Biology (Year 12) - DNA Structure and Function
Image: Transcription and Translation image, Image by Christinelmiller, Sourced Under a Creative Commons 4.0 License from Wiki Commons It is key knowledge that proteins are essential to the structure and function of cells, and thus organisms. Protein synthesis is simply the process of making new proteins from the genetic information encoded in DNA. The two processes that facilitate the flow of information from genes to proteins are transcription and translation. The 'biological machinery' involved in transcription includes RNA polymerase and binding factors/proteins, and in translation is the ribosome; a cell organelle not bound by a membrane.
Image: Graphic Designer, Mackenzie Angell
Transcription occurs in the nucleus in eukaryotes, producing mRNA from DNA. During this step in protein synthesis, a section of DNA otherwise known as a gene is unwound and separated, ready for copying.
The enzyme RNA polymerase moves step by step along the DNA molecule, separating the two strands. It is important to note that only the template strand is copied (seen in the diagram).
The template strand can also be referred to as the antisense or non-coding strand, and the other strand is known as the non-template, sense or coding strand. (This is because the codons being made are the same sequence as the non-template strand, which is why it's called the 'coding' strand.)
The coding strand has the same genetic code as the mRNA, except in RNA, the nitrogenous base thymine is replaced by uracil. The sequences of the DNA nucleotides determine the sequence of the RNA nucleotides, as RNA polymerase attaches the RNA nucleotide that is complementary to each DNA base.
A promoter attaches to help the DNA template strand locally separate from the non-template strand, initiating transcription. RNA polymerase binds to the DNA to begin synthesis, synthesising mRNA in a 5' to 3' direction (antiparallel to the template strand).
The mRNA nucleotide triplets are referred to as codons. After the RNA polymerase enables elongation of the strand, the mRNA molecule detaches as pre-mRNA. Pre-mRNA requires modification before it exits the nucleus via nuclear pores.
Image: Post-transcriptional modification of pre-mRNA image, Image by Kep17, Sourced Under a Creative Commons 4.0 License from Wiki Commons
The strip of mRNA first formed when the DNA code has excess baggage; while it does hold the instructions for making proteins, it also carries extra unnecessary nucleotides.
The unrefined mRNA is called pre-mRNA. Before the mRNA can leave the nucleus of the cell, non-coding regions called introns are spliced out in a process called pre-mRNA splicing. The remaining exons are joined together as the final set of refined instructions, ready to move out of the nucleus via a nuclear pore.
The now refined mRNA is called mature mRNA, and it performs the function of carrying the genetic code to the site of translation (the ribosome) where proteins will finally be synthesised!
Image: Graphic Designer, Mackenzie Angell
Translation is the RNA-directed synthesis of a polypeptide (a protein!). Ribosomes are the organelles that facilitate the interaction of mRNA and tRNA (transfer RNA) to the correct position and connect a specific sequence of amino acids.
Ribosomes are mostly composed of ribosomal RNA (rRNA) which is non-coding. After mRNA moves out from the nucleus through a nuclear pore, it enters the cytoplasm and travels to a ribosome, where it will be read and translated. Translation can be further divided into initiation, elongation and termination.
A ribosome binds to a molecule of mRNA, and reads the mRNA nucleotides in codons (pairs of 3).
The codon AUG is the only start codon, and codes for the amino acid methionine. It signals the start of translation and the beginning of a polypeptide chain.
The tRNA molecule that contains the anticodon UAC is attracted to the start codon and pairs with it, bringing the amino acid methionine.
At initiation, two codons enter and are bound to the ribosome, but after that, only one codon enters at a time for translation.
A tRNA molecule that includes an anticodon is attracted to the corresponding codon on the mRNA due to complementary base-pairing rules.
Each tRNA molecule carries an amino acid specified by the codon that it pairs with. As one codon is read and exits the ribosome, another slides in to be read.
tRNA molecules transfer the amino acids to the mRNA-ribosomal complex in the order which is specified by the mRNA codons.
The ribosomes catalyse the formation of covalent peptide bonds between adjacent amino acids, and the mRNA is moved through the ribosome in only one direction.
Once a tRNA molecule has dropped off its amino acid, it will return to the cytoplasm to reload with the same type of amino acid.
The tRNA is not used up during translation, and some amino acids are coded for by more than one codon.
Elongation continues until a stop codon in the mRNA sequence enters the ribosome.
There are 3 stop codons; UAG, UAA and UGA. They all act as signals to stop translation.
The newly formed polypeptide chain (protein) is released and the mRNA code leaves the ribosome.
Once it is released. the polypeptide may fold (or even join with another polypeptide to fold) to become a structural or functional protein.
The protein will either be used within the cell it was formed in, or it may be transported out of the cell elsewhere for other uses.
Transcription and Translation (Prokaryotes)
In prokaryotic cells, the chromosome is generally in the form of a closed circle that is not wrapped around histone proteins; it is found in the nucleoid region of the cell. In addition to the single chromosome, bacteria may contain plasmids, which are small rings of DNA.
Plasmids code for traits but are not essential to the survival of the cell. Transcription begins when a section of the double-stranded chromosome is separated and enzymes synthesise an mRNA product that is complementary to the template strand.
The key difference is that transcription and translation are simultaneous; translation begins while mRNA is still being synthesised. Numerous ribosomes concurrently translate the mRNA transcripts into polypeptides, whereas a eukaryotic cell performs transcription in the nucleus and translation in the cytoplasm.