Transcription and translation

Genes provide information for building proteins. They don’t however directly create proteins. The production of proteins is completed through two processes: transcription and translation.

Transcription and translation take the information in DNA and use it to produce proteins. Transcription uses a strand of DNA as a template to build a molecule called RNA.

The RNA molecule is the link between DNA and the production of proteins. During translation, the RNA molecule created in the transcription process delivers information from the DNA to the protein building machines.

DNA → RNA → Protein

DNA and RNA are similar molecules and are both built from smaller molecules called nucleotides. Proteins are made from a sequence of amino acids rather than nucleotides. Transcription and translation are the two processes that convert a sequence of nucleotides from DNA into a sequence of amino acids to build the desired protein.

These two processes are essential for life. They are found in all organisms – eukaryotic and prokaryotic. Converting genetic information into proteins has kept life in existence for billions of years.

DNA and RNA

RNA and DNA are very similar molecules. They are both nucleic acids (one of the four molecules of life), they are both built on a foundation of nucleotides and they both contain four nitrogenous bases that pair up.

A strand of DNA contains a chain of connecting nucleotides. Each nucleotide contains a sugar, a nitrogenous base and a phosphate group. There is a total of four different nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C).

A strand of DNA is almost always found bonded to another strand of DNA in a double helix. Two strands of DNA are bonded together by their nitrogenous bases. The bases form what are called ‘base pairs’ where adenine and thymine bond together and guanine and cytosine bond together.

Adenine and thymine are complementary bases and do not bond with the guanine and cytosine. Guanine and cytosine only bond with each other and not adenine or thymine.

There are a couple of key differences between the structure of DNA and RNA molecules. They contain different sugars. DNA has a deoxyribose sugar while RNA has a ribose sugar.

While three of their four nitrogenous bases are the same, RNA molecules the have a base called uracil (U) instead of a thymine base. During transcription uracil replaces the position of thymine and forms complementary pairs with adenine.


Transcription

Transcription is the process of producing a strand of RNA from a strand of DNA. Similar to the way DNA is used as a template in DNA replication, it is again used as a template during transcription. The information that is stored in DNA molecules is rewritten or ‘transcribed’ into a new RNA molecule.

Sequence of nitrogenous bases and the template strand

Each nitrogenous base of a DNA molecule provides a piece of information for protein production. A strand of DNA has a specific sequence of bases. The specific sequence provides the information for the production of a specific protein.

Through transcription the sequence of bases of the DNA are transcribed into the reciprocal sequence of bases in a strand of RNA. Through transcription the information of the DNA molecule is passed onto the new strand of RNA which can then carry the information to where proteins are produced. RNA molecules used for this purpose are known as messenger RNA (mRNA).

A gene is a particular segment of DNA. The sequence of bases in for a gene determines the sequence of nucleotides along an RNA molecule.

Only one strand of a DNA double helix is transcribed for each gene. This strand is known as the ‘template strand’. The same template strand of DNA is used every time that particular gene is transcribed. The opposite strand of the DNA double helix may be transcribed for other genes.

RNA polymerase

An enzyme called ‘RNA polymerase’ is responsible for separating the two strands of DNA in a double helix. As it separates the two strands, RNA polymerase builds a strand of mRNA by adding the complementary nucleotides (A, U, G, C) to the template strand of DNA.

A specific set of nucleotides along the template strand of DNA indicates where the gene starts and where the RNA polymerase should attach and begin unravelling the double helix. The section of DNA or the gene that is transcribed is known as the ‘transcription unit’.

Rather than RNA polymerase moving along the DNA strand the DNA moves through the RNA polymerase enzyme. As the template strand move through the enzyme it is unravelled and RNA nucleotides are added to the growing mRNA molecule.

As the RNA molecule grows it is separated from the template strand. The DNA template strand reforms the bonds with its complementary DNA strand to reform a double helix.

In prokaryotic cells, such as bacteria, once a specific sequence of nucleotides has been transcribed then transcription is completed. This specific sequence of nucleotides is called the ‘terminator sequence’.

Once the terminator sequence is transcribed RNA polymerase detaches from the DNA template strand and releases the RNA molecule. No further modifications are required for the mRNA molecule and it is possible for translation to begin immediately. Translation can begin in bacteria while transcription is still occurring.

Modification of mRNA in eukaryotic cells

Creating a completed mRNA molecule isn’t quite as simple in eukaryotic cells. Like prokaryotic cells, the end of a transcription unit is signalled by a certain sequence of nucleotides. Unlike prokaryotic cells however, RNA polymerase continues to add nucleotides after transcribing the terminator sequence.

Proteins are required to release the RNA polymerase from the template DNA strand and the RNA molecule is modified to remove the extra nucleotides along with certain unwanted sections of the RNA strand. The remaining sections are spliced together and the final mRNA strand is ready for translation.

In eukaryotic cells, transcription of a DNA strand must be complete before translation can begin. The two processes are separated by the membrane of the nucleus so they cannot be performed on the same strand at the same time as they are in prokaryotic cells.

