Inheritance is the transfer of properties from ancestors to decedents via the process of reproduction (either sexual or asexual reproduction). Genes are containing the complete data about the structures and functions of an organism in the form of codons. This coded information is decrypted at the location of ribosomes by combined effects of ribosome and RNAs and results in the formation of a specific protein. This process is called gene expression.
History of inheritance study
Inheritance has always been a topic of interest, but the striking progress was made in the 19th century. However, it was thoroughly studied by an Austrian scientist, Gregor Johann Mendel when he scientifically studied the “inheritance of traits”.
Imre Festetics, a Hungarian geneticist was the first person to use the term “Genetics”.
He took pea plant for experiments and after a lot of studies, he came with an idea that there are some small units that control the process of heredity. That specific unit was given the name of “Gene”.
Theory of inheritance
The chromosomal theory of inheritance, given by Walter Sutton (an American geneticist) and Theodor Boveri (a German biologist) stated that:
The properties are transferred from parents to their children via chromosomes.
What are the laws of inheritance?
Inheritance is the process of transfer of properties from ancestors to descendants. The inheritance process is completely described by Mendel when he studied the pea plant and came out with the three laws of inheritance. These laws are the backbone of biological sciences. They are described as:
Law of Dominance:
Mendel’s first law, known as law of dominance, states that only those traits will be inherited by offspring, that are associated with dominant alleles leaving behind the recessive alleles.
Law of segregation:
According to this law, allele pairs are separated during the formation of gamete and then united again during fertilization.
An allele is actually a version of a gene and each gene consists of two alleles. So, an offspring gets one allele from each parent during fertilization to make a pair and that gene pair controls the specific trait.
Law of independent assortment:
This is Mendel’s second law of inheritance. According to this law, the alleles of one gene that are arranged in the gamete, do not affect the alleles of another gene, and express their associated traits independently.
Inheritance process is completely described by the laws proposed by Gregor John Mendel. These laws are known as, the law of dominance, law of segregation, and law of independent assortment.
What are the components of the genetic system?
The genetic system comprises of following components:
Chromosomes are threadlike structures made up of nucleic acids and proteins. They are present in the nucleus of living cells and carry the genetic information for every trait e.g. height and the color of the eye.
They consist of DNA, present in the form of coils around a basic protein called Histone. Each human cell contains approximately 23 pairs of chromosomes giving a total of 46.
Genes are discrete units of inheritance and consist of nucleotides (DNA). Nucleotides are made up of a nitrogenous base, a sugar, and a phosphate group. Gene is a basic and functional unit of inheritance.
Genes are present on chromosomes and chromosomes act as the residence of genes.
Each gene consists of two copies, that are known as alleles and two alleles are present together at genetic locus (a specific location at chromosome).
One allele is inherited from each parent and then combined to complete a gene. These alleles actually control that which trait would be inherited by offspring.
If both of these alleles are the same at a single genetic locus, the organism is known to be “homozygous” and if both of them are different from each other, they organism s called “heterozygous”.
A single chromosome can contain thousands of genes and about 30,000 genes are present in a single human cell.
Genetic code or genetic codon is a set of instructions, existing in the form of a trinucleotide sequence of either DNA or RNA within the gene, that guides the cell about the formation of a particular protein.
In other words, the arrangement of nucleotides in a genetic code serves as a recipe to form a specific protein.
This sequence consists of nucleotide bases named Adenine (A), Thymine (in case of RNA it’s Uracil), Cytosine (T), or Guanine (G). It can be expressed as AGG, UAA, UAG, UCA, ATT, etc.
Genetic system comprises of chromosomes, genes, alleles, and genetic codon. Chromosomes are the residence of genes and each gene consists of two copy alleles. A set of three nitrogenous bases comprises a genetic codon.
How the genetic code is expressed?
To clearly understand the process of inheritance, first we should understand that how does DNA is transcribed into messenger RNA.
All the information required for the shape and structure of a living being is present in DNA.
This information is present in the form of a specific sequence of nucleotides. This sequence is known as a genetic codon. This specific sequence designs the anatomy (internal structure) morphology (external form) and physiology (how it works) of an organism.
