SC.912.L.16.5: Explain the basic processes of transcription and translation, and how they result in the expression of genes.
Structure of RNA
Transcription is the conversion of the cell’s DNA message to a similar chemical message called RNA (ribonucleic acid). RNA is almost always single stranded unlike DNA, and an RNA nucleotides also use a ribose sugar instead of a deoxyribose sugar.
RNA nucleotides consist of
- pentose sugar (ribose)
- phosphate group
- nitrogen base
Generally speaking small segments of the DNA code are often converted into an even smaller segment of RNA code. This segment of RNA is frequently used by a cell to encode for instructions to create a chain of amino acids referred to as a “protein”. This idea is called “The Central Dogma” of biology, DNA > RNA> Protein. This is true for the most part though, there are exceptions to this such as RNA retroviruses like HIV.
Transcription
What is the purpose of transcription?
In a process that is similar to DNA replication enzymes are involved in converting the dna message into a RNA message. The double-stranded DNA molecule is opened up by transcription enzymes that use the dna nucleotides as a template to construct a complementary strand of RNA nucleotides.
Types of RNA
Messenger RNA (mRNA): RNA molecule that is responsible for coding for amino acids. mRNA must be modified through splicing, non-usable sections that do not encode an amino acid will be removed, leaving behind only the usable sections called Exons.
Ribosomal RNA (rRNA): RNA that is combined with small subunits of proteins to form a ribosome. The rRNA is necessary to form ribosomes in both prokaryotes and eukaryotes.
Transfer RNA (tRNA): RNA molecule that has a single amino acid attached to them. Each amino acid has a unique sequence of tRNA associated with them.
Process of Transcription
- Genes of DNA is transcribed into mRNA molecule
- pre-mRNA is edited, removing introns and leaving behind full matured mRNA containing only Exons
- mRNA molecule will leave the nucleus through nuclear pores to begin translation in the cytosol or bound
Translation
What is translation?
Translation is the process of using an mRNA gene sequence to code for an amino acid sequence (protein). This process happens exclusively outside of the nucleus of a eukaryotic cell and involves the interaction between mRNA, tRNA/amino acids, and the ribosome. By fine tuning the process of translation a cell can upregulate (increase) or downregulate (decrease) the expression levels of various proteins so the cell can meet what ever needs it has at the moment.
The process of translation
Before translation can begin the mRNA goes through a process called mRNA editing. mRNA editing involves the removal of sections called introns which leaves behind the sections called exons. Introns are interrupting segments that do not code for the amino acids needed to form a functional protein, while exons are the sections that code for the amino acids that are a part of the final protein product. The process of removing the introns is also called Splicing.
Once a fully matured and edited mRNA transcript is ready for translation the process will occur in 3 basic steps; Initiation, Elongation, and Termination.
During initiation the ribosome will assemble around the first codon of the mRNA, at the same time the first tRNA is attached to the first codon by its anticodon.
Elongation occurs as the ribosome moves along the mRNA transcript codon by codon as each corresponding tRNA/amino acid is joined at the complimentary anticodon. As each tRNA is brought in their respective amino acids are joined to create a longer chain of amino acids.
As a stop codon on the mRNA is reached the ribosome will come apart and release the mRNA transcript to be reused by the next ribosome. This marks the end of translation as the protein chain that was formed is released to perform its function or be further modified by the cell.
Insert picture of termination
The protein code
Every 3 letters of an mRNA segment is called a codon. That codon ultimately codes for a specific amino acid building block of a protein. When the mRNA is associated with a ribosome each codon is complementary to an anticodon that is only found on a tRNA molecule. Each codon of an mRNA can only be bound to a specific tRNA and since each tRNA carries only a specific amino acid this allows the cell to create a highly regulated coding system to build proteins!
This coding system has been thoroughly studied and the information is available in what is called an Amino Acid Chart.
