding site for a
tRNA molecule for the next codon.
Translation
Translation
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Like an assembly line worker who attaches
one part to another, the ribosome forms a
polypeptide bond between the first and second
amino acids, methionine and phenylalanine.
[see diagram]
At the same time, the ribosome breaks the
bond that had held the first tRNA molecule to
its amino acid and releases the tRNA
molecule.
The ribosome then moves to the third codon,
where a tRNA molecule brings it the amino
acid specified by the third codon.
Translation
Translation
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The polypeptide chain continues to grow until
the ribosome reaches a stop codon on the
mRNA molecule.
When the ribosome reaches a stop codon, it
releases the newly formed polypeptide and
the mRNA molecule, completing the process.
Translation
Central Dogma of Molecular Dogma