Transcription vs Translation Worksheet
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Differential gene expression
Believe it or not, all of the cells within a multicellular organism (excluding red blood cells and gametes) contain exactly the same DNA. In which case, why is a heart composed of cardiomyocytes? Or the liver hepatocytes? How can cells containing the same DNA be so physiologically different?
The answer lies in differential gene expression – the combination of genes that are turned on (expressed) or turned off (repressed) in particular cells – this is what makes each cell unique.
Gene expression is regulated by both internal and external factors – a perfect interplay between the genome and the environment.1
The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. These steps differ in prokaryotic and eukaryotic cells. Here, we will focus on eukaryotic cells.
DNA vs RNATo understand fully the different processes involved in gene expression, it is key that you can know the differences between DNA and RNA.
What is transcription?Transcription is the synthesis of any type of complimentary RNA from a DNA template: note, several types of RNA can be encoded by a DNA strand. Here, we focus specifically on transcription that leads to pre-mRNA, mRNA and eventually proteins.
In the process of gene expression, transcription involves the production of messenger RNA (mRNA) from a DNA template. It takes place in the nucleus of a cell and is catalyzed by the enzyme RNA polymerase II.
- RNA polymerase I transcribes rRNA genes
- RNA polymerase II transcribes mRNA, miRNA, snRNA, and snRNA genes
- RNA polymerase III transcribes an array of RNA genes, including but not limited to tRNA and 5S rRNA genes.2
The steps of transcriptionThe process of transcription entails several steps:
What do we mean by 5' and 3'?
This refers to the carbon numbers in DNA and RNA's backbone. The 5' carbon ribose ring frequently has a phosphate group attached, and the 3' carbon end has a hydroxyl (-OH) group attached. The asymmetry gives the DNA and RNA strands a "direction".
The DNA strand moves through the RNA polymerase II enzyme. In the region behind where the nucleotides are being added to form the pre-mRNA strand, the DNA helix re-forms. This means that the pre-mRNA produced is eventually released from the DNA template a single strand.
From pre-mRNA to mRNA
The process of translation occurs in three main stages:
The small unit of the ribosome binds to the start of the mRNA sequence, at the location of the start codon. In all mRNA molecules, the start codon has a sequence of AUG, which codes for the amino acid methionine. The tRNA carrying the anticodon recognizes this sequence and caries the amino acid methionine to the mRNA. Then, the large subunit of the ribosome binds to form the initiation complex.
In this stage of translation, the ribosome continues down the mRNA strand translating each codon in turn. The corresponding amino acids are added by tRNA in a growing chain, linked together by peptide bonds. This continues until the entire sequence of codons is read, and the ribosome reaches a "stop" codon.
DNA replication via DNA polymerase:
Note that transcription and translation are different to DNA replication. DNA replication is the process by which the genome is conserved for the next generation. It involves the replication of a single DNA strand into two daughter strands via the enzyme DNA polymerase. Each daughter strand containing half of the original DNA double helix.
Summary of comparisons chart
Purpose and product
|To use genes as a template to create several forms of RNA (such as mRNA as discussed in this article).
|To synthesize proteins from an RNA template.
|RNA polymerase protein binds to the promoter region in the DNA and forms the transcription initiation complex.
|Takes place when ribosome recognizes AUG start codon and binds the mRNA.
|RNA polymerase travels in the 5' to 3' direction and builds an RNA strand.
|tRNA with complimentary anticodons to the codons within mRNA binds to mRNA and builds a chain of amino acids joined by peptide bonds.
|The RNA transcript is released. RNA polymerase detaches from DNA and the DNA rewinds back into a double helix.
|Ribosome encounters stop codon. No tRNAs are able to recognize stop codons and the ribosome thus dissembles tRNA and releases the polypeptide that has been built.
- Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Differential Gene Expression. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10061/
- Carter R, Drouin G. Structural differentiation of the three eukaryotic RNA polymerases. Genomics. 2009;94(6):388-396. doi:10.1016/j.ygeno.2009.08.011
- Alberts B, Johnson A, Lewis J, et al. From DNA to RNA. Molecular Biology of the Cell. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26887/
- Ramanathan A, Robb GB, Chan SH. mRNA capping: biological functions and applications. Nucleic Acids Res. 2016;44(16):7511-7526. doi:10.1093/nar/gkw551