Gene Technology is the altering of genetic materials by direct interventions, biomolecular behaviors, and changing genetic processes, to manufacture new medicines or to improve the functions of existing organisms.
The interesting part of genetics is how genes control the cell functions, the body's height weight, complexion the resemblance of a child to its parents, cell functions, various enzymes, antibodies, through the cell protein synthesis.
Proteins are large macromolecules made out of polypeptides that have major roles in various important functions to keep our bodies healthy.
For example, insulin is a protein to regulate our body's glucose metabolism.
A protein is known as 'ferritin' that helps the body to store iron in the liver, marrow, and spleen.
Antibodies protecting us from infections are all various kinds of proteins.
Most of our body enzymes are made out of proteins.
Actin is the protein envelope that gives flexibility to our cells.
Our body cells contain many organelles including cell nucleus, nucleolus, endoplasmic reticulum, mitochondria that are all made of proteins.
Each and every cell of our body is a powerful factory of protein synthesis. The process involves central dogma, and gene expression to result in protein synthesis.
Synthesis of Proteins in Eukaryotic Cells:- |
| Fig-1 |
A simple eukaryotic cell has been shown above (Fig-1) that differs from the prokaryotic cell by having a defined nucleus and nucleolus. All multicellular organisms including human beings are eukaryotes. Transcription
The first step of protein synthesis is the transcription that is carried out in the nucleus. "The codon copying of the protein-coding genes by using DNA as a template into an mRNA which contains the message to prepare a particular protein is known as Transcription.
This is similar to copying a single recipe for a particular food from the original book of recipes.
In the nucleus, the genetic material presents as a chromosome that is a tightly twisted thread made of double-stranded DNA. Every strand of DNA is stitched with the basic units known as the nucleotides. There are four types of nucleotides used to stitch the DNA strand. They are labeled as adenine (A), guanine (G), thymine (T), and cytosine (C) nucleotides. |
| Fig-2 |
The four types of nucleotides are having the structures as shown in the figure (Fig-2)
Adenine and Guanine are known as purines
Thymine and cytosine are known as pyrimidines.
Nucleotides are formed and bonded as follows:-
Adenine and thymine nucleotides are coupled with double hydrogen bonds and guanine and cytosine nucleotides are coupled with triple hydrogen bonds in a DNA strand. All these nucleotides are stitched in as a strand fitted on a backbone contains a deoxyribose sugar ring bonded alternatively with a phosphate ion to the 5th carbon atom of one sugar ring on the upper side and to the 3rd carbon atom of another sugar ring at the lower side. See Fig-2)
There are billions of nucleotide units in each of our human body cells embedded in the chromosomes.
These nucleotides are grouped as triplets known as codons like ACC, ATC, ATG, TAG, TAC, and so on of a total of 64 codons and each represents one amino acid. Amino acids are the basic units of protein and hence to synthesize a particular protein there are a number of amino acids are required. To do this job a group of codons is to be transcribed from a particular portion of the chromosomes embedded by the DNA thread. That particular portion is known as Gene. |
| Fig-3 |
There are 23 pairs of a total of 46 chromosomes in every cell of our human body.
Transcription:- |
| Fig-4 |
A portion of DNA is stretched from the chromosome out. An enzyme is known as DNA dependant RNA polymerase that unwinds the double-stranded DNA apart with the help of some cofactors. One of the unwinded single strands of the DNA (Template strand) is used by the RNA polymerase to transcribe the DNA codes to mRNA as shown in Fig-5 below. |
| Fig-5 |
DNA nucleotide sequence is arranged from tail to head (3' to 5') and the RNA polymerase moves and reads the codons from head to tail (5' to 3') to prepare mRNA. |
| Fig-5A |
Thymine in DNA is replaced by Uracil in mRNA by the RNA polymerase without any change in the genetic information.
This is because thymine and uracil both are pyrimidines and have similar structures. Thymine is methyl uracil that needs more energy to move. Since DNA is unmovable and fixed within the nucleus but RNAs are mobile and must carry the genetic copy of DNA to do their job outside the nucleus they must contain lightweight uracil instead of thymine to save much energy.
Once the raw mRNA is prepared in the nucleolus they contain uncoded portions also. These portions are known as introns that are cut and trimmed out and the rest of the coded portions are joined together as an exon and exit the nucleus into the cytoplasm.
Translation:-  |
| Fig-6 |
This is the next step in the process of Central Dogma or gene expression or protein synthesis. While the step of transcription is carried out inside the nucleolus of the nucleus, the step of translation is carried out in the cytoplasm outside the nucleus.
