Video: Protein synthesis

I see them. They're building so many proteins. Let's get to those ribosomes and insert our DNA so they'll produce our proteins. What's your problem? Yeah.

You got a better plan? You're wasting your time. The DNA is in the nucleus. What? Amateurs.

Let me tell you about protein synthesis. Most cell functions rely on proteins. These molecules are so important that the instructions for building them are locked up in the cell nucleus. A section of DNA that contains the instructions to build a protein or another functional molecule like RNA is called a gene. Humans have many genes.

Each cell of their body contains all genes, collectively known as the genome. However, each cell expresses only a fraction of its genes. The active genes determine which proteins the cell produces and therefore its function. Some genes are always active, like those regulating the expression of proteins necessary for essential functions. Others can be silenced and reactivated when needed.

In our DNA, the instructions to build proteins are written as nucleotides with four nitrogenous bases: adenine, thymine, guanine, and cytosine. RNA, another nucleic acid involved in protein synthesis, has the same bases but has uracil instead of thiamine. Human cells have twenty primary amino acids. How can only four types of nucleotides result in twenty amino acids? If each base coded for a single amino acid, DNA would only code for four amino acids.

That's not good. Instead, the bases are combined, similar to how we combine letters to form words. Pairing up four bases results in sixteen unique combinations, and that's still not enough. That is why amino acids are coded by triplets of nucleotides. These triplets can result in sixty four combinations, and most of them code for an amino acid.

During protein synthesis, the sequence of bases in DNA triplets is copied onto complementary RNA nucleotides called codons. Scientists have decoded how each RNA codon matches with an amino acid. This dictionary is called the genetic code and can be used to translate the language of RNA codons into the language of amino acids and vice versa. For example, the start codon for most proteins in humans is AUG, adenine, uracil, guanine. This codes for the amino acid methionine.

While each codon corresponds to a single amino acid, most amino acids are coded by multiple codons. For instance, ACA always codes for threonine, but this amino acid is coded by multiple codons. Right. Codons can be translated into amino acids, But proteins are more than single amino acids. How are they produced?

Protein synthesis requires four steps. Step one, transcription. DNA copies its information onto a messenger RNA. Step two: mRNA processing mRNA gets trimmed and modified. Step three: translation ribosomes use the mRNA as instructions to link amino acids into a polypeptide.

Step four, post translational modifications. The polypeptide is modified and folded to make it a fully functional protein. Let's dive into these phases one by one, starting from transcription. During transcription, the cell synthesizes RNA from DNA. This is necessary because most of the cell's DNA is located in the nucleus, while our protein factories, the ribosomes, are on the nuclear envelope, rough endoplasmic reticulum, and in the cytosol.

DNA is a very large molecule, too large to leave the nucleus, so it can't communicate with the ribosomes directly. This is why the cell transcribes DNA information into smaller RNA molecules that fit through the nuclear pores. This RNA is called messenger RNA or mRNA. Transcription has three phases, initiation, elongation, and termination. To start, a transcription factor protein binds to the promoter, which is a DNA region before the gene that needs to be transcribed.

This attracts RNA polymerase, which binds to DNA and unwinds the two strands. DNA has two strands: a coding strand, which contains the triplets that code for different amino acids, and the template strand, which contains bases complementary to the coding strand. RNA polymerase transcribes the template strand. In the elongation phase, the polymerase adds nucleotides to mRNA, lengthening the strand. It ensures that each RNA base complements that of DNA.

Cytosine with guanine, guanine with cytosine, thymine with adenine, and adenine with thi wait. Why is this not work oh, right. Adenine with uracil. Since the polymerase copy is the DNA template strand, the sequence of mRNA codons ends up being the same as that of the DNA coding strand, but with uracil instead of thymine. A special series of nucleotides, known as the terminator sequence, indicates the end of transcription.

When RNA polymerase reaches this point, both the enzyme and the mRNA strand detach from DNA. The mRNA strand is free, but it's not ready for the ribosomes yet. Before it leaves the nucleus, it has to undergo mRNA processing. Not all the information present in the messenger RNA is necessary to build proteins. This freshly transcribed mRNA, sometimes called pre mRNA, needs to be modified to be easily read by ribosomes.

During the mRNA processing phase, non coding segments called intrins are removed by a process called splicing. Most of the remaining mRNA consists of exons, which code for specific amino acids. Additional mRNA processing includes alternative splicing. Sometimes, a single pre mRNA can give rise to multiple different proteins. This occurs through alternative splicing, in which different combinations of exons are joined together to form distinct, mature mRNA molecules.

Once the exons are spliced together, the trimmed messenger RNA is ready to zip through the nuclear pores to initiate translation. When mRNA reaches ribosomes, either free in the cytosol or bound to the rough endoplasmic reticulum or to the nuclear envelope, it meets up with two of its cousins. One is called ribosomal RNA, and it is found in the ribosome along with ribosomal proteins. Ribosomes are made up of a small and a large ribosomal subunit. Another type is transfer RNA, which is a clover leaf shaped strand of RNA.

At one end, it carries an amino acid. At the opposite end, it contains an anticodon, which is a sequence of bases that indicates which amino acid the transfer RNA carries. More about this later. More about this later. More about this later.

During translation, all three types of RNA work together to build a chain of amino acids known as a polypeptide. Translation can also be split into the same three phases as transcription initiation, elongation, and termination. Translation starts when a small ribosomal unit binds to an mRNA strand and starts sliding along it. The initiator tRNA looks for the start codon. This is possible because its UAC anticodon is complementary to the start codon AUG.

Since tRNAs with anticodon UAC always carry methionine, mRNA codons with bases AUG always result in the addition of the amino acid methionine, just as the genetic code demands. Then the large ribosomal unit binds so that the initiator tRNA is in the middle slot known as the p site. This translation complex has two other sites a and e. Let's see what they're needed for. During the elongation phase, tRNAs bring in other amino acids.

The tRNA anticodons ensure that the amino acid sequence is the one specified in the mRNA. Each new amino acid is linked to the previous one. Once complete, the complex slides forward, moving the empty tRNA to the e site to be released. Its job is done. The second tRNA moves to the p site, leaving the a site empty for the next tRNA.

As this process is repeated over and over, the polypeptide becomes longer and longer. Translation ends when an mRNA stop codon is found. At this point, a protein called a release factor enters the A site, releasing the RNAs and the polypeptide. The mRNA may remain in the cytosol to be translated again or be degraded if the cell has enough of the protein it encodes. The polypeptide is the newly synthesized protein almost ready to become fully functional.

After translation, it needs to undergo post translational modifications. Post translational modifications often take place in the Golgi apparatus, rough endoplasmic reticulum, and cytosol. Freshly synthesized proteins have the correct amino acid sequence, but their structures still need some work before they become functional proteins. Some modifications include folding into a three-dimensional shape, creation of crosslinks between amino acids, and addition of other molecules or groups. Cells often use a special tagging system to move proteins to the organelle that modifies them or to their final destination.

Some of the proteins synthesized on free ribosomes are not tagged, so they stay in the cytosol. Protein synthesized on bound ribosomes are often tagged with sorting signals that direct them deeper into the rough endoplasmic reticulum and to the Golgi apparatus. Once modified and folded, the proteins are now finally ready to get to work. So we got a plan. Get to the nucleus.

Hijack the DNA. And take over protein production. You three, stop right there. To learn more about protein synthesis, check out the quiz and study unit. See you next time.