Transcription and RNA Processing

To understand how genetic information is transcribed and translated we must first return the basic idea of proteins.

What are proteins? Proteins are polypeptides consisted up of amino acids. These amino acids are linked via peptide bonds, which are covalent bonds joining together to form unique proteins.

This allows for gene expression, by which DNA coordinates the synthesis of proteins. These two stages include, transcription and translation which are recurring in ALL organisms. DNA is transcribed into RNA, and then translated into protein.

What are transcription and translation exactly?

Transcription is the synthesizing DNA information to form RNA. This allows for "message" of the DNA to be transcribed, occurring namely in the nucleus. 

Translation is the synthesizing of RNA information to form polypeptide bonds thus forming proteins. This allows for a nucleotide sequence to become and amino acid sequence, occuring namely at the ribosome.

Types of RNA

Through exploring transcription, and translation we will begin to learn three key RNA molecules within these processes.

(tip: use the abbreviations of each function to memorize the RNA)

Messenger RNA (mRNA)

Messenger RNA is formed during transcription from a DNA template. mRNA carries information from the DNA at the nucleus to ribosomes in the cytoplasm. As the name suggests, it is a messenger that carries genetic information. DNA at nucleus to ribosomes at cytoplasm, synthesized during transcription.


Transfer RNA

Transfer RNA molecules are important in the processes of translation. Each specific tRNA can carry a unique amino acid. This enables attachment to mRNA via the anticodon (a complementary codon to mRNA). Thus allowing information to be translated into a peptide sequence.


Ribosomal RNA

Ribosomal RNA assists with the formation of ribosomes. This subsequently helps link amino acids together. 


The Genetic Code

DNA contains the sequence of nucleotides that code for proteins (eg. AGU). This sequence is read of groups of three called  the triplet code. 

During transcription, only one DNA strand is being transcribed. This strand being transcribed is known as the template strand, also known as the noncoding strand, minus strand or even antisense strand. 

mRNA molecules are formed antiparallel, and complementary to DNA nucleotides. The base pairing: A=U and C=G of RNA have mRNA nucleotide triplets named codons. Codons as the name suggests, code for amino acids. 

There are 64 different codon combinations. 61 out of the 64 codes for amino acids. Contraily 3 are stop codons, universal to all life. Redundancy describes how more than one codon codes for each amino acid. This essentially means an amino acid is not limited to one specific code, namely so "redundant". 

Reading frame describes how codons on the mRNA must be read in correct groupings during translation to synthesize correct proteins. If the reading frames shifts by even ONE minute letter, will lead to the production of a completely different outcome. 

Transcription step-by-step

There are three key steps in transcription.


Step 1: Initiation

Transcription begins when RNA polymerase molecules attach to a promoter region of DNA. The attachment can occur without the use of a primer. Promoter regions are upstream (5' end) of the gene being transcribed. 

In eukaryote cells, the promoter region is called the TATA box, and transcription factors help RNA polymerase bind. 

In prokaryote cells, RNA polymerase can directly bind to promoter.

Key tip: the promoter is always upstram of the gene of interest being transcribed. 

Step 2: Elongation

RNA polymerase opens the DNA and reads the triplet code of template strand. Contrastly, this step moves in the 3' to 5' direction. Although mRNA transcript elongates 5' to 3'. 

RNA polymerase moves downstream, opening only small sections of DNA at a time This pairs complementary RNA nucleotides, with growing mRNA strand peeling away from the DNA template strand. Thus the DNA double helix is reformed.

A single gene can be transcribed simultaneously by multiple RNA polymerase molecules. This helps increase the magnitude of mRNA being synthesized, thus increasing protein production.

Step 3: Termination

In prokaryotes transcription occurs via a termination sequence. This causes a termination signal, which causes RNA polymerase to detach. Coupled with mRNA transcript being released and proceeds to translation. mRNA does NOT need modifications.


In eukaryotes, RNA polymerase transcribes a sequence of DNA called the polyadenylation signal sequence. Coding for a polyadenylation signal (AAUAAA), releases the pre-mRNA from the DNA. Modifications must be undergone prior to translation.

Pre-mRNA Modifications

There are three modifications to occur in eukaryotic pre-mRNA before it is ready for translation.

1, The 5' cap (GTP) describes the 5' end of the pre-mRNA which receives a modified guanine nucleotide "cap".

2. The Poly-A tail describes the 3' end of the pre-mRNA which receives 50-250 adenine nucleotides.

Similarities: Both the 5' cap and the Poly-A tail function to,

3. RNA Splicingoccurs within sections of the pre-mRNA called introns. These introns are removed and exons are joined together.

Why does splicing occur?

A single gene can code for more than one kind of polypeptide, this is known as alternative splicing. 


Once all modifications have been completed, the pre-mRNA is now considered mature mRNA and enabled to leave the nucleus. It is then exported to the cytoplasm for translation at the ribosomes, as mentioned previously.


In our next topic, we will delve into the process of translation, following the transcription process.