RNA and Protein Synthesis | Notes on the A-Level Biology Review (2023)

RNA and protein synthesis go hand in hand. RNA serves as a template for translating the genetic information from DNA into proteins. The wide variety of proteins found in living systems can be divided into two main groups:

1. Structural Proteins

These are the proteins that make up the building blocks of living organisms. Major types of fibrous proteins that serve as structural elements include actin and myosin in muscles and other contractile systems, collagens that form connective tissues in the body, keratin that forms protective coverings such as skin, hair, hooves, horns, feathers, and more.

2. Regulatory proteins

These are the proteins that regulate numerous processes and activities for the body. These include enzymes that modulate chemical reactions, antibodies that provide immunity against infection, hormones, and a host of other substances that ensure that each life form responds appropriately to its ever-changing internal and external environments.

• There is a common pattern for the thousands of types of proteins found in living systems. The 20 types of naturally occurring amino acid monomers are linked together in unbranched linear polymer chains of proteins.
• All 20 amino acids have the same basic structure. There is a carboxyl group, COOH, and an amino group, NH_2,bonded to the first atom (α-carbon). and a hydrogen atom as the third moiety bonded to that carbon in all amino acids.
• However, glycine has a second hydrogen atom attached to the α-carbon.
• The other 19 amino acids also have a fourth group (R) that differs from amino acid to amino acid.

  

• In the presence of the right enzymes, amino acids are linked together by peptide bonds in a dehydration reaction, resulting in the polypeptide polymers that make up the protein. Differences between proteins are due to differences in the number, type and arrangement of amino acids in polymers. It is also attributed to the number of polymers that make up the protein, in addition to the weak hydrogen bonds that give the protein molecule its particular shape. Protein synthesis is a very complicated process and involves the interaction of different types of molecules. The most important are RNA.

Types of RNA are involved in protein synthesis

There are three main types of RNA involved in protein synthesis:

1. Messenger RNA (mRNA)

• Transcription of DNA into RNA begins when the enzyme RNA polymerase binds to a series of nucleotides in the DNA called a promoter. Then the two strands of DNA are separated and one strand serves as a template for the formation of a complementary strand of RNA. RNA polymerase moves along the DNA, sequentially joining complementary nucleotides to the growing RNA strand.
• The enzyme only works in the 3 to 5 direction in its DNA template and assembles the RNA in the 5 to 3 direction. This process is very similar to DNA replication, with one important difference: once DNA replication starts, it normally copies all of the cell's DNA, whereas RNA synthesis only transcribes selected portions of the DNA.
• Because DNA molecules are double-stranded, virtually any piece of DNA can be transcribed into different RNA molecules, one complementary to each strand. In practice, only one of the strands is transcribed in each DNA segment. The promoter's orientation dictates which strand to transcribe.
• At the beginning of each mRNA is a ribosome binding site: a series of nucleotides that bind to a ribosome in such a way that the first codon (AUG) is correctly positioned for translation and the last codon (the stop codon) can be one. It consists of three codons: UAA, UAG and UGA. At the other end of an mRNA molecule is a polyadenine (ploy-A) tail, composed of up to 200 adenine residues. This tail appears to protect the mRNA from being degraded by enzymes in the cytoplasm.

Read more about mRNA

2. Ribosomal RNA (rRNA)

• Ribosomes, the organelles of protein synthesis, are made up of several types of rRNA and about 70 types of polypeptides.
• In eukaryotes, ribosome production occurs in a region of the cell nucleus called the nucleolus, where several hundred thousand Ribosomes are produced hourly. Such rapid production is only possible because the DNA of a eukaryotic cell contains up to 600 copies of the ribosomal RNA genes from which rRNA is transcribed.
• There are four different types of rRNA involved with the protein involved in the structure of the ribosome.
• A functional ribosome consists of two subunits, one large and one small. When not involved in protein synthesis, the subunits separate and move independently. Each can join another subunit of the opposite type the next time it participates in protein synthesis.
• In eukaryotes, polypeptides are made from ribosomes in the cytoplasm and then pass through the nuclear envelope and enter the nucleolus where rRNA and polypeptides are assembled into ribosomal subunits. During protein synthesis, an interaction between mRNA and rRNA occurs.