Proteins are mainly the determinants of the form of an organism, for example, hair proteins, muscle proteins, etc.
While the compounds other than the proteins (e.g. fats or carbohydrates) are all formed under the influence of proteins, that act as catalysts (known as enzymes) or they are their placement in specific sites inside the body is controlled by proteins. So, proteins are the basic ingredient in the recipe of an organism.
Here starts the action of genes. Genes are the basic determinants of the protein’s structure and function and proteins further control the structure and function of a whole organism.
These genes express their instructions in the form of genetic code. Although, in most cases, a single protein is coded by a single gene, yet there are some proteins that require the collaboration f more than one gene.
So, the flow of information from a DNA to proteins follows several paths.
DNA is transcribed into mRNA inside the nucleus. That copy of the genetic message is carried by mRNA to the ribosomes that reside in the cytoplasm. The message is decrypted to form the proteins by the assistance of ribosomal and transfer RNAs.
How many codons are used to design one amino acid?
As 4 nucleotides are present in DNA (A, G, T, C) and RNA (A, G, U, C) and these 4 can design only 4 amino acids if they are considered individually.
While there are 20 different types of amino acids that build proteins by arrangement in different manners. So there is a need for a group of nucleotides working together to design all 20 types of amino acids.
If each code would have consisted of just two nucleotides, then they would make only 16 types of codes that are surely not enough to completely synthesizing the whole set of proteins.
While, if 3 nucleotides are arranged in alternate manners (e.g. UAA, AGC, CUG, etc.) then they form 64 code words that are a reasonable number to design all 20 amino acid.
Hence the triplet code is used in genetics instead of singlet or doublet.
However, all 64 codons are not specified for amino acids, 61 are for amino acid synthesis, and the remaining three codons act as stop codons and act as the full stop at the end of the amino acid chain.
It’s not necessary that one amino acid is designed by one codon, instead most of the amino acids may require more than one codon.
Out of 20 amino acids, just two (Methionine, Tryptophan) require a single codon, while some are specified by more than 5 codons.
There are 64 codons that are involved in the synthesis of 20 amino acids that are required for the synthesis of proteins within the cell.
What is Transcription?
Transcription is the process in which a specific sequence of nucleotides is copied from double-stranded DNA to single-stranded RNA.
- This RNA copy of a gene is formed at a specific location of the chromosome where that particular gene is present.
- All this transcription process takes place in the nucleus, which is the membrane bound organelle inside the cell and controls the growth and reproduction.
- This RNA that contains a copy of the information is known as messenger RNA and it can be described as a duplicate of original DNA.
- The only difference in these original and copied form is that the RNA contains Uracil instead of DNA, in which it is Thymine. But it doesn’t affect the message or information that is copied from DNA because both of these bases form the same type of hydrogen bond with the opposite base.
What are the steps of transcription?
Transcription takes place in the following steps:
- An enzyme known as RNA polymerase binds at the specific region of the gene (called promoter) and triggers the uncoiling of DNA molecule that is normally present as a coiled spring.
- Now every single strand of DNA is available for the enzyme to read the nucleotide sequence, for making a strand of mRNA.
- Strand of DNA that is being copied is known as template strand. Both these copies are not identical because the enzyme copies the nucleotides from parent to daughter copy in a complementary sequence of base pairs.
During elongation, the addition of nucleotides is being continued to the strand of mRNA by RNA polymerase.
This is the end stage of transcription in which RNA polymerase adds a specific termination sequence of nucleotides to mRNA that indicates that the mRNA strand is complete. The mRNA formed in this way is actually a preform of mRNA with some undesired segments in it.
4. Processing and Splicing
- Before moving outside the nucleus, pre-mRNA is processed by several ways to remove the undesirable parts known as introns to create a final and mature copy of mRNA.
- Normal base pairs are present as Adenine-thymine and Guanine-cytosine while in mRNA uracil is present instead of thymine.
- Now final and mature molecule of mRNA is all set to move outside the nucleus to reach its destiny for conveying the message.
Ribosomes are smaller particles present in the cytoplasm which act as a destiny of mRNA and act as a factory for protein synthesis.
Transcription takes place in the cell nucleus and consists of four steps. These steps include initiation, elongation, termination, and processing. In the end, mRNA is spliced to get rid of extra parts known as introns.
