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  • DNA is very long and cumbersome.

  • The strands of DNA also code for many different proteins.

  • To get the code for the desired protein from the nucleus to the ribosomes in the cytoplasm,

  • a copy of the small necessary section of DNA will be made.

  • The copying process is called transcription and it occurs in the nucleus.

  • It's the first step in protein synthesis--the process of making protein.

  • The second step is called translation.

  • A portion of the DNA is unzipped so that the mRNA can be made from the DNA like a template--that's

  • transcription.

  • Nucleotides of RNA match with one strand of DNA and make mRNA.

  • RNA polymerase unzips the DNA and puts RNA nucleotides into the right place.

  • C and G nucleotides match up.

  • A and T nucleotides match up, but any time the mRNA places a compliment to an A on the

  • DNA it places a U instead of a T on the RNA.

  • Remember, RNA uses Uracil instead of Thymine.

  • So, if the DNA strand looked like this, the complimentary mRNA strand would be the compliment

  • but each time there would normally be a Thymine there's a Uracil in its place.

  • The mRNA also doesn't copy the entire strand of DNA.

  • It only copies the portion of DNA that codes for the protein it wants at the time.

  • In fact, most of DNA doesn't code for anything.

  • Only about two percent of it codes for protein.

  • So this small section of coding DNA has the start and stop signal.

  • After the mRNA is formed, it moves out of the nucleus through a nuclear pore and goes

  • into the cytoplasm.

  • Remember, there are three different types of RNA.

  • The purpose of the mRNA is to get the protein code out of the nucleus without pulling the

  • DNA out of the nucleus.

  • Then the mRNA will need to find a ribosome in the cytoplasm.

  • Ribosomes can be found on the rough ER or floating in the cytosol.

  • mRNA is read three bases at a time and these three bases are called codons.

  • Now, the ribosomes are made of protein and RNA.

  • And the ribosomes are the protein making machines that read the mRNA code and add the correct

  • amino acid using tRNA.

  • Remember, tRNA stands for: transfer RNA and that's because its purpose is to transfer

  • the right amino acid to the ribosome to build the protein.

  • The tRNA has an anti-codon on one end which will match a specific codon on the mRNA and

  • has a specific amino acid on the other end.

  • Together these three parts will make translation happen.

  • Translation occurs in the cytoplasm of the cell wherever there is a ribosome.

  • To get translation started, mRNA attaches to a ribosome and a start codon must be read.

  • It's usually AUG and I remember that school usually starts in August and that helps me

  • remember AUG.

  • The first amino acid is brought in by tRNA.

  • The anti-codon on tRNA matches up to the codon on mRNA then the next the next tRNA molecule

  • moves in and matches up with the mRNA codon.

  • This time the amino acids form a peptide bond and link together, then the first tRNA can

  • detach and the mRNA shifts through like ticker tape and the next tRNA molecule can come in.

  • The protein grows until a stop codon is reached.

  • Once the stop codon is read, the protein is formed and ready to finish folding to become

  • functional.

  • And that's the end of protein synthesis.

  • Now we can figure out the sequence of amino acids using the mRNA and amino acid chart

  • like this one.

  • Let's use an example.

  • Starting with the first three letters "AUG" we can find the amino acids.

  • The first letter is A which narrows it down to this row.

  • The second letter is U which narrows it down to this column and the third letter is G which

  • means that this is methionine which is the starting amino acid.

  • Next is "CCC" which is the second row, second column and second row on the right.

  • But we don't even have to look at the third letter in this case because every third letter

  • will bring the same amino acid: proline.

  • Next is "GGC".

  • Fourth row, fourth column, second row on the right.

  • That gives us glycine.

  • And lastly is "UAA".

  • Row one, column three, row three and that gives us STOP.

  • There's no amino acid here.

  • It just means the protein will detach and it marks the end of protein synthesis.

  • Thanks for watching this episode of Teacher's Pet!

  • Don't forget to like and subscribe and follow me on Twitter @sciencepet

DNA is very long and cumbersome.


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B2 中高級

蛋白質的合成 (Protein Synthesis)

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    nowayout 發佈於 2021 年 01 月 14 日