This video is covering the topic of protein synthesis and please be aware that it is a very summarised version of this topic so it's a basic summary geared towards the Irish Leaving Certificate so be mindful of that. It all starts with genes and a gene is a length or a section of DNA that codes for a particular protein. So protein synthesis is a complex process and when you look at the cell structure you'll understand why. So let's look at this eukaryotic cell and Remember the DNA and genes are sections of DNA. is found within the nucleus of those eukaryotic cells.
But proteins are made in ribosomes, these little organelles in the cytoplasm. So somehow we have to get the code on that gene from the nucleus, out of the nucleus and into that ribosome. But remember, DNA is a double helix and the molecule is too large to fit out through the nuclear pores. And so to overcome this problem, the code that's contained within that gene is copied onto a long single-stranded molecule.
It's similar to DNA, but it's single-stranded so it can fit out through the nuclear pores and make its way to those ribosomes, those protein-making factories in the cytoplasm. I always discuss protein synthesis as four stages. Initiation, transcription, translation and protein folding. And the way in which I remember each of the four stages and the sequence in which they happen is I say, I talk to people. Step one in protein synthesis is initiation.
This is where the DNA unwinds at the gene. and enzymes are involved in breaking the strands apart to expose the bases of the gene. So step number two is transcription. This is making messenger RNA using DNA as a template, and you have to say using DNA as a template.
So in transcription, the DNA was used as a template, and this strand of messenger RNA is made. This is formed by messenger RNA nucleotides that enter the nucleus from the cytoplasm and those that are complementary. to the exposed bases of the DNA align. So these are the exposed bases on a DNA strand and you can see those complementary messenger RNA bases align and RNA polymerase connects them together to form messenger RNA. The bases in DNA are A, T, G and C but in RNA there is no T, there's no thymine, instead there is uracil.
This means that adenine on DNA will be transcribed as uracil onto messenger RNA. So at the end of transcription, you end up with messenger RNA. And messenger RNA is just this molecule that has all of these bases arranged in groups of threes.
So a group of three bases on messenger RNA is known as a codon. And there are three types of codon. There are start codons, codons that code for amino acids, and then there are stop codons.
So now the messenger RNA will leave the nucleus and go to the ribosomes. So step three is translation. making a protein using messenger RNA code. So the messenger RNA leaves the nucleus and it becomes attached to a ribosome at the start codon. Ribosomes are made of two subunits that join together and they're made of ribosomal RNA.
Outside of the ribosome floating free in the cytoplasm are transfer RNA molecules. These will carry the correct amino acid back to the ribosome where it will be connected using peptide bonds in the right way. So the ribosome has attached to the messenger RNA at the start codon, but we also have amino acid codons and finally the stop codon.
Floating free in the cytoplasm are those transfer RNA molecules with complementary anti-codons to the messenger RNA codons. So transfer RNA molecules with complementary anti-codons and carrying the correct amino acid now enter the ribosome. And because they attach or pair up based on complementary codons and anti-codons, This ensures that the correct amino acid is added in the right way. So translation continues and the chain of amino acids is getting ever longer. The ribosome is moving down the messenger RNA and then eventually will reach this stop codon where no more additional amino acids are added.
So translation ends when the stop codon is reached and then we have the final stage, stage four, which is protein folding. This is just where that chain of amino acids is folded into its functional shape, which is so important for proteins. So I do it in four stages.
Initiation, transcription, translation and protein folding. I talk to people. But some books will only mention transcription and translation. They'll have the other two steps incorporated into both of those. So please note these videos do not replace using your textbook.
They're not made for monetary gain and they're not intended for commercial use. Do pass papers. Best of luck.