What is Translation?
Translation is the second main step in the process of gene expression.
During translation, the message that was taken by mRNA in the form of genetic code is read and expressed in the form of proteins.
- Which nucleotide sequence will make which type of protein? It is confined in the genetic code.
- Sequence of DNA or nucleotides is related to the sequence of amino acids that are basic structural units of protein.
- A group of three nitrogenous bases (triplet) makes a single codon, and each codon is for a specific amino acid.
- In this way, amino acids are arranged in the form of chains according to the sequence of triplet codons in mRNA, leading towards the synthesis of protein.
Where does Translation Occur?
The whole translation factory is present in minute cell particles within the cytoplasm, which are called as Ribosomes.
In eukaryotic cells, mRNA after maturation leaves the nucleus and travels to the ribosomes, while in prokaryotes that have no specific membrane-bound nucleus, ribosomes can approach mRNA even in the transcription phase and gets attached to it.
What are ribosomes?
In both prokaryotes and eukaryotes, the ribosome is an organelle that is made up of two parts or subunits.
- A large subunit called 50S subunit
- Small subunit called as 30S subunit.
“S” stands for the Svedberg unit, a unit of sedimentation velocity.
- These the 50S and 30S subunits are not present together, rather they are present separately within the cell sap and come together to join each other at mRNA.
- Two other types of RNA that take part in protein synthesis, called ribosomal RNA (rRNA) and transfer RNA (tRNA) are present at these ribosomal subunits.
What are tRNA and rRNA?
Transfer RNA is actually an adapter molecule having two ends. At one end they can decode the message of mRNA while at the other end the particular amino acid is attached.
Francis Crick, a British biologist and discoverer of DNA structurer along with James Watson, gave the concept that transfer RNA acts as an adapter molecule.
A specific type of tRNA is associated with each amino acid and that specific tRNA transfers amino acid to the leading end of the growing chain if the upcoming codon at mRNA has a code for that specific amino acid.
Ribosomal RNA (rRNA) combine with some proteins to shape a Ribosome. These tRNA act as a catalyst during the alignment of amino acids to form a specific protein.
Transfer RNA and several other important molecules that are required for protein synthesis are also attached to rRNA. Each subunit of a ribosome contains its own type of rRNA
What steps are involved in translation?
1. Initiation phase:
The translation process starts with the formation of the initiation complex at mRNA. This complex consists of a small ribosomal subunit (with an initiator complex of 3 proteins i.e. IF1, IF2, IF3) and tRNA molecule carrying methionine.
This ribosomal subunit and tRNA molecule are attached near the start codon (AUG) at mRNA to form that initiation complex. A small ribosomal subunit contains three sites:
• Amino acid site (A)
• Polypeptide site §
• Exit site (E)
- Methionine carrying tRNA attaches to AUG and this methionine will be the first amino acid to be incorporated into the polypeptide chain.
- Although methionine is the first amino acid that is incorporated, it’s not the first amino acid in each protein. In most cases it is removed after the process of translation is completed.
2. Elongation phase:
During the elongation phase, the elongation factor G assists in translocating the ribosome along with mRNA from 5’ end to 3’ end of mRNA.
Hence the codons are being read by tRNA, one by one, and amino acids are continuously attached to the growing end until there comes the stop sequence.
- When ribosomes come in front of the stop codon (UAG, UAA, or UGA) they are not recognized by tRNA.
- Hence, instead of tRNA certain release factors (RF1, RF2, etc.) get fitted in the P site and help to release the mRNA from the ribosome leading to the dismantling of ribosomes.
- The released chain of amino acid is the specific protein that was being synthesized.
What are gene mutations?
- Because of the complexity of DNA and the large number of cell divisions that are always taking place during the life span of an organism, there is a huge probability of genetic errors.
- Genetic error can start at the level of translation of a genetic message from DNA into RNA and if not corrected by certain correction points within the body, these errors can affect the whole genetic makeup of the organism by altering the genetic code
A mutation is an event that causes the interchange of one allele to another.
- Varying upon the level of change, it can either be so small that just affects one nucleotide pair or can be so large to affect hundreds of nucleotides.
