What
is Protein Synthesis?
Protein synthesis is the production of proteins using
information found in DNA.
All about DNA!
Lets start out by reviewing a little bit
about DNA. DNA (deoxyribonucleic acid)
is the hereditary information of a cell. It is responsible for what you look
like. Your genetic makeup is called your genotype, and your phenotype is the
physical traits you have because of your DNA.
The
Structure of DNA
DNA consists of two chains of information, coiled in the shape
of a double helix (as shown in the pictures above). The two strands of DNA run in opposite directions. DNA consists of four nucleotide bases. Thymine (T) always pairs with Adenine (A),
and Guanine (G) always pairs with Cytosine (C), when the two strands of DNA
connect. Hydrogen bonds link the bases
together, allowing the DNA to maintain its double helix shape. Phosphate and deoxyribose units hold the
individual strands of DNA together; this makes up a backbone for the DNA.
For more
pictures of DNA you can go to the following link:
Transcription
Transcription is the first step in protein synthesis. It is the process in which DNA acts as a
template in the synthesis of RNA within chromosomes.
Transcription
has three steps:
- Initiation - Binding of RNA polymerase to double-stranded DNA. RNA polymerase binds at a sequence of
DNA called the promoter
- Elongation – The addition of nucleotides, held together by covalent bonds, to
the 3' end of the growing polynucleotide chain. This involves the development of a short stretch of DNA that
is transiently single-stranded.
- Termination - The recognition of the transcription termination sequence and
the release of RNA polymerase.
There is one major
difference with RNA then DNA. RNA has
the nucleotide Uracil (U) instead of Thymine (T). This means, when the RNA is making a strand using the DNA as a
template, whenever there is an Adenine (A) in the DNA, the RNA will pair a
Uracil (U) nucleotide instead of a Thymine (T) nucleotide.
The picture below shows the overall process
of transcription. The Process of RNA
being created from a DNA template. You
can observe the base pairings; especially that RNA has the nucleotide Uracil
(U), instead of Thymine (T). It is also
easy to notice that the two strands run in opposite directions. Also you will see the polymerase is
responsible for the process.
RNA
Processing
RNA
processing is the part of transcription in which the newly transcribed RNA
becomes an mRNA strand, and ready to set out for a ribosome. Pre-mRNA is processed into mature RNA (mRNA)
in three steps: capping,
polyadenylation, and splicing.
Capping is when a methylgylatuane cap is added to the 5’ end of the
pre-mRNA. Polyadenylation is the
addition of a poly-a-tail to the 3’ end of the pre-mRNA. Lastly splicing is when the introns are
removed from the pre-mRNA and the exons are spliced together. The image below should give you a better
understanding of RNA processing.
After
processing the mRNA will leave the nucleus and enter the cytoplasm of the cell
where it will meet a ribosome and undergo translation.
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TRANSLATION
of RNA
Translation involves taking the message that's in the
messenger RNA and decoding that message from the language of nucleic acids to
the language of proteins or polypeptides. For translation to occur, the
messenger RNA goes to the cytoplasm where it is attached to a cellular a
ribosome. Ribosomes are two part molecular assemblies consisting of various
proteins plus a special kind of RNA called ribosomal RNA.
Another kind of RNA, called tRNA, carries amino
acids to the mRNA when it is attached to a particular part of the ribosome's
small subunit, called a binding site.
An important feature of mRNA and how it is translated is the fact that
each three nucleotides in the mRNA are called codons, and it is the codon that
is translated. Thus the sequence of
codons corresponds to the sequence of amino acids in the polypeptide. You will
see that the tRNA molecules have a set of three nucleotide bases at one end
that are complementary to a corresponding codon, the are called anti-codons.
This is critical because the anti codons make the connection between the codons
and the correct amino acids that go with each codon. Ultimately the amino acids are joined together by peptide bonds,
forming a polypeptide chain.
Steps in Translation:
|
|
Initial steps:
Messenger RNA is bound to ribosome with the start codon (AUG) at the P site.
A transfer RNA molecule with the amino acid methionine (M) and the anticodon
UAC has bound to the exposed start codon. The codon UCA is exposed at the A
site. |
|
|
A second transfer
RNA molecule, with the anticodon AGU and the amino acid serine (S) has bound
to the A site. The 2 amino acids are close enough to form a peptide bond
between them. |
|
|
A peptide bond has
formed between M and S and the peptide is bound to the A site. The methionine
transfer RNA leaves, and the P site is exposed. |
|
|
The ribosome has
moved along the messenger RNA one codon, bringing the peptide to the P site.
This exposes the A site and the next transfer RNA, carrying alanine (A) is
about to bind. |
Below is a picture of a tRNA:
Another picutre of Translation
Now that you know all about translation take the
following QUIZ
to see how much you have learned!
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PROTEIN PROCESSING
Why do proteins go through
post-translational processing?
Protein processing adds functionality to proteins,
effects targeting of proteins, regulates protein activity, and increases the
proteins mechanical strength.
During protein processing multiple polypeptide chains
are folded into a proteins 3D shape, as shown below:
Protein Structure
Polypeptides and proteins can be found in four levels
of structure.
These four levels are:
-Primary Structure
-Secondary Structure
-Tertiary Structure
-Quaternary Structure
Primary Structure- This is the sequence of amino acids in the polypeptide chain.
Secondary Structure-
This
is composed of an arrangement of amino acids, stabilized by hydrogen
bonds. The 2 main secondary structures
are the alpha helix and the beta-pleated sheet. There are structures other than these, but these two are
the most stable. A single protein could
contain many secondary structures.
Alpha Helix Beta-Pleated Sheet
Tertiary
Structure- This is the 3D arrangement of atoms in a
polypeptide chain, maintained my disulfide bonds (also called disulfide
bridges).
Quaternary Structure- This is a term
used to describe proteins that are made up of multiple polypeptide molecules
(also called monomers). The arrangement
of these monomers in the 3D protein is the quaternary structure. This structure is held together and
stabilized by hydrophobic interactions.
You can click HERE
to review everything you have learned on this website:
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Works Cited
http://www.lewport.wnyric.org/JWANAMAKER/animations/Protein%20Synthesis%20-%20long.html
http://web.jjay.cuny.edu/~acarpi/NSC/12-dna.htm
http://library.thinkquest.org/C0122628/media/images/ProteinSynthesis.jpg
http://www.accessexcellence.org/AE/AEC/CC/DNA_structure.html
http://en.wikipedia.org/wiki/DNA
http://library.thinkquest.org/C004535/QZ_rna_translation.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Translation.html