The mRNA exited out from the nucleus reaches one of the scattered numerous numbers of mini organelles known as ribosomes embedded in the hard endoplasmic reticulum as well as free unbounded in the entire cytoplasm.
Ribosomes are tiny organelles made of 60% rRNA (ribosomal RNA) and 40% proteins. This is the protein-cooking stove. It cooks protein according to the DNA codon recipe copied in the mRNA with the help of tRNA (transfer-RNA). The tRNAs contain anticodons to read the mRNA recipe and bring the necessary amino acid ingredients to be cooked into proteins.
The rRNA present in the ribosome cooker bonds together the amino acids into a peptide chain.
The tRNAs start to read the mRNA from head to tail (5' to 3') direction. The first codon at the start point in the mRNA is mostly AUG that represents the amino acid methionine (Met). In the end, there is a stop codon that does not represent any amino acid with which the cooking process ends.
There are three stop codons namely, UAA, UAG, UGA is available. Anyone of them can stop the protein synthesis process.
There are 64 codons formed by the nucleotide triplets in the main DNA template. Out of that 61 codons are represented by 20 amino acids. No two amino acids are indicated by the same codon but one amino acid may have been indicated by different codons. (e.g)
Glycine is indicated by the codons GUU, GUC, GUA, GUG.
Similarly, leucine is indicated by six codons namely UUA, UUG, CUU, CUC, CUA, and CUG.
How to remember the codons and the corresponding amino acids in an easy way will be dealt with in a separate post.
Translocation:
Protein synthesis is a cyclic process. The first tRNA that brought methionine the first amino acid come into the ribosome's A-compartment followed by moving into the P-compartment and locate the aminoacid by decoding the AUG codon in the mRNA strand by its on code UAC and after that empty tRNA moves the exit E-compartment and exit from there. By that time the ribosome moves one codon downstream to locate the next tRNA to moe in from the A site to the P site compartment to locate the amino acid which it carries and the process is going on up to the stop codon reached. During this process, one by one amino acids is spindled together as polypeptides and released out from the ribosome. The polypeptides finally polymerized together to form the final shape of a protein. These proteins are sent to the proper locations within the cell or outside the cell for utilization.
SUMMARY:-
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| Fig-7 |
In the eukaryotic cells, the protein-coding genes express their protein-synthesizing action by the above equation (Fig-7) DNA, the hard copy of the whole cookbook is present inside the nucleus as chromosomes. The enzyme RNA polymerase locates the gene and reads its codons for synthesizing a particular protein. It copies the codons as mRNA. This copying process is known as the Transcription.
The mRNA swims out of the nucleus like a warm with a head and tail into the cytoplasmic colloidal fluid.
There are numerous numbers of organelles known as ribosomes scattered freely in the entire cytoplasm as well as in the endoplasmic reticulum outside the nucleus.
The ribosomes are of 80S unit which is divided into a large 60s and 40s subunits. When they unite together they will look like a hand with a clenched fist. But always they remain separated. The ribosomes contain 60% rRNA and 40% proteins. The small 40s subunit unctions to carry the mRNA on its upper platform and facilitate its codons to be readable. The large 60s subunit functions to receive the tRNA and helps it to process with bonding and fixing the aminoacids blocks brought by the tRNAs. (Fig-6).
The mRNA enters on the upper platform of the small 40s subunit ribosome on a longitudinal direction with all its codons projecting upwards. The larger 60s subunit of the ribosome receives a tRNA with an anticodon UAC at its bottom tail end and the corresponding amino acid methionine (see Fig-6) (M) at its aminoacyl head in its A cage. After this, the tRNA moves into the P-cage where it reads the mRNA code AUG in 5' to 3' direction while the mRNA moves in the opposite 3' to 5' direction (Fig-6)
The anticodon UAC decode the codon AUG and the amino acid (M) block is removed from the tRNA head and fixed at the top of the ribosome. The emptied tRNA moves into the E cage and from there it exits out. The process continues until at least 20 to 20 amino acids are bonded by rRNA to build-up into polypeptides. As the amino acid blocks are received the rRNA translocates them in order as the first received is on the second, and the second received on the third, and so on (Fig-6).
Finally, a set of amino acid blocks are stitched to form a polypeptide and further processed into a protein molecule.
Thus synthesized proteins are either utilized intracellularly for various cell unctions or extracellularly various body functions.
The following table gives the synthetic sites and usage of some proteins:
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| Fig-8 |
A Cartoon Analogy to Explain Protein Synthesis in Eukaryotes:-
