Transcript Chapter 17 Presentation

Chapter 17
From Gene to Protein
The Bridge Between DNA and
Protein
 DNA
contains the genes that make us who
we are.
 The characteristics we have are the result of
the proteins our cells produce during the
process of transcription and translation.
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Main Questions:
 Somehow
the information content in DNA-- -the specific sequence of nucleotides along
the DNA--strands needs to be turned into
protein.
 How
does this information determine the organism’s
appearance?
 How is the information in the DNA sequence
translated by a cell into a specific trait?
The Bridge Between DNA and
Protein
 RNA
is the single stranded compound that
carries the message from the DNA to the
ribosome for translation into protein.
 Recall,
 The
DNA = A,T,C,G; RNA= A,U,C,G
order of these bases carries the code
for the protein which is constructed from any
or all of the 20 amino acids.
RNA
 RNA
is used because it is a way to protect
the DNA from possible damage.
 Many copies of RNA can be made from one
gene, thus, it allows many copies of a
protein to be made simultaneously.
mRNA and RNA Polymerase
 mRNA
is the “messenger” or vehicle that
carries the genetic information from the DNA
to the protein synthesizing machinery.
 RNA polymerase pries apart the DNA and
joins RNA nucleotides together in the 5’-->3’
direction (adding, again, to the free 3’ end).
 RNA polymerase is just like DNA
polymerase, but it doesn’t need a primer.
Transcription and Translation
 The
process of going from gene to mRNA is
called transcription.
 Translation is the process that occurs when
the mRNA reaches the ribosome and
protein synthesis occurs.
Transcription and Translation
 The
mRNA produced during transcription is
read by the ribosome and results in the
production of a polypeptide.
 The polypeptide is comprised of amino
acids.
 The specific sequence of amino acids is
determined by the genetic code on the
DNA.
Transcription
 The
gene determines the sequence of
bases along the length of the mRNA
molecule.
 One of the two regions of the DNA serves
as the template.
 The DNA is read 3’-->5’ so the mRNA can
be synthesized 5’-->3’
 Not all regions of DNA codes for protein.
Transcription
 There
are numerous segments of
DNA to which transcription
factors bind.
 These govern the synthesis of
mRNA and regulate gene
expression.
 Promoter
sequence
 Termination sequence
 Enhancers
Other Functions of Non-Coding
DNA
 Other
regions of non-coding DNA are
involved in regulating gene expression,
coding for tRNA molecules, and ensuring
that the DNA maintains its length
(telomeres).
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tRNA Structure and Function
 tRNA,
like mRNA, is made
in the nucleus and is used
over and over again.
 tRNA binds an aa at one
end and has an anticodon
at the other end.
 The anticodon acts to
base pair with the
complementary code on
the mRNA molecule, and
delivers an aa to the
ribosome.
Transcription and Translation
 Additionally,
in eukaryotes, once genes get
transcribed, the RNA that is produced is
often modified before getting translated.
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Post Transcriptional Modification
 In
eukaryotes, once the primary transcript is
made, it is spliced and modified before
getting translated into protein.
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mRNA Modification
The initial transcript (~8000
bp) is reduced (to ~1200 on
average).
 The large, non-encoding
regions of the DNA that get
transcribed are spliced out.
 Introns--intervening
regions are removed.
 Exons--expressed
regions are kept.

mRNA Modification
 Some
untranslated regions of the exons are
saved because they have important
functions such as ribosome binding.
Translation
 mRNA
triplets are called codons.
 Codons are written 5’-->3’
 Codons are read 5’-->3’ along the mRNA
and the appropriate aa is incorporated into
the protein according to the codon on the
mRNA molecule.
 As this is done, the protein begins to take
shape.
Protein Synthesis
 Many
copies of protein can be made
simultaneously within a cell using a single
mRNA molecule.
 This is an efficient way for the cell to make
large amounts of protein in times of need.
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Polyribosome
 Here
you can see an
mRNA transcript being
translated into many
copies of protein by
multiple ribosomes in a
eukaryote.
 This is a way in which
the cell can efficiently
make numerous copies
of protein.
Polyribosome
 Here
it is again in a
prokaryote.
 The process essentially
the same between
prokaryotes and
eukaryotes.
 The main exception is
where it occurs.
One Main Difference
 Between
prokaryotes and eukaryotes, there
is one main difference between transcription
and translation. The two processes can
occur simultaneously in prokaryotes
because they lack a nucleus.
 In eukaryotes, the two processes occur at
different times. Transcription occurs in the
nucleus, translation occurs in the cytoplasm.
Translation
 So
how, exactly, does the cell translate
genetic code into protein?
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The Genetic Code
 Scientists
began wondering how the genetic
information contained within DNA instructed
the formation of proteins.
 How could 4 different base pairs code for 20
different amino acids?
 1:1 obviously didn’t work; a 2 letter code
didn’t work either; but a 3 letter code would
give you more than enough needed.
The Genetic Code
 Codons
are composed
of triplets of bases.
 61 of the 64 codons
code for amino acids.
 3 of the codons code
for stop codons and
signal an end to
translation.
 AUG--start codon
Genetic Code
 The
genetic code is said to be redundant.
 More than one triplet codes for the same
amino acid.
 One triplet only codes for one amino acid.
 The reading frame is important because any
error in the reading frame codes for
gibberish.
Ribosomes
 rRNA
genes are found on chromosomal
DNA and are transcribed and processed in
the nucleolus.
 They are assembled and transferred to the
cytoplasm as individual subunits.
 The large and small subunits form one large
subunit when they are attached to the
mRNA.
Ribosomes


The structure of ribosomes fit their
function.
They have an mRNA binding site,
a P-site, an A-site and an E-site.




A-site (aminnoacyl-tRNA) holds the
tRNA carrying the next aa to be
added to the chain.
P-site (peptidyl-tRNA) holds the tRNA
carrying the growing peptide chain.
E-site is the exit site where the tRNAs
leave the ribosome.
Each of these are binding sites for
the mRNA.
The 3 Stages of Protein Building
 1.
Initiation
 2. Elongation
 3. Termination
 All three stages require factors to help them
“go” and GTP to power them.
1. Initiation
Initiation brings together
mRNA, tRNA and the 2
ribosomal subunits.
 Initiation factors are
required for these things
to come together.
 GTP is the energy source
that brings the initiation
complex together.

1. Initiation
Initiation brings together
mRNA, tRNA and the 2
ribosomal subunits.
 Initiation factors are
required for these things
to come together.
 GTP is the energy source
that brings the initiation
complex together.

2. Elongation
 The
elongation stage is
where aa’s are added
one by one to the
growing polypeptide
chain.
 Elongation factors are
involved in the addition
of the aa’s.
 GTP energy is also
spent in this stage.
3. Termination
Termination occurs when a stop codon on the mRNA
reaches the “A-site” within the ribosome.
 Release factor then binds to the stop codon in the “A-site”
causing the addition of water to the peptide instead of an aa.
 This signals the end of translation.

Polypeptide Synthesis
 As
the polypeptide is being synthesized, it
usually folds and takes on its 3D structure.
 Post-translational modifications are often
required to make the protein function.
 Adding
fats, sugars, phosphate groups, etc.
 Removal of certain proteins to make the protein
functional.
 Separately synthesized polypeptides may need to
come together to form a functional protein.