Course Calendar

December 1-4

Biology 12

QUIZ - DEC 3/4

- describe the structure of DNA
- Describe the three steps of DNA semi conservative replication
- Identify the purpose and site of DNA replication


Homework: Please select a SHORT article or story about genetic engineering, gene therapy or a genetic disorder involving proteins (ex. Cystic Fibrosis). And e-mail it to my e-mail address by December 4th.

Check out Scientific American, Discover, BBC, CBC, etc...

Protein Synthesis Research Notes
The connection between genes and proteins.
It was believed since the early 1900s that genes determine the way an organism looks
through enzymes that catalyze specific chemical reactions in the cell. In other words, some
diseases are caused by missing or defective enzymes. In the 1930s George Beadle and Edward Tatum were able to definitively establish the link between genes and enzymes in their exploration of the metabolism of a bread mold. They bombarded the mold with X-rays and screened the survivors for mutants that differed in their nutritional needs.

The normal mold could grow on agar containing very little nutrients. Beadle and Tatum identified mutants that could not survive on this minimal medium, because they were unable to synthesize certain essential molecules from the minimal ingredients. However, most of these nutritional mutants were able to survive on a complete growth medium that includes all 20 amino acids and a few other nutrients.
They suggested that the mutants were unable to survive because they lacked an enzyme necessary to make the particular nutrient missing from the medium. By providing the missing nutrient, the mutants were able to survive.

Their results provided strong evidence for the one gene–one polypeptide hypothesis.
Proteins carry information in their amino acid sequence and nucleic acids carry information
in their nucleotide sequence.

To get from DNA (in nucleic acid language) to protein (in amino acid language) requires
two steps:

During transcription, a DNA strand provides a template for the synthesis of a
complementary RNA strand. This RNA molecule is called mRNA.

The use of mRNA provides protection for the genetic information contained in DNA.

Also, more protein can be made simultaneously because many mRNA copies of a gene can be made. Lastly, each mRNA can be translated many times.

During translation, the instructions are converted from nucleic acid language to amino acid language.

The genetic code:

There are only 4 bases but 20 amino acids so it is not sufficient for one nucleotide to
represent one amino acid.

The code must be a series of triplets (three bases) which indicate a particular amino acid. The genetic instructions for a polypeptide chain are written in DNA as a series of non overlapping three-nucleotide words called codons.

Sixty-one of the 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 are “stop” signals marking the termination of translation.
Some amino acids are coded for by two or more codons but a given codon ALWAYS codes for only one amino acid. i.e. there is redundancy but no ambiguity. e.g., GAA and GAG both mean glutamic acid, but never mean any other amino acid.

The process of transcription is much like DNA replication except that DNA is acting as a template for the construction of mRNA rather than new DNA


RNA polymerase separates the DNA strands at a particular sequence called the promoter and bonds the RNA nucleotides as they base-pair along the DNA template.

Like DNA polymerase, RNA polymerase can only assemble a polynucleotide in its 5’ to 3’ direction.

Unlike DNA polymerase, RNA polymerase is able to start a chain from scratch; it does not need a primer.

As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at time, adding bases by base pairing rules to form mRNA.

Remember that in RNA, U rather than T is paired with A.

Behind the point of RNA synthesis, the double helix re-forms and the mRNA molecule peels away.

If there is high demand for a protein, the cell can have several RNA polymerases transcribing the same gene simultaneously to produce several mRNAs.

Transcription proceeds until RNA polymerase transcribes a terminator sequence

The mRNA is then released from the RNA polymerase and sent to the cytosol.

A transcription unit is the sequence between the start and stop sequences – generally speaking, one gene.

In the process of translation, a cell interprets a series of codons along an mRNA molecule
and builds a polypeptide. That is, mRNA is translated from nucleic acid language to amino
acid language.

The interpreter is transfer RNA (tRNA), which carries amino acids to a ribosome. The
ribosome then adds each amino acid carried by tRNA to the growing end of the polypeptide chain.
Each tRNA carries a specific amino acid attached to the amino acid attachment site.

At the other end of the tRNA is a group of three nucleotides called the anticodon that binds by complementary base pairing to the nucleotides of a codon.

E.g., if the codon on mRNA is UUU, a tRNA with an AAA anticodon and carrying phenylalanine will bind to it.

The tRNA molecule is a translator, because it can read a nucleic acid word (the mRNA codon) and translate it to a protein word (the amino acid).

Each ribosome has a binding site for mRNA and three binding sites for tRNA molecules.

(1) The P site holds the tRNA carrying the growing polypeptide chain.
(2) The A site carries the tRNA with the next amino acid to be added to the chain.
(3) A tRNA that has dropped off its amino acid leaves the ribosome at the E (exit) site.

A ribosome binds to mRNA and begins looking for the start codon (AUG)

To extract the message from the genetic code requires specifying the correct starting point.

This establishes the reading frame; subsequent codons are read in groups of three nucleotides.

When it is found, it is displayed in the P site so tRNA molecules can attempt to recognize it by complementary base pairing with their anticodon. When this occurs, the two ribosomal subunits come together to form the functional ribosome.

The tRNA with the anticodon complementary to AUG always carries methionine (met) so it is always the first amino acid in every protein. The tRNA carrying methionine enters the P site.

The ribosome now displays the next codon in the A site and waits for the tRNA with the complementary anticodon to recognize it.

If the cellular demand for a protein is high, several ribosomes can translate the same
mRNA simultaneously

Elongation (~ 60 ms per peptide bond)
The tRNA with an anticodon complementary to the next codon enters the A site

The growing polypeptide chain on the tRNA at the P site, now one amino acid longer, is transferred to the tRNA at the A site. The ribosome forms a new peptide bond by transferring the amino acid from tRNA in the P site to the amino acid on the tRNA in the A site

The ribosome moves over one codon on the mRNA so that the next codon is now displayed in the A site.
The tRNA (now empty) that had been in the P site is moved to the E site and then leaves the ribosome.
the appropriate tRNA moves in and the ribosome attaches the amino acids (now 2 of them) from the tRNA in the P site to the amino acid on the tRNA in the A site. The chain is now three amino acids in length

These steps of elongation continue to add amino acids codon by codon until the polypeptide chain is completed.

The elongation process continues until one of the three stop codons is reached displayed in the A site.

There is no tRNA which recognizes any of the stop codons but a release factor binds to the stop codon and hydrolyzes the bond between the polypeptide and its tRNA in the P site.

This frees the polypeptide, so it is released from the ribosome.

A ribosome requires less than a minute to translate an average-sized mRNA into a polypeptide.
During and after synthesis, a polypeptide coils and folds to its three-dimensional shape spontaneously.


Protein Synthesis Animations & quizzes

Gene Expression
Stages of Transcription
How Translation Works
Protein Synthesis