3. Transmission of RNA (tRNA)

• The third type of RNA involved in protein synthesis is transfer RNA (tRNA), which transports amino acids to the ribosomes. For each type of amino acid there is a specific type of tRNA molecule that recognizes and transports it. (Amino acids, for which there are different codons, have more than one specific type of tRNA.)
• Transfer RNA is transcribed from tRNA genes, which are usually arranged in groups of seven or eight tRNA genes on the same part of the DNA molecule. All tRNA molecules have the same general shape. Parts of the molecule fold into characteristic loops, held in shape by base pairs between different parts of the molecule.
• There are two sites on a tRNA molecule that are important for protein synthesis. The first location is where the amino acid is attached to the molecule. This site consists of the three bases (CCA) at the 3' end of the molecule. The other site is the anticodon site, whose base pairs with the correct mRNA codon on the mRNA-ribosome complex. This temporarily links the tRNA to the mRNA, allowing the amino acid carried by the tRNA to be incorporated into the polypeptide at the correct location.
  

  

• Protein synthesis begins when the mRNA binds to a ribosomal small subunit and the first codon (AUG) is positioned correctly for initiation. The AUG codon then pairs with the anticodon of the tRNA that carries the methionine. This methionine eventually becomes the first amino acid in the polypeptide chain.
• A large ribosomal subunit then binds to the complex and the protein synthesis reaction can begin.
• A ribosome has two sites where tRNA can attach. As a result of the events described above, the initiation codon (AUG) of the mRNA molecule is located at the first of these sites on the ribosome: the peptidyl (P) site.
• The mRNA codon for the second amino acid is aligned at the second site: the aminoacyl site. From that point, the polypeptide chain elongates in a three-step cycle.
• The first step is the binding of the next tRNA to an anticodon complementary to the next codon of the mRNA. The amino acid carried by this tRNA is the next amino acid in the polypeptide chain.
• The second step is the peptidyl transferase reaction, which leads to the formation of peptide bonds. The peptidyltransferase enzyme that catalyzes the reaction is an integral part of the large ribosomal unit. This enzyme joins the first amino acid to the second via a peptide bond, so the first tRNA is now empty and the second contains both amino acids.
• The third step of the cycle moves the ribosome along the mRNA. This brings the next codon into the P site of the ribosome, and the cycle begins again when the anticodon of the correct tRNA attaches to the codon, placing the third amino acid in the A site. The growing polypeptide chain is attached to the newly arrived amino acid of this third tRNA and the sequence repeats.
• Protein synthesis stops when the ribosome hits a stop codon on the mRNA. A special protein called a release factor binds to the stop codon and causes the mRNA to come off the ribosome. The ribosomal subunits separate when the 5' end of the mRNA leaves the ribosome and can attach to another small ribosomal subunit, which starts protein synthesis again.
• Attached to each mRNA molecule are typically more than 100 ribosomes, which transcribe your message as it travels. An mRNA with many ribosomes attached forms a cluster called a polyribosome or polysome.
  

  

common questions

What steps are involved in protein synthesis?

Protein synthesis is carried out in two main steps;

transcription

translation

What organelles are involved in protein synthesis?

Protein synthesis occurs primarily on ribosomes. Ribosomes are also called protein factories.

What is the role of tRNA in protein synthesis?

The tRNA is primarily responsible for transporting the amino acid from the cytoplasm to the ribosomes (where proteins are synthesized).

What molecules are needed for protein synthesis?

The following molecules are required for protein synthesis;

amino acids

DNS

RNS

Difficult