Proteins - Biology

Protein synthesis starts in the nucleus with transcription. Transcription is the process where DNA is transcribed into mRNA. Translation occurs in the cytoplasm within ribosomes where mRNA is translated and becomes a protein with the help of rRNA and tRNA.

Enzymes are biological catalysts that increase the rate of a chemical reaction. This is accomplished by lowering the activation energy for the reaction. Enzymes increase the rate of a reaction, but do NOT increase the amount of products formed in the reaction. They simply cause the products to be formed faster.

Keratin is a fibrous protein found in hair, nails, and the epidermis of skin. Myosin is one of the major proteins of muscle. Trypsin is an enzyme in the small intestine that helps digest proteins. Collagen is a fibrous protein found in connective tissue, such as bone and cartilage. Fibrin is a clotting protein found in the blood in response to injury.

Collagen in skin and keratin in hair and nails provide the body with structure. Actin and myosin are found in muscle and facilitate movement. Antibodies defend the body against microbes. Numerous enzymatic proteins, such as pepsin and lipase, act as catalysts. Peptide hormones, such as insulin and oxytocin, help relay signals in the body.

tRNA is responsible for bringing individual amino acids to the ribosome in order to be incorporated into the protein. It has an anticodon that attaches to a specific codon found on the mRNA. Once the tRNA and mRNA are bound, a peptide bond if formed between the amino acid residue from the tRNA and the amino acid chain on the ribosome. This is how amino acids are added to the growing protein.

The genetic code is considered both degenerative and unambiguous. A codon will only code for one amino acid, making the code unambiguous. In contrast, multiple codons can code for the same amino acid, making it degenerative. For example, UGU will always code for cysteine, but UGC also codes for cysteine.

Nearly every living organism uses the same genetic code. tRNA anticodons are complementary to mRNA codons; they are not the same code.

As amino acids are added to a polypeptide during translation, tRNA molecules will enter the A site of the ribosome. The tRNA is then transferred to the P site, where a peptide bond is formed between the amino acid residue and the amino acid chain. Finally, the tRNA moves to the E site to release its tRNA and exit the ribosome.

The small subunit of eukaryotic ribosomes is 40S and the large subunit is 60S. These combine to form the ribosome, which is 80S in sedimentary size.

For prokaryotes, the ribosome subunits are 30S and 50S to form a total of 70S total unit.

The ribosome contains three sites: the A, P, and E sites.

The A site is where activation occurs, starting translation. This is where a tRNA molecule enters the ribosome and matches its anticodon to the mRNA codon.

The tRNA then shifts over to the P site to attach the amino acid. The ribosome facilitates the formation of a peptide bond, adding the amino acid to the chain.

At the E site, the empty tRNA exits the ribosome and dissociates from mRNA.

DNA contains genetic information that is transcribed into mRNA. This process is known as transcription, and occurs in the nucleus. After modification in the nucleus, mRNA exits the nucleus and enters the cell cytoplasm. In a process called translation, mRNA (in conjuction with tRNA and a ribosome) is used as a template to join amino acids to form specific polypeptides.

In summary, DNA is transcribed into RNA, which is translated into protein.

During initiation, the first tRNA molecule will bring the first amino acid to the ribosome. Although the following amino acids will enter at the A site, the first amino acid is positioned in the middle P site. The large ribosomal subunit will then attach, and translation can begin.

During the subsequent elongation phase of translation, tRNA/amino acid complexes will enter the ribosome at the A site, transfer to the P site, and then exit through the E site. Only the first complex will begin in the P site, during initiation.

At the beginning of translation, mature mRNA will travel into the cytosol and attach to the small ribosomal subunit first. This signals the larger subunit to come and attach in order to begin elongation of the polypeptide.

Note that ribosomes do not have an "active site." Active sites are the region of a protein that will bind a substrate and initiate a catalytic change. Ribosomes are not proteins; they are composed of ribosomal RNA (rRNA), and thus do not have active sites.

During translation, the ribosome binds to mRNA and to the appropriate tRNAs. On the ribosome, the mRNA condons are translated into the amino acid sequence of a protein with the help of the tRNA anticodons.

Transcription refers to the synthesis of an RNA molecule from a DNA template. Transformation occurs when a bacterium is able to absorb and incorporate genetic material from the extracellular environment. Transfusion is the transfer of blood from a donor to a recipient. Transfection is the infection of bacteria by phage DNA.

The tRNA carries the amino acid specified by its anticodon. The anticodon base pairs with the codon on the mRNA to ensure the correct amino acid is added to the new protein that is being made. Thus there is a specific tRNA for each codon.

Ribosomes are made up of rRNA and proteins. rRNA synthesis, and ribosome assembly takes place in the nucleolus.

While one might quickly calculate that 90 nucleotides/3 would yield a 30 amino acid sequence, it is important to remember that while the first 3 nucleotides will encode for an amino acid to start translation (methionine), the last 3 nucleotides do not. They simply stop translation and signal for the growing polypeptide chain to be released from the ribosome-tRNA translation complex without actually adding another amino acid to the end of the chain.

The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes, and a modified methionine (f-Met) in prokaryotes. The most common start codon is 5' AUG 3'. The start codon is preceded by a untranslated region which includes the ribosome binding site in prokaryotes.

5' UAA 3', 5' UGA 3', 5' UAG 3' are all stop codons.

Translation refers to the processing of an mRNA script into a protein. This process utilizes ribosomes and tRNA. In translation, messenger RNA (which is produced by transcription from DNA) is decoded by a ribosome to produce a specific amino acid chain, or polypeptide. The polypeptide is later folded into an active protein.

Translation is the process of creating protein from an mRNA template, and consists of initiation, elongation, and termination. Modification of protein strands may occur after translation, but the three defined steps of translation are initiation, elongation, and termination. Many types of transcribed RNA, such as tRNA, rRNA (ribosomal), and small nuclear RNA, do not undergo translation in order to become functional proteins.

Transfer RNA—tRNA—is a small folded RNA molecule (i.e. 80-90 nucleotides in length) utilized during translation. On the other hand, tRNA serves as a link between the messenger RNA—mRNA—and the growing protein’s amino acid sequence. The tRNA carries an amino acid to the ribosome directed by the mRNA’s codon—a three-nucleotide sequence. The tRNA carries an anticodon, which is also a three-nucleotide sequence that matches the genetic code, which complements the mRNA. Last, rRNA is associated with the ribosome.