Introduction:
Proteins determine the structure and function of all the cells. DNA, or deoxyribonucleic acid, contains the genes to determine the identity. The sequence of amino acids decides the structure of the protein. Instructions for forming proteins and the correct amino acid sequence are encoded in the DNA.
DNA is present in chromosomes. In eukaryotic cells, the chromosome is in the nucleus, but proteins are produced at ribosomes in the rough endoplasmic reticulum or on the cytoplasm.
How Does DNA Carry Out Protein Synthesis Outside the Nucleus?
A type of nucleic acid is responsible, such as RNA or ribonucleic acid, a small molecule that squeezes through pores in the nuclear membrane and carries the information from the nuclear DNA to a ribosome in the cytoplasm. They help the formation of the protein.
DNA → RNA → Protein
Discovering this sequence of events is an essential element in molecular biology. The two processes in the central dogma are transcription and translation.
What Is Transcription?
Transcription is the first phase of the central dogma of molecular biology.
DNA → RNA.
It involves the transfer of genetic instructions in DNA to messenger RNA. Transcription takes place in the nucleus of the cell. During transcription, a strand of mRNA forms complementary to a DNA strand called a gene. A gene can be easily identified from the DNA sequence. A gene contains three primary regions: a promoter, a coding sequence, and a terminator.
What Are the Steps of Transcription?
Transcription occurs in three steps: initiation, elongation, and termination.
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Transcription begins with initiation: It starts with the enzyme RNA polymerase binding to a region in a gene called the promoter. This sends signals to the DNA to unwind so that the enzyme reads the bases in either DNA strand. The enzyme makes a strand of mRNA with a complementary sequence of bases. The promoter is not a part of the mRNA produced.
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Elongation involves the addition of nucleotides to the mRNA strand.
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Termination ends the transcription: RNA polymerase transcribes the terminator and detaches from DNA. The mRNA strand completes.
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Processing mRNA: The new mRNA is not immediately ready for translation in eukaryotes. It is called pre-mRNA and needs processing before leaving the nucleus as mature mRNA. The processing includes adding a 5' cap, splicing, editing, and 3' polyadenylation tail. These processes modify the mRNA in different ways. These modifications allow a single gene to be used to make more protein.
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5' cap helps in the protection of mRNA in the cytoplasm and attaches them to the ribosome, which causes further translation.
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Splicing eliminates introns from the protein-coding sequence of mRNA. Introns do not have coding for the protein. The remaining mRNA has exons that have code for the protein.
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A human protein helps transport lipids in the blood and has two forms; one form is smaller than the other because editing causes an early stop signal in mRNA.
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Polyadenylation adds a tail to the mRNA. The tail has a string of adenine bases. It signals the end of mRNA. It exports mRNA from the nucleus and protects mRNA from enzymatic breakdown.
What Is Translation?
The translation is the second phase of the central dogma of molecular biology:
RNA to Protein:
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In this phase, the genetic code in mRNA is read to form a protein. As mRNA leaves the nucleus, it moves towards a ribosome consisting of rRNA and proteins. Translation occurs on the ribosomes from the cytosol or the ribosomes that are attached to the rough endoplasmic reticulum. The ribosome can detect the sequence of codons in mRNA, and the tRNA molecules bring amino acids to the ribosome in the proper sequence.
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Each tRNA molecule carries an anticodon for the amino acid. An anticodon is secondary to the codon for every amino acid. The amino acid called lysine has the codon AAG, and the complementary anticodon is UUC. Hence, lysine is carried by a tRNA molecule with the anticodon UUC. When the codon AAG appears in mRNA, a UUC anticodon of tRNA binds it temporarily. When bound to mRNA, tRNA gives away its amino acid. With the help of rRNA, bonds are formed between amino acids as these are brought to the ribosome and create a polypeptide chain. The chain of amino acids keeps growing till a stop codon is obtained.
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Ribosomes made out of rRNA, or ribosomal RNA and protein, are classified as ribozymes as the rRNA has enzymatic activity. The rRNA is necessary for the peptidyl transferase activity that binds all the amino acids. Ribosomes have two subunits, which consist of rRNA and protein. The larger subunit has three active sites called E, P, and A. These sites play a vital role in the catalysis of ribosomes.
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Like mRNA synthesis, protein synthesis is divided into three phases: initiation, elongation, and termination. Along with the mRNA template, other molecules, like ribosomes, tRNAs, and various enzymatic factors, contribute to translation.
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Translation Initiation:
The small subunit binds to a site on the 5' side of the start of the mRNA. Then, it scans the mRNA in the 5'-->3' direction and reaches the START codon (AUG). Next, the large subunit further attaches, and the initiator tRNA carries methionine (Met) and binds to the P site on the ribosome.
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Translation Elongation:
The ribosome shifts codon, catalyzing each process at the three sites. A charged tRNA then enters the complex, the polypeptide enlarges by one amino acid, and uncharged tRNA is lost. The bond energy between amino acids is obtained from GTP, like ATP. The ribosomes interact with RNA molecules to form chains of amino acids called polypeptide chains and have a peptide bond between amino acids. In the ribosome, three sites involved in the translation process are A, P, and E.
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Translation Termination:
Translation termination occurs due to a stop codon (UAA, UAG, or UGA). When the ribosome contacts the stop codon, the growing polypeptide releases with the help of releasing factors, and the ribosome subunits separate from the mRNA. After the ribosomal translation is completed, the mRNA is degraded, and the nucleotides can be reused in some transcription reactions.
After the synthesis of a polypeptide chain, it undergoes different processes. First, it forms a folded tertiary shape by interacting with the amino acids. It binds with polypeptides or dissimilar molecules, like lipids or carbohydrates.
How Does Protein Synthesis Form Hair?
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DNA's primary purpose is to make proteins in the cell. Different cells carry out other activities. The genes control the organism and protein synthesis in the cell. Protein synthesis aims to form a polypeptide chain of amino acids.
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In a hair follicle cell, a protein called keratin forms. Many ribosomes work on a single strand of mRNA. Protein synthesis is a fast process. A protein chain of 400 amino acids forms in 20 seconds. The keratin formed by the hair follicle makes long fibers. The cells growing under the scalp die, and the keratin remains. This keratin emerges from the scalp as hair.
Conclusion:
Protein synthesis is the process of formation of proteins. It occurs in the transcription and translation stages. Transcription involves transferring genetic information in DNA to mRNA. It includes initiation, elongation, and termination. After the mRNA is processed, data is sent to cytoplasm ribosomes. The translation is carried out at the ribosome by rRNA and proteins. In translation, the information in mRNA is read, and tRNA is responsible for the proper sequence of amino acids to the ribosome. rRNA forms amino acid bonds to form a polypeptide chain. As a result, a polypeptide chain is synthesized. Further processing of the polypeptide chain forms the protein.