Rate of transcription

If a certain protein is required in large numbers, one gene can be transcribed by several RNA polymerase enzymes at one time. This makes it possible for large amount of proteins to be produced from multiple RNA molecules in a short time.

 


Translation

Translation is the process where the information carried in mRNA molecules is used to create proteins. The specific sequence of nucleotides in the mRNA molecule provide the code for the production of a protein with a specific sequence of amino acids.

Much like how RNA is built from many nucleotides, a protein is formed from many amino acids. A chain of amino acids is called a ‘polypeptide chain’ and a polypeptide chain bends and folds on itself to form a protein.

During translation, the information of the strand of RNA is ‘translated’ from RNA language into polypeptide language i.e. the sequence of nucleotides is translated into a sequence of amino acids.

Translation occurs in ribosomes

Ribosomes are small cellular machines that control the production of proteins in cells. They are made from proteins and RNA molecules and provide a platform for mRNA molecules to couple with complementary transfer RNA (tRNA) molecules.

Each tRNA molecules are bound to an amino acid and delivers the necessary amino acid to the ribosome. The tRNA molecules bind to the complementary bases of the mRNA molecule.

The bonded mRNA and tRNA are fed through the ribosome and the amino acid attached to the tRNA molecule is added to the growing polypeptide chain as it moves through the ribosome.

Nucleotide bases are translated into 20 different amino acids

RNA molecules only contain four different types of nitrogenous bases but there are 20 different amino acids that are used to build proteins. In order to turn four into 20, a combination of three nitrogenous bases provide the information for one amino acid.

CodonsEach three base ‘word’ is called a ‘codon’ and the series of codons holds the information for the production of the polypeptide chain. There are a total of 64 different codons and more than one codon translates into each amino acid.

A strand of mRNA obviously has multiple codons which provide the information for multiple amino acids. A tRNA molecule reads along one codon of the mRNA strand and collects the necessary amino acid from the cytoplasm.

The tRNA returns to the ribosome with the amino acid, binds to the complementary bases of the mRNA codon and the amino acid is added to the end of polypeptide chain as the RNA molecules move through the ribosome.

tRNA

There is a different tRNA molecule for each of the different codons of the mRNA strand. Each tRNA molecule contains three nitrogenous bases that are complementary to the three bases of a codon on the mRNA strand.

The three bases of the tRNA molecule are known as an anticodon. For example, an mRNA codon with bases UGU would have a complementary tRNA with an anticodon AGA.

The opposite end of the tRNA molecule has a site where a specific amino acid can bind to. When the tRNA recognises its complementary codon in the mRNA strand, it goes to collects its specific amino acid. The amino acid is bonded to the tRNA molecule by enzymes in the cytoplasm.

As the tRNA molecule returns with the amino acid, the anticodon of the tRNA binds to the codon of the mRNA and moves through the ribosome. Each tRNA molecule can collect and deliver multiple amino acids. One codon at a time, amino acids are brought to the ribosome and the polypeptide chain is built.

Ribosome binding sites

Ribosomes have three sites for different stages of interaction with tRNA and mRNA: the P site, A site and E site. The P site is where the ribosome holds the polypeptide chain and where the tRNA adds its amino acid to the growing chain.

The A site is where tRNA molecules bind to the codons of the mRNA strand and the E site or exit site is where the tRNA is released from the ribosome and the mRNA strand.

Translation begins when a ribosome bonds to an mRNA strand and an initiator tRNA. The initiator tRNA delivers an amino acid called ‘methionine’ directly to the P site and keeps the A site open for the second tRNA molecule to bind to.

The strand of mRNA moves through the ribosome from the A site to the P site and exits at the E site. Molecules of tRNA bind to the codons of the mRNA at the A site before moving to the P site where their amino acid is attached to the end of the growing polypeptide chain.

Once tRNA molecules have released their amino acids they move into the E site and are released from the mRNA and ribosome. As one tRNA molecules moves from the P site into the E site another tRNA molecule moves from the A site into the P site and delivers the next amino acid to the polypeptide chain.

Termination of translation and modification of the polypeptide

Translation ends when a stop codon on the mRNA strand reaches the A site in the ribosome. The stop codon doesn’t have a complementary tRNA or anticodon.

Instead a protein called a ‘release factor’ binds to the stop codon and adds a water molecule to the polypeptide chain when it moves into the P site. Once the water molecule is added to the polypeptide, the polypeptide is released from the ribosome.

It is common for multiple strands of mRNA to be translated simultaneously by multiple ribosomes. This greatly increases the rate of protein production.

A polypeptide chain must fold on itself to create its final shape as a protein. As the polypeptide is being made it is already folding into a protein. Other proteins are used to guide the polypeptide to fold into the correct shape.

Often a polypeptide chain will need to be modified before it is able to perform properly. A range of molecules, such as sugars and lipids, can be added to the polypeptide. Likewise the polypeptide chain may be split into smaller chains or have amino acids removed.

Last edited: 18 March 2016

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