Types of gene mutations
There are the following types of mutations:
- Silent mutations
- Missense mutations
- Non-sense mutations
- Frameshift mutations
1. Silent mutations
If there is the replacement of one base pair by another (e.g. AT with GC) such mutations are called point mutations.
Sometimes these alterations do not express themselves even at the level of protein synthesis because the codon may come out to be the same as was required for a specific amino acid, hence known as silent mutations.
2. Missense mutations
In the case where the mutation is not silent and a different codon arises as a result of altered base pair, it may add a different amino acid in the growing protein chain during translation resulting in a protein that may have an altered function or no function at all.
Such kind of mutations is called “missense” mutations.
3. Nonsense mutations
This is the most serious type of mutation.
In this type of mutations, a stop codon is added at a point where it is not needed and hence terminates the protein synthesis before completion. This premature protein certainly has no functionality.
4. Frameshift mutations
An important type of mutation in which protein function is completely lost is frameshift mutation.
- In frameshift mutations, either the DNA bases are added or deleted undesirably. If there is a missing pair or an extra pair of nucleotides in the reading frame of codons, it will cause the shifting of the reading frame by one base pair resulting in an alteration of all further codons.
- These kinds of mutations result in a protein with a functional part before the mutated pair of bases and the functionless tail part after the mutation.
The disease is known as “fragile-X syndrome” is caused by the repetition of CGG for about 1000 times in the gene located on the X chromosome.
Can mutations result in a tumor?
- If mutations occur in a specific type of genes known as proto-oncogenes, it can result in unstoppable cell divisions giving rise to the defective cells causing a tumor.
- These mutations can be repaired by some repairing enzymes but if they bypass the special checkpoints, they will be further manipulated.
Translation is the process by which the genetic message is decrypted and proteins are formed in the cytoplasm. It consists of initiation, elongation, and termination phases. Sometimes gene mutations can also occur and if bypass the checkpoints, they result in disease of offspring. Gene mutations have several types e.g. silent, missense, nonsense, and frameshift.
Frequently asked questions
1. What is gene expression?
By the process of inheritance, genetic information is transferred from parents to children by a series of events, finally resulting in the accumulation of favorable traits that are necessary to be evolved. Gene expression is the process by which the coded data of a gene is decoded in the form of amino acid sequences that result in the formation of specific proteins.
2. What is the difference between genes and chromosomes?
Gene is a basic hereditary unit that directs the formation of a whole organism.
There are two versions of each gene that are called alleles of that gene. These alleles can either be the same or different from each other and control the specific trait that will be transferred from parents to offspring.
If both alleles are different, the trait associated with the dominant allele will be expressed in the offspring masking the effect of the recessive allele. Chromosomes are threadlike structures present in the nucleus of an organismic cell.
Each chromosome has two arms known as sister chromatids, that are joined at the center called the centromere.
Each human cell contains 23 pairs of chromosomes that make a total of 46. Different species contain a different number of chromosomes. Genes are located at the chromosomes in the form of nucleotides
3. What is the possible length of DNA?
THE human DNA of one cell can be approximately 2 meter-long (6 feet) if it is uncoiled and placed end to end. Similarly, if the DNA of the whole human body is arranged in this way, it will reach about 67 billion miles which is about double the diameter of the solar system.
4. What are the steps of gene expression?
Gene expression takes place in two basic steps.
During transcription, the DNA acts as the original copy, and data is copied from that DNA in the form of mRNA by the action of RNA polymerase enzyme.
RNA polymerase acts to uncoil the helical shaped double-stranded DNA so that both the strands are exposed and available to copy.
Messenger RNA is then processed to acquire a full and final neat copy of RNA which is then transferred to the ribosome in the cytoplasm to start the process of translation.
The translation is the process by which the coded message carried by mRNA is decrypted and the information is expressed in the form of amino acids that are basic structural units of protein.
Inheritance is the process by which the offspring get the properties from their parents. The expression of these properties takes place via gene expression. Gene expression is the process of converting the genetic message into specific proteins by the involvement of DNA and RNA. Transcription and translation are the main events taking place in the process of gene expression. Transcription takes place in the nucleus and translation occurs in the cytoplasm.